1 Full Notes

1PHYS0608PHYS0608

Aleksandra Djurii, Room 3152859 7946

[email protected]

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Course OutlineCourse Outline Basic food molecules pH value Flavors Foams and bubbles Sauces Cakes, bread and cookies Culinary curiosities Kitchen Tools Cleaning

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Reference ListReference List9 P. Barham, The Science of Cooking, Springer, 20019 P. L. Couteur and J. Burreson, Napoleons Buttons, Jeremy P. Tarcher/ Penguin, 20049 B.G. Osborne , T. Fearn, Near Infrared Spectroscopy in Food Analysis, Longman

Scientific & Technical, 19889 H. McGee, The Curious Cook, Wiley Publishing, Inc. , 19909 A. Gardiner , S. Wilson, with the Exploratorium, The Inquisitive Cook, Henry Holt and

Company, 19989 R. L. Wolke, What Einstein Told His Cook, W. W. Norton & Company, 20029 R. L. Wolke, What Einstein Told His Cook 2, W. W. Norton & Company, 20029 T. Lister, Kitchen Chemistry, Royal Society of Chemistry, 20059 S. Perkowitz, Universal foam, Walker Publishing Company, Inc., 20009 T. P. Coultate, FoodThe chemistry of its components, Royal Society of Chemistry,

19899 D. Weaire and S. Hutzler, The physics of foams, Oxford University Press, 19999 L. L. Schramm, Emulsions, foams, and suspensions, WILEY-VCH Verlag GmbH & Co.

KGaA Weinheim, 20059 C. H. Snyder, The extraordinary chemistry of ordinary things, John Wiley & Sons, Inc,

19989 O. R. Fennema, Food chemistry, Marcel Dekker, Inc., 19969 H. McGee, Food and Cooking, Hodder and Stoughton Ltd, 20049 A. Vantal, Book of coffee, Hachette Livre, 19999 A. P. Robert, Book of tea, Hachette Livre, 1999

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Chapter 1Chapter 1Basic Food MoleculesBasic Food MoleculesWater, Carbohydrates,

Proteins & Fats

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smallest and simplest of the basic food molecules, just three atoms: H2O, two hydrogens and an oxygen

each water molecule is electrically unsymmetrical or polar (has a positive end and a negative end) because oxygen atom exerts a stronger pull than the

hydrogen atoms on the electrons they share hydrogen atoms project from one side of the

oxygen to form a kind of V shape: so theres an oxygen end and a hydrogen end to the water molecules, and the oxygen end is more negative than the hydrogen end

CarbohydratesWater FatsProteins

WaterWater

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WaterWater

the diagram shows the orientation of the 2 hydrogen atoms and the 2 other pairs of electrons around the central oxygen atom within a water molecule

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WaterWater

each gray spoke radiating out from the oxygen represents a pair of electrons, so oxygen has filled its outer shell by sharing electrons with the 2 hydrogens

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the significance of having 2 hydrogen atoms on one side of a water molecule is that the oxygen is able to pull the electrons shared with the hydrogen towards it

the result is an unequal sharing of the electrons this makes a water molecule polar, in that the

oxygen end of the molecule has more electrons (a negative charge), while the hydrogen end has a slightly more positive end (as the electrons are found there less frequently)


WaterWater

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the positive end is able to attract negative ions or the negative end of other polar molecules and vise versa

because water does this very well, it is able to dissolve many substances, and it is thus called a universal solvent[]


WaterWater

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the molecules in ice and liquid water are participating in 1-4 hydrogen bonds at any given moment

hydrogen bonds in liquid water are fleeting[], and are constantly being formed and broken since the motion of the molecules in the liquid is forceful enough to overcome the strength of hydrogen bonds and break them

the natural tendency of water molecules to form bonds with each other has a number of effects in life and in the kitchen


WaterWater

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water forms hydrogen bonds not only with itself, but with other substances that have at least some electrical polarity, some unevenness in the distribution of positive and negative electrical charges

major food molecules are much larger and more complex than water, for example, both carbohydrates and proteins

have polar regions


WaterWater

313

water molecules are attracted to these regions and cluster around them

when they do this, they effectively surround the larger molecules and separate them from each other

if they do this more or less completely, so that each molecule is mostly surrounded by a cloud of water molecules, then that substance has dissolved in the water


WaterWater

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under normal conditions, water exists in one of three states: solid, liquid, and gas

the solid state of water is commonly known as ice

pure water at sea level freezes at 0C liquid water is most often used in cooking,

and the temperature varies between 0C and 100C

water boils at 100 C and becomes steam (the gaseous state)

the transition between two states is called a phase transformation


Thermodynamics of Water Thermodynamics of Water Three States of WaterThree States of Water

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Heating curve of water

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if a cup of water is heated under atmospheric conditions, the heating curve was obtained as shown in previous slide

horizontal axis shows time (or more energy added), and the vertical axis is temperature

notice that the temperature of water stays constant with added energy during phase transformation

this is because all of the energy is used to transform one state to another that no energy is available for heating the water

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transitions between solid, liquid, and gaseous phases typically involve large amounts of energy compared to the specific heat

if heat were added at a constant rate to an ice to take it through its phase changes to liquid water and then to steam, the energies required to accomplish the phase changes (called the latent heat of fusion and latent heat of vaporization ) would lead to plateaus in the temperature vs time graph


FreezingFreezing

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boiling water is one of the most commonly used heat source in cooking

boiling water is undergoing liquid-to-gas transition, and because of this it stays at a constant temperature of approximately 100C

this provides a convenient standard for us to control the cooking process

as liquid water is heated, the molecules become increasingly mobile


BoilingBoiling

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some of the molecules acquire enough energy to escape as vapor

these molecules exert a force on the atmosphere, called the vapor pressure[]

the vapor pressure is opposed by another force(atmospheric pressure[]), created by a column of air pushing down on the pan

water begins to boil when the vapor pressure overcomes the atmospheric pressure


BoilingBoiling

Pressure and boiling

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this means that the majority of water molecules have become energetic enough to escape the surface

the temperature at which water boils is related to the vapor pressure required for boiling, which is equal to the atmospheric pressure

the implication of this is that as the atmospheric pressure changes, the boiling point of water changes as well


BoilingBoiling

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Interesting Questions in CookingInteresting Questions in Cooking

Why does water boil at a lower temperature at altitude?

Does food cook faster at a higher altitude?

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the temperature of boiling water at sea level is 100C

the atmosphere of air surrounding the earth creates a pressure against us and all objects on earth

as you increase altitude, the density of the air becomes thinner and exerts LESS pressure (the column of air pushing down is smaller)


Cooking at High AltitudesCooking at High Altitudes

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therefore, water boils at a lower temperature, and food takes longer to cook

for every 1000 ft. in altitude, the boiling point of water decreases by about 1 C

a clever appliance designed to take advantage of the pressure-boiling point relation is the pressure cooker

if less heat is required, then less temperature is required, then the water will boil at a lower temperature


Cooking at High AltitudesCooking at High Altitudes

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the boiling point of water can also be changed by adding impurities, such as salt or sugar, in the water

generally, impurities increase the boiling point of water because impurities dilute the concentration of water the number of molecules that can vaporize at

any give temperature decreases the result is that a higher temperature is

required to achieve the same vapor pressure

CarbohydratesWater FatsProtein

Impurities and Boiling PointImpurities and Boiling Point

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for example, concentrated sugar-water solutions that are used for making candies and caramel boil at temperatures exceeding 150 C


Impurities and Boiling PointImpurities and Boiling Point

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water freezes when the molecules have slowed down enough to develop bonds upon collision

the rate at which freezing occurs is governed by nucleation and growth

nucleation [] is the formation of small solids in a liquid

the clusters of solids are called the nuclei the rate at which new nuclei form (number

of nuclei per second) is the nucleation rate


FreezingFreezing

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once the nuclei have formed, they become the landing sites for other molecules to attach onto

The growth rate is the rate at which the radius of a nucleus grows after formation

the solidification rate is determined by the combination of nucleation and growth rates

the size of crystals formed during solidification is determined by the nucleation/growth processes

a solidification process with fast nucleation rate and/or slow growth rate will result in many small crystals forming


FreezingFreezing

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larger crystals form from slow nucleation rate most liquids decrease in volume upon

solidification; water has a rather unique property of expanding during liquid-to-solid transformation

this property comes from the hexagonal structure of ice crystals; water molecules form a hexagonal crystal structure, which actually takes up more volume than if the molecules were freely slipping past one another water expands when it freezes because of its

molecular structure, in tandem with the unusual elasticity of the hydrogen bond and the particular lowest energy hexagonal crystal conformation that it adopts under standard conditions


FreezingFreezing

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consequently, ice cubes float in water water has highest density at ~4C the freezing point of water at sea level is 0 C

this temperature can be changed, however, by adding impurities in water Sprinkling salt on road surfaces on an icy day

melts the ice by lowering the melting temperature


FreezingFreezing

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normally the solid phase of a given substance is denser than the liquid phase

as the molecules attraction for each other becomes stronger than their movements, the molecules settle into a compact arrangement determined by their geometry

in solid water, however, the molecular packing is dictated by the requirement for even distribution of hydrogen bonds


FreezingFreezing

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the result is a solid with more space between molecules than the liquid phase has, by a factor of about one-eleventh because water expends when it freezes that

water pipes burst when the heat fails in winter; that bottles of beer put in the freezer for a quick chill and then forgotten will pop open


FreezingFreezing

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due to the hydrogen bonding between water molecules, liquid water has a high specific heat

the amount of energy required to raise its temperature by a given amount i.e. water absorbs a lot of energy before its temperature rises

in the time that it takes to get an iron pan too hot to handle on the stove, water will have gotten only tepid[]


Liquid water is slow to heat upLiquid water is slow to heat up

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before the heat energy added to the water can cause its molecules to move faster and its temperature to rise, some of the energy must first break the hydrogen bonds so that the molecules are free to move faster

in the kitchen, it means that a covered pan of water will take more than twice as long as a pan of oil to heat up to a given temperature longer and conversely, it will hold that temperature longer after the heat is removed


Liquid water is slow to heat upLiquid water is slow to heat up

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as water starts to boil (approaching 100C in temperature), you can see little bubbles of vapor form on the bottom where the water is closest to the fire

these bubbles of gaseous water, or steam, rise to the surface and erupt into the air

more and larger bubbles form until the entire surface of the liquid is rolling

at this temperature, the water changes state and leaps from a liquid into a gaseous form

liquid water absorbs a lot of heat as it vaporizes into steam


BoilingBoiling

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hydrogen bonding gives water an unusually high latent heat of vaporization[], or the amount of energy that water absorbs without rise in temperature as it changes from a liquid to a gas

cooks take the advantage of it when bake delicate preparations like custards gently by

partly immersing the containers in an open water bath

oven-roast meats slowly at low temperatures simmer stock in an open pot

in these cases, vaporization removes energy from the food or its surroundings and causes it to cook more gently


BoilingBoiling

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Steam releases a lot of heat when it condenses into water

when water vapor hits a cool surface and condenses into liquid water, it gives up that same high heat of vaporization

this is why steam is such as effective and quick way of cooking foods compared with plain air at the same temperature

in bread baking, an initial blast of steam increases the doughs expansion, or oven spring, and produces a lighter loaf


CondensationCondensation

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if you leave water in a pot at room temperature it will gradually all vaporize into the air, leaving the pot empty and dry

this process is called evaporation it takes place at any temperature factors affecting the rate of evaporation:

temperature movement of air humidity of surrounding air surface area of the liquid


EvaporationEvaporation

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Carbohydrates[] have the general formula [CH2O]n

they function in short-term energy storage, such as sugar as intermediate-term energy storage, such as

starch for plants and glycogen[] for animals as structural components in cells, such as

cellulose[] in the cell walls of plants and many protists[], and chitin[] in the exoskeleton[] of insects and other arthropods[]


CarbohydratesCarbohydrates

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sugars are structurally the simplest carbohydrates.

they are the structural unit which makes up the other types of carbohydrates

Monosaccharides[] are single (mono=one) sugars

if several sugar rings are joined together, the resulting molecule is called an oligosaccharide[] (oligo means few in Greek)



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Common carbohydrates 42

Disaccharides[] are formed when two monosaccharides are chemically bonded together

examples are sucrose[]and lactose[]


DisaccharidesDisaccharides

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oligosccharides are carbohydrate molecules made up of fewer than ten monosaccharide units, and are still generally referred to as sugars rather than starches

the three- and four-unit oligosaccharides raffinose[] and stachyose[]are present in beans but are not digestible by humans


OligosaccharidesOligosaccharides

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polysaccharides are large molecules composed of individual monosaccharide units

many sugar molecules are joined together to form long strings, e.g. cellulose, amylose[]and amylopectin[]

a common plant polysaccharide is starch cellulose is the constituent of plant cell walls

that gives them stiffness and strength which very few organisms can break and digest

in contrast, starch forms a staple food of many plants and animals which possess enzymes that allow them to digest starch


PolysaccharidesPolysaccharides

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food sugars are made up of rings of 4 or 5 carbon atoms and one oxygen atom, with 1 or 2 more carbon atoms attached on the side of the ring

the single ring sugars will generally release more energy than the multiple ring sugars when burnt

there are different enzymes for the reduction of different sugars

most plants produce sucrose while most mammals tends to produce lactose


SugarsSugars

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Glucose[] is also called dextrose it is a simple sugar, and the most common

sugar from which living cells directly extract chemical energy

it is found in many fruits and in honey, but always in a mixture with other sugars

glucose is the major component of sucrose a chain of two glucoses is called maltose[]


GlucoseGlucose

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it is made up of a single ring and called monosaccharide it normally exists in a different form other than

straight chain formcyclic/ ring structures the ring of glucose consists of five carbon atoms

and one oxygen atom There are two versions of glucose-glucose

and -glucose for -glucose, the OH at carbon number 1 is below the ring for -glucose, if the OH at carbon number 1 is above the ring


GlucoseGlucose

The left one is -glucose while the right one is -glucose 48

cooks encounter it most often as the sweet substance in corn syrup, which is made by breaking starch down into individual glucose molecules and small glucose chains

compared to table sugar, or sucrose, glucose is less sweet, less soluble in water, and produces a thinner solution

it melts and begins to caramelize[]at around 150C


GlucoseGlucose

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Fructose[] is also called levulose it has the same formula as glucose C6H12O6 fructose and glucose are isomers

compounds that have the same chemical formula but different arrangements of these atoms

like glucose, fructose is found in fruits and honey, and certain corn syrups are treated with enzymes to convert their glucose into fructose

it is sold in pure crystalline form


Fructose (fruit sugar)Fructose (fruit sugar)

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it is the sweetest of the common sugars, the most soluble in water, and absorbs and retains water most effectively

our bodies metabolize fructose more slowly than glucose and sucrose, so it causes a slower rise in blood glucose levels, a quality that makes it preferable to other sugars for diabetics

it melts and begins to caramelize at a much lower temperature than the other sugars do, just above the boiling point of water at 105C



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fructose molecule exists in several different shapes when dissolved in water, and the different shapes have different effects on our sweet receptors the sweetest shape, a six-corner ring, predominates

in cold, somewhat acid solutions; in warm solutions, sweet five-corner ring dominates

it is a useful substitute for table sugar in cold drinks, where it can provide the same sweetness with half the concentration and a calories savings approaching 50%

in hot coffee, however, its sweetness drops to the level of table sugar



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it the scientific name for table sugar it is a composite molecule made of one

molecule each of glucose and fructose consist of two rings joined together and

called disaccharides green plants produce sucrose in the process

of photosynthesis, and we extract it from the stalks of sugar cane and the storage stems of sugar beets


SucroseSucrose

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It is composed of two simple monosaccharides it contains equal amounts of glucose and fructose

but not as a mixture of two different molecules The glucose and one fructose are joined together

through the removal of a molecule of water between the OH at carbon number 1 of -glucose and the OH on carbon number 2 of -glucose


SucroseSucrose

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SucroseSucrose

Removal of a molecule of H2O between glucose and fructose forms sucrose. The fructose molecule has been turned 180 and inverted in these diagrams.

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it is the second sweetest, after fructose, but is alone in having a pleasant taste even at the very high concentrations found in candies and preserves; other sugars can seem harsh

it is also the second most soluble sugartwo parts can dissolve in one part of room-temperature water and it produces the greatest viscosity, or thickness, in a water solution

sucrose begins to melt around 160C and caramelizes at around 170C


SucroseSucrose

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when a solution of sucrose is heated in the presence of some acid, it breaks apart into its two subsugars

breaking sucrose into glucose and fructose is often referred called invert sugar or invert syrup

Inversion refers to a difference in optical properties between sucrose and a mixture of its components parts

invert syrups are about 75% glucose and fructose, 25% sucrose


SucroseSucrose

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invert sugar only exists as a syrup, since the fructose component wont fully crystallize in the presence of glucose and sucrose

sucrose inversion and invert sugars are useful in candy making because they help in limit the extent of sucrose crystallization


SucroseSucrose

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lactose is the sugar found in milk it is a disaccharide formed from one unit of

glucose and one unit of another monosaccharide, galactose[] which is an isomer of glucose: the only difference is that in galactose the OH group at carbon number 4 is above the ring and not below the ring as it is in glucose


Lactose (milk sugar)Lactose (milk sugar)

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having an OH above or below the ring may seem like a very minor difference, but for those people who suffer from lactose intolerance, it is not

to digest lactose and other disaccharides or large sugars, we need specific enzymes called lactase that initially break down these complex molecules into simpler monosaccharides

insufficient lactase makes the digestion of milk and milk products difficult and causes the symptoms associated with lactose intolerance, e.g. diarrhea[]



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-galactose has C#4 OH above the ring while -glucose has C#4 OH is below the ring. These two molecules combine to form lactose.

Structure of the lactose molecule

Galactose on the left is joined through C#1 to C#4 of glucose on the right.

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a sweet taste indicates that fruit is ripe Sour taste tells us there are still lots of acids

present unripe fruit may cause a stomachache bitter taste in plants often indicates the

presence of a type of compound, known as an alkaloid, which is often poisonous

the relationship between chemical structure and sweetness is complicated


SweetnessSweetness

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one simple model, known as the A-H,B Model suggests that a sweet taste depends on an arrangement of a group of atoms within a molecule


SweetnessSweetness

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SweetnessSweetness

these atoms (A and B in the diagram) have a particular geometry, allowing atom B to be attracted to the hydrogen atom attached to atom A

this results in the short-term binding of the sweet molecule to a protein molecule of a taste receptor, causing a generation of a signal (transmitted through nerves) informing the brain, this is sweet

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there are many sweet compounds other than sugar, and not all of them are good to eat, such as ethylene glycol[]: it has a sweet taste but

it is very poisonous glycerol: it has a very similar structure to ethylene

glycol and also tastes sweet but it is safe to be consumed in moderate amount it is used as an additive in many prepared foods

because of its viscosity and high water solubility it occurs naturally in wine when you swirl a glass of wine, the legs that form on

the glass are due to the presence of glycerol increasing the viscosity and smoothness characteristic of good vintages


SweetnessSweetness

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the difference between cellulose and starch is in the geometry of the way they are joined: cellulose molecules are joined in a way that they

make stiff molecules that pack tightly together and are held in place by internal hydrogen bonds

the geometry of the bonds between the sugar rings in the starch molecules leads to a more open helical structure and fewer internal bonds


Starch and CelluloseStarch and Cellulose

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Starch and CelluloseStarch and Cellulose

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starch is formed by many plants in small granules

from a cooking point of view, the amount of protein and its location in the starch granules is crucial starch granules with a high protein content will

absorb a lot of moisture at room temperature while those with low protein contents absorb little water


StarchStarch

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starch consists of molecules of the complex carbohydrates amylose and amylopectinpacked into a starch granule

when you heat flour in liquid, the starch granules absorb water molecules, swell and soften when the temperature of the liquid reaches

approximately 60C the amylose and amylopectinmolecules inside the granules relax and unfold, breaking some of their internal bonds (bonds between atoms on the same molecule) and forming new bonds between atoms on different molecules


StarchStarch

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the result is a network that traps and holds water molecules

the starch granules then swell, thickening the liquid

if you continue to heat the liquid (or stir it too vigorously), the network will begin to break down, the liquid will leak out of the starch granules, and the sauce will separate


StarchStarch

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StarchStarch

Starch molecules inside

Protein molecules around the outside

Starch granuleStarch granule

Swollen granuleSwollen granule

Protein molecules absorb water and expand

Aggregated GranulesAggregated Granules

Wet protein molecules are sticky and hold granules together

Stretched aggregateStretched aggregate

Gluten sheets form where the proteins are stretched

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cellulose is a polymer of glucose it is a polysaccharide found in plant cell walls it forms the fibrous part of the plant cell wall in terms of human diets, cellulose is indigestible,

and thus forms an important, easily obtained part of dietary fiber

as compared to starch and glycogen, which are each made up of mixtures of - and -glucoses, cellulose (and the animal structural polysaccharide chitin) are made up of only -glucoses


CelluloseCellulose

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the three-dimensional structure of the structural polysaccharides is thus constrained into straight microfibrils by the uniform nature of the glucoses, which resist the actions of enzymes (such as amylase) that breakdown storage polysaccharides (such a starch)


CelluloseCellulose

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long cellulose chains pack tightly together, forming the rigid, insoluble fiber of which plant cell walls are constructed

cellulose chains lie side by side on bundles the shape of a linkage confers on the

structure allows the cellulose chains to pack closely enough to form these bundles which then twist together to form fibers visible to the naked eye


CelluloseCellulose

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on the outside of the bundles are the OH groups that have not taken part in the formation of the long cellulose chain, and these OH groups can attract water molecules, thus cellulose can take up water

human and all other mammals lack the digestive enzymes needed to break down linkages in these structural polysaccharides so we cannot use them as a food source


CelluloseCellulose

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but we do have a digestive enzyme that splits an linkage

the configuration is found in the storage polysaccharidesstarch and glycogen

storage polysaccharide in plant are amylose and amylopectin

storage polysaccharide in animals is glycogen formed mainly in the cells of the liver and skeletal muscle


CelluloseCellulose

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CelluloseCellulose

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Differences in branchingDifferences in branching

The diagram shows different branching in starch compared with glycogen. The greater the branching, the greater the number of chain ends for enzymes to break down the linkages and the faster glucose can be metabolized

Amylose

Amylopectin

Found in plants

Glycogen

Found in animals

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Cellulose FibersCellulose Fibers

Cellulose Fibers from Print Paper (SEM x1,080)

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in the kitchen, sugar is a versatile ingredient because sweetness is one of a small handful of basic taste sensations

sugar interferes usefully with the coagulation of proteins and so tenderizes the gluten[] network of custards and creams

if we heat sugar enough to break its molecules apart, it generates both appealing colors and an increasing complexity of flavor: no longer just sweetness, but acidity, bitterness, and a full, rich aroma


Cooking SugarCooking Sugar

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honey is largely invert sugar, a mixture of glucose and fructose

secondary sources include nectary[]elsewhere on the plant and honeydew, the secretions of a particular group of bugs

some nectars are mostly sucrose, some are evenly divided among sucrose, glucose and fructose

the most concentrated natural source of sweetness is honey

its major ingredient is sugars


HoneyHoney

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the stored food of certain species of bees which reaches 80% sugars

the principal raw material of honey is the nectar collection from flowers which produce it in order to attract insects and birds

the most important sources of nectar []are flowers of plants in the bean family especially clover[]

most honey is made from a mixture of nectars from different flowers


HoneyHoney

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in the hive[], the bees concentrate the nectar to the point that it will resist bacteria and molds and so keep until it is needed

house bees pump the nectar in and out of themselves for 15-20 minutes, repeatedly forming a thin droplet under their proboscises from which water can evaporate until the water content of the nectar has dropped to 50% or 40%

the bees then deposit the concentrated nectar in a thin film on the honeycomb which is a waxy network of hexagonal cylinders about 0.2 inch/ 5 mm across


HoneyHoney

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with the workers keeping the hive air in continuous motion by fanning their wings, the nectar loses more moisture until it is less than 20% water

this process is known as ripening[] it takes about three weeks the ripening of honey involves both

evaporation and the continuing work of evaporation and the continuing work of bee enzyme


HoneyHoney

84


HoneyHoney

Honeycomb, and the anatomy of the worker bee. Worker bees hold freshly gathered nectar in the honey sac, together with enzymes form various glands, until they return to the hive.

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with its syrup-like viscosity, glossiness, and range of brown shades, it makes an attractive topping for pastries and other foods

it can be substituted for sugar 1 measure of honey equivalent of 1.25-1.5

measures of sugar because it is more hygroscopic or water

attracting than table sugar, honey will keep breads and cakes moister than sugar and even absorbing it on humid days


Honey in cookingHoney in cooking

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due to its antioxidant[] phenolic[]compounds, it slows the development of stale flavors in baked goods and warmedover flavors in meats

bakers can use its acidity to react with baking soda and leaven quickbreads

its reactive reducing sugars accelerate desirable browning reactions and the development of flavor and color in the crusts of baked goods in marinades and glazes and other preparations



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honey has not been refined the way table sugar and is chemically complex

its vitamin content is negligible since the bees get most of theirs from the pollen

its antibacterial properties which led early physicians to use it to dress wounds, are due largely to hydrogen peroxide, one of the products of glucose-oxidizing enzyme



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honey should not be fed to children less than a year old since it often carries the seed-like dormant spores of the botulism bacterium which are able to germinate in immature digestive systems and infant botulism[ ] can cause difficulty in breathing and paralysis



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the cane stalks were first crushed and pressed, and the resulting juice was cleared of many organic impurities by heating it with lime and a substance such as egg white which would coagulate and trap the coarse impurities in a scum, that could be skimmed off

the remaining liquid was then boiled down in a series of shallow pans until it had lost nearly all of its water, and poured into cone-shaped clay molds

then it was cooled, stirred, and allowed to crystallize into raw sugar, a dense mass of sucrose crystals coated with a thin layer of syrup


Making sugarMaking sugar

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the clay cones were left to stand inverted for a few days, during which time the syrup film would run off through a small hole in the tip

in the final phase, a fine wet clay was packed over the wide end of the cone, and its moisture was allowed to percolate through the solid block of sugar crystals for eight to ten days

the resulting sugar was generally yellowish



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the sucrose is whitened by the technique of decolorization[], in which granular carbona material like activated charcoal []that can absorb undesirable molecules on its large surface areais added to the centrifuged, redissolved sugar

after it absorbs the last remaining impurities, the charcoal is filtered out

the final crystallization process is carefully controlled to give individual sugar crystals of uniform size

table sugar consists of purely 99.85% sucrose



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Making SugarMaking Sugar

Sugar caneSugar cane

1. wash, mill

Cane juiceCane juice

1. heat, clarify

2. heat under vacuum, evaporate water and concentrate

Dark brown syrupDark brown syrup1. crystallize

2. centrifuge

Raw sugarRaw sugar

1. wash

2. dissolve

3. clarify, decolorize

4. evaporate, crystallize

5. centrifuge

Refined sugarRefined sugar

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Under a microscope, you can see that sugar crystals arent cubes, exactly, but

oblong and slanted at both ends.


SugarSugar

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Caramelization[] is the chemical reactions that occur when any sugar is heated to the point that its molecules begin to break apart

this destruction triggers a remarkable cascade of chemical creation

the more the sugar is cooked, the less sugar and sweetness remain, and the darker and more bitter is gets


CaramelizationCaramelization

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sugar helps set preserves by interacting with pectin, a carbohydrate that forms an invisible network that sets jams and jellies

without sufficient sugar, pectin molecules would be more likely to bond with water than with each other

but sugars affinity for moisture takes some water out of circulation, leaving pectin[]molecules free to reach each other


JamJam

96

the successful setting of jams and jellies depends on a balance between pectin, acid, and sugar

if you want a low-calorie jam, try low-sugar pectin they gel differently as pectin combines with

calcium and very little sugar less sugar means jams and jellies spoil more easily,

so consider making your low-calorie jam the freezer variety, or process it in a hot water bath

the considerable amount of sugar in regular jams also acts as a preservative, to keep microorganisms from growing


JamJam

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Protein[] is a special class of polymers made up by joining together amino acids[]

there are more than 20 different amino acids and each amino acid is made up of about 20 atoms

possible to select from the almost infinite range of possible protein molecules ones that have particular shapes and perform specific tasks

ProteinsProteins


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the shape of the proteins gives them particular biological function, e.g. the protein haemoglobin is designed to

carry oxygen around in the blood the shapes of protein molecule is

determined both by the sequence of amino acids along its length and by internal bonds between different amino acids

there are several types of internal bonds that can form links between the amino acids in proteinsdisulphide bridge or hydrogen bond

ProteinsProteins


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ProteinsProteins


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ProteinsProteins


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the polypeptide backbone is folded into a spiral that is held in place by hydrogen bonds (black dots) between backbone oxygen atoms and hydrogen atoms

note that all the hydrogen bonds have the same polarity

the outer surface of the helix is covered by the side-chain R groups

Model of the Model of the --helixhelixCarbohydratesWater FatsProteins

102

Denaturation[] is the process that the internal bonds of the proteins are broken and thus their shape changes from that in the natural state

heat in cooking is a cause of denaturation all molecules vibrate all the time amplitude of these vibrations increases as

the temperature is increased if the vibrations are strong enough, the

molecules can literally shake itself free of its internal bondsuse this property of proteins to fight infection viruses are complex molecules which can be

very sensitive to heating

DenaturationDenaturation


18

103

most proteins are denatured at temperatures around 40C

when the proteins are heated to higher temperatures, they start to undergo chemical reactions that can cause them to break up or to join together into even larger molecules e.g. when cooking an egg, the egg proteins

denature once the temperature is above 40C and they start to react together to cook the egg once the temperature is above about 75C which makes the egg changes from liquid solution of proteins into solid mass

DenaturationDenaturation


104

the wheat kernel contains several proteins, including gliadin[] and glutenin[]

when you mix flour with water, gliadin and glutenin clump together in a sticky mass

kneading the dough relaxes the long gliadinand glutenin molecules, breaking internal bonds between individual atoms in each gliadin and glutenin molecule and allowing the molecules to unfold and form new bonds between atoms in different molecules

the result is a network structure made of a new gliadin-glutenin compound called gluten

GlutenGluten


105

gluten is very elastic the gluten network can stretch to

accommodate the gas (carbon dioxide) formed when you add yeast to bread dough or heat a cake batter made with baking powder or baking soda (sodium bicarbonate), trapping the gas and making the bread dough or cake batter rise

when you bake the dough or batter, the gluten network hardens and the bread or cake assumes its finished shape

GlutenGluten


106

GlutenGluten


gluten (gliadin + glutenin)

gliadin

glutenin

107

Effects on egg proteins by heatEffects on egg proteins by heat


c prepare a baked custard mixture, pour it into individual serving dishes, and place all except one in a shallow pan with hot water halfway up the sides of the custard on the oven rack beside the water bath, and bake all the custards the same length of time

d compare this custard to one cooked in the water bath

e when they are cooled, turn each onto a plate and examine it carefully

108

chances are the custard without the benefit of the water bath is full of little holes and oozing liquid

taste it and you will find the texture is quite rubbery that is because the egg proteins have

coagulated, then toughened, squeezing out liquid as theyre exposed to prolonged heat

the same phenomenon occurs when an egg is cooked in a sizzling frying pan



19

109

the white gets tough and rubbery and has little holes throughout where water has leaked from the coagulating white

even for frying eggs, gentle heat is preferable



110

the proteins of egg white are globular proteinstake shapes similar to the micelles that soap and detergents form in water as a result they move around easily in the blood

and other fluids, available to do their chemical work where they are needed

enzymes are examples of globular proteins the hydrogen bonds of egg whites proteins

hold the long protein molecules in their globular form, giving the white its characteristic consistency and transparency

Cooking eggCooking egg


111

heating an egg white, as we do when we cook the egg, overcomes the hydrogen bonds that hold the protein in its higher structures and allows the molecule to unravel it comes in contact with other protein molecules which have also unraveled from their globular shapes

it is a way to denature the native proteins of an egg white

the heat breaks the hydrogen bonds that hold its molecules in their native secondary and higher structures



112

the proteins then regroup on other ways that produce the texture and appearance of the cooked egg

another way to denature proteins is by stretching the molecules happens when liquid is accelerating, all the liquid

becomes stretched out, provided that the flow is fast enough, the proteins dissolved in it can be extended

in a static solution, the proteins are in the natural state and they are tight coils

in the flow, the proteins can stretch out and become long strings



113

fats and oils are members of a large chemical family called lipids

they are invaluable in the kitchen: they provide flavor and a pleasurable and persistent smoothness

they tenderize many foods by permeating and weakening their structure

they are cooking medium that allows us to heat food well above the boiling point of water, thus drying out the food surface to produce a crisp texture and rich flavor


Fats and OilsFats and Oils

114

since they dont mix with water, lipids are well suited to the job of forming boundariesmembranes between watery cells

fats and oils themselves are created and stored by animals and plants as a concentrated, compact form of chemical energy, packing twice the calories as the same weight of either sugar or starch



20

115

fats and oils are members of the same class of chemical compounds, the triglycerides[]

they differ from each only in their melting points: oils are liquid at room temperature, fats are solid

natural fats and oils are triglycerides, a combination of three fatty acid molecules with one molecule of glycerol[]



116

all fats produced by plants and animals are used to store energy

energy is released by a process called oxidation which is the reaction of fat with oxygen

this reaction generates a lot of heat fats consist a short strings of carbon atoms

with hydrogen atoms most fat encountered in cooking have three

strings all joined together at one end (about 10 to 20 carbon atoms in each string)


FatsFats

117

every molecule of fat incorporates their molecules of fatty acids

the fatty acids may be either saturated or unsaturated, and they thereby impart those qualities to the fat as a whole

fatty acids are generally bad-tasting and foul-smelling chemicals

they are tamed by being chemically fastened to a chemical called glycerol, in the ratio of three fatty acid molecules to each glycerol molecule


Fatty acidsFatty acids

118

three fatty acid molecules tied to a glycerol molecule constitute one molecule of fat

fatty acids are the acids that are found as components of fats

they are members of a larger family that chemists call carboxylic acids

they are very weak acids a fatty acid molecule consists of a ling chain

of as many as 16-18 carbon atoms, each one of which carries a pair of hydrogen atoms



119

if the chain contains its full complement of hydrogen atoms, the fatty acid is said to be saturated[]

but if somewhere along the chain one pair of hydrogen atoms is missing, the fatty acid is said to be monounsaturated

if two or more pairs of hydrogen atoms are missing, it is said to be polyunsaturated

some common fatty acids are stearic acid (saturated), oleric acid (monounsaturated) and linoleic and linolenic acids (polyunsaturated)



120

fats are distinguished by degree of saturation a saturated lipid is one whose carbon chain is

saturated with hydrogen atoms: there are no double bonds between carbon atoms, so each carbon within the chain is bonded to two hydrogen atoms

double bonds in unsaturated fats are easier sites for oxidation atmospheric oxygen can react with unsaturated

fats at quite low temperatures which recognizes as the fat going rancid

if a vegetable oil is left open at room temperature, it quickly oxidizes and becomes rancid


Saturated & Unsaturated FatsSaturated & Unsaturated Fats

21

121

why are saturated fats regarded as being less healthy than unsaturated fats? because saturated fats have higher melting

temperature if we should get a build up of saturated fats in our

arteries, the melting point of the particular fat should be close to or even higher than the body temperature

this is a real danger that the fat may solidify in an artery and cut of the flow of bloodleading to high blood pressure or even a stroke


Saturated & Unsaturated FatsSaturated & Unsaturated Fats

122

saturated fats have the maximum possible number of hydrogen atoms attached to the carbon atoms

each carbon atom in the chain is attached to two hydrogen atoms and two carbon atoms

carbon atoms are joined by single bonds


Saturated fatsSaturated fats

123

unsaturated fats have two or more of the carbon atoms joined together by a double bond and have only one hydrogen attached Mono-unsaturated fatstwo carbons are joined

by a double bond Poly-unsaturated fatsseveral pairs of carbons

are joined by double bonds


Unsaturated fatUnsaturated fat

124

omega-3 fatty acids are unsaturated fatty acids whose first double bond begins at the third carbon atom from the end

they are essential in our diet for proper function of the immune and cardiovascular systems

omega-3 is the chemists way of telling exactly how far the first missing pair of hydrogen atoms is from the end of the polyunsaturated molecule: it is three places from the end


OmegaOmega--3 Fatty Acids3 Fatty Acids

125

Omega-6 fatty acids are fatty where the term "omega-6" signifies that the first double bond in the carbon backbone of the fatty acid, counting from the end opposite the acid group, occurs in the sixth carbon-carbon bond

The biological effects of the omega-6 fatty acids are largely mediated by their interactions with the omega-3 fatty acids


OmegaOmega--6 Fatty Acids6 Fatty Acids

126

essential fatty acids (EFAs) are fatty acids that are required in the human diet

they cannot be synthesized by the body from other fatty acids

must be obtained from food there are two closely related families,

omega-3 and omega 6 in the body, essential fatty acids serve

multiple functions


Essential Fatty AcidsEssential Fatty Acids

22

127

in each of these, the balance between dietary omega-3 and omega-6 strongly affects function they are modified to make

the eicosanoids[] (affecting inflammation and many other cellular functions)

the endogenous[] cannabinoids[](affecting mood, behavior and inflammation)

the lipoxins from omega-6 EFAs and resolvins from omega-3 (in the presence of aspirin, downregulatinginflammation)

the isofurans, isoprostanes, epoxyeicosatrienoic acids (EETs) and neuroprotectin D

they form lipid rafts (affecting cellular signaling) they act on DNA (activating or inhibiting

transcription factors for NFB, a pro-inflammatory cytokine)



128

fatty acids are straight chain hydrocarbons possessing a carboxyl (COOH) group at one end (alpha) and a methyl group at the other end (omega)

in physiology, EFAs are named by the position of the first double bond from the omega end For example, the term omega-3 signifies that the

first double bond exists as the third carbon-carbon bond from the terminal CH3 end of the carbon chain



129

some of the food sources of omega-3 and omega-6 fatty acids are fish and shellfish, flaxseed, soya oil, canola oil, hemp oil, pumpkin seeds, sunflower seeds, leafy vegetables, and walnuts

essential fatty acids play a part in many metabolic processes, and there is evidence to suggest that low levels of essential fatty acids, or the wrong balance of types among the essential fatty acids, may be a factor in a number of illnesses



130

termed 'essential' because each was more or less able to meet the growth requirements of rats given fat-free diets

human metabolism requires both omega-3 and omega-6 fatty acids

any omega-3 and any omega-6 can relieve the worst symptoms of fatty acid deficiency

the essential fatty acids are: alpha-Linolenic acid (18:3) - omega-3 Linoleic acid (18:2) - omega-6


What is What is EssentialEssential??

131

these two fatty acids cannot be synthesisedby humans, as humans lack the desaturaseenzymes required for their production

omega-9 fatty acids are not essential in humans, because humans possess all the enzymes required for their synthesis

do not confuse EFAs with essential oils, which are 'essential' in the sense of being a concentrated essence


What is What is EssentialEssential??

132

essential in the sense of being indispensable means simply that the substance is the aromatic

essence they can be obtained in pure form by steam

distillation, or by extraction into cold fat, hot fat, or volatile organic solvents that can be evaporated away

many essential oils are terpenes, a class of unsaturated hydrocarbons e.g. menthol in oil of peppermint, limonene in

orange and lemon oil, and zingerone


Essential OilsEssential Oils

23

133

among food oils, olive oil is unique for being extracted not from a dry grain or nut, but from a fleshy fruit, and for carrying the prominent flavors of that fruit

quality is judged by its overall flavor and by its content of free fatty acids, or fatty carbon chains that should be bound up in intact oil molecules but instead are floating free


Olive OilsOlive Oils

134

There are five retail grades for olive oils[] : Extra-virgin olive oil comes from the first pressing of the olives,

contains no more than 0.8% acidity, and is judged to have a superior taste. There can be no refined oil in extra-virgin olive oil.

Virgin olive oil with an acidity less than 2%, and judged to have a good taste. There can be no refined oil in virgin olive oil.

Olive oil is a blend of virgin oil and refined virgin oil, containing at most 1% acidity. It commonly lacks a strong flavor.

Olive-pomace oil is a blend of refined pomace olive oil and possibly some virgin oil. It is fit for consumption, but it may not be called olive oil. Olive-pomace oil is rarely found in a grocery store; it is often used for certain kinds of cooking in restaurants.

Lampante oil is olive oil not used for consumption; lampantecomes from olive oil's ancient use as fuel in oil-burning lamps. Lampante oil is mostly used in the industrial market.


Olive OilsOlive Oils

1Chapter 2pH Values of

Common Foods

2

the acidity of fruits, vegetables, and other ingredients can be measured using a scale called pH

the pH of a solution is a measure of the molar concentration of hydrogen ions in the solution and as such is a measure of the acidity or basicity of the solution

the letters pH stand for "power of hydrogen" the pH of a solution is defined as the

negative logarithm of the hydrogen ion concentration expressed in mole per liter

pH scalepH scale

3

the logarithm of a number is the exponent, or power, to which 10 must be raised in order to obtain the number

each increment of 1 in pH signifies an increase or decrease in portion concentration by a factor of 10

so there are 1000 times the number of hydrogen ions in a solution of pH 5 as there are in a solution of pH 8

pH scalepH scale

4

the usual range of pH values encountered is between 0 and 14

the pH of neutral, pure water, with equal numbers of protons and OH ions, is set at 7

a pH lower than 7 indicates a greater concentration of protons and so an acidic solution, while a pH above 7 indicates a greater prevalence of protons-accepting groups, and so a basic solution

pH scalepH scale

5

acidic fruits and vegetables, such as lemons and tomatoes, have a pH of less than 7

foods like egg white and baking soda, with a pH greater than 7, are bases, or alkaline

pH scalepH scale

6

27

note that most foods you eat are acidic even bland vegetables, such as potatoes,

are slightly on the acidic side it is a hidden partner in cookingcapable

of exerting a strong influence on color, texture and flavor

larger concentrations are described by smaller negative exponents, so a more acidic solution will have a pH lower than 7, and a less acidic, more basic solution will have a pH higher than 7

pH scalepH scale

8

pure water may contain other combinations of oxygen and hydrogen

water tends to dissociate to a slight extent, with a hydrogen occasionally breaking off from one molecule and rebonding to a nearby interact water molecule

this leaves one negatively charged OH combination, and a positively charged H3O

pH scalepH scale

9

under normal conditions, a very small number of molecules exist in the dissociated state

the number is small but is significant because the presence of relatively mobile hydrogen ions, which are the basic units of positive charge, can have drastic effects on other molecules in solution

humans have a specialized taste sensation to estimate proton concentrationsourness

pH scalepH scale

10

the term used for the class of chemical compounds that release protons into solutions, acids, derives from the Latin, meaning to taste sour

the complementary chemical group that accepts protons and neutralizes them are called bases or alkalis

the standard measure of proton activity in solutions is pH

pH scalepH scale

11

water solutions of all acids taste sour turn litmus[] red react with certain metals to liberate hydrogen gas

water solutions of all bases taste bitter turn litmus blue feel slippery

Acids and basesAcids and bases

12

is an organic acid with antioxidant[]properties

the L-enantiomer of ascorbic acid is commonly known as vitamin C.

the word vitamin comes from a contraction of two wordsvital (necessary) and amine (a nitrogen-containing organic compound)

the C in vitamin C indicates that it was the third vitamin ever identified

Vitamin CVitamin C

Structure of vitamin C

313

the RDA (recommended daily allowance) of vitamin C for an adult is generally given as sixty mg per day, about that found in a small orange

an excess vitamin C is eliminated through the kidneys

swallowing a seventy-milligram ascorbic acid pill may not produce quite the same benefits as the seventy milligrams of vitamin C obtained from eating an average-sized orange

Vitamin CVitamin C

14

the main commercial use of vitamin C today is as a food preservative

ascorbic acid can be used as antioxidant: used in home canning of fruit to

prevent browning Antimicrobial[] agent: the acidity is

increased and can protect against botulism which is the name given to the food poisoning resulting from the toxin produced by the microbe

Vitamin CVitamin C

15

fruits are unique in the way that they progress from inedibility to deliciousness

there are two different styles of ripening among fruits: dramatic undramatic

FruitsFruits

16

Dramatic: when triggered by ethylene, the fruit stimulates

itself by producing more ethylene[], and begins to respireto use up oxygen and produce carbon dioxide

its flavor, texture, and color change rapidly, and afterwards they often decline rapidly as well

such fruits can be harvested while mature but still green, and will ripen well on their own

e.g. bananas, pears and tomatoes

FruitsFruits

17

Undramatic: Nonclimacteric fruits dont respond to ethylene

with their own escalating ethylene production they ripen gradually and usually dont store sugar

as starch once harvested, they get no sweeter, though

other enzyme actions may continue to soften cell walls and generate aroma molecules

e.g. pineapples, citrus fruits and melons

FruitsFruits

18

FruitsFruits

AppleBananaMangoPapayaPeach PearTomatokiwifruit

CherryCitrus fruits (orange, lemon, lime, grapefruit)CucumberGrapePineappleSoft berries (blackberry, raspberry, strawberry)Watermelon

Fruits continue to ripen after picking (climacteric)

Fruits dont ripen after picking (nonclimacteric)

419

these basic styles of ripening determine how fruits are handled in the kitchen

using a process called chromatography[], scientists can determine what chemical compounds a food contains, producing a display called a chromatogram

in chromatograms, showing the flavor-producing compounds of a tree-ripened peach and an artificially ripened one, the difference are striking

FruitsFruits

20

the tree-ripened fruit has about twice as many different volatile flavor-producing compoundsand much, much more of each compound

Each spike in these chromatograms represents a different flavor compound

Tree-ripened peach Artificially ripened peach

FruitsFruits

An artificially ripened peach has fewer flavor compounds and less of each compound than a tree-ripened peach

21

800.216Grape

280.514Cherry1000.110Pear230.49Peach51.79Apricot

570.317Banana121.113Pineapple91.211Orange

33.210Black Currant

180.47Guava

Ratio of sugar content to acid content

Acid Content(% of fresh weight)

Sugar Content(% of fresh weight)

FruitsFruits

Sugar and Acid Content of Fruits 22

as fruits ripen, their sugars accumulate, aromatic compounds form, and the pectin that cements their cells together changes to a more soluble form, so their texture softens

fresh fruits do not generally keep well as the ripening process continues in the

picked fruits, texture becomes more mushy and flavor deteriorates

refrigeration is the key to keep fresh fruits longer as the enzymes that cause ripening work more slowly in the cold

FruitsFruits

23

some produce of tropical origin, such as cucumbers, tomatoes, eggplants, bananas, snap beans, peppers, winter squash, and sweet potatoes, however, is subject to injury if its stored at temperatures below 10C, thus they are best stored at cool, but not cold, temperatures

FruitsFruits

24

many fruits owe their characteristic aroma to chemicals called esters

an ester molecule is a combination of two other molecules, an acid and an alcohol

EstersEsters

525

a typical plant cell contains many different kinds of acids, and several different kinds of alcohol

the alcohols are usually by-products of cell metabolism

fruits have enzymes that join these basic cell materials into aromatic esters

a single fruit will emit many esters, but one or two account for most of its characteristic aroma

EstersEsters

26

chlorophyll has a porphyrin ring system with an Mg2+ ion in its centre

during cooking, this magnesium ion can be replaced by two H+ ions to give a compound called phenophytin

this is brown and is responsible for the color of overcooked vegetables

Color change of vegetables when cookingColor change of vegetables when cooking

chlorophyll molecule

27

the replacement of Mg2+ by two H+ ions takes place most readily in acidic conditions and this is the reason why some cooks add sodium hydrogencarbonate (bicarbonate of soda) when cooking vegetables

sodium hydrogencarbonate, NaHCO3, is the salt of a strong alkali and a weak acid and is therefore alkaline in solution because of its interaction with water

this keeps the cooking water alkaline and minimizes replacement of magnesium ions thus maintaining the green color


28

unfortunately, alkalis catalyze the oxidation of vitamin C (ascorbic acid) to dehydroascorbic acid

so addition of sodium hydrogencarbonate is not ideal as it accelerates the loss of this vitamin

factors that do affect the color of green vegetables during cooking are the acidity or alkalinity of the water the hardness of the water


29

the longer a green vegetable cooks, the more the nature of its chlorophyll changes

cooking green vegetables as quickly as possible lessens the damage

using tin or iron cooking pans also changes the color of chlorophyll to a dull brown as these metals alter chlorophylls molecular configuration


30

chlorophyll is equally sensitive to acids not only the acids that you add in cooking

like lemon juice, but also the plant acids within the vegetables themselves that escape as the tissues soften in cooking

leaving the lid ajar on the saucepan for the first few minutes allows some of the volatile acids to escape, diminishing their effect on the chlorophyll

keeping cooking to a minimum also limits chlorophylls exposure to acids


631

if lemon juice is squeezed onto the spinach[] earlier before serving, the acids have time to react with the chlorophyll, turning the spinach an ugly olive color

not all natures colors are as sensitive as chlorophyll

Carotenoids[], which are responsible for the yellows, oranges, and red-oranges in fruits and vegetables, are very sturdy pigments

the colors of carrots, corn, squash, sweet potatoes, and red peppers change only slightly whether you braise with moisture or bake them in dry heat


32

most cut or crushed fruits and vegetables are discolored by some of their enzymes, which react with the scrambled contents of damaged cells to produce a brownish pigment e.g. avocado and the basil leaf

the torn or cut edges develop a limp margin of darker green

Green and BrownGreen and Brown

33

raw fruits and vegetables turn brown sometimes after you peel and slice them

it happens when compounds called phenols react with oxygen in the presence of plant enzymes to form brown pigments or melanin

it contributes to the natural coloring of raisins, prunes, dates, and figs


34

no browning takes place when susceptible produce is left whole, because enzymes and phenolic compounds are segregated by their cell structures and protected from the air by skin

it isnt until you cut or bite or peel a piece of fruit that the cell walls are damaged, throwing together the compounds responsible for browning


35

browning isnt harmful, but you can control it in several simple ways: refrigerating the cut fruits: it helps prevent

browning because enzymes work more slowly at cool temperatures

using ascorbic acid: it acts as an antioxidant, combines with oxygen before the air has a chance to reach the phenols responsible for browning

using a a sprinkling of lemon juice for small amounts of fruit

Ways to Prevent BrowningWays to Prevent Browning

36

pectins, which are polysaccharides, are present in vegetables, and form water-retaining gels that help to give vegetables their structure

during cooking, pectins become soluble and are extracted into the cooking water making the vegetable go mushy

A section of a pectin molecule

Texture change of Vegetables When CookingTexture change of Vegetables When Cooking

737

calcium ions, Ca2+, found in hard water, can form cross links between pectin molecules, making them less soluble and keeping the vegetable tough

In some countries, the water is relatively hard so: people cook vegetables in bottled water to

reduce this effect and shorten cooking times one of the taps is fitted with a water softener to

reduce the level of Ca2+ ions in the water the calcium ion content of water can affect

the color of cooked vegetables as well as their texture, but indirectly, by its effect on pectin molecules


38

since most vegetables require some softening during cooking, cooking in hard water means that longer is needed to achieve the optimum softening

during this longer cooking time, more chlorophyll is converted to phenophytin and the color of green vegetables becomes browner


39

not surprisingly, lid on or lid off makes no difference

the lid raises the pressure inside the pan and thus increases the boiling point of the water

unless the pan is sealed, there should be no pressure increase and thus no effect on the boiling point

Lid on or lid off?Lid on or lid off?

40

salts in its mineral form is known as halite, formed from the elements sodium and chlorine

salts can be used to add flavor to food to preserve food, such as curing meats

and pickling salty surroundings discourage the growth of

microorganisms responsible for food spoilage and food poisoning as they lose water by osmosis in a salty environment

SaltsSalts

41

For cooks, they think adding salts when cooking beans can: keep the beans green prevent the beans going soggy improve the flavor raise the boiling point of water so the beans cook

faster

Add salts when cooking beansAdd salts when cooking beans

42

For scientists, they think it seems to be no good reason because: only the acidity and calcium content of the

water affect the color of the beans adding the amount of salt used by cooks does

increase the boiling point of water but only by about 0.1 C not enough to make any detectable difference to the speed of cooking

vegetables will go soggy if cooked for too long whether salt is added or not

very little salt is actually absorbed onto the surface of a bean during cookingtypically 1/10000g of salt per bean which is too little to be tasted by most people

Add salts when cooking beansAdd salts when cooking beans

843

sauting softens onions so they meld smoothly into a soup or a sauce

this is particularly important if the next step is to combine them with acidic ingredients, like tomatoes or wine

if onions are not softened first, acids in the other ingredients will keep them in firm and distinct pieces

if you are including onions in a dish such as scalloped potatoes, sauting the onion lets some of their acids evaporate so there are fewer available to curdle the milk in the sauce

SautSaut Onions SeparatelyOnions Separately

1Chapter 3Flavors

2

through chemical reactions, you intensify the flavor of foods by producing volatile compounds that are not present in the uncooked foods

chemical reactions that cause food to brown is known as browning reactions

browning reactions are responsible for the distinctive flavors of roasted coffee beans, maple syrup, toasted bread, nuts, coconut, dry roasted spices, etc.

Creating Flavor By CookingCreating Flavor By Cooking

3

the temperature at which food cooks also affects its flavor

different flavors develop in baking, roasting, frying, or boiling where parts of the food reach appreciably higher temperatures

even when cooking has finished, the temperature to which the food has been raised, its degree of doneness, and amount of waiting time all affect the flavors and aromas that develop

Creating Flavor By CookingCreating Flavor By Cooking

4

we can taste molecules of many sizes however, we can only smell

comparatively small molecules that are swept up in the air into our noses

acids will always taste sour, no matter what their size

the overall flavor of a dish comes from the combination of both taste and smell sensations so is determined by a whole range of molecules

Flavor Comes From Where?Flavor Comes From Where?

5

fruits all have characteristic flavors that are carried by small molecules

many plants want to advertise that they have fruits available

animals then eat the fruits and distribute the seeds as they pass through the digestive systems unharmed

so the need for plenty of flavor to encourage consumption of the fruits is a good evolutionary trend

Flavor Come From Where?Flavor Come From Where?

6

molecules responsible for flavor in vegetables are normally trapped inside the cell walls

during cooking, the cell walls are damaged for two reasons: chemical damage occurs as the cell walls, which

are made of cellulose, break down physical damage occurs as water inside the cells

boils forming steam, and the cell walls break

The Chemistry of FlavorThe Chemistry of Flavor

cell wall

27

cell wall damage results in the loss of flavor moleculesovercooking

water-soluble flavor molecules are lost in the cooking water

for other vegetables, such as broccoli[] or green beans, the flavor molecules are more soluble in oil than in water therefore it makes sense to cook these

vegetables in water rather than oil to retain most flavor in the vegetable

The The ChemsitryChemsitry of Flavorof Flavor

8

it is rare for a single substance to be responsible for the flavor of a particular foodstuff

it is also true that other senses as well as taste and smell may be involved

volatile molecules in food can be separated by gas chromatography[](GC) and then identified by mass spectrometry[] (MS), the combined technique being called GCMS


9

in GC, components are separated as they are carried through a column of absorbent material by a flow of an inert gas

different substances pass through the column at different rates and emerge from it at different times called their retention times

the mass spectrometer is then used identify them the mass spectrometer does this by breaking each

molecule into ionized fragments and detecting these fragments using their charge to mass ratio

each molecule has a specific fragment spectrum which allows for its detection


10

one current area of interest is why certain flavors seem to go well together (like fish and chips or strawberries and cream)

one suggestion is that this occurs when both of the components have a number of important aroma molecules in common


11

reasons for cooking food: kill bacteria and other microorganisms in or on

the food to make it safe to eat improve the texture, e.g. softening tough meat improve the color, e.g. browning of meat or toast improving the flavor and aroma, e.g. developing

the flavor and aroma of cooked meat

Chemical reactions in cookingChemical reactions in cooking

12

there are many important chemical reactions, which help to develop flavor during cooking

the first group contains the enzymatic reactions these are natural chemical reactions that affect the food there are many different enzymes that are present in

different foods all foods contain enzymes which control biochemical

reactions essential for the life of the organism the reactions can continue once the

organisms are being used as food e.g. ripening of fruit, setting of cheese, and breaking

down of proteins in the ageing of meat


313

the second group of reactions are those which affect sugars and carbohydrates when they are heated, many disaccharide and oligosaccharide sugars

will undergo a process known as hydrolysis when heated with some water

the water reacts with the oxygen atom joining the sugar rings and breaks the conversion of sucrose to a mixture of fructose and glucose which occurs during the preparation of boiled sweets

if sugars are heated further, then additional reactions take place and the rings will open up to form new moleculesthese reactions are generally called degradation reactions

degraded sugars form acids


14

if the temperature is increased to a sufficient temperature that sugars melt, then more complex reactions occur which start to oxidize the sugar, these are called caramelisation[] reactions

caramelisation is the name for the browning that takes place when sugar is heated this is how toffee is made

caramelisation begins with the conversion of sucrose to glucose and fructose

CaramelisationCaramelisation

15

the ring structures of these smaller sugars are then broken open through degradation reactions and these smaller molecules recombine to form chain-like molecules

as the complex reactions progress, the color of the system changes from a clear liquid through yellow to dark brown

during caramelisation, a whole range of new small flavor molecules are formed

many of these molecules have been identified as a range of organic acids that are formed along with the brown colored polymers


16

Sugar before (left) and during (right) caramelisation in a pan


17

as the reaction proceeds, so the new molecules that form tend to be more like the alkaloids [] and have increasingly bitter tastes

sugars, with general formula Cn(H2O)n, are decomposed into carbon and water (in the form of steam) Cn(H2O)n nC + n(H2O)


18

reasons for cooking meat: to produce browning on the outside

this improves the appearance and also produces flavor and aroma molecules

at temperatures of above 140 C a group of chemical reactions called Maillard reactions occurs

these take place between carbohydrates and molecules with NH2 groups

they make the meat brown and also volatile flavor molecules that give the aroma and taste of roast meat

Cooking MeatCooking Meat

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to decompose molecules of collagen that form the connective tissue of meat connective tissue is the thin translucent film that

separates layers of muscle it holds muscles together and attaches muscle

to bone too much of it makes the meat tough collagen is a protein in the form of a triple spiral at temperatures above 60C the spiral begins to

unwind and the collagen softens, eventually turning into gelatine a soft material that is a constituent of jellies


20

other undesirable changes may take place during the cooking of meat, e.g. other protein molecules begin to denature at about 40C and cause the meat to harden

meat with a high collagen content (large amounts of connective tissue) will have to be cooked at above 60C to break down the collagen, while meat with little connective tissue would be better cooked at 50C or less to prevent hardening


21

the Maillard reactions do not take place below 140C, which would be far too high a temperature for cooking

one way round this is for cooks to use a blow torch to heat the surface of the meat to over 140C for a few moments enough for Maillard browning to occur on the surface but not long enough for this temperature to occur in the bulk of the meat


22

the size of the piece of meat is also important

meat is a relatively poor conductor of heat so the temperature inside the meat will lag behind that of the surface

this is obvious when you carve a joint that has been roasted in the oven

the centre of the joint could be relatively raw and red while the surface is brown


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they occur between sugars and amino acids the amino acids can come from any proteins

and the sugars from any carbohydrates in the first stage of the reactions, the proteins

and the sugar from any carbohydrates are degraded into smaller sugars and amino acids

then the sugar rings open, the resulting aldehydes and acids react with the amino acids to produce a wide range of chemicals

MaillardMaillard reactionsreactions

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the reactions occur between the carbonyl group of a sugar molecule (in the chain form) and an NH2 group to eliminate a molecule of water

the NH2 group may be part of an amino acid molecule (that was originally part of a protein molecule) or an amino acid that is still part of a protein chain in the latter case, it must be an amino acid that

has an NH2 group as part of its side chain this side chain is different for each amino acid


525

Uncooked meat The Maillard reactions are evident in the appearance of the meat after cooking

products of the Maillard reactions include polymers that are responsible for the brown color of roast meat and small molecules such as maltol that are responsible for aromas


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enzymic browning is responsible for the browning of fruit such as apples and bananas once they have been cut or bruised

it is not considered to improve the food because the browned fruit is thought to be unattractive in appearance and the reaction does not result in any aroma molecules

this browning is caused by the oxidation of compounds related to phenols[](hydroxybenzenes) that are found inside plant cells

EnzymicEnzymic BrowningBrowning

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the oxidation occurs when the cells are damaged by cutting or bruising and become exposed to oxygen in the air and enzymes that catalyse the oxidation

the product molecule then polymerises to form the brown pigment

EnzymicEnzymic BrowningBrowning

apples before (left) and during (right) enzymic browning

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primary fermentation products of heterofermentative[] lactic acid bacteria, such as Leuconostoccitrovorum[], and the combination of acetic acid[], diacetyl[] and acetaldehyde[]provides much of the characteristic aroma of cultured butter and buttermilk

Homofermentative[] lactic acid bacteria produce only lactic acid, acetaldehyde, and enthanol in milk cultures

Lactic AcidLactic Acid--Ethanol FermentationsEthanol Fermentations

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acetaldehyde is the character-impact compound found in yogurt, a product prepared by a homofermentative process

Diacetyl[] is the character-impact compound in most mixed-strain lactic fermentations, and has become universally known as a diary or butter-type flavorant

lactic acid contributes sourness to cultured or fermented diary products

Acetoin[], although essentially odorless, can undergo oxidation to diacetyl


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lactic acid[]bacteria produce very little ethanol, and they use pyruvate[]as the final H receptor in metabolism

yeast produce ethanol as a major end product of metabolism

Malty[] strains of Streptococcus lactis[] and all brewers yeast also actively convert amino acids to volatile compounds through transaminations[ ] and decarboxylation[]

these organisms tend to produce mainly the reduced forms of the derivatives, such as alcohols, although some oxidized compounds, such as aldehydes[], and acids also appear


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wine and beer flavors, which can be ascribed directly to fermentations, involve complex mixtures of these volatiles and interaction products of these compounds with ethanol, such as mixed esters and acetals

these mixtures give r

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