Food chemistryFood chemistry

Prof. Dr . Fahim Shaltout Prof. Dr . Fahim Shaltout

Fats and other lipidsFats and other lipids

Lipids are one of large groups of organic Lipids are one of large groups of organic compounds which are of great importance compounds which are of great importance in the food . in the food .

Occurrence in foodsOccurrence in foods

The American diet unusually rich in fats The American diet unusually rich in fats and other lipidsand other lipids

Edible fats and oilsEdible fats and oils

Prepared edible fats and oils which are Prepared edible fats and oils which are sold in pure state sold in pure state

Fatty acidsFatty acids

Natural fats are mixed glycosides in which Natural fats are mixed glycosides in which the three fatty acids the three fatty acids

Identification of natural fats and oilsIdentification of natural fats and oils

1- physical properties1- physical properties 2- chemical properties2- chemical properties

11 - -physical propertiesphysical properties

A- melting point of fat A- melting point of fat

B- Softening point of fatB- Softening point of fat

Capillary tubes are filled with oil and packed in Capillary tubes are filled with oil and packed in ice over night so that the oil can be solidify and ice over night so that the oil can be solidify and come to equilibrium . come to equilibrium .

The capillary tubes are clamped to a The capillary tubes are clamped to a thermometer and submerged in a beaker of thermometer and submerged in a beaker of water .water .

The temperature is slowly raised and the The temperature is slowly raised and the temperature at which the column of fat rises in temperature at which the column of fat rises in the capillary tube is called the softening point the capillary tube is called the softening point

C – Slipping pointC – Slipping point

Small brass cylinders filled with the solid Small brass cylinders filled with the solid fat are suspended in a bath close to the fat are suspended in a bath close to the thermometer . As the bath is stirred the thermometer . As the bath is stirred the temperature is slowly raised . The point at temperature is slowly raised . The point at which the fat rises in the cylinder or slips is which the fat rises in the cylinder or slips is recorded as slip point.recorded as slip point.

D – Shot melting pointD – Shot melting point

Is the temperature at which a small lead Is the temperature at which a small lead shot will fall through a sample shot will fall through a sample

E – Specific gravityE – Specific gravity

The specific gravity of fat or oils usually The specific gravity of fat or oils usually measured at 25 C. But it may be measured at 25 C. But it may be necessary to use temperature of 40 C or necessary to use temperature of 40 C or even 60 C for high melting fats .even 60 C for high melting fats .

F – Refractive indexF – Refractive index

Is the degree of deflection of a beam of Is the degree of deflection of a beam of light that occurs when it passes from one light that occurs when it passes from one transparent medium to another.transparent medium to another.

Abbe Refractometer at 25 C. with high Abbe Refractometer at 25 C. with high melting point fats 40 C or even 60 C.melting point fats 40 C or even 60 C.

G – Smoke , Flash and Fire points G – Smoke , Flash and Fire points

Smoke point: is the temperature at which a Smoke point: is the temperature at which a fat or oil gives off a thin bluish smoke .fat or oil gives off a thin bluish smoke .

Flash point : is the temperature the Flash point : is the temperature the mixtures of the mixtures of vapors with air mixtures of the mixtures of vapors with air will ignite .will ignite .

Fire point: is the temperature at which the Fire point: is the temperature at which the substance will sustain continued substance will sustain continued combustion combustion

H – Turbidity point H – Turbidity point

The turbidity point of an oil determined by The turbidity point of an oil determined by a mixture of it and a solvent in which it a a mixture of it and a solvent in which it a limited solubility . The mixture is warmed limited solubility . The mixture is warmed until complete solution occurs. And then until complete solution occurs. And then slowly cooled until the oil begins to slowly cooled until the oil begins to separate and turbidity occurs. The separate and turbidity occurs. The temperature at which turbidity first temperature at which turbidity first detectable is known as turbidity point.detectable is known as turbidity point.

Chemical Properties Chemical Properties A number of chemical tests have been A number of chemical tests have been evolved during the years of study of evolved during the years of study of oils and fats which are based on the oils and fats which are based on the partial determination of the chemical partial determination of the chemical composition of the oil or fat . These composition of the oil or fat . These tests serve both to identify the fat and tests serve both to identify the fat and to detect the presence of adulteration .to detect the presence of adulteration .

Reichert Meissl NumberReichert Meissl Number

Is the number of milliliters of 0.1 N alkali Is the number of milliliters of 0.1 N alkali ( such as potassium hydroxide )required to ( such as potassium hydroxide )required to neutralize the volatile water -soluble fatty neutralize the volatile water -soluble fatty acids in 5 g. sample of fat. The Reichert acids in 5 g. sample of fat. The Reichert Meissl test determines the amount of Meissl test determines the amount of butyric and caproic acids which are butyric and caproic acids which are readily soluble in water and the caprylic readily soluble in water and the caprylic and capric acids which are slightly solubleand capric acids which are slightly soluble..

Polenske numberPolenske number

Is the number of milliters of 0.1 N alkali Is the number of milliters of 0.1 N alkali necessary to neutralize the volatile ,water –necessary to neutralize the volatile ,water –insoluble fatty acids which are present in 5 g. insoluble fatty acids which are present in 5 g.

samplesample. .

The Saponification numberThe Saponification number

is defined as the number of milligrams of is defined as the number of milligrams of potassium hydroxide required to saponifypotassium hydroxide required to saponify g. of fat or oil.g. of fat or oil. 1 1

iodine numberiodine number

is the number of grams of iodine or iodine is the number of grams of iodine or iodine compounds absorbed by 100 g. of fatcompounds absorbed by 100 g. of fat..

The Acetyl ValueThe Acetyl Value

is a measure of the amount of hydroxyl fatty is a measure of the amount of hydroxyl fatty acids present in a fatacids present in a fat. .

PHYSICAL AND CHEMICAL PROPERTIESPHYSICAL AND CHEMICAL PROPERTIES

By the use of the physical constants and the By the use of the physical constants and the chemical methods it is possible to chemical methods it is possible to differentiate and identify natural oils and differentiate and identify natural oils and

fatsfats. .

Flavor changes in fats and oilsFlavor changes in fats and oils

When fats and oils are stored they undergo When fats and oils are stored they undergo flavor changes which markedly influence flavor changes which markedly influence their market valuetheir market value..

RancidityRancidity Fats and oils slowly take up oxygen for a period of time before it is Fats and oils slowly take up oxygen for a period of time before it is

possible to detect the flavor of the products of rancidity . This period possible to detect the flavor of the products of rancidity . This period is called the induction period , and it is followed by a second period is called the induction period , and it is followed by a second period in which the uptake is much more rapidin which the uptake is much more rapid. .

Temperature ,moisture ,the amount of air in contact with the Temperature ,moisture ,the amount of air in contact with the fat ,light ,and presence or absence of antioxidants influence the fat ,light ,and presence or absence of antioxidants influence the reactionreaction. .

Long ago farm women learned to store fat in crocks with as small a Long ago farm women learned to store fat in crocks with as small a surface exposed to the air as possiblesurface exposed to the air as possible. .

Vegetable fats especially those from seeds show a marked resistance Vegetable fats especially those from seeds show a marked resistance to the onset of rancidityto the onset of rancidity. .

The uptake of oxygen and the onset of rancidity seems to be related to The uptake of oxygen and the onset of rancidity seems to be related to the unsaturation of the fatthe unsaturation of the fat. . Tocopherols and Vitamin E act as antioxidantsTocopherols and Vitamin E act as antioxidants . .

Sugar in cookies and biscuits appears to have a marked inhibiting Sugar in cookies and biscuits appears to have a marked inhibiting effect on the onset of rancidityeffect on the onset of rancidity. .

Tests for rancidityTests for rancidity

11--Peroxide valuePeroxide value

22--Determination of carbonylDetermination of carbonyl

33--Active oxygen determinationActive oxygen determination

44--Thio-barbituric acid testThio-barbituric acid test

0055--Schaal oven testSchaal oven test

Peroxide valuePeroxide value

measures the amount of iodine released measures the amount of iodine released when potassium iodide reacts with rancid when potassium iodide reacts with rancid fat .The Lea method uses 1 g. of fat and 1 fat .The Lea method uses 1 g. of fat and 1 g. of potassium iodide with an acetic g. of potassium iodide with an acetic acid :chloroform (2:1)solvent .After heating acid :chloroform (2:1)solvent .After heating , the iodine formed is determined by , the iodine formed is determined by titration with standard thiosulfate. Other titration with standard thiosulfate. Other methods modify quantities of solvent , but methods modify quantities of solvent , but the principle remains the samethe principle remains the same

The measurement of carbonyl The measurement of carbonyl compoundscompounds

has followed traditional methods for this has followed traditional methods for this group .2,4 dinitro phenyl hydrazine is group .2,4 dinitro phenyl hydrazine is commonly used in the Lappin-Clark commonly used in the Lappin-Clark

methodmethod. .

Active oxygenActive oxygen

is a method which measures the length of time is a method which measures the length of time required to produce 20 mg peroxide per 1000g. required to produce 20 mg peroxide per 1000g. fat when air is bubbled through fats under fat when air is bubbled through fats under standard conditions . A bubble train is set up so standard conditions . A bubble train is set up so

that cleanthat clean. . Dry air bubbles through the oil at constant rat such Dry air bubbles through the oil at constant rat such as 2.33 ml per secas 2.33 ml per sec..

The oil is heated in a constant –temperature bath. The oil is heated in a constant –temperature bath. Other samples are added at definite time Other samples are added at definite time intervals so that the time required for 20 mg can intervals so that the time required for 20 mg can

be calculatedbe calculated. .

Thio-barbituric acid test (TBA) Thio-barbituric acid test (TBA) methodmethod

for determining rancidity : an oxidized or for determining rancidity : an oxidized or rancid fat will react with 2-thiobarbituric rancid fat will react with 2-thiobarbituric acid (TBA) to form a red color the intensity acid (TBA) to form a red color the intensity of which is proportional to the amount of of which is proportional to the amount of rancidity . In recent years this has been rancidity . In recent years this has been developed into a method for measuring developed into a method for measuring the extent of rancidity in a sample of fat . the extent of rancidity in a sample of fat . In the rancid fat the compound formed In the rancid fat the compound formed which reacts with the 2-thiobarbituric acid which reacts with the 2-thiobarbituric acid is malonaldehyde,CH2(CHO)2is malonaldehyde,CH2(CHO)2..

Schaal oven testSchaal oven test

is widely used in the baking industryis widely used in the baking industry. . It takes little equipment and is very easy to set up . It takes little equipment and is very easy to set up . Biscuits or cookies are stored in beakers or jars Biscuits or cookies are stored in beakers or jars with loose fitting tops at 63 C or 145 F. The with loose fitting tops at 63 C or 145 F. The number of days required to develop rancidity is number of days required to develop rancidity is measured by odor and taste .The temperature is measured by odor and taste .The temperature is slightly above what might be encountered during slightly above what might be encountered during distribution of the food through regular distribution of the food through regular commercial channels. Its greatest use is in commercial channels. Its greatest use is in comparing fats .It is difficult to equate shelf life of comparing fats .It is difficult to equate shelf life of a product with the Oven Test where conditions a product with the Oven Test where conditions

are variableare variable. .

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