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CARBOHYDRATES (or Glucides, from Greek glucos, sweet)
• They are the most abundant and spread organic compounds on earth.
• They have a central role in the metabolism of plants and animals.
• Their biosynthesis in green plants, i.e. photosynthesis, starting from CO2 and H2O, in
presence of light, is the existence base of all the other organisms.
• They are the basic constituents of many foods representing a large portion of the total
intake of nutrients in human diet (40-75% of the daily energy intake).
• Also non digestible carbohydrates are important in a balanced daily diet.
• Carbs have other important functions in foods (beyond the energetic):
-they act as sweetening;
-as gel- and paste- forming;
-as thickening;
-as stabilizers;
-they are forerunners of aromatic substances and colorants which are formed in the food
during production and subsequent processes (also cooking).
Class (DP*) Sub-Group Some components
Sugars (1-2)
Monosaccharides Glucose, galactose, fructose
Disaccharides Sucrose, lactose
Polyols Sorbitol, mannitol
Oligosaccharides (2-9)Malto-oligosaccharides Maltodextrins
Other oligosaccharides Raffinose, stachyose, fructo-oligosaccharides
Polysaccharides (>9)
Starch Amylose, amylopectin, modified starchs
Non-starch polysaccharides Cellulose, Hemicelluloses, Pectins, β -Glucans, Fructans, Gums, Mucilages2
Carbohydrates can be defined as polyhydroxy aldehydes, ketones, alcohols, acids,
their simple derivatives and their polymers having acetal type linkages.
They may be classified according to their degree of polymerization and may be divided
into three principal groups, namely sugars, oligosaccharides and polysaccharides.
Carbs chemical formula: Cn(H2O)n
This is a simplification, there are other molecules having different formula but reacting as
carbs, thus belonging to the same category (e.g. deoxysugars, aminosugars, sugars with
carboxyl moiety).
DP * = Degree of polymerization
CLASSIFICATION
• Monosaccharides: polyhydroxy-aldehydes or -ketones with a linear carbon chain (3 to 8 carbonatoms); e.g.: glucose, fructose and galactose;
• Oligosaccharides: formally derived from the condensation of monosaccharides, with H2O elimination;
disaccharides: sucrose, maltose, lactose; trisaccharides: raffinose; tetrasaccharides: stachyose;
glucose
fructose galactose
lactose
raffinose
3
• Polysaccharides: polymers with high MW,
having different characteristics with respect to
other carbs; often insoluble in H2O, they are
not sweet and they are somewhat inert; e.g.:
starch, cellulose, pectins.
cellulose (portion)
Total sugar in various foods
Food Total sugar (%)
Vaccine milk 4,8
Human milk 7,2
Cheese 0,1-0,9
Yoghurt 7,8
Fruit yoghurt 15,7
Ice cream 22,2
Apples 11,8
Banana 20,9
Grape 15,4
Oranges 8,5
Honey 75
Jam 50-70
Chocolate 60
Beer 1,5-2,3
Wine 0-5
Dessert wine 5-15 4
Sugars distribution in fruit and vegetables
0% 20% 40% 60% 80% 100%
Apples
Bananas
Cabbages
Carrots
Figs
Grapes
Onions
Orange juice
Pineapple
Soy
Mais
Tomatoes Glucose
Fructose
Sucrose
Maltose
Galactose
Stachyose,Raffinose,Verbascose
5
6
MONOSACCHARIDES
NOMENCLATURE
If two carbonyl moieties are present:-the molecule can be a dialdose (two aldehydes groups);-or osulose (one aldehyde and one ketone group);-or diulose (two ketone groups).When –OH is substituted by –H, the molecule is a deoxysugar , when –OH is substituted by –NH2, the molecule is an aminodeoxysugar .
ALDOSES : polyhydroxyaldehydes
deriving formally from glyceraldehyde by
the addition of –CH-OH units (they can
be triose, tetrose …).
KETOSES: polyhydroxyketons deriving
formally from dihydroxyacetone by the
addition of –CH-OH units (they can be
triulose, tetrulose ,… ); the position of
carbonyl is specified by a numeric prefix
(usually is in position 2).
H
O
OHH
CH2OH
CH2OH
CH2OH
Osuffix: -ose suffix: -ulose
The suffix –ose indicates the presence of the carbonyl moiety in an hydroxylated carbon chain.
7
CYCLIZATION
All the monosaccharides starting from tetroses and 2-pentuloses cyclize to five and sixmembered lactols (furanoses from furan, and pyranoses, from pyran) respectively byintramolecular hemiacetal (or hemiketal) formation.
With the exception of erythrose, monosaccharides crystallize in cyclic forms; in solution there is an equilibrium between the open chain and the cyclic forms, the second being predominant.
8
CONFIGURATION
ALDOSES
Glyceraldehyde has a chiral center, thus it exists as a pair of enantiomers, D and L forms.
From D-glyceraldehyde it is possible to obtain a mixture of D-erythrose and D-threose,
while from L-glyceraldehyde a mixture of L-erythrose and L-threose is obtained.
CHO
CH2OH
OHH
D-glyceraldehyde
HCN
CN
OHH
OHH
CH2OH
CN
HHO
OHH
CH2OH
+
1) H+O O
OH OH
OH
H
H
OH
H
OHOH
H
2) NaHg/NaBH4
D-erythrose D-threose
+
Through the cyanhydrin reaction, starting from D-glyceraldehyde two D-tetroses areobtained and from each of them two D-pentoses, and so forth; thus from D-glyceraldehyde 8 hexoses belonging to the same D-series can be obtained.
CHO
CH2OH
OHH
CHO
OHH
OHH
CH2OH
CHO
HHO
OHH
CH2OH
CHO
OHH
OHH
CH2OH
H OH
CHO
HHO
OHH
CH2OH
H OH
CHO
OHH
HHO
CH2OH
H OH
CHO
HHO
HHO
CH2OH
H OH
OHH
OHH
CH2OH
H OH
OHH
OHH
CH2OH
H OH
OHH
CHO CHO
HHO
HHO
OHH
CH2OH
H OH
HHO
OHH
CH2OH
H OH
OHH
CHO CHO
HHO
OHH
HHO
CH2OH
H OH
OHH
HHO
CH2OH
H OH
OHH
CHO CHO
HHO
HHO
HHO
CH2OH
H OH
HHO
HHO
CH2OH
H OH
OHH
CHO CHO
HHO
D-glyceraldehyde (D-glycero-)
D-erythrose (D-erythro-) D-threose (D-threo-)
D-ribose(D-ribo-)
D-arabinose(D-arabino-)
D-xylose(D-xylo-)
D-lyxose(D-lyxo-)
D-allose(D-allo-)
D-altrose(D-altro-)
D-glucose(D-gluco-)
D-mannose(D-manno-)
D-gulose(D-gulo-)
D-idose(D-ido-)
D-galactose(D-galacto-)
D-talose(D-talo-)9
10
Epimers: two molecules differing only for the configuration of a chiral center, e.g. D-
glucose and D-mannose.
CHO
CHOH
R
EtSH, H+CH(SEt)2
CHOH
R
RCOOOHCH(SO2Et)2
CHOH
R
OH-CH2(SO2Et)2
HC
R
O
+
An important aldoses degradation reaction takes place via disulfone formationstarting from the dithioacetal.
Occurence of aldoses
11
KETOSES Name, structure Where is it found?
hexulose D-fructose vegetables, honey
D-psicose residues of fermented molasses
Eptulose, octulose, nonulose
D-manno-2-heptulose avocado
D-glycero-D-manno-2-octulose
"
D-erythro-L-gluco-2-nonulose
"
L series??
SYSTEMATIC NOMENCLATURE
ALDOSE
If the number of C atoms is <=6, traditional name can be used, otherwise the moleculeportion adjacent to the carbonyl is assigned the maximum possible prefix and the remainingportion of the molecule, (if constituted at least by 2 C atoms), another prefix is assigned andthis is named first; then the name is written depending on the total of C atoms.
Examples:
HHO
OHH
CH2OH
H OH
OHH
CHO
HHO
OHH
H OH
OHH
CH2OH
OHH
H OH
HO H
HHO
CHO
H OHH OH
HO H
HHO
H OH
OHH
D-glucose orD-gluco-hexose
D-gluco-
L-manno-
D-glycero-D-glycero-L-manno-heptose
KETOSE
If it contains less than 4 chiral carbons, traditional name can be used, otherwise the groupsadjacent to the carbonyl must be considered:
12
The longest of the two portions adjacent to carbonyl is named first:
HHO
OHH
H OH
HHO
H OH
HHOHHO
HHO
OHH
Fructose
or D-arabino-2-hexulose
D-arabino
L-glycero-
D-threo- D-lyxo-D-threo-L-glycero-3-hexuloseorD-lixo-3-hexulose
When sugar cyclizes to lactol, a new chiral center is formed, thus two diastereomers(anomers) are formed, named α and β anomers.
glucose α-D-glucopyranose β-D-glucopyranose13
14
All the monosaccharides can
exist in solution in five forms:
Cyclic forms are much favoured with respect to open chain forms.
Generally, the favourite cyclic form, more stable, is the pyranosic one.
At equilibrium:[α][α][α][α]20
D= +52,53°°°°
[α][α][α][α]20D=
+110°°°°[α][α][α][α]20
D= +19°°°°
36%64%
<1%
15
Obviously specific rotation for two anomers is different (and even for furanoseand pyranose forms of a same sugar); thus a solution of a pure isomer freshlyprepared has a rotation angle varying during time till it reaches a constant value(at equilibrium among the various forms).
[ ]cl
t
∗∗= αα λ
100
α = deviation angle at T °C;
l = polarimetric tube length (dm);
c = grams of optically active substance in 100 ml of solution;
[αααα]tλλλλ = specific rotation constant;
λλλλ = selected wave length (generally sodium D-line light);
t = temperature at which the measurement is done (usually 20-25°C).
PHYSICAL PROPERTIES
1. OPTICAL ROTATION AND MUTAROTATION
Non racemic chiral compounds deviate polarized light by an angle α proportional to theirconcentration in the solution.
MUTAROTATIONPHENOMENON
16
2. Higroscopicity and solubility
• The amount of water kept by the sugars depends on the sugar structure, the isomers
present and the sugar purity.
• Solubility of mono- and oligosaccharides in water is good.
• Anomers can have very different solubility (e.g. α and β lactose).
• Monosaccharides have low solubility in ethanol and they are insoluble in organic
solvent such as benzene, ethyl ether, chloroform.
17
SENSORY PROPERTIES
• Mono-, oligosaccharides and their alcohols are sweet (few exceptions);
• Main sweeteners: sucrose, glucose, fructose, invert sugar (glucose and fructose),lactose and alcohols (sorbitol, mannitol, xylitol).
• Sugars differ in the quality of sweetness and taste intensity.
• As oligosaccharides dimension increases, their sweetness power decreases.
• The taste intensity can be quantificated by determining the minimum level ofconcentration at which the sweet taste is still detected or referring to a referencesolution (usually sucrose).
Sugar Limit of detection %
Fructose 0.24 Glucose 1.17 Lactose 2.60 Maltose 1.36 Sucrose 0.36
Sugar Relative sweetness Sugar Relative
sweetness
Sucrose 100 D-Mannitol 69 D-Glucose 69 D-Mannose 59 D-Galactose 63 Raffinose 22 D-Fructose 114 D-Ramnose 33 Invert sugar 95 D-Sorbitol 51 Maltose 46 Xylitol 102 Lactose 39 D-Xylose 67
• The minimum value depends on the affinity between the substance’s structureand the chemoreceptor sites for sweetness .
• Further parameters influencing the quality and intensity of sweetness are: pH,temperature, presence of other compounds .
fructose
glucose
galactosemaltose
Temperature ( °°°°C)
Rel
ativ
esw
eetn
ess
Temperature dependance of relative sweetness of some sugars
• There is also a relation between sugar contentand volatiles compounds.
• Also the color of the solution can influence theorganoleptic evaluation.
• Composition and concentration of sweetenermust be carefully evaluated in each foodformulation to give an optimal sensory result.
Need of an AH (H donor) B (H acceptor) X (hydrophobic site) system in a substance in order to give sweet taste.
D-glucopyranose 18
19
REACTIVITY
1) REDUCTION to ALCOHOLS
• NaBH4
• electrolysis
• catalytic hydrogenation
Alcohol name : in the sugar name –ulose or –ose is substituted with –itol .
Xylitol (pentose), sorbitol (naturally found in many fruits), D-mannitol are used in dietformulations, to decrease water activity, as softeners, etc. They afford 2,4 Kcal/g .
2) a. OXIDATION to ALDONIC ACIDS
NAME: ALDOSE ALDONIC ACID
β-D-glucopyranose
20
NAME: ALDOSE ALDARIC ACID (dicarboxylic acid)
2) b. OXIDATION to ALDARIC ACIDS
Stronger conditions (e.g. HNO3) allow oxidation of both the terminal carbons ofaldose:
It can form mono or dilactones
2) c. OXIDATION to URONIC ACIDS
To oxidize saturated terminal carbon only, the carbonyl moiety of the aldosemust be protected; then, after deprotection, the uronic acid is obtained.
21
Uronic acids are widespread in nature, forming polysaccharides (e.g. pectines) havingindustrial applications as gel-forming.
3) REACTION in BASIC and/or ACIDIC MEDIA
• Monosaccharides are stable in a pH range of 3-7 (if compounds with aminogroups are not present).
• At low pH enolization followed by H2O loss, predominates.
• At very high pH, enolization followed by chain fragmentation, predominates.
22
3) a. REACTION in STRONGLY ACIDIC MEDIUM
• Disaccharides and oligosaccharides are formed (intermolecular glycosidicbond ).
• When the monosaccharide conformation is proper a glycosidic intramolecularbond can be formed.
• Warming in acidic medium, enolization, dehydratation , formation ofsubstituted furans and pyrans take place:
HC
OHH
HHO
OHH
OHH
CH2OH
O HC
OH
HHO
OHH
OHH
CH2OH
O
H
HC
O
H
OHH
OHH
CH2OH
O
H
HC
O
H
H
OHH
CH2OH
O
O
OHOHC
CH2OH
O
CHO
CH2OH
H
- H2O - H2O - H2O
Where is the mistake???
HMF (HydroxyMethyl Furfural)
23
3) b. REACIONS in STRONGLY BASIC MEDIUM
• In strongly basic medium aldose and ketose enolise quickly, thus fructose, mannose and
glucose equilibrate by the formation of the shared 1,2-enediol.
• In presence of O 2 or other oxidants (Cu 2+) the double bond C,C breaks formingcarboxylic acids.
The method is applied to the quali- quantitative determination of reducing sugars.
• Anyway also other transformations can take place leading to formation of several volatilecompounds.
• Acetic acid
• Hydroxyacetone
• Hydroxybutanone
• Furfurylalcohol
• 5-Methyl-2-furfurylalcohol
• γ-butyrolactone
• Various cyclopentenolones
O
HO
H3C
Cyclopentenolones are typical compounds with “caramel like” aroma
Some of the volatile compounds formedwarming up fructose syrup atpH 8-10 for 3 h.
24
O
OHRNH2
NR
OH
NHR
OH
HO HO HO
NR
OH
CH2
O
O-H2O -NH2R O
H
O
HOH2C
NHR
O
HO
NON ENZYMATIC BROWNING
Sugars contribute to the organoleptic characteristics of a food not only by their presence, butalso by the products of their degradation.
Caramelization (neutral or basic/acidic catalysis, high temperature, sugars)
Maillard reaction (neutral or basic/acidic catalysis, high temperature, sugars, amino groups )
glucide Schiff base enaminol
Browning is due to the polymerization of
many molecules having low MW.
Polymers formed (melanoidins ) have
structures as the following:
Amadori compound
HMF
NR
NR
NR
X XX=O, NR
25
Aroma compounds are formed: lactones, furanons, pyranons, aldehydes, etc.
O
OOO
O
O
CH3
OH
CH3H3C
HOCH3
CH3HO
Sotolon (typical aroma of brown sugar).
Negative aspects:
• Milk browning;
• Loss of essential aminoacids;
• Formation of potentially carcinogenic compounds (heterocyclic amines);
• Formation of aroma not always good (acrolein, piruvic aldehyde, glyoxal, etc.).
Positive aspects:
Formation of desirable aroma (toasting of coffee, cooking of food, etc.);
Formation of colour compounds (cooking of bread, of meat);
Formation of antioxidant compounds that protect the food against oxidation.