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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

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• 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

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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.

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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.

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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

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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

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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:

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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

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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%

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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

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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.

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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

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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

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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.

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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.

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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)

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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.

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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

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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.