Carbohydrates

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In the human diet, carbohydrates are mainly present as starch. Well-known sources are cereals, potatoes and pulses.

Celluloses and other polysaccharides in the plant cell wall do not serve as energy sources. They cannot be digested by humans and contribute mainly to the dietary fiber intake. Fiber appears to play an important role in the maintenance of gastro-intestinal function, metabolism and health. Carbohydrates are useful food components because of their sweetness, solubility, cristallization behavior, water activity, hygroscopic behavior and rheological properties.

6.2.2.1 Changes in dietary carbohydrates during manufacturing and storage of food

Introduction. Reducing sugars may undergo a well-known (non-enzymatic) browning reaction, the so-called Maillard reaction in which sugars condensate with amino acids. Pentoses are usually more reactive than hexoses. The mechanism underlying this reaction has not yet been fully elucidated. The Maillard reaction is a sequence of reactions, resulting in the formation of a mixture of insoluble dark-brown polymeric pigments, known as melanoidins. In the early steps of the reaction, a complex mixture of carbonyl compounds and aromatic substances is formed. These products are water-soluble and mostly colorless. They are called premelanoidins.

Initial steps of the Maillard reaction. The first step in the Maillard reaction is the condensation of sugars with amino groups of amino acid moieties. The initial products, glycosylamines, are quite unstable and undergo Amadori rearrangement (Figure 6.6). The course of the condensation depends on the water content of the food. Further, these reactions proceed more rapidly upon heating, especially under neutral to alkaline conditions.

lysine residue (free amine)

RNH2 ^

► RN ^ ►

RNH

+

H2O or

— C = O

H — C strong acid h

— c-1

(CHOH)4

(CHOH)4

(CHOH)3 o

CH2OH glucose

ch2oh H

(reducing sugar)

transamination

CH2OH

N - substituted aldosylamine (glycosylamine)

Schiff s base

Amadori rearrangement -►

(reducing sugar)

transamination oxoacid -amino sugar

Strecker degradation later stages of browning

CH2OH

N - substituted aldosylamine (glycosylamine)

1 - amino -1 - deoxy - 2 - ketose (1, 2 - enol form)

enolization dehydration

Reductones

Furfurals unsaturated polycarbonyl compounds p RNH CH2

(CHOH)3

CH2OH

N - substituted - 1 -amino - 1 - deoxy -2 - ketose (keto form) (Amadori product, ketosamine, hexoseamino acid)

scissions, aldol condensations, Strecker degradations, polymerization, etc.

later stages of browning (advanced Maillard compounds and polymers)

weak acid

Schiff s base

Figure 6.6 Condensation of glucose with lysine, followed by Amadori rearrangement.

The classic example of a foodstuff in which the initial steps may occur is drum-dried milk powder in which a 10 to 40% decrease in availability of lysine has been reported. If spray-drying is used, the availability of lysine does not decrease.

Subsequent steps of the Maillard reaction. In the following steps, reactive unsaturated polycarbonyl compounds such as reductones, and heterocyclic compounds such as pyrazines, are formed. These compounds bind to a-terminal, e-amino and other amino groups of different polypeptide chains to form colored, high-molecular mass, highly cross-linked carbohydrate-protein polymers of low solubility, low digestibility, and low nutritional value. These steps may be followed by breakage of the polypeptide chains, and decarboxylation and ultimately deamination of the amino acid moieties (Strecker degradation, Figure 6.7).

COOH

COOH

:valine-

:leucine-

methylpropanal methylbutanal

CO2 t

CO2 t

R

R"

C =

= N -

-CH

C =

O

H

R'

Figure 6.7 Strecker degradation of amino acids.

The last steps of the Maillard reaction may take place to a considerable extent on heating of the food, especially chocolate and baked products such as bread, biscuits, and cakes. The initial steps of the Maillard reaction do not cause marked changes in the color and flavor of the foodstuff. However, as mentioned above, the course of the Maillard reaction depends on the water content of the food. As a result, the deterioration may be intensified by concentration and dehydration of protein-containing food, e.g., concentration and drum-drying of milk, dehydration of egg white, and drying of oilseed products. The last steps are largely responsible for the desired color and flavor of baked products.

Animal studies have indicated that premelanoidins inhibit growth, disturb reproduction and cause liver damage. Further, certain types of allergic reactions have been attributed to Maillard reaction products. Maillard reactions can be prevented by using the additive power of the carbonyl group in reducing sugars. A reaction characteristic of aldehydes and some ketones is addition of sodium bisulfite. The addition products are crystalline salts, very soluble in water. Further, Maillard reactions may be inhibited by regulating the temperature, pH, and water content.

6.2.3 Proteins

Proteins are the source of essential amino acids. They are the only dietary source of nitrogen for protein synthesis. Denaturation of protein occurs when food is heated during preparation. This is a desirable effect, as denaturated protein is more readily digested. Apart from their nutritional value, proteins are important to the physical properties of foods. Their solubility and dispersability, hygroscopic behavior, viscosity, and stabilizing properties determine the structure and texture of foods.

6.2.3.1 Changes in proteins during processing of raw materials, and during manufacturing, preparation and storage of food A technique that is increasingly used in the processing of proteins is treatment with alkali. It involves solubilization and purification of proteins. Protein concentrates are prepared this way. Alkali cooking of maize is a traditional Mexican technique to increase its digestibility. Treatment with sodium hydroxide is advocated for the peeling of grain. The use of gaseous ammonia has been proposed to free peanut and cottonseed products from afla-toxin.

Intensive treatment of proteins with alkali is known to result in advanced degradation of several amino acids, cystine, arginine, threonine, and serine being the most sensitive. Under mild alkaline conditions and at moderate temperatures, products may be formed that have been found to be nephrotoxic in rats, e.g., lysinoalanine (LAL), ornithinoalanine (OAL) and lanthionine. LAL is the condensation product of dehydroalanine and lysine (Figure 6.8). Dehydroalanine is formed on alkaline desulfurization of cystine. Sulfur is released as H2S. Treatment of serine and phosphoserine moieties with alkali also yields dehydroalanine.

CO— Cystinyl residue

Seryl residue

(Dehydroalanyl residue)

Lysinoalanyl residue

Figure 6.8 Formation of lysinoalanine (LAL).

The extent of LAL formation depends on the nature of the protein, the duration and temperature of the reaction, and the alkali concentration. Different legume proteins from mung beans, cow peas, and peanuts have been shown to produce considerable amounts of LAL when treated with 0.05 to 0.075 N NaOH at 20°C for 30 min. The LAL content may range from 200 to 800 mg/100 g of protein. Some legumes, such as kidney beans, lima beans, and vetch were stable under these conditions. After treatment with alkali at 80°C, however, all legumes contained LAL. Their LAL content decreased on prolonged treatment at higher temperatures. In several other foods, LAL is formed during cooking in the absence of alkali (Table 6.4). Chicken meat which was free from LAL before cooking, contained 200 |g/g after cooking in a microwave oven. Egg white free from LAL when fresh, contained 270 to 370 |g/g after boiling for 10 to 30 min, and 1.1 mg/g if pan-fried at 150°C for 30 min.

Table 6.4 Lysinoalanine content of heated proteins and some protein-containing food products

Protein or food LAL (|ig/g)

Sausage after boiling in water for 10 min 50

Corn chips 390

Pretzels 500

Tortillas 200

Evaporated milk 590-860

Simulated cheese 1070

Egg white solids 160-1820

Hydrolyzed vegetable proteins 40-500

Whipping agent 6500-50,000

Soya protein isolates 0-370

The reactions of proteins with oxidation products of oils and fats and with reducing sugars (Maillard reaction) have already been discussed in Sections 6.2.1.2.2 and 6.2.2.1 respectively.

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