Introduction to the Chemistry of Foods and Forages János Csapó Introduction to the Chemistry of Foods and Forages

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Introduction to the Chemistry of Foods and Forages

János Csapó

Introduction to the Chemistry of Foods and Forages

János Csapó

Table of Contents

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1. Water and Watery solutions Error: Reference source not found

2. Minerals Error: Reference source not found

3. Carbohydrates Error: Reference source not found

4. Amino Acids, Peptides and Proteines Error: Reference source not found

5. Lipids Error: Reference source not found

6. Vitamins Error: Reference source not found

7. Natural Food Colorants Error: Reference source not found

8. Flavor Compounds Error: Reference source not found

9. Enzymes in the Food Industry Error: Reference source not found

10. Food Technological Additives Error: Reference source not found

11. Toxic Compounds in Food Error: Reference source not found

12. Food Items Error: Reference source not found

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3. Egg Error: Reference source not found

4. Fats and Oils Error: Reference source not found

5. Cereals and Cereal Products Error: Reference source not found

6. Vegetables and Fruits Error: Reference source not found

7. Sweeteners and Chocolate Error: Reference source not found

8. Alcoholic Beverages Error: Reference source not found

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Food chemistry

Educational supplement for students of MSc courses of Nutrition and Feed Safety and Animal Science

All rights reserved. No part of this work may be reproduced, used or transmitted in any form or by any means – graphic, electronic or mechanical, including photocopying, recording, or information storage and retrieval systems - without the written permission of the author.

Food chemistry


Csapó, János DSc, university professor (Kaposvár University)

Varga-Visi, Éva PhD assistant professor (Kaposvár University)

© Kaposvár University, 2011

All rights reserved. No part of this work may be reproduced, used or transmitted in any form or by any means – graphic, electronic or mechanical, including photocopying, recording, or information storage and retrieval systems - without the written permission of the author.
Manuscript enclosed: 25 July 2011

Responsible for content: TÁMOP-4.1.2-08/1/A-2009-0059 project consortium

All rights reserved. No part of this work may be reproduced, used or transmitted in any form or by any means – graphic, electronic or mechanical, including photocopying, recording, or information storage and retrieval systems - without the written permission of the author.

Responsible for digitalization: Agricultural and Food Science Non-profit Ltd. of Kaposvár University

All rights reserved. No part of this work may be reproduced, used or transmitted in any form or by any means – graphic, electronic or mechanical, including photocopying, recording, or information storage and retrieval systems - without the written permission of the author.

Chapter 1. Water and Watery solutions

Water is an elemental and often predominant constituent in many foods. It supports chemical reactions, and also reactant in hydrolytic processes. In the point of view of food preservation an important feature is that the removal or binding of water retards many reactions and therefore inhibits the growth of microorganisms hence the shelf lives of foods improves. Water also contributes significantly to the texture of food because interacts with its other constituents e.g. polysaccharides, proteins, lipids and salts.

The state of the water in food can be free or bound. The chemically bound water is attached to the organic or inorganic constituents of food with strong forces.

The physically-chemically bound water can be trapped with two different ways:

  1. Adsorption water . The water is bound on the surface of the constituents in hydration shell. It cannot be eliminate with mechanical processes (e.g. pressing) but it can be evaporated by heating or/and with low moisture content air.

  2. Water bound by osmosis . Microcavities in food created by macrocomponents contain small molecular weight soluble materials that induce osmosis and water is trapped osmotically in these cavities.

The mechanically bound water can be present between the higher structural units of food cells (structural water) and also in capillaries and on the surface of the food (wetting with adhesion).

The storage life of the raw materials and also processed food products basically does not depend on the absolute value of the water content. It depends on the water content available for the microorganisms. Microbes can not utilize the chemically bound water and that of frozen from food. The value of water activity (aw) represents the ratio of the available water content within the whole water content of food.

Where P = partial vapor pressure of food moisture at temperature T

P0 = saturation vapor pressure of pure water at T

ERH = equilibrium relative humidity at T

The sorption isotherm determines the relationship between the water content and water activity of a food. The sorption isotherm of a food with high water content (Fig 1.) differs from that of food with low water content (Fig 2.). In the second case when the food contains less than 50% water, minor changes in this parameter led to major changes in water activity. Moreover, the relationship between aw and water content depends on whether the food adsorb (wetting) or desorb (drying) the water. This phenomenon is called hysteresis and it is represented with the hysteresis loop in the sorption isotherm (Fig 2.). When the food absorbs water, the same water content means higher water activity related to drying. In the case of wetting small changes in the water content may result in large increment in water activity and can increase the risk of acceleration of food deterioration.

When aw is decreased the growth of microorganisms is retarded. The reaction rate of non-enzymatic browning and enzyme catalyzed reactions is slower. However dried food systems with low aw are prone to lipid autoxidation (Fig 3.).

Foods that possess aw between 0.6 - 0.9 (Intermediate Moisture Foods, IMF) are largely protected against microbial spoilage.

In order to improve the shelf life of food aw can be decreased by drying. Using humectants (i.e. additives with high water binding capacities e.g. salt, sucrose, glycerol and sorbitol) is also a matter of choice.

Chapter 2. Minerals

Minerals are inorganic compounds. They remain back after incineration of food of animal and plant origin. They can be sorted according to the magnitude of their intake requirements for human beings. The main elements occurring in foods are Na, K, Ca, Mg, Cl, P and S. They are essential for human in amounts more than 50 mg/day. Iron, I, Mo, Co, F, Zn, Se, Cu, Mn, Cr, Ni assigned to the group of trace elements. They are essential in concentrations less than 50 mg/day. The necessity of ultra-trace elements such as Al, As, Ba, Bi, B, Br, Cd, Cs, Ge, Hg, Li, Pb, Rb, Sb, Si, Sm, Sn, Sr,Tl, Ti, and W has also been proven in animal experiments. Their absence resulted in deficiency symptoms. If their biochemical function is revealed ultra-trace elements are replaced to the group of the trace elements.

The knowledge of the mineral content and their bioavailability in foods is important in order to estimate whether food intake provides for the micro and macroelement needs of the organisms. Minerals have widespread biological role. They can be present both in inorganic form in solutions (e.g. electrolytes constituents) or building materials (e.g., in bones) and essential part of organic macromolecules (e.g. enzyme prosthetic group).

Minerals exert also an effect on the food quality e.g. they can activate or inhibit enzyme catalyzed reactions or nonenzymic reactions in food and therefore they can have an impact on organoleptic properties (color and flavor).

The mineral content of a certain type of food depends on several factors. The most important traits to be considered are:

  • Genetic factors

  • Climate and soil composition

  • Agricultural procedures

  • Ripeness of the harvested crops

  • Food processing procedures (Table 1.)

The mineral supply depends on both food intake and the bioavailability of the mineral elements. The ratio of which a given mineral component of food can be utilized depends both on the digestive system of the organism and several food-relating factors. Redox potential and pH value of food effect valency state of the minerals and therefore influences solubility and absorption. Several sorts of food constituents are prone to bind minerals and therefore affect their absorption, e.g., lignin, phytin, organic acids, proteins, peptides, amino acids, polysaccharides and sugars. Minerals can be present both in the form of solubilized ions and in the form of organic iron compounds and the rate of absorption can differ significantly (e.g. for organic and inorganic iron compounds).

Sodium maintains the osmotic pressure of the extracellular fluid and also can be an enzyme activator. An excessive intake can lead to hypertension that can be avoided with nonsalty diet or using diet salt.

Potassium is the most abundant cation in the intracellular fluid regulating osmotic pressure within the cell. Some foods have low levels of potassium (e.g. white bread) if their consumption is predominant potassium deficiency may occur.

Magnesium influences the activity of several enzymes and also can be an enzyme constituent.

Calcium participates in the control of several processes (e.g. muscle contraction, blood clotting and brain cell activation) and it is involved in the structures of bones and the muscular system. The prerequisite of the adequate absorption of calcium is an adequate intake of vitamin D. The main sources of calcium are milk and milk products, fruits and vegetables. The intake of chloride is correlated with that of sodium. Chloride is a counter ion for hydrogen ions in gastric juice.

Phosphorus is present in the body mostly in the form of phosphates having an essential role in the metabolisms (e.g. energy conversion, enzyme composition/activation) and in the composition of several constituents (e.g. nucleic acid, bones). The amount of Ca and P in molar basis should be about equal in the consumed food. The food additive polyphosphates can be absorbed after hydrolysis.

Most of the trace elements has a role as constituent of metalloproteines or/and activate/inhibit enzymes.

Iron is a component of oxygen-binding proteins in muscle tissue (myoglobin) and blood (hemoglobin) and also present in several enzymes. The highest iron absorption was detected from meat and the less utilizable sources are cereals and vegetables.

Copper is together with iron can enhance unwanted reactions during food processing moreover copper (II)-ions are taste bearing.

Chromium has an important role in the glucose metabolisms: its deficiency can cause a decrease in glucose tolerance and therefore the risk of cardiovascular disease enhances.

Adequate supply of selenium is contributed to the sufficient function of antioxidant activity of the body (e.g. glutathione peroxidase).

High zinc intake obtaining the level of toxicity was occurred when soured food kept in zinc-plated metal containers was consumed.

Manganese is relatively nontoxic even in higher amounts.

Molybdenum is present in bacterial nitrate reductase that is involved in meat curing and pickling processes.

Nickel has been shown to enhance insulin activity.

Fluorine in the form of fluoride inhibits the growth of microorganisms responsible for the development of dental caries. Builts in the material of teeth (fluoroapatite) and therefore retards the solubilization of tooth enamel in an acidic pH.

The occurrence of goiter has been shown to correlate with the iodine content of drinking water. Most foods contain very low levels of this element. The most abundant sources are milk, eggs and seafood.

Minerals present in food can be originated from raw materials or acquired during food processing and storage. Metal ions have an impact on the nutritional value of food because they catalyze reactions that results in the loss of some important nutrients, e.g., the oxidation of ascorbic acid. Their presence can modify the visual appearance of food e.g. color fading of fruit and vegetable products. They initiate lipid peroxidation processes and therefore facilitate the formation of off-flavors in the products.
Chapter 3. Carbohydrates

This group of macromolecules is the most widespread and also can be found in the highest quantities among organic compounds in the Earth. Besides carbohydrates provide nutritional energy (17 kJ/g) they have got several functions in food. Nondigestible carbohydrates are called as dietary fiber and have an important role in the balanced daily nutrition. In the point of view of food processing some carbohydrates can be considered as sweeteners, others potential gel-forming and thickening agents or stabilizers. Carbohydrates are also precursors for the formation of aroma and coloring substances during food processing.

Monosaccharides are polyhydroxy-aldehydes (aldoses) or polyhydroxy-ketones (ketoses) according to chemical structure. They have chiral center(s) with the exception of the simplest aldoketose, dihydroxyacetone.

The mono- and oligosaccharides possess good solubility in water. The solubility of the anomers may differ substantially. These groups of carbohydrates together with sugar alcohols are sweet, with a few exceptions. The most important sweeteners are saccharose (sucrose), starch syrup (a mixture of glucose, maltose and malto-oligosaccharides), invert sugar (from the hydrolysis of saccharose), fructose-containing glucose syrups (high fructose corn syrup), glucose, fructose, lactose and sugar alcohols {e.g. sorbitol (D-glucitol), xylitol (pentitol), D-mannitol (hexitols)}. Sugars differ in quality of sweetness and taste intensity. The taste of saccharose is pleasant even at high concentrations but not that of the other sugars. In the case of the oligosaccharides the taste intensity is inversely proportional to the chain length.

The taste intensity can be determined with the determination of the recognition threshold value (the lowest concentration of sugar at which sweetness is still perceived) or with isosweet concentrations (the concentration of the examined sugar at which the same sweetness is provided as in the case of the reference sugar at a given concentration). Saccharose is usually chosen as reference substance. The last method is better used for practical purpose. Taste intensity varies greatly among sweet compounds as can be seen on Table 2. Taste reception parameters that influence taste quality and intensity are the structure of the compound, the temperature, pH and the presence of other sweet or non-sweet compounds in the matrix of food.

Monosaccharides can be reduced to the corresponding alcohol. Sugar alcohols are used as sugar substitutes in dietetic food formulations to decrease water activity in IMF.

Aldoses can be oxidized to aldonic acids under mild conditions. The resulting lactones (e.g. glucono-δ-lactone) are used in food when a slow acid release is required, as in baking powders, raw fermented sausages or dairy products.

Monosaccharides are relatively stable in the pH range 3–7 in the absence of amine components. In an acidic medium enolization and subsequent elimination of water with retention of the carbon-chain predominate (Fig 4. and Fig 5.). These processes occur during heating e.g. pasteurization of fruit juices and baking of rye bread. Among the products of these reactions 5-hydroxymethyl furfural (HMF) is used as an indicator for the heating of food (e.g. honey).

The rate of the enolization reactions is much higher in an alkaline medium and therefore the rate of aldose-ketose isomerization (e.g. lactose to lactulose) is increased. The enediols can oxidize to carboxylic acids. These molecules can also be enolized and hydroxyacids can be formed.

In alkaline medium the carbohydrate skeleton can be degraded following enolization and hydroxyaldehydes and hydroxyketones are formed by chain cleavage due to retroaldol reaction (Fig 6.). Lots of the resulting products of the above reactions in alkaline medium are volatiles and aroma active compounds.

Caramelization can occur both in of acidic and/or alkaline medium when food with sugar content is heated. This process led to aroma formation and brown pigment accumulation at a different rate depending on the nature of the precursors and pH.

Maillard reaction (nonenzymatic browning) is the collective noun of groups of reactions. The primary reactants are reducing sugars (mainly glucose, fructose, lactose and maltose) and amino acids that form N-glycosides (Fig 7.) following numerous consecutive reactions. Amino acids with a primary amino group are more important than those with a secondary amino group. In the case of proteins mostly the ε-amino groups of lysine reacts.

The reactions are accelerated by high temperature and low water activity.

The most important Maillard reaction products are volatile compounds, brown pigments (melanoidins) and reductons.

Volatiles can contribute to the desirable aroma pattern of cooked, baked, roasted or fried food, but some of them possess unpleasant aroma. These off-flavors especially form during storage of food in the dehydrated state, but heat treatment (pasteurization, sterilization) can also result in them.

The presence of melanoidins is desired in several cases (e.g. baking and roasting), but not in foods which have other color of their own (e.g. tomato soup).

Reductones (presence of an enediol structure element in the α-position to the oxo function, (Fig 8.)) possess highly reductive properties therefore they can promote the preservation of food against oxidative deterioration.

There are some feedbacks of the Maillard reaction. Compounds with potential mutagenic properties can be arisen. The nutritional value of protein can decrease through the direct deterioration of some essential amino acids (lysine, arginine, cysteine, methionine) or through the formation of cross-linkage of proteins.

In the course of food technology processes when Maillard reaction is undesirable it can be inhibited with the application of the lowest possible temperatures and lower pHs, with the avoidance of the critical water contents. Other possibilities are the addition of non reducing sugar instead of reducing ones and addition of sulfite.

Oligosaccharides contain up to about 10 monosaccharide residues bound to each other by glycosidic linkages. Dissacharides can bear reducing or nonreducing properties depending on whether the glycosidic linkage is established between one lactol group and one alcoholic hydroxyl group (reducing) or between the lactol groups of two monosaccharides (nonreducing).

Saccharose can be hydrolyzed to equimolar mixture of glucose and fructose. The resulting mixture is called invert sugar because the specific rotation changes.

The decomposition of starch with α-amylase results in maltodextrins. β-cyclodextrin consists of hydrophobic cavity that is sterically suitable for apolar compounds therefore this material is used for stabilizing lipophilic aroma substances and vitamins and for neutralizing the taste of bitter substances.

Polysaccharides can be homoglycans (containing one type of sugar structural unit) or heteroglycans (several types of sugar units are bound with glycosidic linkages). The monosaccharides can be attached in a linear pattern (as in amylose and cellulose) or in a branched fashion (amylopectin, glycogen).

Polysaccharides in food products often preserve their natural roles as skeletal substances (fruits and vegetables) and assimilative nutritive substances (cereals, potatoes, legumes). They can be used in isolated form as water-binding substances (e.g. agar, pectin and alginate in plants; mucopolysaccharides in animals).

Isolated polysaccharides possess highly variable properties. They can be insoluble (cellulose) or bear good swelling power and solubility in hot and cold water (starch, guaran gum). The viscosities of the solutions can be low even at high concentrations (gum arabic) or high even at low concentrations (guaran gum). They are used by the food industry to a great extent as gel-setting or thickening agents, stabilizers for emulsions and dispersions, inert fillers to enhance the ratio of indigestible ballast materials in a diet, and protecting agents for sensitive food compounds.

Perfectly linear polysaccharides (one type of monosaccharide residue with one type of linkage e.g. cellulose, amylose) readily precipitate from solution and insoluble in water.

Branched polysaccharides (e.g. amylopectin, glycogen) are more soluble in water since the chain–chain interaction is less pronounced and there is a greater degree of solvation of the molecules. They can be readily rehydrated and solutions have lower viscosity. They are not prone to precipitation. At higher concentrations they compose a sticky paste that makes them suitable as binders.

Linearly branched polysaccharides (long ‘backbone’ chain and many short side chains, e.g. alkyl cellulose) possess the combined properties of perfectly linear and branched polymers. Their solutions have got high viscosity owing to the long ‘backbone’ chain. The interactions between the chains are weakened by the numerous short side chains therefore the rehydration rates of the molecules is fast and their solubility is good.

Polysaccharides with carboxyl groups (e.g. pectin, alginate, carboxymethyl cellulose) have good solubility in the neutral or alkaline pH range. The molecules are stretched and resist intermolecular associations due to their negative charges owing to carboxylate anions. If pH is below three, precipitation occurs, since electrostatic repulsion ceases to exist and gel formation occurs.

Polysaccharides with strongly acidic groups (e.g. carrageenan, modified starch) have good solubility in water and the viscosity of their solutions is very high. Their solutions are stable even at lower pHs.

Modified polysaccharides can be sorted according to the nature of their substituents and the dergee of substitution. In the case of neutral substituents (e.g. ethyl, ethyl and hydroxypropyl cellulose) the solubility in water, the viscosity and stability of the solutions increase. The hydration is facilitated by the interference of the alkyl substituents in chain interactions.

Acidic substituents (e.g. carboxymethyl or phosphate groups) increase the solubility and viscosity of the solution. Some acid-modified polysaccharides, when wetted, have a pasty consistence.

Chapter 4. Amino Acids, Peptides and Proteines

The nutritional role of the above group of compounds is widespread. Their most important function is probably that they supply consumer’s needs from the required building blocks of biosynthesis of proteins and other bioactive materials derived from the amino acids. Proteins are primary not utilized as energy source. The nutritional energy value of proteins is the same as that of carbohydrates. They contribute to the flavor of food as more of them are precursors for aroma compounds and colors. The physical properties of food also depend on the presence or absence of these constituents. They can affect the formation and stabilization of gels, emulsions, foams, and fibrillar structures.

Amino acids on the one hand can be classified according to the chemical properties of their side chains: amino acids with nonpolar, uncharged side chains; amino acids with polar, uncharged side chains; amino acids with charged side chains. The other sort of assignment based on their nutritional/physiological roles. Essential amino acids should be uptaken from the food (valine, leucine, isoleucine, phenylalanine, tryptophan, methionine, threonine, lysine and arginine for human, histidine (essential for infants)). Nonessential amino acids can be synthesized by the organism (glycine, alanine, proline, serine, asparagine, glutamine, aspartic acid and glutamic acid for human).

Amino acids are chiral molecules with the exception of glycine. In aqueous solution they are present as cations, zwitterions or anions, depending on pH. Their solubility in water is highly variable.

In oligopeptides 10 or less amino acid residues are bound together through an amide linkage.

The molecular weight of polypeptides is about 10 kdal, (that corresponds to approx. 100 amino acid residues). The molecular weight of proteins is usually higher than this value. The primary structure of protein is the amino acid sequence that also determines the total structure. On the whole of the secondary and tertiary structures means the conformation of the protein molecules. The secondary structure reveals the possible arrangements of the peptide chain in space; the tertiary structure shows how these arrangements realized on the entire peptide chain. The individual protein molecules often form aggregates and the geometric structure of the subunits gives the quaternary structure .

The taste quality of the amino acids depends on configuration. The D-amino acids usually generate a sweet taste. In the case of peptides there is no relationship to configuration. Peptides are neutral or bitter in taste – with the exception of sweet aspartic acid dipeptide esters. The methyl ester of the aspartic acid/phenylalanine dipeptide is a sweetener (aspartame). Peptides exhibiting salty taste, e.g. ornithyl-β-alanine hydrochloride can be used as substitutes for sodium chloride. The taste intensity depends on the hydrophobicity of the side chains; moreover the amino acid sequence also is an important factor. In foods when proteolytic reaction occur (e.g. ripened cheeses) the amount of bitter tasting peptides can arise.

The chemical reactions of amino acids during food processing can concern of both carboxyl and amino groups and the side chain. With the decomposition of carboxyl groups biogen amines are formed (Fig 9.). Free amino acids that are used for the fortification of foods can be N-acilated (e.g. N-acetyl-L-methionine, N-acetyl-L-threonine) in order to prevent their decomposition (e.g. methionine → methional) during heat treatment. The nutritional values of N-acetyl-L-methionine, N-acetyl-L-threonine are equal to those of the free amino acids.

Amino acids can react with dicarbonyl compounds. These reactant are generated by the Maillard reaction. Strecker degradation (Fig 10.) resulted in aldehydes that are potent aroma compounds (Strecker aldehydes). The ninhydrin reaction is a special case of the Strecker degradation.

The ε-amino group of lysine can be protected in the form of ε-N-benzylidene-L-lysine and ε-N-salicylidene-L-lysine (Fig 11.). Using these derivatives the rate of nonenzymatic browning was reduced related to free lysine and modified lysine proved to be as effective as free lysine in feeding tests with rats.

Mild oxidation of cysteine (e.g. with thiol reagents) result in cystine. The formation of sulfonic acid group in the side chain occurs when oxidation with stronger agents (e.g., with performic acid) is carried out. During food processing methionine is readily oxidized to the sulfoxide and then to the sulfone causing losses of this essential amino acid in food.

Acrylamide is produced in reactions of asparagine with reductive carbohydrates. Acrylamide is both toxic and volatile compound. Its formation occurs during heat treatment of food. Cysteine and methionine also form acrylamide in the presence of glucose but the yields are considerably lower.

The incorrect application of some cooking techniques (e.g. barbecuing) may results in the formation of mutagenic heterocyclic compounds . If excess heat is applied and the surface of meat or the fish is charred some part of the amino acids present in the food pyrolyze and pyridoindoles, pyridoimidazoles and tetra-azo-fluoroanthenes are formed.

The nature and the degree of the chemical changes of proteins during food processing depend on the composition of food and the applied conditions. To sum, it can be concluded that owing to these reactions the biological value of proteins may be decreased. Essential amino acids can be converted into derivatives which are not utilizable by the organism or the digestibility of proteins can be decreased by intra- or interchain cross-linking. Toxic compound can also be formed. The changes induced by processing of food should be evaluated in its complexity. Nutritional, physiological and toxicological factors should also be considered.

The tripeptyde glutathione (γ-L-glutamyl-L-cysteinyl-glycine) is involved as a coenzyme in many redox-type reactions and in the active transport of the amino acids. Its presence exerts an effect on the rheological properties of wheat flour dough. If reduced glutathione are present in higher quantities it reduces the disulfide bonds of wheat gluten and the molecular weight of protein decreases.

Some unique peptides are used for the authentication of meat. Carnosine (β-alanyle-L-histidine) is characteristic for beef muscle tissue. Anserine (β-alanyle-1-methyl -L-histidine) is present in chicken meat and balenine (β-alanyle-3-methyl-L-histidine) in the muscle of whales.

The peptide nisin active against several Gram-positive microorganisms (e.g. Streptococci, Bacilli, Clostridia) as a broad-spectrum bacteriocin. Its use as a preservative is permitted in several countries. Nisin contains unusual amino acids and produced by strains of Streptococcus lactis. With the application of nisin the butyric acid fermentation in cheeses due to the presence of Clostridia can be blocked. It can also be used in combined preservation technologies for canned vegetables parallel with mild sterilization conditions.

The rate of nonenzymic browning has been shown to reduce if lysine is present in the form of dipeptydes. Some dipeptyde of lysine proved to be as effective as lysine in rat feeding tests hence they can be applied for lysine fortification in foods which contain sugar and must be heat treated.

The denaturation is the reversible or irreversible change of the native conformation of protein. Covalent bonds of protein are not cleaved during denaturation with the exception of the disulfide bridges, but the cleaving of hydrogenbridges, ionic or hydrophobic bonds can occur. Conditions that can cause denaturation of proteins are high temperature, too low or too high pH, increment of interface area, addition of organic solvents, salts, or detergents.

In the case of reversible denaturation the denaturing agent stabilizes the peptide chain in its unfolded state. When the denaturation is irreversible the unfolded peptide chain is stabilized by interaction with other chains (e.g. thiol groups form disulfide bonds). If globular proteins are denaturated irreversibly their solubility or swellability is reduced and the peptide chains are aggregated. In contrast, with the denaturation of fibrous proteins the destruction of their highly ordered structure occurs that results in increased solubility.

The activity of biologically active proteins is ceased or diminished owing to denaturation. In the point of view of the nutrition denaturation can be advantageous because the denatured food proteins are more readily digested by proteolytic enzymes. If harsh conditions are applied during food processing (e.g. elevated temperature for longer times) protein deterioration can occur i.e. the side chains of the individual amino acids can be modified resulting the loss of nutritional value of proteins.

Proteins can be used as foam forming or stabilizing components. Protein molecules diffuse into interfaces of gas and liquid and denatured there, forming flexible and cohesive films around the gas bubbles (e.g. proteins of egg white). Lipids and organic solvents displace proteins from the gas bubble surface due to their hydrophobicity and hence destroy foams (e.g. lecithins in egg yolk).

Proteins are also often involved in gel formation. Gels are disperse systems in which the disperse phase in the dispersant forms a cohesive network hence they show lack of fluidity and elastic deformability. There are two basic types of gels. In polymeric networks fibrous molecules form a three-dimensional network via partly ordered structures (e.g. gelatin gels). These gels possess thermo-reversible character that is the gels are formed when a solution cools, and they melt again when it is heated. Besides cooling gel formation can be caused by setting a certain pH or by adding certain ions. The explanation of their thermo-reversible character is that the aggregation of the molecules of disperse phase takes place mostly via intermolecular hydrogen bonds which easily break when the gel is heated.

The disperse phase of aggregated dispersions is composed by globular proteins. During the formation of these sorts of gels the peptide chains are unfolded due to the effect of heat and the amino acid side chains are released. The deliberated side chains develop new intermolecular interactions. Disulfide bonds can be formed between released thiol groups and also intermolecular ionic bonds between proteins with different isoelectric points in heterogeneous systems (e.g. egg white). These gels have thermoplastic (thermo-irreversible) character i.e. they do not liquefy when heated but soften or shrink.

Proteins owing to their amphipathic nature (contain both hydrophilic and hydrophobic moieties) also has an emulsifying effect. They can form interface films between two immiscible phases. They can stabilize oil in water type emulsions (e.g. fat in milk) (Fig 12.). The formation of these sorts of emulsions is thermodynamically favored because the hydrophobic amino acid residues can escape the hydrogen bridge network of the water molecules, moreover the water molecules are displaced by the proteins from the hydrophobic regions of the oil-water bound. The ideal features for a protein to fulfill its role as an emulsifier are hydrophobe surface, low molecular weight, good solubility in water and balanced amino acid composition.

Chapter 5. Lipids

The chemical structure of this compound group shows a great diversity. The common feature of these molecules is that they are soluble in organic solvents but not in water. Some lipid molecules are amphiphilic and hence they have surface-active properties.

Lipids can be classified according to the hidrofobicity characteristics (neutral – polar). Another sort of grouping is based on if they can be saponified or not. The bulk majority of the food lipids (96-98%) belong to the group of triacylglycerols (triglycerides).

Lipids provide high nutritional energy (37 kJ/g for triacylglycerols) to the organisms hence they are considered as fuel molecules. They are also important sources of vitamins and essential fatty acids, moreover some of them are precursor for bioactive compounds. Amphiphilic lipids are building blocks of biological membranes. Their quantities by weight are less than 2% in food but their high reactivity may exert a strong effect on the organoleptic properties.

The most important source of the edible oils and fats are the storage tissues of plants and animals when triacylglycerols (triglycerides) are deposited.

Fats notably contribute to the enrichment of the nutritional quality of food. The presence of fat provides a specific mouthfeel and pleasant creamy or oily taste, moreover important for the achievement of the desired texture. The fat content of food is also important for the retention of the aroma substances as they can be solvents for taste and odor substances. Beyond this role they act as aroma substances (e.g. short chain fatty acids) or aroma precursors themselves. Amphiphilic lipids can be used as food emulsifiers. Lipids that possess color (fat- or oilsoluble pigments) and occur in raw materials can be applied as natural food colorants.

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