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

Milk fat present in 0.1–10 μm droplets in milk consisted of mainly triglycerols (95–96%). The lipid core (triglycerols) of these globules is surrounded with a membrane containing phospho- and glycolipids and protein double layer. Milk fat globule membrane proteins (MFGM proteins) are dangerous for the health of sensitized persons.

When the milk is homogenized it is forced through small passages under pressure. As the result of turbulence, cavitation and shear forces fat droplets disintegrate into smaller droplets (<1 μm) and cream will not be separated even after prolonged standing. The membranes of the fat globules prevent lipolysis. When the milk is homogenized and fat globule surface area is increased casein also participates in membrane formation and enzyme proteins are also dominant. Lipases binding to the fat globules can hydrolyze the triacylglycerols at a high rate and rancid off-flavor may be formed therefore pasteurization is required before milk homogenization.

Low carbon number fatty acids occur at relatively high ratio in the milk fat in the milk of ruminants. The proportion of linoleic acid is low: the bulk amount of linoleic acid originated from feed is hydrogenated in the rumen; however it can be enhanced by the addition of encapsulated plant oil to the feed. Odd-carbon-number, branched-chain and oxo-fatty acids are also present in small quantities.

Milk is an important source of A (retinol), B₂ (riboflavin) and B₁₂ (cobalamin) vitamins and a rich source of calcium and phosphorus. The daily needs of these elements of an adult person can be supplied with 7 dl milk. The absorption of calcium from milk is very efficient because calcium mostly present in protein-bound form and lactose, vitamin D and citric acid also promote absorption.

Heat treatments applied on milk in order to extend the shelf life of the product can lead to the deterioration of valuable milk constituents in different degree depending on the heating conditions. While sterilization accompanied with 20-100% loss of thiamine, pyridoxine cyano-cobalamine, folic acid and ascorbic acid, the losses owing to pasteurization, UHT-technology or boiling were moderate (0-30%).

Sterilization can induce reactions between lactose and free amino groups resulting Maillard reaction products. This process can be detected with the increase of the level of hydroxymethyl furfural (HMF) or browning of milk. In the presence of hydroxy- or keto-fatty acids δ-lactone is formed with pleasant odor, but methyl-ketones also can be formed with undesirable, unpleasant odor. Owing to the excessive heat impact calcium phosphate precipitate on casein micelles and the membranes of milk fat globules are modified therefore cream separation properties change.

Heat treatment can cause thiol-disulfide exchange reaction between κ-casein and β-lactoglobulin therefore κ-casein become less susceptible to hydrolyze due to the action of chymosin and the rennet coagulation of heated milk is retarded.

Casein is not a heat coagulable protein; it coagulates only at high temperatures. Sodium or calcium caseinate solutions are dephosphorylated at 120 ºC for 5 h. The pH exerts an important effect on the coagulation properties of casein because at lower pH the coagulation temperature is lower.

The coagulation of whey proteins is significantly affected by both temperature and pH. At higher temperatures with longer heating the whey proteins start to denature. If the pH is in the range of their isoelectric points due to souring whey proteins denatured together with casein (coprecipitation). This coagulation process is important e.g. in cottage cheese production.

2. Meat and Meat Products

From food legislation aspect meat is the parts of warm-blooded animals that are suitable for human consumption, both in fresh or processed form. In informal language it means the skeletal muscle tissue containing more-or-less adhering fat. In the point of view of nutrition meat is a very valuable source of protein, fat, vitamin and minerals. Its carbohydrate content is low. The organic acids have a role in the development of the characteristic aroma of meat. Meat contains in an average 76% of moisture. Among the compounds of dry matter the ratio of protein is abundant relatively to the others (89% of dry matter is protein in an average) – but significant deviations can be detected from that value owing to the high variation in fat content. Factors effecting the base chemical composition of meat are genetic (species, breed), feeding management, age, sex and part of the carcass.

The differences in meat color can be attributed to the alterations in myoglobin content and the ratio of connective tissue. White muscle (poultry and fish) are rich in myofibrils and poor in sarcoplasm while red muscle has high ratio of sarcoplasm to myofibrils.

Proteins in the muscle tissue can be divided into three groups: proteins of the contractile apparatus that are extractable with concentrated salt solutions; water soluble proteins; and insoluble proteins (in connective tissue and membrane).

The members of the first group are myosin, actin, tropomyosin, troponin and titin.

Water soluble proteins are the enzymes and myoglobin . If the partial pressure of oxygen is high oxymyoglobin is present giving the bright red color of fresh meat cuts. When the partial pressure of oxygen is low the oxidation is slow resulting brown metmyoglobin. Details on the formation of coloring compounds of meat (including the effect of curing) are discussed in Chapter 7. in the part on ’Metal bearing ringed tetrapyrrole derivatives’.

Mostly the connective tissue proteins belong to the fraction of insoluble proteins. Collagen gives 20-25% of the total protein in mammals. Collagen is denaturated due to the action of lactic acid formed post mortem from glucose and it can be cleaved by lysosomal enzymes (e.g. lysosomal collagenase and cathepsin B1). The intact collagen fiber shrinks when heated. When heating exceeds shrinkage temperature, a transition from the highly ordered structure of the fibrous protein into random coils occurs and the solubility of the resulting protein increases. The extent of gelatinization (transition of collagen to gelatin) during the cooking and roasting of meat depends on the age of the animal (the degree of collagen cross-linking); and the applied conditions (temperature, time, pressure). Food gelatin obtained from bones or skin is used as gelling agent.

Biogenic amines are formed on the decarboxylation of amino acids (mostly by the enzymes of microbes) therefore their presence is an indicator of microbial quality. In beef and pork: histamine, tyramine, putrescine and cadaverine may occur.

Creatine and creatinine are typical constituents of muscle tissue (creatine phosphate: energy reservoir). Its presence indicates the inclusion the meat extract in a food product.

In stress impaired hog ATP decomposes very rapidly in the muscle prior to slaughtering therefore the rate of glycolysis is accelerated resulting rapid pH decrease and body temperature increase to 40–41 ºC. These processes lead to the formation of pale, soft, and exudative meat (PSE meat). Due to the light scattering of precipitated proteins the meat appears paler although the myoglobin content is unchanged. Owing to the disintegration of the cell membranes the meat has low water holding capacity and therefore there is an extensive loss of water (drip loss <15% in 3 days vs. 4% in normal meat).

The other defect in pork is called dark and firm and dry meat (DFD meat). In this case glycogen is largely used up due to stress and the amount of the lactic acid formed after slaughtering is low therefore the pH hardly falls. The higher pH the more water is bound by myofibrils therefore the texture is dry. The oxymyoglobin is more stabile at higher pH values causing a dark color. These meats with high pH value are susceptible to microbial infection.

In case of beef muscle PSE effect is not significant because fat oxidation provide enough energy in tissues and glycogen breakdown can occur slowly.

3. Egg

Egg has high quality nutrients and it is called one of nature’s nearly perfect foods. It can be utilized in many ways both in the food industry and the home. Eggs of hens and other birds (ducks, geese, plovers, seagulls and quail) are also consumed.

The microstructure of egg yolk is a fat-in-water emulsion containing 50% dry matter including 65% lipids and 31% proteins. The main proteins are phosvitin and lipovitellins. Phosvitin is a relatively heat stable glycophosphoprotein acting like a polyelectrolyte (polyanion). It very strongly binds multivalent cations; due to the trapping of heavy metal ions, phosvitin can synergistically support antioxidants.

Lipovitellins are high density lipoproteins (HDL) that are covalently bound to oligosaccharides (from mannose, galactose, glucosamine, sialic acid units). They are present as a complex with phosvitin and heat stable.

Lipids in egg yolk are closely associated with the proteins occurring in yolk (lipoproteins). Their fatty acid composition can be effected by feeding but the fatty acid pattern of the feed is reflected more clearly in the triglyceride fraction of egg lipids than in the polar lipids. Most of the sterols are cholesterol being present 2.5% of the dry matter and 4% of the egg lipids therefore it exceeds by far that in all other foods and can be used as an indicator of the addition of eggs.

Albumen (egg white) is a 10% aqueous solution of different proteins.Albumen proteins with biological activity give a protection of the egg from microbial spoilage e.g. enzymes (e.g., lysozyme), enzyme inhibitors (e.g., ovomucoid, ovoinhibitor) or complex-forming agents for coenzymes (e.g., flavoprotein, avidin).

Ovalbumin is the main albumen protein that is relatively readily denatured with shaking or whipping its aqueous solution. Conalbumin (ovotransferrin) retard the growth of microorganisms while ovomucoid inhibits bovine but not human trypsin activities. Lysozyme is can be found in many animal tissues and secretions and hydrolyzes the cell walls of Gram-positive bacteria.

Egg colorants are used to color of the yolk in order to improve quality characteristic. Carotenoids and xanthophylls (preferably lutein) absorbed from the feed. The color of the yolk can be also intensified by feed additives. During storage of eggs the pH, the density and the viscosity of the eggs change and other properties important for utilization of egg products also fluctuatie (e.g. foaming ability). Other important factors effecting production are emulsifying effect and thermal coagulation.

4. Fats and Oils

Edible fats and oils contain mostly triacylglycerides with different fatty acid composition. The ratio of unsaponifiable fraction is usually low (below 3%). Fat generally designates a solid at room temperature while oil is liquid – these denominations are confusing because many fats are neither solid nor liquid, but semi-solid.

Oils rich in lauric and myristic acids are coconut and palm seed oils. These oils are solid at room temperature and used in vegetable margarines. As shelf life stability is reflected in fatty acid compositions and linoleic acid is present in negligible amounts the autoxidative changes are retarded.

Oils rich in palmitic and stearic acids are cocoa butter and fats called ’cocoa butter interchangeable fats’. They are relatively hard. Cocoa butter has a narrow melting range. It gives a pleasant, cooling sensation in the mouth when it is melted. It has a resistance to autoxidation and microbiological deterioration and utilized in the manufacturing of chocolates, candy and confections.

Oils rich in palmitic acid are cottonseed oil and cereal germ oils. Corn (maize) oil used widespread in kitchen and in the food industrial production. Wheat germ oil has additional nutritive value because of its high tocopherol content. Rice germ oil consumed mostly in Asia while pumpkin oil in southern Europe.

Oils low in palmitic acid and rich in oleic and linoleic acids are important raw materials for margarine production and a large number of oils from diverse plant families. Sunflower is the most cultivated oilseed plant in Europe. Its refined oils used as salad or frying oil and raw material for margarine production. Soybean oil is at the top of the world production of edible oils of plant origin. Its shelf life can be improved by partial hydrogenation. Soybean genotypes with low linolenic and high oleic acid content are considerably more stable to oxidation and partial hydrogenation is no longer required. Rapeseed oil of old rape cultivars contained 45–50 [w/w%] erucic acid that can cause heart muscle damage. Nowadays ’zero’ erucic acid cultivars and ’double zero’ cultivars (low levels of erucic in the oil and goitrogenic compounds in the seed meal) are used. Linseed oil readily autoxidizes owing to its high linolenic acid content. Mostly the cold pressed oil is utilized as edible oil.

The major fatty acids of animal fats are oleic, stearic and palmitic. The fatty acid composition is greatly varying depending on the breed of animal and feeding conditions. Beef fat is friable with brittle consistency and its fatty acid composition is not influenced greatly by feed intake. Hog fat (lard) is the most consumed animal fat after tallow and butter. Its consistency is grainy and oily. The fatty acid profile is influenced by breed and feeding.

The factors that are affecting the quality of fats and oils are the extent of lipolysis and that of oxidative deterioration. These processes have been discussed in Chapter 5.

5. Cereals and Cereal Products

The major cereals are wheat, rye, rice, barley, millet and oats. Cereals are amongst the most important staple foods of mankind. In industrial countries nutrients provided by bread consumption covers 50% of the daily requirement of carbohydrates, one third of the proteins and 50–60% of vitamin B. Cereals are also an important source of minerals and trace elements.

Starch is the major storage carbohydrate form of cereals occurring in the endosperm cells. Starch granules have an amorphous layer (mainly amylose) and semicrystalline layers (amylopectin). Their swelling and gelatinization shows a characteristic temperature range depending on the cereal source. The other polysaccharides are primarily constituents of cell walls therefore their amount in flour increases as the extent of grinding increases. Soluble and insoluble polysaccharides other than starch and lignin are called dietary fiber .

The proteins of cereals contain low ratio of lysine and methionine. The amount of methionine is particularly low in wheat, rye, barley, oats and corn. Enzyme proteins are present in the albumin and globulin fraction while storage proteins belong to the fractions prolamins and glutelins. Wheat, rye and barley have similar amino acid composition. The amino acid composition of only the prolamins can be correlated to the botanical genealogy of cereals.

The gluten proteins of wheat together with associated lipids are responsible for the cohesive and viscoelastic flow properties of dough. Gluten is responsible for plasticity and dough stability. These rheological properties determine the dough gas-holding capacity during leavening and the porous, spongy product with an elastic crumb after baking.

In the case of rye and other cereals gluten is not formed. Pentosans and some proteins swell after acidification that contribute to gas-holding properties and affect the baking quality.

Cereal kernels contain relatively low levels of lipids that are preferentially stored in the germ and in a smaller extent in the aleurone layer. The fatty acid composition of cereal lipids do not differ significantly. Linoleic acid predominates among fatty acids. Wheat lipids greatly influence baking quality. Polar lipids positively influence the gas-holding capacity of doughs and the baking volume. Nonpolar lipids generally negatively influence the backing result with most varieties of wheat.

In the presence of yeast the optimum activities of α- and β-amylases are desirable in dough making in wheat and rye.

The lipase activity in dormant seed (with the exception of oats) is usually low. During flour storage the level of free fatty acids can rise due to the action of these enzymes.

The partial hydrolysis of phytate (inositol-hexaphosphate) with phytase is desirable from a nutritional physiological point of view because less phosphorylated inositols do not form such stable complexes with cations as phytate and the absorption of zinc, iron, calcium and magnesium ions is not impeded.

Lipoxygenase from wheat cooxidizes carotenoids therefore a loss of yellow color in pasta products can occur. Wheat lipoxygenase has to be inactivated during the preparation of these products. Polyphenoloxidases can cause browning in whole-meal flours.

6. Vegetables and Fruits

Vegetables are fresh parts of plants which are suitable for human nutrition - raw, cooked, canned or processed in some other way. Ripe seeds (peas, beans, cereal grains, etc.) are not considered to be vegetables. Their chemical composition varies significantly depending upon the cultivar, agriculture and origin. Most vegetables contain mostly carbohydrates within the dry matter fraction. Vitamins, minerals and flavor substances are important secondary constituents.

Starch is widely occurs as a storage carbohydrate and present in large amounts in some root and tuber vegetables. Inulin is the storage carbohydrate in Compositae. The predominant sugars in the carbohydrate fraction are glucose and fructose and sucrose. Pectin has distinct role in the tissue firmness of vegetables.

The protein fraction of vegetables consists to a great extent of enzymes that can exert a beneficial or a detrimental effect on processing. They may contribute both to the development of the typical flavor of the product or the formation of undesirable flavors, tissue softening and discoloration.

The lipid content is usually low. Carotenoids are occasionally found in large amounts (e.g. in green bell and paprika peppers and tomato). Some of the aroma substances characteristic to the given vegetable is originated from the enzymatic oxidation of unsaturated lipids or other sources (e.g. from glucosinolates in Brassicaceae or alliin in garlic). Aromas present in the raw vegetables undergo significant conversion during heat treatments. The effects of enzymes catalyzing aroma defect reactions can be eliminated by blanching.

There is a significant variation in vitamin content. The most important factors effecting values are vegetable cultivar, climate and elapsed time of storage. The most abundant mineral constituent is potassium.

The major constituents of fruits are sugars, polysaccharides and organic acids. Mostly glucose and fructose are present in greatly varying ratios. The total pectin content decreases during ripening while the ratio of soluble pectin fraction increases. Starch is present in unripe fruits with few the exceptions (e.g. bananas).

Phenolic compounds mostly present as glycosides, partly also as esters. They are antioxidatively active and contribute to the color and taste of many types of fruit. In processing they can cause discoloration by the formation of metal complexes or turbidity by complexation of proteins. The oxidation of phenols by phenol oxidase enzymes is called enzymatic browning. Discoloration can be avoided by inactivation of enzymes by heat treatment or using reductive agents (SO₂ or ascorbic acid) or to remove the available oxygen.

7. Sweeteners and Chocolate

Among the sugars occurring in the nature only a few is used as sweeteners. The most important sugars used worldwide are sucrose (saccharose), glucose (starch sugar or starch syrup), invert sugar (equimolar mixture of glucose and fructose); moreover maltose, lactose and fructose. The degree of the application of sweeteners depends mostly on the next factors: the nutritional, physiological and processing properties of the molecules, cariogenicity as compared to sucrose, economic impact and the quality and intensity of the sweet taste.

The most important physical properties affecting the use of the sweeteners are solubility, viscosity of the solutions, and hygroscopicity. Among chemical properties, pH is very important because the stability of sugars depends on pH. In mildly acidic solutions monosaccharides are stable, but disaccharides hydrolyze to yield monosaccharides. In stronger acidic solutions dehydration reactions prevail. In mildly alkaline solutions reducing sugars are unstable while nonreducing disaccharides (e.g. sucrose) have stability maxima. The thermal stability of sugars is also different.

In nutritionally point of view, sucrose, maltose and isomaltose type oligosaccharides hydrolyze in the gastrointestinal tract of the human organism while lactase enzyme is lacking in some adults. All metabolized monosaccharides can be interconverted. Only glucose can enter the insulin-regulated and -dependent energy metabolism and be utilized by all tissues. Galactose is rapidly transformed into glucose therefore it is nutritionally equal to glucose. Oral intake of glucose and galactose causes rapid increase in blood sugar levels and insulin secretion. Other monosaccharides primarily metabolized by the liver and do not directly affect glucose status or insulin release. The blood sugar raising effect of carbohydrates is quantified with the use of Glycemic Index (GI).

Artificial sweeteners are discussed in Chapter 10. among food technological additives.

Honey is the oldest sweetener has been used since ancient times. It can not be regarded simply as a concentrated aqueous solution of glucose and fructose, because it contains several other compounds (e.g. enzymes, minerals, organic acids and aroma substances). Its water content should be less than 20% in order to avoid deterioration by osmophilic yeast. It has more than 20 sorts of di- and oligosaccharides with maltose predominating. The bacteriostatic effect of honey can be explained by the gluconic acid content that formed from glucose by glucose oxidase.

Chocolate is made of cocoa liquor (milled cocoa nib), sucrose, cocoa butter, aroma and other ingredients (milk ingredients, nuts, coffee paste, etc.). The fat of cocoa butter is rich in palmitic and stearic acid, relatively hard and has a narrow melting range (30-40 ºC). Owing to its fatty acid composition it is resistant to autoxidation and microbiological deterioration; moreover during consumption gives a pleasant, cooling sensation (’melting in the mouth’ effect). The phenolic compounds being present in cocoa powder have been shown to contribute to the chemoprevention against tumor development.

8. Alcoholic Beverages

The fermentation technology of beer and wine production has been known to early civilizations while distillation processes for liquor production were developed much later.

Wine is obtained by full or partial alcoholic fermentation of fresh, crushed grapes or grape juice (must). The must of ripe grapes contains glucose and fructose in equimolar ratio while fructose predominates in overripe or botrytised berries. In a good vintage L-tartaric acid predominates among organic acids whilst L-malic acid is the main component when unripe grapes are fermented. Phenolic compounds like tannins occur in stems, skin and seeds. Color pigments of red wine are anthocyanidins and anthocyanins (formed through the glycosylation of aglicone anthocyanidin). The glycoside form is more stabile. The composition of the pigments depends on grape cultivar.

During the