A knowledge of the constituents of foods and their properties is central to food science. The advanced student of food science, grounded in the basic disciplines of organic chemistry, physical chemistry, and biochemistry, and biochemistry, can visualize the properties and reactions between food constituents on a molecular basic. The beginning student is not yet so equipped. This chapter, therefore, will more concerned with some of the general properties of important food constituents, and how these underlie practices of food science and technology.
Foods are made up mostly biochemical (i.e. edible biochemical) which are mainly derived from living sources such as plants and animals. There are three main groups of constituents in foods: carbohydrates, proteins, and fats, and derivatives of these. In addition, there are inorganic and mineral components, and a diverse group of organic substances present in comparatively small proportions that include such substances as vitamins, enzymes, emulsifiers, acids, oxidants, antioxidants, pigments and flavors. There is also the ever-present and very important constituent, water. These components are arranged in different foods to give the foods their structure, texture, flavor, color, and nutritive value. In some instances, foods also contain substances that can be toxic if consumed in large amounts. The general composition of food as well as the way in which the components are organized give a food its individual characteristics. For example, whole milk and fresh apples have about the same water content, but one is a solid and the other a fluid because of the way the components are arranged.
The above constituents occur in foods naturally. Sometimes we are not satisfied with the structure, texture, flavor, color, nutritive value, or keeping quality of foods, and so we add other materials to foods to improve one or more properties. These may be natural or synthetic. For example, we may add natural or synthetic fruit flavors to beverages.
Carbohydrates
Carbohydrates (from “hydrates of carbon”) are organic compounds with the basic structure C4(H20). Among the most important types of carbohydrates in foods are the sugars, dextrin’s, starches, celluloses, hemicelluloses, pectin’s, and certain gums. Chemically, carbohydrates contain only the elements carbon, hydrogen, and oxygen.
Simple carbohydrates are called sugars. One of the simplest carbohydrates is the six-carbon sugar glucose. Glucose and other simple sugars form ring structures of the following form:
Formula of glucose, mannose and galactose
These simple sugars each contain 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms [CX(H20) Y where x=6; y=6]. They differ in the positions of oxygen and hydrogen around the ring. These differences in the arrangement of the elements result in differences in the solubility, sweetness, rates of fermentation by microorganisms, and other properties of these sugars.
Two glucose units may be linked together with the splitting out of a molecule of water. The result is the formation of a molecule of a disaccharide, in this case maltose:
Formula
Common disaccharides formed in similar fashion are sucrose (e.g., cane or beet sugar) made from glucose and fructose (a five-membered ring), maltose or malt sugar from two molecules of glucose, and lactose or milk sugar from glucose and galactose. These disaccharides also differ from one another in solubility, sweetness, susceptibility to fermentation, and other properties.
A larger number of glucose units may be linked together in polymer fashion to form polysaccharides (i.e., “many sugars”). One such polysaccharide is amylose, an important component of plant starches. A chain of glucose units linked together in a slightly different way forms cellulose.
Thus, the simple sugars are the building blocks of the more complex polysaccharides, the disaccharides and disaccharides, trisaccharides, the dextrin, which are intermediate in chain length, on up to the starches, celluloses, and hemicelluloses; molecules of these latter substances may contain several hundred or more simple sugar units. Chemical derivatives of the simple sugars linked together in long chains likewise yield the pectin and carbohydrate gums.
The disaccharides, dextrin, starches, celluloses, hemicelluloses, pectin, and carbohydrate gums are composed of simple sugars, or their derivation. Therefore, they can be broken down or hydrolyzed into smaller units, including their simple sugars. Such breakdown in the case of amylose, a straight chain fraction of starch, or amylopectin, a branched chain fraction (Fig. 3.1), yield dextrin of varying intermediate chain length, the disaccharide maltose, and the monosaccharide glucose. This breakdown or digestion can be accomplished with acid or by specific enzymes, which are biological catalysts. Microorganisms, germinating grain, and animals including humans possess various such enzymes.
The chemically reactive groups of sugars are the hydroxyl groups (---OH) around the ring structure, and when the ring is opened, the
Figure 3.1. Straight chain amylose and branched chain amylopectin fractions of starch. Courtesy of Northern Regional Research Laboratory.
Sugars that possess free aldehyde or ketone groups are known as reducing sugars. All monosaccharides are reducing sugars. When two or more monosaccharides are linked together through their aldehyde or ketone groups so that these reducing groups are not free, the sugar is no reducing. The disaccharide maltose is a reducing sugar; the disaccharide sucrose is a no reducing sugar. Reducing sugars particularly can react with other food constituents, such as the amino acids of proteins, to form compounds that affect the color, flavor, and other properties of foods. In like fashion, the reactive groups of long chain sugar polymers can combine is a cross-linking fashion. In this case the long chain can align and form fibers, films, and three-dimensional gellike networks. This is the basis for the production of edible films from starch as a unique coating and packaging material.
Carbohydrates play a major role in biological systems and in foods. They are produced by photosynthesis in green plants and are nature’s way of storing energy from sunlight. They may serve as structural components as in the case of cellulose, be stored as energy reserves as in the case of starch in plants and liver glycogen in animals, and function as essential components of nucleic acids as in the case of ribose, and as components of vitamins such as the ribose of riboflavin. Carbohydrates can be oxidized to furnish energy. Glucose in the blood is a ready source of energy for animals. Fermentation of carbohydrates by yeast and other microorganisms can yield carbon dioxide, alcohol, organic acids, and a host of other compounds.
Some Properties of sugars
Such sugars as glucose, fructose, maltose, sucrose, and lactose all share the following characteristics in varying degrees:
1. They are usually used for their sweetness
2. They are soluble in water and readily form syrups
3. They form crystals when water is evaporated from their solutions (this is the way sucrose is recovered from sugar can juice)
4. They supply energy
5. They are readily fermented by microorganisms
6. They prevent the growth of microorganisms in high concentration, so they may be used as a preservative
7. They darken in color or caramelize on heating
8. Some of them combine with proteins to give dark colors, known as the browning reaction
9. They give body and mouth feel to solutions in addition to sweetness.
A very important advance in sugar technology has been the development of enzymatic processes for the conversion of glucose to its isomer, fructose. Fructose is sweeter than glucose or sucrose. This has made possible the production of sugar syrups with the sweetness and certain other properties of sucrose starting from starch. Commonly, corn starch is hydrolyzed to provide the glucose, which is the isomerized. The United States produces enormous quantities of corn and with this technology has become less dependent on imported sucrose, the availability and price of which can fluctuate greatly.
Some properties of Starches
The starches important in foods are primarily of plant origin and exhibit the following properties:
1. They are not sweet.
2. They are not readily soluble in cold water.
3. They form pastes and gels in hot water.
4. They provide a reserve energy source in plants and supply energy in nutrition.
5. They occur in seeds and tubers as characteristic starch granules swell due to water uptake and gelatinize.
This increase the viscosity of the suspension and finally, a paste is formed which, on cooling, can form a gel. Because of their viscosity, starch pastes are used to thicken foods, and starch gels, which can be modified by sugar or acid, are used in puddings. Both pastes and gels can revert or retrograde back to the insoluble form on freezing or ageing, causing changes in food texture. Partial breakdown of starches yields dextrin’s, which are intermediate in chain length between starches and sugars and exhibit other properties intermediate between these two classes of compounds.
In recent years much has been learned about modifying the properties of natural starches by physical and chemical means. This has greatly increased the range of uses for starch as a food ingredient, especially with respect to controlling the texture of food systems and permitting the manufacture of food items that require minimum heating to achieve desired viscosity. This technology has been used to make such products as instant puddings which do not require cooking.
Modification techniques include reduction of a starch’s viscosity by chemically or enzymatically breaking the molecules at the glucosidic linkages or by oxidation of some of the hydroxyl groups. The swelling properties of starch heated in water also can be slowed down by cross-linking agents that react with hydroxyl groups on adjacent starch molecules to form chemical bridges between linear chains. The viscosity of such cross-linked starch also is less likely to break down in acid foods and at high temperatures as in cooking and canning. Starch further may be modified by reacting its hydroxyl groups with a range of reagents that form ester, ether, acetyl, and other derivatives. A major effect of this type of modification is to interfere with the tendency of linear molecules to associate or retrograde to the insoluble form on freezing and ageing. Starch granules also may be precooked to produce a starch that will swell in cold water.
Some properties of Celluloses and Hemicelluloses
Celluloses and hemicelluloses, which are abundant in the plant kingdom and act primarily as supporting structures in plant tissues, are relatively resistant to break-down. They are insoluble in cold and hot water and are not digested by man, so do not yield energy. They are important, however, as dietary fiber. Long cellulose chains may be held together in bundles forming fibers, as in cotton and flax; such structures make celery “stingy” and are often reputed by the growth of ice crystals when vegetables such as lettuce are frozen. The fiber in food that produces necessary dietary roughage is largely cellulose, and the hard parts of coffee beans and nut shells contain cellulose and hemicellulose. These materials can be broken down to glucose units by certain enzymes and microorganisms. For example, cellulose from plants and from waste paper can be enzymatically converted to glucose, supplemented with nitrogen, and used for the growth of yeast and other microorganisms as an animal feed supplement or as a source of protein for humans.
Some properties of pectin and Carbohydrate Gums’
Pectin and carbohydrate gums-sugar derivatives usually present in plants in lesser amounts than other carbohydrates exhibit the following characteristics:
1. Like starches and celluloses, pectin is made up of chains of repeating units (but the units are sugar acids rather than simple sugars)
2. Pectin are common in fruits and vegetables and are gum like (they are found in and between cell walls and help hold the plant cells together.
3. Pectin are soluble in water, especially hot water.
4. Pectin in colloidal solution contribute viscosity to tomato paste and stabilize the fine particles in orange juice from settling out.
5. Pectin in solution form gels when sugar and acid are added and this is the basis of jelly manufacture.
Other carbohydrate gums from plants include gum Arabic, gum karaya, and gum tragacanth (seaweeds yield the gums agar-agar, carrageenan, and algin). In addition to their natural occurrence, pectin and gums are added to foods as thickeners and stabilizers.
PROTEINS
Proteins are made by linking individual amino acids together in long chains. Amino acids are made up principally of carbon, hydrogen, oxygen, and nitrogen. Some amino acids also contain other elements such as sulfur.
Proteins are essential to all life. In animals they help form supporting and protective structures such as cartilage, skin, nails, hair, and muscle. They are major constituents of enzymes, antibodies, many hormones, and body fluids such as blood, milk, and egg white.
Typical amino acids have the following chemical formulas.
Formula of leucine, lysine, isoleucine, valine,
Amino acids have the NH2 or amino group, and the COOH or carboxyl group attached to the same carbon atom. These groups are chemically active and can combine with acids, bases, and a wide range of other reagents. The amino and carboxyl groups themselves are basic and acidic, respectively, the amino group of one amino acid readily combines with the carboxyl group of another. The result is the elimination of a molecule of water and formation of a peptide bond, which has the following chemical representation:
Formula of peptide bond
In this case, two amino acids have reacted, a dipeptide is formed, with the peptide bond at the center. The remaining free amino and carboxyl groups at the ends can react in like fashion with other amino acids forming polypeptides. These and other reactive groups on the chains of different amino acids can enter into a wide range of reactions with many other food constituents. There are 20 different major amino acids and a few minor ones that make up human tissues, blood proteins, hormones, and enzymes. Eight of these are designated essential amino acids since they cannot be synthesized by human in adequate amounts to sustain growth and health and must be supplied by the diet. The remaining amino acids also are necessary for health but can be synthesized by humans from other amino acids and nitrogenous compounds and so are designated as nonessential. The essential amino acids are leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. To this list of eight is added histidine to meet the demands of growth during childhood. The nonessential amino acids are alanine, arginine, aspartic acid, cysteine, cysteine, glutamic acid, glycine, hydroxyproline, proline, serine, and tyrosine. The list of essential amino acids differs somewhat for other animal species.
The complexity of amino acid polymerization to form protein chains is illustrated for the protein of human hemoglobin. There is enormous opportunity for variation among proteins. This variation arises from combinations of different amino acids, from differences in the sequence of amino acids within a chain, and from differences in the shapes the chains assume. That is whether they are straight, coiled, or folded. These differences are largely responsible for the differences in the taste and texture of chicken muscle, and milk curd.
Protein chains can be oriented parallel to one another like the strands of a rope as in wool, hair, and the fibrous tissue of chicken breast. Or they can be randomly tangled like a tangled bunch of string. Thus, proteins taken from different foods such as egg, milk, and meat may have a very similar chemical analysis as to C, H, O, and N, and even with respect to their particular amino acids, yet contribute remarkably different structures to the foods containing them.
Further, the complex and subtle configuration of a protein can be readily changed, not only by chemical agents but by physical means. A given protein in solution can be converted to a gel or precipitate. This happens to egg white when it is coagulated by heat. Or the process can be reversed: a precipitate transformed to a gel or solution as in the case of dissolving animal hoofs with acid or alkali to make glue.
When the organized molecular or spatial configuration of a protein is disorganized, we say the protein is denatured. This can be done with heat, chemicals, excessive stirring of protein solutions, and acid or alkali. When egg white is heated, it becomes a solid rather than a liquid because it is irreversibly denatured.
These changes in food proteins are easily recognized in practice. When meat is heated; the protein chains shrink, and so steak shrinks’ ion cooking. When milk is coagulated by acid and heat, protein precipitates, forming cheese curd. If the heat or acid is excessive, the precipitated curd shrinks and becomes tough and rubbery.
Protein solutions can form films and this why egg white can be whipped. The films hold entrapped air, but if your over whip you denature the protein, the film break, and the foam collapses.
Like carbohydrate polymers, proteins can be broken down to yield intermediates of various sizes and properties. This can be accomplished with acids, alkalis, and enzymes. The product of protein degradation in order of decreasing size and complexity are protein, proteases, peptones, polypeptides, peptides, amino acids, NH3, and elemental nitrogen. In addition, highly odorous compounds. Such as mercaptans, skatole, putrescine, and H2S4, can form during decomposition.
Controlled cheese ripening involves a desirable degree of protein breakdown. Putrefaction of meat is the result of excessive protein breakdown accompanying other changes. The deliberate and unavoidable changes in proteins during food processing and handling are among the most interesting aspects of food science. Today, animal, vegetable, and microbial proteins are being extracted, modified, and incorporated into numerous manufactured food products. In addition to their nutritional value, they are selected for specific functional attributes including dispensability, solubility, water sorption, viscosity, cohesion, elasticity, emulsifying effects, foam ability, foam stability, and fiber formation. Additional properties of proteins are discussed in chapter dealing with nutrition, and in subsequent chapters.
FATS AND OILS
Fats differ from carbohydrates and proteins in that they are not polymers of repeating molecular units. They do not form long molecular chains, and they do not contribute structural strength to plant and animal tissues. Fats are smooth, greasy substances that are insoluble in water.
Fat is mainly a fuel source for the animal or plant in which it is found, or for the animal that eats it. It contains about 21/4 times the calories found in an equal dry weight of protein or carbohydrate. For this reason, reduction in the caloric content of foods is often accomplished by replacing fat with protein or carbohydrate. Fat always his other substances associated with it in natural foods, such as the fat-soluble vitamins A, D, E, and K; the sterols, cholesterol in animal fats and ergo sterol in vegetable fats; and certain natural lipid emulsifiers designated phospholipids because of the presence of phosphoric acid in their molecules.
A typical fat molecule consists of glycerol combined with three fatty acids. Glycerol and butyric acid, a common fatty acid found in butter, have the following chemical formulas:
Formula of glycerol butyric acid
Glycerol has three reactive hydroxyl groups, and fatty acids have one reactive carboxyl group. Therefore, three fatty acid molecules can combine with each glycerol molecule, eliminating three molecules of water. Such fats are called triglycerides.
There are about 20 different common fatty acids that are connected to glycerol in natural fats. These fatty acids differ in length and in the number of hydrogen atoms they contain formic acid (HCCOH), acetic acid (CH3COOH), and propionic acid (CH3CH2COOH) are the shortest of the fatty acid. Stearic acid (C17H35COOH) is one of the longer common fatty acids. Some of the opportunities for variations in natural fats can be seen from the formula for a typical triglyceride:
Formula of Typical triglyceride
In this case, the fatty acids reacting with glycerol from top to bottom are lauric acid, stearic acid, and oleic acid, with carbon chain lengths of 12, 18, and 18, respectively. Stearic and oleic acids, although of similar length, differ with respect to the number of hydrogen atoms in their chains. Stearic acid is said to be saturated with respect to hydrogen. Oleic acid with two fewer hydrogen atoms in said to be unsaturated. Another 18 carbons unsaturated fatty acid with four fewer hydrogen atoms and two points of unsaturation is linoleic acid. This unsaturated fatty acid is a dietary essential for health. The degree of unsaturation also affects the physical properties, such as melting temperature, of fats.
Fat molecules can differ with respect to the lengths of their fatty acids, the degree of unsaturation of their fatty acids, the position of specific fatty acids with respect to the three carbon atoms of glycerol, orientation in the chains of unsaturated fatty acids to produce spatial variations within these chains, and in still other ways.
Fat molecules need not have all three hydroxyl groups of glycerol reacted with fatty acids as in a triglyceride. When two are reacted, the molecule, the resulting fat is a monoglyceride. Diglycerides and monoglyceride have special emulsifying properties.
Natural fats are not made up of one type of fat molecule but are mixtures of many types, which may vary in any of the ways previously described. This complexity of fat chemistry today is well understood to the point where fats of very special properties are custom produced and blended for specific food uses.
The chemical variations in fats lead to widely different functional, nutritional, and keeping quality properties. The melting points of different fats are an example of this functional variation. The longer fatty acids yield harder fats, and the shorter fatty acids contributes to softer fats. Unsaturation of the fatty acids also contributes to softer fats. An oil is simply a fat that is liquid at room temperature. This is the basis of making solid fats from liquid oils. Hydrogen is added to saturate highly unsaturated fatty acids, a process known as a hydrogenation. More will be said about changes in fat consistency in the chapter on fats and oils (Chapter 16)
Some additional properties of fats important in food technology are the following:
They gradually soften on heating, that is, they do not have a sharp melting point, since fats can be heated substantially above the boiling point of water, they can brown the surfaces of foods.
When heated further, they first begin to smoke, then they flash, and then burn. The temperatures at which these occur are known as the smoke point, the flash point, and the fire point, respectively. This is important in commercial frying operations.
Fats may become rancid when they react with oxygen or when the fatty acids are liberated from glycerol by enzymes.
Fats form emulsions with water and air. Fat globules may be suspended in a large amount of water as in milk or cream, or water droplets may be suspended in large amount of fat as in butter. Air may be trapped as an emulsion in fat as in butter cream icing or in whipped butter.
Fat is a lubricant in foods that is, butter makes the swallowing of bread easier.
Fats has shortening power; that is, it interlaces between protein and starch structures and makes them tear apart easily and short rather than allow them to stretch long. In this way, fat tenderizes meat as well as baked goods.
Fats contribute characteristic flavors to foods and in small amounts produce a feeling of satiety or loss of hunger.
ADDITIONAL FOOD CONSTITUENTS
Whereas, proteins, and fats often are referred to as the major food constituents due to their presence in substantial amounts, there are other groups of substances which play in important role, out of proportion to their relatively small concentration in foods.
Natural Emulsifiers
Materials that keep fat globules dispersed in water or water droplets dispersed in fat are emulsifiers. Without emulsifiers, mayonnaise would separate into water and oil layers. The mayonnaise emulsion is stabilized by the presence of egg yolk, but the active ingredients in egg yolk stabilizing the emulsion are phospholipids, the best known of which is lecithin. There are many lecithin differing in their fatty acid contents. Chemically, a typical lecithin would have the following formula:
Formula of Typical lecithin
Lecithin are structurally like fats but contain phosphoric acid. Most important, they have an electrically charged or polar end (the + and – at the bottom) and a no charged or nonpolar end at the top. The polar end of this and similar molecules is water-loving or hydrophilic and easily dissolved in water. The uncharged or nonpolar end is fat-loving or hydrophilic and easily dissolved in fat or oil. The result in water oil mixture is that the emulsifier dissolves part of itself in water and the other part in oil. If the oil is shaken in an excess of water, the oil will form small droplets. Then the nonpolar ends of lecithin molecules orient themselves within the fat droplets and the polar ends stick out from the surface of the droplets into the water phase. This has the effect of surrounding the oil droplets with an electrically charged surface. Such droplets repel one another rather than having a tendency to coalesce and separate as an oil layer. The emulsion is thus stabilized, such phenomena are common in foods containing oil and water. Lecithin and other phospholipid emulsifiers are present in animal and plant tissues and in egg, milk, and blood. Without them we could not have stable mayonnaise, margarine, or salad dressings. The monoglyceride and Diglycerides mentioned earlier are also highly effective emulsifiers as are certain proteins.
Emulsifiers belong to a broader group of chemicals known as surface active agents, designated as such because they exert their effects largely at surfaces. Today, a large number of natural and synthetic emulsifiers and emulsifier blends suitable for food use are available. Selection is based largely on the type of food system to be emulsified. With water and oil, one can have oil-in water or water-in-oil emulsions. In an oil-in water emulsion, water is the continuous phase and oil is the dispersed or discontinuous phase; mayonnaise is an example of this type of emulsion. In a water-in-oil emulsion, oil is the continuous phase and water is the dispersed phase; margarine is an example. Generally, the phase present in greater amount becomes the continuous phase of the food system. In choosing an effective emulsifier for a manufactured food, oil-in water emulsions are best stabilized with emulsifiers that have a high degree of water solubility (along with some oil solubility), whereas water-in-oil emulsions are best prepared with emulsifiers having considerable oil solubility and lesser water solubility.
Analogs and New Ingredients
In response to the desire to reduce the caloric content or improve the flavor of many foods, considerable effort has been directed at developing analogs of fat, sugar, and other food components. These analogs have the common objective of mimicking the functional properties such as flavor, mouthfeel, texture and appearance of the indigenous components while at the same time reducing the caloric content of the food. Often these analogs are used to replace high-calorie sweeteners such as sugar or to replace fat. Replacing fat is especially desirable because as pointed out in Chapter 4, on an equal weight basis, fat contains more than twice the calories of other food components.
The use of fat replacers in ice cream is a good example of the use of such analogs. Fat contributes smoothness, creaminess, and flavor to ice cream. A process has recently been developed where egg or milk proteins are formed into very small hard spheres. When suspended at eh proper concentration in a liquid, these protein spheres also have a creamy or smooth texture. This is similar to chocolate in which the finely ground cocoa produces smoothness. These protein spheres can then replace some or all of the fat in ice cream or other products. Because protein on an equal-weight basis has fewer calories than fat, the net result is a reduction in caloric content of the ice cream.
Other analogs or substitutes have been developed. Fat substitutes which can be used as a cooking oil, yet are not absorbed by humans have been developed. This means that fried foods such as potato chips could have their caloric content substantially reduced. However, government approval of these substitutes is still pending. Aspartame, which is made of amino acids, is an example of a sugar substitute. On an equal-weight basis, aspartame has about the same caloric content as sucrose but is 200 times sweeter. Therefore, less is used and the caloric content of the food is reduced.
Several challenges remain in the use of analogs. For example, analogs may mimic some important functions but often do not behave in exactly the same way as the food component they are substituting. Fat replacers may have the mouthfeel of fat but do not carry the fat-soluble flavors or vitamins of the natural fat. Sometimes, sugar substitutes can leave undesirable after tastes. Some fat or sugar substitutes are not neat stable and decompose when heated.
Organic Acid
Fruits contain natural acids, such as citric acid of oranges and lemons, malic acid of apples, and tartaric acid of grapes. These acids give the fruits tartness and slow down bacterial spoilage.
We deliberately ferment some foods with desirable bacterial to produce acids and thus give the food flavor and keeping quality. Examples are fermentation of cabbage to produce lactic acid and yield sauerkraut, and fermentation of apple juice to produce first alcohol and then acetic acid to obtain vinegar. In the manufacturer of cheese, a bacterial starter culture is added to milk to produce lactic acid. This aids in curd formation and in the subsequent preservation of the curd against undesirable bacterial spoilage.
Besides imparting flavor and aiding in food preservation. Organic acids have a wide range of textural effects in food systems due to their reactions with proteins, starches, pectin, gums, and other food constituents. The rubbery or crumbly condition of cheddar cheese depends largely on acid concentration and Ph.as does the stretch ability of bread dough, the firmness of puddings, the viscosity of sugar syrups, the spread ability of jellies and jams, and the mouthfeel of certain beverages. Organic acids also influence the colors of foods, as may plant and animal pigments are natural Ph. indicators. Acids are also important inhibitors of bacterial spoilage in foods, particularly of bacterial which can cause human disease. For example, under anaerobic conditions and slightly above a ph. of 4.6, clostridium botulinum can grow and produce lethal toxin. C. botulinum does not grow in foods high enough in organic acids to have a ph. of 4.6 or less, so it is not a hazard.
Oxidants and Antioxidants
Many food constituents are adversely affected by oxygen in the air. This is so of fats, oils, and oily flavor compounds which may become rancid on excessive exposure to air, carotene, which yields vitamin A, and ascorbic acid, which is vitamin C, also are diminished in vitamin activity by oxygen. Oxygen is an oxidant; it causes oxidation of these materials. Oxygen is always present in and around foods, although it may be minimized by nitrogen or vacuum packaging.
Certain metals such as copper and iron are strong promoters or catalysts of oxidation. This is one of the reasons why copper and iron have largely been replaced in food processing equipment by stainless steel. Many natural foods, however, contain traces of copper and iron, but they also contain antioxidants.
An antioxidant, as the term implies, tends to prevent oxidation. Natural antioxidants present in foods include lecithin (which also is an emulsifier), vitamin C and E, and certain sulfur containing amino acids. However, the most effective antioxidants are synthetic chemicals approved by the Food and Drug Administration for addition to foods.
Enzymes
Enzymes are biological catalysts that promote a wide variety of biochemical reactions. Amylase found in saliva promotes digestion or breakdown of starch in the mouth. Pepsin found in gastric juice promotes digestion of protein. Lipase found in liver promotes breakdown of fats. There are thousands of different enzymes found in bacteria, yeast, molds, plants, and animals. Even after a plant is harvested or an animal is killed, most of the enzymes continue to promote specific chemical reactions, and most foods contain a great number of active enzymes. Enzymes are large protein molecules which, like other catalysts, need to be present in only minute amounts to be effective.
Enzymes function by lowering the activation energies of specific substrates. They do this by temporarily combining with the substrate to form an enzyme-substrate complex that is less stable than the substrate alone. This overcomes the resistance to reaction. The substrate thus excited plunges to a still lower energy level by forming new products of reaction. In the course of reaction, the enzyme is released unchanged. The release of the enzyme so that it can continue to act explains why enzymes are effective in such trace amounts.
The reactions catalyzed by a few enzymes of microbial origin are indicated in table 3.1. Some properties of enzymes important to the food scientist are the following:
In living fruits and vegetables, enzymes control the reactions associated with ripening;
After harvest, unless destroyed by heat, chemicals, or some other means, enzymes continue the ripening process, in many cases to the point of spoilage such as soft melons and overripe bananas;
Because enzymes enter into a vast number of biochemical reactions in foods, they may be responsible for changes in flavor, color, texture, and nutritional properties;
The heating processes in food manufacturing are designed not only to destroy microorganisms but also to inactivate food enzymes and thereby extend the storage stability of foods;
When microorganisms are added to foods for fermentation purposes, the important agents are the enzymes the microorganisms produce;
Enzymes also can be extracted from biological materials and purified to a high degree.
Such commercial enzymes preparations may be added to foods to break down starch, tenderize meat, clarify wines, coagulate milk protein, and produce many other desirable changes. Some of these additional changes are indicated in Table 3.2.
Some we wish to limit the degree of activity of an added enzyme but cannot readily inactivate the enzyme without adversely affecting the food. One way to accomplish this is to immobilize the enzyme by attaching it to the surface of a membrane or another inert object in contact with the food being processed. In this way reaction time can be regulated without the enzyme becoming part of the food.
Such immobilized enzymes are presently being used to hydrolyze the lactose of milk into glucose and galactose, to isomerize the glucose from corn starch into fructose, and in many other industrial food processes.
Pigments and colors
Foods may acquire their color from any of several, sources. One major source is natural plant and animal pigments. For example, chlorophyll imparts green color to lettuce and peas, carotene gives the orange color to carrots and corn, lycopene contributes to the red of tomatoes and watermelons, anthocyanin contributes purple to grapes and blueberries, and ox myoglobin gives the red color to meat.
These natural pigments are highly susceptible to chemical change as in fruit ripening and meat ageing. They also are sensitive to chemical and physical effects during food processing. Excessive heat alters virtually all natural food pigments. Chopping and grinding also generally change food colors because many of the plant and animal pigments are organized in tissue cells and pigment bodies, such as the chloroplasts which contain green chlorophyll. When these cells are broken, the pigment leach out and are partially destroyed on contact with air.
Not all food color comes from true plant and animal pigments. A second source of color comes from the action of heat on sugars. This is referred to as caramelizing. Examples of caramelization are the darkening of maple sugar on heating, the color of roasting bread, and brown color of caramel candy.
Third, dark colors result from chemical interactions between sugars and proteins, referred to as the browning reaction or the Millard reaction. In this case, an amino group from a protein combines with an aldehyde or ketone group of a reducing sugar is produce a brown color – an example is the darkening of dried milk on long strong.
Complex color changes also occur when many organic chemicals present in foods come in contact with air examples are the darkening of a cut surface of an apple and the brown color of tea from tea tannins. These oxidations generally are intensified by the presence of metal ions.
In many foods and in cooking, final color is the result of a combination of several of the above, which adds to the complexity if the field of food color.
Not to be overlooked is the intentional coloring of food by the addition of natural or synthetic colors as in the coloring of gelatin desserts or the addition of vegetable dyes to Cheddar cheese to make it orange.
Flavors
If food color is complex, then the occurrence and changes that take place in food flavors are certainly no less complex.
In coffee alone over 800 constituents have been identified, many of which may contribute to the flavor and aroma, although the contribution of many of them may be quite small. These organic chemicals are highly sensitive to air, heat, and interaction with one another. The flavor and aroma of coffee, milk, cooked meats, and most foods is in a continuing state of change-generally becoming less desirable as the food is handled, processed, and stored. There are exceptions, of course, as in the improvement of flavor when cheese is ripened, wine is aged, or meat is aged.
It is important to recognize that flavor often has a regional and cultural basis for acceptance. Not only do many Orientals prize the flavors of “100-years eggs” and sauces made from aged fish, but in the United States different blends of coffee are favored in the South and in the North, and sour cream is not as popular in the Midwest as in the East.
The detailed discussion of the chemistry of flavor is beyond the scope of this book. However, flavors are one of the major areas of interest among food chemists. Much progress has been made in this area from use of analytical methods such as gas chromatography and mass spectrometry. In gas chromatography, aroma compounds are separated from one another on the basis of relative volatility by a special column though which gas is passed. Each compound gives a specific peak on a recording chart.
The peaks corresponding to aroma compounds, obtained from two kinds of apples are shown in Fig. 3.5. Although such methods are highly sensitive, for many flavor and aroma compounds they are not as sensitive as the human nose. Furthermore, the instrumental approach does not tell whether a flavor is liked or disliked. Therefore, subjective methods of study also are used. These employ various kinds of taste panels. Because the results are subjective, conclusions are generally based on the judgments of several people making up the panel.
Vitamins and Minerals
Vitamins and minerals are essential pars of food because they are required for normal health, there are a wide range of minerals and vitamins that are important in the diet, and the effects of food formulation and processing on vitamins and minerals must be understood. This will be discussed in detail in Chapter 4 on the nutrients of food.
Natural Toxicants
Over the centuries, plants have evolved the ability to form many compounds which play no direct biochemical role in the plant but may serve to protect the plant or help ensure reproduction. These secondary metabolites may attract pollinating insects or reject predators which attack the plant. It is not surprising that some of these compounds can toxic. For example, some species of mushrooms have poisonous properties due to specific nitrogen-containing bases or alkaloids that, depending on concentration, can produce marked physiological effects. Many other natural foods also contain substances that can be harmful if consumed in sufficient quantities, but are not a threat at the low concentrations present in our usual diets.
Similarly, soil and water normally contain the potentially harmful metals lead, mercury, cadmium, arsenic, zinc, and selenium, and so traces of these metals occur in foods and always have. At their low levels of occurrence, however, not only are these natural components of foods harmless but zinc, selenium, and possibly others are essential nutrients.
Many harmful substances are not normal components of foods but can become part of food; these include industrial contaminants, toxins produced in food by microorganisms, and additives whose safe-use levels are exceeded. These kinds of materials are dealt with in subsequent chapters.
In addition to heavy metals, some of the better known toxicants occurring naturally in foods include low levels of the alkaloid sol nine in potatoes, cyanide-generating compounds in lima beans, safrole in spices, prussic acid in almonds, oxalic acid in spinach and rhubarb, enzyme inhibitors and hemagglutinins in soybean and other legumes, gossypol in cottonseed oil, goitrogen in cabbage that interfere with iodine binding by the thyroid gland, tyramine in cheese, avid in egg white which is antagonistic to the growth factor biotin, thiamine is fish which destroys vitamins A and D and essential amino acids such as methionine also exhibit toxic effects in excessive concentration.
Several of these materials and certain other natural toxicants are largely removed or inactivated when foods are processed. Thus, the heat of cooking destroys enzymes inhibitors and hemagglutinins of beans, avid in of egg white, and thiaminase of fish. Water soaking and fermentation also remove some cyanogenic compounds. Removal of gonads, skin, and parts of certain fish eliminates toxins concentrated in these tissues. Breeding and selection also have lowered concentrations of toxicants in certain plant foods. Further, in the course of evolution, man has developed physiological mechanisms to detoxify low levels of many potentially dangerous substances and has learned to exclude clearly toxic species as food sources.
Although much more remains to be learned, a varied diet of the conventional foods of a region or culture pose small risk from natural toxicants to normally healthy individuals. Departures from conventional food sources and time honored processes without adequate testing, microbial toxins, and harmful levels of industrial chemicals generally present greater dangers. With respect to all substances that may be normal constituents of food or become part of a food, it is important to recognize that such substances are not harmless or harmful per se but only so in terms of their concentrations.
Water
Water is present in most natural foods to the extent of 70% of their weight. Fruits and vegetables may contain 90% or even 95% water. Cooked meat from which some of the water has been driven off still contains about 60% water. Water greatly affects the texture of foods a raisin is a dehydrated grape, and a prune, a dried plure. The form in which water occurs in foods to a large extent dictates the physical properties of the food. For example, fluid milk and apples contain approximately the same amount of water but have different physical structures.
Water greatly affects the keeping qualities of food, which is one reason for removing it from foods, either partially as in evaporation and concentration, or nearly completely as in true food dehydration. When foods are frozen, water as such also is removed since water is most active in foods in its liquid from. As a liquid in foods, it is the solvent for numerous food chemicals and thus promotes chemical reactions between the dissolved constituents. It also is necessary for microbial growth.
The other reason for removing water from food (in addition to preservation) is to reduce the weight and bulk of the food and thus save on packaging and shipping costs.
A great deal of food science and food technology can be described in terms of the manipulation of the water content of foods: its removal, its freezing, its emulsification, and its addition in the case of dissolving or reconstituting dehydrated foods.
Water exists in foods in various ways as free water in the case of tomato juice, as droplets of emulsified water in the case of butter, as water tied up in colloidal gels in gelatin desserts, as a thin layer of adsorbed water on the surface of solids often contributing to caking as in dried milk, and as chemically bound water of hydration as in some sugar crystals.
Some of these bound water forms are extremely difficult to remove from foods even by drying, and many dehydrated foods with as little as 2-3% residual water have their storage stability markedly shortened.
Close control of final water content is essential in the production of numerous foods: as little as 1-2% of excess water can result in such common defects as molding of wheat, bread crusts becoming tough and rubbery, soggy potato chips, and caking of salt and sugar. Many skills in food processing involve the removal of these slight excesses of water without simultaneously damaging the other food constituents. On the other hand, even where a dehydrated product is involved, it is possible to remove too much water. In some cases, the storage stability of a dehydrated item is enhanced by leaving a trace of moisture, equivalent to a monomolecular layer of water then may serve as a barrier between atmospheric oxygen and sensitive constituents in the food which otherwise would be more easily oxidized.
It is obvious that the purity of water used in foods or associated with the manufacture of foods is of utmost importance. It is less obvious, however, that suitable drinking water from a municipal water supply may not be of adequate purity of certain food uses.
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