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二、Carbohydrates


Lesson 2 Carbohydrates

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The food scientist has a many-sided interest in carbohydrates. He is concerned with their amounts in various foods, availability (nutritional and economic), methods of extraction and analysis, commercial forms and purity, nutritional value, physiological effects, and functional properties in foods. Understanding their functional properties in processed foods requires not only knowledge of the physical and chemical properties of isolated carbohydrates, but also knowledge of the reactions and interactions that occur in situs between carbohydrates and other food constituents and the effects of these changes upon food quality and acceptance.

This is a tall order for knowledge. Because processing affects both nutritional and esthetic values of food, knowledge of the changes that carbohydrates undergo during milling, cooking, dehydration, freezing, and storage is especially important.

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Students are advised to study the fundamental chemistry underlying useful carbohydrates properties. Of service will be an understanding of the association of polar molecules through hydrogen bonding, ionic effects, substituent effects, chelation with inorganic ions, complexing with lipids and proteins, and decomposition reaction. This background will provide an understanding of properties that affect the texture and acceptance of processed foods (e.g., solubility, hygroscopicity, diffusion, osmosis, viscosity, plasticity, and flavor production), properties that enable the formation of highquality pastries, gels, coatings, confections, and reconstitutable dehydrated and frozen foods.

? Ability to predict what changes in functional properties are likely to ensue from incorporating various types of carbohydrates into processed foods is a practical goal of the food scientist.2)Such forecasting requires either a wealth of experience with trial-and-error methods or a deep knowledge of carbohydrate properties as related to structure — perhaps both. However, scientific knowledge of cause and effect is highly respected when it shortens industrial development time.

Source, Types, and Terminology
? The layman ’ s conception of carbohydrates generally involves only the sugars and starches of foods — those that generate calories and fat. The food chemist knows many other types that are ingested.

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Because most people enjoy the sweetness of sugars and the mouthfeel of cooked starches, they become familiar by association with table sugar (sucrose), invert sugars (hydrolyzed sucrose), corn syrup sugars (Dglucose and maltose), milk sugar (lactose), and the more starchy foods. These carbohydrates are nutritionally available; i.e., they are digested (hydrolyzed to component monosaccharides) and utilized by the human body. The carbohydrates of dietary fiber (cellulose, hemicellulose, pentosans, and pectic substances), in contrast, tend to be overlooked because they are largely unavailable. They are not hydrolyzed significantly by digestive enzymes; nevertheless, they may be quite important for human health.

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The carbohydrates of natural and processed foods are divided into available and unavailable types. The available carbohydrates vary in degrees of absorption and utilization depending upon quantities ingested, accompanying food types, and human differences in complements of digestive enzymes and intestinal transport mechanisms. Malabsorption difficulties and adverse physiological effects are known for all the available carbohydrates but gelatinized starches give little or no trouble.

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It is important to realize that in ruminants the unavailable and most abundant polysaccharide cellulose is partially hydrolyzed to the same highly available sugar that starch provides upon digestion; i,e., D-glucose. Grazing animals do it through the cellulases generated by the microorganisms of their rumen. Cellulose is, therefore, a contributing source of valuable animal protein. The efficiency and economics of the ruminant’s conversion of cellulose to nutrients probably can be improved upon by food chemists.

Development of cellulases that are stable outside the cells of microorganisms enables the culturing of fungi and yeasts on cellulose hydrolyzates. Fungi (e.g., mushrooms) can produce protein with the biological value of animal protein. The conversion of cellulose wastes to animal feed and human food is an intriguing prospect for limiting environmental pollution and for feeding the world’ expanding population.

? Carbohydrates were first named according to their natural sources; e.g., beet sugar, cane sugar, grape sugar, malt sugar, milk sugar, cornstarch, liver glycogen, and sweet corn glycogen. Trivial names were then formed, in English terminology, often from a prefix related to the source followed by the suffix “-ose” to denote carbohydrate. Names arising in this way, for example, are fructose, maltose, lactose, xylose, and cellulose.

? These short, well-established names are still commonly used. They furnish no information on the chemical structures, however, so a definitive carbohydrate nomenclature has been developed. From the definitive names, structural formulas can be written. Some of the terms involved in the definitive nomenclature are explained in the following paragraphs.

? The simple sugars (monosaccharides) are basically aliphatic polyhydroxy aldehydes and ketones: HOCH2- (CHOH) n-CHO and HOCH2(CHOOH) n-1-C-O-CH2OH, called “aldoses” and “ketoses,” respectively. However, it must be understood that cyclic hemiacetals of those open-chain forms prevail in solids and at equilibrium in solutions. In the definitive nomenclature, the suffix “ose” is appended to prefixes denoting the number of carbon atoms in the monosaccharide; e.g., trioses (n=1), tetroses (n=2), pentoses (n=3), hexoses (n=4).

To distinguish aldoses from ketoses, ketoses are designated as “-uloses.” Thus, the simplest ketose, HOCH2-C:O-CH2OH, is a triulose; the most common ketose, D-fructose (levulose), is a hexulose. To designate the configurations of hydroxyl groups on the asymmetric carbon atoms of monosaccharides, the prefixes D and L are used together with prefixes derived from the trivial sugar names (e.g., D-glycero-, L-arabino-, D-xylo-) followed by pentose, hexose, hexulose, etc..

? As open-chain hydroxy aldehydes and hydroxyl ketones, the monosaccharides are very reactive. They readily enolize in alkaline solutions to reduce ions such as Cu2+ and Fe (CN)63-. Therefore, they are called “reducing sugars”. Plants protect the reactive monosaccharides for transport and storage by condensing them, with loss of water, into less reactive sugars, e.g., Dglucose and D-fructose, are condensed in such a way that their reactive functions are lost to form the disaccharide nonreducing sugar, sucrose.

The less reactive sucrose is then transported to all parts of the plant for enzymic syntheses of oligoand polysaccharides. From thousands or more Dglucose moieties of sucrose, the glucans, starch and cellulose, are built. From the D-fructose moiety of sucrose, fructans, such as inulin are assembled. Other polysaccharides are formed from other sugars which arise by enzymic transformations of phosphorylated hexoses and sugar nucleotides.

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The prefix “glyc,” is used in a generic sense to designate sugars and their derivatives; e.g., glycoses, glycosides, glycosans, glyconic, glyceric, and glycuronic acids. The generic name for polysaccharides is “glycans”. Homoglycans are composed of a single monosaccharide; for example, the D-glucans, cellulose and starch, release only D-glucose by hydrolysis.

Other homoglycans (e.g., the hexosans, Dgalactan and D-manan, and the pentosans, Larabinan and D-xylan) are uncommon in nature. Heteroglycans, composed of two or more different monosaccharides, are more widely distributed than the homoglycans that are not glucans. Galactomannans, glucomannans, arabinogalactans, and arabinoxylans are common diheteroglycans (composed of two sugars). The glycans prevail over free glycoses in natural foods.

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The reducing sugars are readily oxidized. Mild oxidation of aldoses yields aldonic acids, HOCH2(CHOH)n-COOH; e.g., gluconic acid(n=4). Oxidation of both ends of the aldose molecule yields aldaric acids, HOOC-(CHOH)n-COOH; e.g., tartaric acid(n=2). Oxidation of the terminal CH2OH group of hexoses without oxidation of the reducing function (protected) produces hexuronic acids, HOOC-(CHOH)4-CHO. The hexuronic acids are common monosaccharide constituents of many heteroglycans. For example, they are found in acidic hemicelluloses, pectic substances, algin , plant exudate gumes, and the mucopolysaccharides of mammalian tissues. Penturonic acids have not been found in nature.

? Reduction of aldoses or ketoses yields sugar alcohols, properly called “alditols”, HOCH2(CHOH)n-CH2OH. The suffix “ -itol ” is applied to the trivial prefixes to denote different alditols; e.g., D-glucitol, D-manniitol, xylitol. The triitol, glyceritol (by common usage, glycerol, n=1), is the alditol moiety of fats. Glycerol and Dglucitol (sorbitol) are acceptable and useful food additives because they are glucogenic and keep foods moist by virtue of their strong affinity for water. Pentitols (n=3) and hexitols (n=4) are found in small amounts in many fruits, vegetables, and mushrooms. The heptitol, perseitol (n=5), and an octitol have been isolated from avocados. Some alditols are nutritionally available; others are not.

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Other types of carbohydrates found in food are the condensed N-acetylated amino sugars of mucopolysaccharides, glycoproteins, and chitin; the condensed deoxy sugars of gums, mucilages, and nucleotides; glucosides (sugars condensed with nonsugars); glucosinolates (toxic thioglycosides); cyclitols (myoinositol, phytic acid); and reductone, L-ascorbic acid. ? Complex carbohydrates, such as cellulose and hemicellulose, are largely indigestible, as are a number of oligosaccharides, certain other carbohydrates, gums, and fibrous matter found in foods of plant origin.

Carbohydrate Composition of Foods
? Dieticians need more exact information on the carbohydrate composition of foods . Food processors also make practical use of carbohydrate composition data. For example, the reducing sugar content of fruits and vegetables that are to be dehydrated or processed with heat is frequently an indicator of the extent of nonenzymic browning that can be expected during processing and storage. The possible hydrolysis of sucrose to reducing sugars during processing also is to be considered. The natural changes in carbohydrate composition that occur during maturation and postharvest ripening of plant foods is therefore of particular interest to food chemists.

? Citrus fruits, which normally ripen on the tree and contain no starch, undergo little change in carbohydrate composition following harvest. However, in fruits that are picked before complete ripening (e.g., apples, bananas, pears), much of the stored starch is converted to sugars as ripening proceed. The reducing sugar content of potatoes also increases during cold storage. According to the activity of endogenous invertase during the sun drying of grapes and dates, sucrose is converted to Dglucose and D-fructose; accordingly, the color of the dried products is deepened by nonenzymic browning reactions.

? Green peas, green beans, and sweet corn are picked before maturity to obtain succulent textures and sweetness. Later the sugars would be converted to polysaccharides, water would be lost, and tough textures would develop. In soybeans, which are allowed to mature completely before harvest, the starch reserve is depleted as sucrose and galactosylsucroses (raffinose, stachyose, verbascose, etc.) are formed. In the malting of cereal grains, rapid conversions of reserve carbohydrate to sugars occur as enzymes are strongly activated.

? In foods of animal origin, postmortem activity of enzymes must be considered when carbohydrate composition data is obtained. The glycogen of animal tissues, especially liver, is rapidly depolymerized to D-glucose after slaughter, and immediate deep freezing is required to preserve the glycogen. Mammalian internal organs, such as liver, kidney, and brains, also eggs and shellfish, provide small amount of D-glucose in the diet. Red fresh meats contain only traces of available carbohydrate (D-glucose, D-fructose, and Dribose) and these are extracted into bouillons and broths. Dairy products provide the main source of mammalian carbohydrate. Whole cow ’ s milk contains about 4.9% carbohydrate and dried skim milk contains over 50% lactose.

? Examination of food composition tables shows that, in general, cereals are highest in starch content and lowest in sugars. Fruits are highest in free sugars and lowest in starch. On a dry basis, the edible portions of fruits usually contain 80-90% carbohydrate. Legumes occupy intermediate portions with regard to starch and are high in unavailable carbohydrate.

? Glycosides of many types are widely distributed in plants. Certain biologically active thioglucosides, properly called “glucosinolates”, are found in significant amounts in crucifers. Mustard oils, nitriles, and goitrins are released by enzymic hydrolysis. Their suspected goitrogenic in humans have been investigated, but the amount of glucosinolates normally consumed in food such as fresh cabbage (300-1000ppm), cauliflower, brussels sprouts, turnips, rutabagas, and radishes are not now believed to cause adverse physiological effects.

? Cyanogenic glycosides, which release hydrogen cyanide by enzymic hydrolysis under certain condition of vegetable maceration, are known to be sources of acute toxicity in certain animal feeds; however they are not active in the customary foods of developed countries. Certain foreign varieties of lima beans and manioc root (cassava) may yield up to 0.3% hydrogen cyanide by weight, but lima beans distributed in the United States yield less than 0.02%. Saponins, the surface-active glycosides of steroids and triterpenoids, are found in low concentrations in tea leaves, spinach, asparagus, beets, sugar beet (0.3%), yams, soybeans (0.5%), alfalfa (2-3%), and peanuts and other legumes.


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