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 valve, 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.

    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, plastity, and flavor production), properties that enable the formation or high quality 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.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.

    Because most people enjoy the sweetness of sugars and the mouth feel of cooked starches, they become familiar by association with table sugar (sucrose), invert sugar’s hydrolyzed sucrose, corn syrup sugars (D-glucose 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。Carbohydrates of dietary fiber (cellulose, hemicelluse,  pentosans, and pectic substances), in contrast, tend to be overlooked because they are largely unavailable. Digestive enzymes do not hydrolyze them significantly; nevertheless, they may be quite important for human health.

    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 defective 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.

    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 celluloses generated by the microorganisms of their rumen. Cellulose is, therefore, a contributing source of voluble animal protein. Food chemists probably can improve upon the efficiency and economics of the ruminant’s conversion of cellulose to nutrients. Development of celluloses that are stable outside the cells of microorganisms enables the culturing of fungi and with 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’ expending 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 (monosaccharides0 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 I 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 nomosaccaride; 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 hexlose. 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 ketenes, the monosaccharides are very reactive. They readly enolize in alkaline soluions 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., D-glucose and D-fructose, are condensing in such a way that their reactive functions are lost to form the disaccharide no reducing sugar, sucrose. The less reactive sucrose is then transported to all parts of the plant for enzymin syntheses of oligo-and polysaccharides. From thousands or more D-glucose 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 sugar, which rose by enzymic transformations of phosphorylated hexoes and sugar nucleotides.

    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 “glycan”homoglycansare composed of single monosaccharide; for example, the D-glucans, cellulose and starch, release only D-glucose by hydrolysis. Other homoglycans (e.g., the hexcsans, D-galactan and D-manan, and the pentosans, L-arabinan and D-xy-lan) are uncommon in nature. Heteroglycans, composed of two or more different monosaccharides, are widely distributed than the homoglycans that are not glucans. Galactomnnans, glucomammans, arabinogalactans, and arabinoxylans are common diheteroglycans(composed of two sugars).the glycant vail over free glycoses in natural foods.

    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)-CHO. The hexuronic acids are common monosaccharide constituents of many heteroglycans .for example, they are found in acidic hemicelluloses, pectic substances, alginpl and exudate gumes, and the mucopolysaccharides of mammalian tissues. Penturonic acids have not been found in nature.

    Reduction of aldoses or ketoses yield 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, gllyceritol (by common usage, glycerol, n=1), is the alditol moiety of fats.Glycerol and D-glucitol(sorbitol) are acceptable and useful food addiaffinity for water. Pentitols(n=3) and hexutols(n=4) are found in small amount in many fruits, vegetables and hexitol, perseitol (n=5), and an octitol have been isolated from avocados. Some aditols are nutritionally available; others are not.

    Other types of carbohydrates found in food are the condensed N-acetylated amino sugars of mucopolysaccharides, glycoproteins, and chitin; the condense deoxy sugars of gum, mucilages, and nucleotides; glcosides (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 origins
 
    Carbohydrate Composition of Foods
 
    Detains need more exact information on the carbohydrate compassion of foods. Food pressers 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 browing that can 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 post harvest 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 fruit that are picked before complete ripening (e.g., apples, bananas, pears), much of the stored starch is converted to sugars as ripening process. The reducing sugar content of potatoes also increase during the sun drying of grapes and dates, sucrose is converted to D-glucose 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 soybean, which is allowed to mature completely before harvest, the starch reserve is depleted as sucrose and galactosy lsucroses (raffinose, stachyose, verbascose, etc.) are form 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 D-ribose) 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% carbohydrates 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. Fruit are highest in free sugars and lowest in starch .on a dry basis, the edible portions of fruit usually contain 80-90% carbohydrate. Legumes occupy intermediate portion 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 amount in crucifers. Mustard oils, nitriles, and goitrins are released by enzymic hydrolysis. Their suspected goitrogenic in humans have been investigated, but the amount of glucosnolates normally consumed in food such as fresh cabbage (300-1000ppm), cauliflower, Brussels sprouts, turning, rutabagas, and radishes are not now believed to cause adverse physiological effects. Cyan genetic 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 manic 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 tealeaves, spinach, asparagus, beets sugar beet (0.3%), yams, soybeans (0.5%), alfalfa (2-3%), and peanuts and other legumes.

    食品科学家对碳水化合物有着多方面的兴趣。他们关心碳水化合物在各种食品中的含量，关心它的可利用性（营养上和经济上），它的提取方法和分析方法，它的商品形式和纯度，它的营养价值、生理效用以及在食品中的功能特性。要了解碳水化合物在加工食品中的功能特性，不仅要有单离态碳水化合物的物理、化学性质的知识，还要有碳水化合物与其它食物成分之间在加工食品中就地所发生的反应和相互作用的知识，以及这些反映变化对食品雷竞技百科 和食品可接受性的影响的知识。显然要了解这些知识是不容易的。由于食品加工过程既影响食品的营养价值，也影响它的美学价值。所以了解碳水化合物在研磨、热处理、脱水、冷冻和贮藏过程中所经受的变化就显得特别重要。

    建议学员们学习基础化学，这时认识碳水化合物有用性质的基础。了解和认识极性分子通过氢键、离子效应、取代基效应、与无机离子螯合、与脂类和蛋白质络和以及分解反应等的缔和作用将具有重要的意义。这些基础知识将有助于我们了解影响加工食品的质构和可接受性的性质（例如溶解度、吸湿性、扩散性、渗透性、粘度、可塑性、风味形成），了解使优质糕点、凝胶食品、糖衣、糖果和可复原脱水食品、冷冻食品得以形成的性质。

    食品科学家实际工作目标之一是能够预测在把各种各样碳水化合物掺到加工食品中之后，可能会发生什么样的功能性质变化。作这样的预测要有丰富的用试探法摸索的经验，或者要具备有关结构的碳水化合物的性质的深奥知识，也可能上述两者都要具备。不过，科学的因果关系知识只有在它缩短了工业产品试制周期之时才受到高度的重视。

    碳水化合物的来源、种类和技术名称 

    外行人的碳水化合物概念一般仅包括食物中能产生热量和脂肪的糖和淀粉，而食品科学家还知道许多摄入的他种碳水化合物。

    由于大多数人喜欢糖的甜味和熟淀粉的口感，所以它们由于经常打交道而对食糖、（蔗糖）、转化糖（蔗糖水解产物）、淀粉糖浆（D-葡萄糖和麦芽糖）、乳糖和含淀粉多的食品十分熟悉。这些碳水化合物都有很好的营养价值，即它们可分为人体所消化（水解成单糖成分）和利用。相反，食用纤维类碳水化合物（纤维素、半纤维素、戊聚糖、果胶物质）因其大部分不能为人体所利用而往往被忽视。食用纤维类碳水化合物不能有效的被消化酶水解；尽管这样，他对人体的健康可能相当重要。

    天然食物和加工食物中的碳水化合物被分成人体可利用和不可利用的两类。可利用的碳水化合物在吸收、利用的程度上也有差别，要看他的摄入量、伴随食物的种类以及人体在消化酶互补情况上和肠道输送机制上的差异而定。大家都知道，除了糊化淀粉大致上没有问题以外，所有可利用碳水化合物都有吸收不良的问题和有害的生理影响。

    重要的是要意识到，在反刍动物方面，数量最丰富的人类不可利用的多糖类纤维素受到了部分水解，变成了淀粉消化时所形成的高度可利用糖，即D-葡萄糖。食草动物利用它们瘤胃中微生物所产生的纤维素酶来水解纤维素。因此，纤维素对有价值的动物蛋白而言是有一定贡献的资源。反刍动物将纤维素转化为营养素的效率和经济性有可能经过食品化学家来改进。开发在微生物细胞外部能稳定存在的纤维素酶使真菌和酵母在纤维素水解产物上培养有了可能。真菌（如蘑菇）能产出具有动物性蛋白质生理效价的蛋白质。对于控制环境污染和供应增长着的世界人口的食品来说，将纤维素废料转化为动物饲料和人类食物有着诱人的前景。

    碳水化合物最初是按照它们的天然来源来命名的，例如甜菜糖、甘蔗糖、葡萄糖、麦芽糖、乳糖、玉米淀粉、肝糖原、甜玉米糖原。以后的英语名称中就形成了唱以前缀表示来源，加后缀“-ose”表示碳水化合物的俗名。由此法产生的名称，有如：fructose（果糖）、maltose（麦芽糖）、lactose（乳糖）、 xylose （木糖）、cellulose（纤维素）等。这些简短而明确的名称现在仍通用。可是，这些名称不反映其化学结构。于是就产生了碳水化合物的定形命名法。国际定形名称，可以写出其结构式。下面几段将对某些涉及定形命名法的专门术语作出解释。

    单糖本质上是脂族的多羟基醛和酮，即HOCH2-(CHOH)2-CHO和HOCH2-(CHOH)n-1-C:O-CH2OH,分别称为醛糖和酮糖。不过要知道，固体中和平衡时溶液中的单糖是以开链型环状半缩醛占多数。在定形命名法中，单糖名由表示单糖碳原子数的前缀加后缀“-ose”构成。例如丙糖（trioses，n=1）、丁糖（tetroses，n=2）、戊糖（pentoses，n=3），己糖（hexose，n=4），为区别醛糖和酮糖，就把酮糖后缀加成“-ulose”。这样，最简单的酮糖HOCH2-C：O-CH2OH即为triulose（丙酮糖）；最常见得酮糖D-果糖（左旋糖）即为hexulose（己酮糖）。为表明单糖不对称碳原子上羟基的构型，可将前缀D和L与取自俗名的前缀一起使用（例如D-甘油-、L-阿拉伯-、D-木-），后面在接上戊糖、己糖、己酮糖等等。

    以开链的羟基醛和羟基酮形式 存在的单糖非常活泼。它们在碱性溶液中易于烯醇化而使Cu2+、Fe（CN）63+之类的离子还原，因此称它们为“还原糖”。植物依靠脱水缩和方式将活泼的单糖变成不大活泼的糖类来保存单糖的还原活性，以便输送和贮存。例如，D-葡萄糖和D-果糖缩合成无还原性双糖——蔗糖时，变失去了活泼的功能。不太活泼的蔗糖然后被输送到植物的各个部位，由酶合成低聚糖和多聚糖。由数千或更多的半个蔗糖分子D-葡萄糖构成葡聚糖、淀粉和纤维素；而由另半个蔗糖分子D-果糖聚合成为果聚糖，如菊粉。其它多聚糖则是由磷酸化己糖和核苷酸糖通过酶作用生成的其它糖聚合形成的。

    前缀“glyc”（甘、糖）在一般意义上用来表示糖及其衍生物。例如：glycoses（单糖）、glycosides（糖苷）、glycosans（聚糖）、glyconic acids (糖酸) 、glyconic acids（甘油酸）、glycuronic acids（糖醛酸）。多聚糖的俗名是“聚糖”。均聚糖由一种单糖构成。例如D-葡聚糖、纤维素和淀粉，它们水解时只产生D-葡萄糖。其它均聚糖（例如己聚糖D-半乳聚糖和D-甘露聚糖；戊聚糖有L-阿拉伯聚糖D-木聚糖）在自然界中很少见到。杂聚糖由两种或多种单糖构成。它们在自然界的分布比均聚糖（除葡聚糖外）广泛 。半乳甘露聚糖、葡甘露聚糖、阿拉伯半乳聚糖、阿拉伯木聚糖时常见的双杂聚糖（由两种糖组成）。天然食物中的聚糖大大多余游离单糖。

    还原糖易于氧化。醛糖经轻度氧化可产生醛糖酸，HOCH2-（CHOH）n-COOH，如葡萄糖酸（n=4）。醛糖分子两端氧化形成糖二酸，HOOC-（CHOH）n-COOH，例如酒产生的糖醛酸，HOOC-（CHOH）4-CHO。己糖醛酸是组成多种杂聚糖的常见单糖成分，例如在酸性半纤维素、果胶物质、藻酸、植物渗出胶和哺乳动物组织中的粘多糖中就有己糖醛酸。戊糖醛酸还未在自然界中发现。

    醛糖或酮糖被还原后变产生糖醇，确实的叫法为“多羟糖醇”（alditols），HOCH2-（CHOH）n-CH2OH。其后缀“-itol”（糖醇）接在俗名前缀之后表示不同的多羟糖醇，例如D-葡糖醇、D-甘露糖醇、木糖醇。丙糖醇，即甘油醇（常称为甘油，n=1），是脂肪的多羟基糖醇部分。甘油和葡糖醇（山梨醇）成为受欢迎的有用的食品添加剂，因为它们能生成葡萄糖，并依靠其强烈的亲水作用保持食品是湿润。许多水果、蔬菜和菇类中存在有少量的戊糖醇（n=3）和己糖醇（n=4）。庚糖醇、甘露庚糖醇（n=5）和某种辛糖醇已从鳄梨中分离出来。有些多羟糖醇是有营养的，有些则无营养。

    在食物中发现它种碳水化合物还有：粘多糖、糖蛋白、壳多糖之类的缩聚N-乙酰氨基糖；植物胶、粘浆、核苷酸之类的缩合脱氧糖；糖苷（糖与非糖缩合）；芥子油苷（毒性的硫代糖苷）；环多糖（肌醇、植酸）；L-抗坏血酸（一种还原酮）。

    复杂碳水化合物如纤维素、半纤维素基本上是不能消化的，就象植物性食物中所找到的许多低聚糖、某些其它碳水化合物、树胶和纤维质材料一样。

    食物中碳水化合物组成

    营养学家需要有较为严格的有关食物碳水化合物组成的知识。食品加工者也把碳水化合物组成的知识用于实际工作。例如，欲待脱水或热处理的水果和蔬菜，其还原糖含量常常时一项预示加工和贮藏过程中非酶褐变程度的指标。当然也要考虑加工过程中蔗糖有可能水解变成还原糖。因此，食品化学家对植物性食物在成熟和后熟期间所发生的碳水化合物组成的自然变化特别感兴趣。

    在果树上正常成熟且不含淀粉的柑桔类水果，其碳水化合物组成在摘下后很少变化。可是，在许多完全成熟前摘下的水果（如苹果、香蕉、梨）中，贮存的大部分淀粉会在继续成熟时转化为糖。马铃薯在冷藏间还原糖含量也会增加。葡萄和海枣在晒干期间，随其内源蔗糖酶活性的不同，会不同程度地使蔗糖转化D-葡萄糖和D-果糖。因此，其干制品的颜色也会因非酶褐变而加深。

    豌豆、青刀豆和甜玉米在成熟前摘下是为了获得多汁鲜嫩的质构和甜味。迟了，所含的糖就会转成多聚糖，水分会失掉，就会产生韧性的质构。熟透后在摘收的大豆，其淀粉储量随蔗糖和类半乳蔗糖（棉籽糖、水苏糖、毛蕊花糖等）的形成而耗尽。用谷物加工麦芽糖使，由于酶被极度活化而使得贮存的碳水化合物迅速转化成糖。

    在动物性食物方面，当获得碳水化合物组成数据时，必须仔细估量动物宰后酶的活性。动物组织（尤其是肝脏）中的糖原在宰后会迅速解聚成D-葡萄糖，所以，要保存糖原，就得立即进行深度冷冻。哺乳动物内部器官（如肝、肾、脑和蛋类、贝类所提供的D-葡萄糖只占饮食中D-葡萄糖的一小部分。红色的新鲜肉类仅含极少可利用的碳水化合物（D-葡萄糖、D-果糖和D-核糖），而这些碳水化合物被溶出进入肉汁和肉汤。乳制品提供了哺乳动物碳水化合物的主要来源，牛全乳含碳水化合物约4.9%，脱脂乳粉含乳糖量超过50%。

    查看一些食物成分表便可知道，通常谷类的淀粉含量最高而糖含量最低。水果类游离糖最多，淀粉最少。以干基计算，水果的可食部分通常含80～90%的碳水化合物。豆类含淀粉不多不少，但含有大量的不可利用的碳水化合物。

    在植物界，广泛分布着多种糖苷。在十字花科植物中大量存在着某些具有生理活性的硫葡糖苷（确切称之为“芥子油柑”）。芥子油柑经酶水解产生芥子油、腈和甲状腺肿素。人们已经研究了它在人体中可疑的甲状肿病源性质。但目前还不认为食用新鲜卷心菜（其中芥子油柑量为300～1000ppm）、花椰菜、抱子甘蓝、芜菁、芜菁甘蓝、小萝卜之类食物中的芥子油苷数量会引起有害的生理效应。生氰苷在一定蔬菜浸渍条件下，通过酶水解能释放出氰化氢。人们都知道生氰苷是某些动物饲料剧毒性的主要来源。可是在发达国家的平常食物中，生氰苷没有活性。某些外国品种的菜豆和木薯可产生高达0.3%的氰化氢（按重量计），但遍布在美国的菜豆产氰化氢低于0.02%。皂角苷类（类固醇和三萜系化合物的表面活性糖苷）会以低浓度出现在茶叶、菠菜、芦笋、甜菜、糖用甜菜（0.3%）、甘薯、大豆（0.5%）、苜蓿（2～3%）、花生及其它豆类。