Chapter 1. Food ScienceThe scientific study of food is one of man‘s most important endeavors, mainly because food is his most important need. It is necessary for his survival, his growth, his physical ability, and his good health. Food processing and handling is the largest of all of man‘s industries. Many factors require that those scientists who choose to study foods be prepared to absorb as much of the physical and life sciences and as much engineering as possible. Among these are the chemical complexity of foods, their vulnerability to spoilage, their role as a disease vector, and the varied sources of food. The availability nutritional adequacy, and wholesomeness of foods are also quite varied.Whether we now have enough of the facts to trace the development of food science from the beginning is questionable,. History reports that the Romans realized more than the Greeks, Egyptians, or any of the prior civilization, that agriculture was a prime concern of government. The Romans, as the Egyptians and the Greeks before them, were able to preserve a variety of foods by holding then them in vinegar (with or without brine), in honey, or in pitch. Some foods were also dried either by the sun or over a fire. Ancient civilizations produced cheeses and wines. Yet it is generally believed that until the later part of the 18thcentury the preservation of foods had evolved as an art handed down from generation to generation, .Its development was slow, depending on accidental discovery, observation, trial and error, and attempts to reproduce and pit into practice the newly found techniques. Drying, freezing, smoking, fermenting, cooking and baking had been practiced for centuries-even by illiterates. Foods frozen accidentally in cold climates and foods dried accidentally in dry climates were observed to have a longer shelf-life than foods which were neither frozen nor dried. Foods that might have been put over a fire to hasten drying could easily have led to the smoking process . Thus, chance occurrences led to preservation methods that permitted man to conserve foods during times of glut so that he might survive the leaner spells. It can be said, then, that those who made the observations and realized their impact and put their interpretations to the test, until the new practice was proven, were the first food scientists. Spallanzani(1765) and Apart(1795) were among the first to apply the quasi-scientific methods for preserving foods, and in 1809 Appert won a prize from the French Government for developing a thermal processing technique for foods to be used by the military. Appert is credited with developing the canning process. Because of the scarcity of sc ientific inf ormation, Appert had to employ trial and error tactics, but his records attest to the accuracy of his observations and conclusions and show that he applied the scientific approach to gain his outstanding achievement, even though he did not know why his method worked.It was not until the discoveries of Pasteur in 1850 and the work of other microbiologists such as pest and Underwood in 1895, that man learned that bacteria spoiled food and why thermal processing prevented food spoilage.By 1875 man had learned to preserve foods by artificial refrigeration using first natural ice, and later manufactured ice, to preserve fish and this enabled him to freeze foods. By 1890 mechanical refrigeration came into wide use, opening the way to the frozen storage of foods. Quick freezing wasFood Sciencefirst used in 1924 to preserve fish. During the period 1932-1934 Clarance Bird eye, with laboratories in Gloucester, Mass., developed over 100 different frozen food items, and this achievement won for him the reputation and the credit for the beginning of the quick-frozen food industry. One of the most important ensuing technological developments was the invention of the fish blacks by Brrdseye technologists. This is considered by many to have revolutionized the fish processing industry.In 1898 it was noted that bacteria were destroyed by exposure to radioactive salts of radium and uranium. By 1930 the use of ionizing radiation to preserve food was patented by O.Wust. But the irradiation preservation of foods was not actively investigated until 1943 by the team of Proctor, Van de Graaf, and Fram from the Massachusetts Institute of Technology.Modern technology has made possible the controlled, automated drying processes and sophisticated modifications such as freeze-drying, drum-drying, spraying-drying, fluidize-bed drying, etc. Controlled, automated versions of thermal and refrigeration processes have also been developed. Radiation processing (by election-, X-, and gamma-rays), microwave processing, and aseptic canning have also been introduced.Though many of the food processes alter foods in such a manner that the finished product is more palatable or otherwise more acceptable (to some at least) than original raw material (sauerkraut, tuna, wine, Roquefort cheese, etc.), in many cases it is desirable that preservation processes do not alter the food (fish fillets, beef steak, pork chops etc.)Only refrigeration can preserve most foods without altering them substantially.Words and Expressionsvinegar (n.) 醋brine (n./v.) 盐水；用盐水浸pitch (n.) 沥青smoke (v.) 熏制（肉，鱼）ferment (v.) 发酵fermentation (n.)shelf life 货架期preserve (v.) 保藏，保存preservation (n.)glut (n) 供过于求，过剩lean (a.) 欠收，不足thermal (a.) 热的canning process 罐藏加工bacteria (n.) 细菌microbiologist (n.) 微生物学家radioactive (a.) 放射性的radium (n.) 镭uranium (n.) 铀ionizing radiation 离子辐射drum-drying 转鼓干燥spray-drying 喷雾干燥fluidize-bed drying 流化床干燥microwave 微波aseptic (a.) 无菌的palatable (a.) 可口的sauerkraut (n.) 泡菜tuna (n.) 罐藏金枪鱼肉beef steak 牛排pork chops 猪排Chapter 2. CarbohydratesClassificationCarbohydrates are usually defined as polyhydroxy aldehydes and ketones or substances that hydrolyze to yield polyhydroxy aldehydes and ketones.Monosaccharides are classified according to (1)the number of carbon atoms present in the molecule and (2) whether they contain an aldehyde or keto group. Thus a monosaccharide containing six carbon atoms is called hexose; a monosaccharide containing an aldehyde group is called an aldose; and one containing a keto is called a ketose.The most important representatives of monosaccharides are glucose, arabinose, galactose, mannose, ribose, and fructose. Glucose is usually used as a carbon source for fermentation. Because the glucose in refined form such as crystallin form or as syrup form is more expensive, glucose in fermentation medium is mostly produced by directly enzymatic conversion of starch.The oligosaccharides can be classified into disaccharides and trisaccharides. The most important representatives of disaccharides are sucrose (from beet or cane), lactose, maltose and cellobiose. The most important representatives of trisaccharides is raffinose which occurs in sugar beet.Sucrose is available for use in fermentation processes either in crystallin form or in crude form as raw juice or mollasses ( a by-product of sugar manufacture). The sucrose contained in molasses is obviously cheaper, but the composition of molasses varies greatly with sources (cane or beet), quality of the crop and the nature of the sugar refining process. The molasses should be pretreated before being used as a raw material for fermentation medium.Lactose is present in whey (a by-product of cheese making that arise following the separation of curds, the solidified casein and butter fat) at a concentration of 4%-5% and whole whey or deproteinized whey is used as a cheap source of carbohydrate in some alcohol production process.Polysaccharides are constructed from monasaccharide unit and their derivatives, and have ten to several thousands units. D-glucose is the most common units. They are insoluble and nonreducing. The most important representatives are starch, glycogens, and cellulose.Starch is the most important carbohydrate used in fermentation processes. It is from plants such as corn, rice, wheat, potatoes and cassava.The extent of starch hydrolysis required varies with fermentation process and depends on considerations as to whether or not the microbial strain to be used produces amylase and whether product synthesis is subject to catabolite repression. For citric acid production , because the A.niger has the ability to synthesize glucoamylase ( or amyloglucosidase: a enzyme that catalyze the removal of one glucose molecule at a time from the terminal end of dextrins, breaking 1,4-links), the starch slurry is gelatinized by cooking at high temperature, then the gelatinized starch is liquefied and dextrinized by cooking at high temperature, and the saccharification step is not necessary. The soluble dextrin hydrolysate is used as a raw material for fermentation medium.CarbohydratesCarbohydrate Composition of FoodsDieticians Dietitians need more exact information on the carbohydrate composition of food. Food processors also make practical use of carbohydrate composition data. For example, the reducing sugar content of fruits and vegetables that are to 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 sugar 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 fruits that are picked before complete ripening (eg. apple, bananas, pears), much of the stored starch is converted to sugars as ripening proceeds. 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 D-glucose and D-fructose; accordingly, the color of the dried products is deepened by nonenzymic browning reactions.Greens peas, green beans, and sweet corn are picked before maturity to obtain succulent texture 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 galactosy lsucroses(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 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% 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 highests in free sugars and lowest in starch. On a dry basis, the edible portions of fruits usually contain 80-90% carbohydrate. Legumes occupy intermediate positions with regard to starch and are high in unavailable carbohydrate.Words and Expressionscarbohydrate (n.) 碳水化合物polyhydroxy (a.) 多羟基的aldehyde (n.) 醛Carbohydratesketone (n.) 酮hydrolyze (v.) 水解hydrolysis (n.) 水解hydrolysate (n.) 水解产物saccharide (n.) 糖monosaccharide (n.) 单糖oligosaccharide (n.) 寡糖polysaccharide (n.) 多糖hexose (n.) 己糖aldose (n.) 醛糖ketose (n.) 酮糖glucose (n.) 葡萄糖arabinose (n.) 阿拉伯糖galactose (n.) 半乳糖mannose (n.) 甘露糖ribose (n.) 核糖fructose (n.) 果糖refine（v.）精制crystalline (n.) 结晶syrup (n.) 糖浆enzymztic（a.）酶作用的sucrose (n.) 蔗糖beet (n.) 甜菜cane (n.) 甘蔗lactose (n.) 乳糖maltose (n.) 麦芽糖cellobiose (n.) 纤维二糖raffinose (n.) 棉子糖mollasses (n.) 糖蜜sugar-refining（a.）糖的精制whey (n.) 乳请curd (n./v.) 凝乳；凝结casein (n.) 酪蛋白butter fat 乳脂deproteinize (v.) 去除蛋白质deproteinized whey 脱蛋白乳请nonreducing（a.）非还原性的starch (n.) 淀粉glycogen (n.) 糖原cellulose (n.) 纤维素cassava (n.) 木薯microbial strain 微生物菌株catabolite repression 分解代谢抑制A.niger黑曲霉glucoamylase (n.) 糖化酶amyloglucosidase (n.) 淀粉葡糖糖苷酶thermostable (a.) 耐热的amylase (n.) 淀粉酶dextrin (n.) 糊精link（n.）键slurry (n.) 浆gelatinize (v.) 糊化，使成胶状saccharification (n.) 糖化medium (n.) 培养基dietician (n.) 营养学家composition (n.) 组成，成分reducing sugar 还原糖dehydrate (v.) 脱水nonenzymic browning 非酶褐变post harvest 采后ripen (v.) 成熟pear (n.) 梨endogenous (a.) 内源的invertase (n.) 转化酶，蔗糖酶date (n.) 海枣pea (n.) 豌豆green bean (n.) 青刀豆soybean (n.) 大豆galactosy lsucrose (n.) 类半乳蔗糖stachyose (n.) 水苏糖verbascose (n.) 毛葱化糖postmortem (a.) 屠宰后depolymerize (v.) 分解mammalian (a./n.) 哺乳动物（的）（mammal 哺乳动物）extract (v.) 萃取bouillon (n.) 肉汁broth (n.) 肉汤dairy product 奶制品skim milk 脱脂牛奶cereal (n.) 谷物legume (n.) 豆类Chapter 3. Amino Acids and ProteinsProteins are molecules of great size, complexity, and diversity. They are the source of dietary amino acid, both essential and nonessential, that are used for growth, maintenance, and the general wellbeing of man. These macromolecules, characterized by their nitrogen contents, are involved in many vital processes intricately associated with all living matter. In mammals, including man, proteins function as structural components of the body. Muscles and many internal organs are largely composed of proteins. Mineral matter of bone is held together by collagenous protein. Skin, the protective covering of the body, often accounts for about 10% of the total body protein.Some proteins function as biocatalysts (enzymes and hormones) to regulate chemical reactions within the body. Fundamental life processes, such as growth, digestion, and metabolism, excretion, conversion of chemical energy into mechanical work, etc., are controlled by enzymes and hormones. Blood plasma proteins and hemoglobin regulate the osmotic pressure and pH of certain body fluids. Proteins are necessary for immunological reactions. Antibodies, modified plasma globulin proteins, defend against the invasion of foreign substances or microorganisms that can cause various diseases. Food allergies result when certain ingested proteins cause an apparent modification in the defense mechanism. This leads to a variety of painful, and occasionally drastic, conditions in certain individuals.Food shortages exist in many areas of the world, and they are likely to become more acute and widespread as the world,s population increases. Providing adequate supplies of protein poses a much greater problem than providing adequate supplies of either carbohydrate or fat. Proteins not only are more costly to produce than fats or carbohydrates but the daily protein requirement per kilogram of body weight remains constant throughout adult life, whereas the requirement for fats and carbohydrates generally decrease with age.As briefly described above, proteins have diverse biological functions, structures, and properties. Many proteins are susceptible to alteration by a number of rather subtle changes in the immediate environment. Maximum knowledge of the composition, structure, and chemical properties of the raw material, especially proteins, is required if contemporary and future processing of foods is to best meet the needs of mankind. A considerable amount of information is already available, although much of it has been collected by biochemists using a specific food component as a model system.Amino AcidsAmino acids are the ―building blocks‖ of proteins. Therefor e, to understand the properties of proteins, a discussion of the structures and properties of amino acids is required. Amino acids are chemical compounds which contain both basic amino groups and acidic carboxyl groups. Amino acids found in proteins have both the amino and carboxyl groups on the α-carbon atom, α-Amino acids have the following general structure.: NH2R-C-COOHHAt neutral pH values in aqueous solutions both the amino and the carboxyl groups are ionized. The carboxyl group loses a proton and obtains a negative charge, while the amino group gains a proton and hence acquires a positive charge. As a consequence, amino acids possess dipolar characteristics. The dipolar form of amino acids has the following general structure:+NH3R-C-COO-HSeveral properties of amino acids provide evidence for this structure: they are more stable in water than in less polar solvents; when present in crystalline form they melt or decompose at relatively high temperature; and they exhibit large dipole moments and large dielectric constants in neutral aqueous solutions.The R group, or side chains, of amino acids exert important influences on the chemical properties of amino acids and proteins. These side chains may be classified into four groups.Amino acids with polar-uncharged (hydrophilic) R groups can hydrogen-bond with water and are generally soluble in aqueous solutions. The hydroxyls of serine, threonine, and tyrosine; the sulfhydryl or thiol of cysteine; and the amides of asparagine and glutamine are the funcitional moieties present in R groups of this class of amino acids. Two of these, the thiol of cysteine and the hydroxyl of tyrosine, are slightly ionized at pH 7 and can lose a proton much more readily than others in this class. The amides of asparagine and glutamine are readily hydrolyzed by acid or base to aspartic acids and glutamic acids, respectively.Amino acids with nonpolar (hydrophobic) R groups are less soluble in aqueous solvents than amino acids with polar uncharged R groups. Five amino acids with hydrocarbon side chains decrease in polarity as the length of the side chain is increased. The unique structure of proline (and its hydoxylated derivative, hydroproline) causes this amino acid to play a unique role in protein structure.The amino acids with positively charged ( basic ) R groups at pH 6-7 are lysine, arginine, and histidine. The amino is responsible for the positive charge of lysine, while arginine has a positively charged quanidino group. At pH 7.0, 10% of the imidazole groups of histidine molecules are protonated, but more than 50% carry positive charges at pH 6.0.The dicarboxylic amino acids, aspartic and glutamic, possess net negative charges in the neutral pH range. An important artificial meal-flavoring food additive is the monosodium salt of glutamic acid.Protein StructureProteins perform a wide variety of biological functions and since they are composed of hundreds of amino acids, their structures are much more complex than those of peptides.Enzymes are globular proteins produced in living matter for the special purpose of catalyzing vital chemical reactions that otherwise do not occur under physiological conditions. Hemoglobin andmyoglobin are hemo-containing proteins that tranport xoygen and carbon dioxide in the blood and muscles. The major muscle proteins, actin and myosin, convert chemical energy to mechanical work, while proteins in tendons (collagen and elastin) bind muscles to bones. Skin, hair, fingernails, and toenails are proteinaceous protective substances. The food scientist is concerned about proteins in foods since knowledge of protein structure and behavior allows him to more manipulate foods for the benefit of mankind.Nearly an infinite number of proteins could be synthesized from the 21 natural occurring amino acids. however, it has been estimated that only about 2000 different proteins exist in nature. The number is greater than this if one considers the slight variations found in proteins from different species.The linear sequence of amino acids in a protein is referred to as ―primary structure‖. In a few proteins the primary structure has been determined and one protein (ribonuclease) has been synthesized in the lab. It is the unique sequence of amino acids that imparts many of the fundamental properties to different proteins and determines in large measure their secondary and tertiary structures. If the protein contains a considerable number of amino acids with hydrophobic groups, its solubility in aqueous solvents is probably less than that of proteins containing amino acids with many hydrophilic groups.If the primary structure of the protein were not folded, protein molecules would be excessively long and thin. A protein having a molecular weight of 13,000 would be 448 A long and 3.7 A thick. This structure allows excessive interaction with other substance, and it is not found in nature. The three-dimensional manner in which relatively close members of the protein chain are arranged is referred to as ―secondary structure‖. Examples of secondary structure are the α-helix of wool, the pleated-sheet configuration of silk, and the collagen helix.The nature structure of a protein is that structure which possesses the lowest feasible free energy. Therefore, the structure of a protein is not random but somewhat ordered. When the restrictions of the peptide bond are super-imposed on a polyamino acid chain of a globular protein, a right-handed coil, the α-helix, appears to be one of the most ordered and stable structures feasible.Theα-helix contains 3.6 amino acid residues per turn of the protein backbone, with the R groups of the amino acids extending outward from the axis of the helical structure. Hydrogen bonding can occur between the nitrogen of one peptide bond and the oxygen of another peptide bond four residues along the protein chain. Hydrogen bonds are nearly parallel to the axis of the helix, lending strength to the helical structure. Since this arrangement allows each peptide bond to form a hydrogen bond, the stability of the structure is greatly enhanced. The coil of the helix is sufficiently compact and stable that even substances with strong tendencies to participate in hydrogen bonding, such as water, cannot enter the core.A secondary structure found in many fibrous proteins is the β-pleated sheet configuration. In this configuration the peptide backbone forms a zigzag pattern, with the R groups of the amino acids extending above and below the peptide chain. Since all peptide bonds are available for hydrogenbonding, this configuration allows maximum crosslinking between adjacent polypeptide chains and thus good stability. Both parallel pleated sheet, where the polypeptide chains runs in the same direction, and antiparallel pleated sheet, where the polypeptide chains run in opposite directions, are possible. Where R groups are bulky or have like charges, the interactions of the R groups do not allow the pleated-sheet configuration to exist. Silk and insect fibers are the best examples of theβ-sheet, although feathers of birds contain a complicated form of this configuration.Another type of secondary structure of fibrous proteins is the collagen helix. Collagen is the most abundant protein in higher vertebrates, accounting for one third of the total body protein. Collagen resists stretching, is the major component of tendons, and contains one-third glycine and one-fourth proline or hydroxyproline. The rigid R groups, and the lack of hydrogen bonding by peptide linkages involving proline and hydroxyproline, prevents formation of anα-helical structure and forces the collagen polypeptide chain into an odd kinked-type helix. Peptide bonds composed of glycine form interchain hydrogen bonds with two other collagen polypeptide chains, and this results in a stable triple helix. This triple-helical structure is called ― tropocollagen‖ and it has a molecular weight of 300,000 daltons.The manner in which large portions of the protein chain are arranged is referred to as tertiary structure. This involves folding of regular units of the secondary structure as well as the structuring of areas of the peptide chain that are devoid of secondary structure. For example, some proteins contain areas whereα-helical structure exists and other areas where this structure cannot form. Depending on the amino acid sequence, the length of theα-helical portion varies and imparts a unique tertiary structure. Those folded portions are held together by hydrogen bonds formed between R groups, by salt linkages, by hydrophobic interactions, and by covalent disulfide (-S-S-) linkages.The structures discussed so far have involved only a single peptide chain. The structure formed when individual (subunit) polypeptide chains interact to form a native protein molecule is referred to as ―quaternary structure‖. The bonding mechanisms that hold protein chains together are generally the same as those involved in tertiary structure, with the possible exception that disulfide bonds do not assist in maintaining the quaternary structures of proteins.Properties and Reactions of ProteinThe primary, secondary, and tertiary structure of proteins affect both how the proteins react in the preparation and processing of foods and how they are affected by the various treatments involved in food preparation.The amphoteric property of proteinsProteins are amphoteric---- they have both acidic and basic characteristics because they can exist as hybrid ions, or zwitter ions. The ionizable hydrogen ion can transfer from the acidic carboxyl group to the basic amino group. If the amino acid glycine is used to represent a protein, then the zwitter ion would be formed as follows:H H OH H H O-H—N—-C—C=O ↔ H—N—-C—C=O+If a hygrogen ion is added to the zwitter ion, it adds on to the carboxyl group as shown below:H H O-H H OHH—N—-C—C=O ↔H—N—-C—C=OH+H H+ HIf a hydroxyl ion is added to the zwitter ion, it removes the hydrogen ion from the amino group to form a molecule of water, as shown below:H H O-H H O-H—N—-C—C=O + OH- ↔H—N—-C—C=O + H2OH+H HThus, proteins can act as buffers because the addition of acid or base does not change the pH of the protein until all of the carboxyl or amino groups are undissociated depending on whether acid or base is added.Fresh milk, for instance, is pH 6.6, and the casein protein carries a net negative charge; that is, there are more ionized carboxyl groups on the protein than ionized amino groups. If acid (H+) is added to milk, no visible change occurs initially with small additions of hydrogen ion, but when the pH reaches 5.2, the milk curdles. At the isoelectric point, pH4.6, the milk protein casein has equal numbers of positive and negtive charges. The pH of the isoelectric point differs for each protein depending on the ratio of the free carboxyl to free amino groups in the protein.the Water-Binding Capacity of ProteinAnother property of proteins that contributes to their ability to form colloidal dispersions is their attraction for water. Molecules of water bind to both the backbone and the polar R groups of protein. These water molecules form a layer of water molecules around the protein molecules and contribute to maintaining the stability of a colloidal dispersion because the water molecules all carry the same charge, and this causes the hydrated protein molecules to repel each other and to remain dispersed,. Proteins vary in the number of sites on the protein molecule that will permit the bonding of water.Proteins that bind water readily are said to be hydrated; an example is ovalbumin, a protein in egg white. Casein is less readily hydrated; it does not bind water readily. Both the layers of water molecules bound to the surface of the protein and the repulsion between the like charges on the protein molecules aid in keeping the protein dispersed and in contributing stability to the colloidal dispersion. Denaturation of ProteinsDenaturation has been defined as a disordered arrangement of the structure of the protein molecule.。
