Proteins are molecules of great size, complexity, and diversity. They are the source of dietary amino acids, both essential and nonessential, that are used for growth, maintenance, and the general well-being of man. These macromolecules, characterized by their nitrogen contents, are involved in many vital processes intricately associated with all living matter. In mammals 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 protein function as biocatalysts (enzymes and hormones) to regulate chemical reactions within the body. Fundamental life process, 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 immunology reactions. Antibodies, modified plasma globulin proteins, defend against the invasion of foreign substances of 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. providingadequate 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 bodyweight remains constant throughout adult life, whereas the requirements 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 materials, 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 Acids 
 
    Amino acids are the “building blocks” of proteins. Therefore, to understand the properties of proteins, a discussion of the structures and properties o f 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 a-carbon atom; a-amino acids have the following general structure:

    NH2   
    │   
R ─ C ─ COOH
    │   
    H    

    At 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, or zwitterions, form of amino acids has the following general structure:

    NH3++   
    │   
R ─ C ─ COO-
    │   
    H    

    Several properties of amino acids provide evidence for this structure: they are more soluble in water than in less polar solvents; when present in crystalline form they melt or decompose at relatively high temperatures (generally above 200): and they exhibit large dipole moments and large dielectric constants in neural aqueous solutions. 

    The R groups or side chains, of amino acids and proteins. these side chains may be classified in to four groups.

    Amino acids with polar-uncharged (hydrophilic) r groups can hydrogenbond with water and are generally soluble in aqueous solutions. The hydroxyls of serine, heroine, and tyrosine; the sulfhydryl of thinly of cysteine, and the amides of asparagines and glutamine are the functional moieties present in r groups of the class of amino acids. Two of these, the toil of cysteine and the hydroxyl of tyrosine, are slightly ionized at PG 7 and can lose a proton much more readily than others in this class. The amides of asparagines and glutamine are readily hydrolyzed by acid or base to aspartic 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 praline (and its hydoxylated derivative, hydroxyproline) 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; argiine has a positively charged quanidino group. At ph 7.0 10% of the imidazole groups of histidine molecules are prorogated, but more than 50% carry positive at ph 6.0. 

    The dicarboxylic amino acids, asparic glutamic, possess net negative charges n the neutral ph range. An important artificial meal-flavoring food additive is the monosodium salt of glutamic acid.
 
    Peptides 
 
    When the amino group of one amino acid reacts with the carboxyl group of another amino acid, a peptide bond is formed and a molecule of water is released. This can bond joins amino acids together to form proteins 

    蛋白質(zhì)錯(cuò)綜復(fù)雜、多種多樣的大分子物質(zhì),是食物必須氨基酸和非必須氨基酸的來(lái)源。人體利用這些氨基酸以滿足生長(zhǎng)發(fā)育、修復(fù)組織和維持正常健康生活的要求。這些大分子以含氮為其特征，參與了許多與各種有生命物質(zhì)有復(fù)雜聯(lián)系的生命過(guò)程。在包括人類在內(nèi)的哺乳動(dòng)物中，蛋白質(zhì)起著機(jī)體改造成分的作用，肌肉和許多體內(nèi)器官主要由蛋白質(zhì)構(gòu)成。骨骼中的礦物質(zhì)靠膠原蛋白得以保持在一起。機(jī)體的保護(hù)層—皮膚中的蛋白質(zhì)通常占機(jī)體蛋白質(zhì)總量的10%的左右。

    有些蛋白質(zhì)有生物催化劑（酶和激素）的作用，以調(diào)節(jié)體內(nèi)的化學(xué)反應(yīng)�；�本的生命過(guò)程如生長(zhǎng)、消化、代謝、排泄、化學(xué)能轉(zhuǎn)變成機(jī)械功等等都受酶和激素的控制。某些體液的滲透壓和pH值受制于血漿蛋白和血紅蛋白。蛋白質(zhì)對(duì)免疫反應(yīng)是必不可少的�？贵w（改性的血漿球蛋白能引起疾病的外來(lái)雜質(zhì)和微生物的入侵。當(dāng)某些攝入的蛋白質(zhì)使防御機(jī)制產(chǎn)生明顯的變化時(shí)，便發(fā)生人體的生物過(guò)敏。這就導(dǎo)致某些個(gè)體身上出現(xiàn)各種各樣的疾病，且有時(shí)是急劇的病情。

    食物短缺現(xiàn)象在世界許多地區(qū)存在。隨著人口的增加，這個(gè)問(wèn)題很可能愈來(lái)愈尖銳、愈普遍。而蛋白質(zhì)供應(yīng)不足問(wèn)題遠(yuǎn)比碳水化合物或脂肪供應(yīng)不足更為嚴(yán)重。蛋白質(zhì)不僅它的產(chǎn)出費(fèi)用要比碳水化合物或脂肪的產(chǎn)出費(fèi)用為高，而且每千克每天所需的蛋白質(zhì)量造整個(gè)成年期是恒定的，而每天所需的脂肪和碳水化合物量一般都隨著年齡的增長(zhǎng)而逐漸減少。

    正如上面簡(jiǎn)述的一樣，蛋白質(zhì)有多種不同的結(jié)構(gòu)、性質(zhì)和生理功能。許多蛋白質(zhì)容易受周圍環(huán)境的一系列微妙變化的影響而發(fā)生變化。要想使現(xiàn)在和將來(lái)的食品加工能理想的滿足人類的需要，就必須徹底了解原料特別是蛋白質(zhì)的組成結(jié)構(gòu)和化學(xué)性質(zhì)。目前，已經(jīng)有這方面的大量資料可供利用，不過(guò)其中大部分是生物化學(xué)家利用某一特定食物成分作為模擬物系加以收集的。
  
    氨基酸

    氨基酸是蛋白質(zhì)的“結(jié)構(gòu)單元”。因此，要了解蛋白質(zhì)的性質(zhì)，舊需要討論氨基酸的結(jié)構(gòu)和性質(zhì)。氨基酸是既含氨基又含酸性羧基的化合物。蛋白質(zhì)中的氨基酸在α-碳原子上同時(shí)有氨基和羧基。α-氨基酸具有如下的一般結(jié)構(gòu)：

      NH2     
      │     
R ─ C ─ COOH
      │     
      H      

    在中性pH水溶液中，氨基和羧基都呈離子狀態(tài)。羧基失去一個(gè)質(zhì)子而帶負(fù)電荷，同時(shí)氨基得到一個(gè)質(zhì)子而帶正電荷。結(jié)果氨基酸便具有偶極的特性。氨基酸的這種偶極形式（即兩性形式）有如下的一般結(jié)構(gòu)：

      NH3+     
      │      
R ─ C ─ COO-
      │     
      H      

    氨基酸有好幾種性質(zhì)都反映了這種結(jié)構(gòu)，這些性質(zhì)是：它們易溶于水而不易溶于極性很小的溶劑：當(dāng)以晶體形式存在時(shí)，它們要在較高溫度（一般在200℃以上）下熔化或分解；它們?cè)谥行匀芤悍N顯示出很大的偶極矩和介電常數(shù)。

    氨基酸的側(cè)鏈R基團(tuán)對(duì)氨基酸和蛋白質(zhì)的化學(xué)性質(zhì)產(chǎn)生重大的影響。這些側(cè)鏈可以分為四類。

    帶有極性非荷電的（親水的）R基團(tuán)的氨基酸能與水形成氫鍵，通常能溶于水溶液。絲氨酸、蘇氨酸和酪氨酸的羥基，半胱氨酸的硫氫基（即硫醇）以及天冬酰胺和谷氨酰胺的酰胺基時(shí)出現(xiàn)在這類氨基酸R基團(tuán)中的功能部分，其中半胱氨酸的硫羥基和酪氨酸的羥基在pH7時(shí)能輕度離子化，因而比這類中其它氨基酸更容易失去質(zhì)子。天冬酰胺和谷氨酰胺的酰胺基容易被酸和堿水解，分別形成天冬氨酸和谷氨酸。

    帶有非極性（疏水的）R集團(tuán)的氨基酸在水溶液中的溶解性比帶有極性非荷電的R基團(tuán)的氨基酸要小得多。帶有烴側(cè)鏈的五種氨基酸，其側(cè)鏈隨側(cè)鏈長(zhǎng)度增加而降低。脯氨酸（以及其烴基衍生物羥脯氨酸）的獨(dú)特結(jié)構(gòu)使這種氨基酸在蛋白質(zhì)結(jié)構(gòu)中有獨(dú)特的地位。

    pH6～7時(shí)帶正電荷（堿性的）R基團(tuán)的氨基酸有賴氨酸、精氨酸和組氨酸。賴氨酸帶正電的原因主要在于氨基，而精氨酸則具有帶正電荷的胍基。pH7時(shí)組氨酸分子中的咪唑基有10%質(zhì)子化，但在pH6時(shí)則有50%以上帶正電荷。

    二羥基氨基酸（天冬氨酸和谷氨酸）在中性pH范圍內(nèi)帶凈負(fù)電荷，谷氨酸的一鈉鹽是一種重要的膳食調(diào)味用的人造食品添加劑。

    肽

    當(dāng)一個(gè)氨基酸分子的氨基與另一個(gè)氨基酸分子的羧基起反應(yīng)時(shí),便形成一個(gè)肽鍵，同時(shí)釋放出一分子水。這種C-N鍵把眾多的氨基酸連接在一起形成蛋白質(zhì)。