Lesson 4

Reading Material Protein

Proteins play a central role in biological systems. They are vital to basic cellular and body functions，including cellular regeneration and repair，tissue maintenance and regulation，hormone and enzyme production，fluid balance，and the provision of energy. In addition，proteins are important constituents of food and one of major nutrients. They directly contribute to the flavor of food and are precursors for aroma compounds and colors formed during thermal or enzymatic reactions in production，processing and storage of food. Other food constituents，such as carbohydrates，also take part in such reactions. Proteins also contribute significantly to the physical properties of food through their ability to build or stabilize gels，foams，emulsions and fibrillar structures.

Composition and Structure

Proteins are very complex and many have been purified and characterized. Proteins vary in molecular mass，ranging from approximately 5000 to more than a million Daltons. They are composed of elements including carbon，hydrogen，oxygen，nitrogen and sulfur.α-amino acids are the basic structural units of proteins. These amino acids consist of anα-carbon atom covalently attached to a hydrogen atom，an amino group（—NH₂），a carboxyl group（—COOH），and a side-chain R group as indicated below：

The R may be a hydrogen atom or a more complex group，giving rise to variety of about 23 or more amino acids present in plant and animal proteins. Some of these amino acids cannot be synthesized in the body，but are essential for maintenance in human beings. These amino acids that have to be supplied in the food are called essential amino acids. The remaining amino acids，which can be synthesized from others in the body，are termed as non essential amino acids，because our body does not have to depend for their supply on the foods we eat.

Animal proteins have a more balanced amino acid profile and contain notable amounts of both essential and nonessential amino acids. The fish proteins contain abundant essential amino acids. Plant proteins generally have lower content of some essential amino acids such as lysine and methionine. Some nonessential amino acids such as glutamic acid and aspartic acid are abundantly found in all proteins.

The differences in structure and function of these thousands of proteins arise from the sequence in which the amino acids are linked together via amide bonds. Literally，billions of proteins with unique properties can be synthesized by changing the amino acid sequence，the type and ratio of amino acids，and the chain length of polypeptides.

Proteins can be classified by their composition，structure，biological function，or solubility properties. For example，simple proteins contain only amino acids upon hydrolysis e. g. albumin，globulin，prolamin，and glutelin，but conjugated proteins also contain non-amino-acid components e. g. nucleoprotein and lipoprotein.

Source of Proteins

Plants are the primary source of proteins as they synthesise protein by combining nitrogen and water from soil with the carbon dioxide from air. Most animals and fish depend on plants to provide them proteins.

For practical purposes，food proteins may be defined as those that are easily digestible，nontoxic，nutritionally adequate，functionally usable in food products，and available in abundance. Traditionally，milk，meats（including fish and poultry），eggs，cereals，legumes，and oilseeds have been the major sources of food proteins. In China and some other Asian countries，tofu is the largest source of food protein. It is consumed fresh or dried，or fried in fat and seasoned with soy sauce. Wheat proteins，another source of food protein，containing albumins，globulins，gliadins，and glutenins，these four basic proteins depending on their varied solubility in different solvents.

Food sources of proteins are presented in Table 4.1.

However，because of the burgeoning world population，nontraditional sources of proteins for human nutrition need to be developed to meet the future demand. The suitability of such new protein sources for use in foods，however，depends on their cost and their ability to fulfill the normal role of protein ingredients in processed and home-cooked foods. The functional properties of proteins in foods are related to their structural and other physicochemical characteristics. A fundamental understanding of the physical，chemical，nutritional，and functional properties of proteins and the changes these properties undergo during processing is essential if the performance of proteins in foods is to be improved，and if new or less costly sources of proteins are to compete with traditional food proteins.

Protein properties

Food preferences by human beings are based primarily on sensory attributes such as texture，flavor，color，and appearance. The sensory attributes of a food are the net effect of complex interactions among various minor and major components of the food. Proteins generally have a great influence on the sensory attributes of foods. For example，the sensory properties of bakery products are related to the viscoelastic and dough-forming properties of wheat gluten；the textural and succulence characteristics of meat products are largely dependent on muscle proteins（actin，myosin，actomyosin，and several water-soluble meat proteins）；the textural and curd-forming properties of dairy products are due to the unique colloidal structure of casein micelles；and the structure of some cakes and the whipping properties of some dessert products depend on the properties of egg-white proteins.

On an empirical level，the various functional properties of proteins can be viewed as manifestations of two molecular aspects of proteins：hydrodynamic properties and protein surface -related properties. The functional properties such as viscosity（thickening），gelation，and texturization are related to the hydrodynamic properties of proteins，which depend on size，shape，and molecular flexibility. Functional properties such as wettability，dispersibility，solubility，foaming，emulsification，and oil flavor binding are related to the chemical and topo-graphical properties of the protein surface.

However，when proteins are exposed to heat，light or change in pH and other processing conditions，structural changes occur. These changes in structure of proteins are known as denaturation. Denaturation leads to change in solubility of proteins. It may be reversible，if conditions which cause it are mild，but mostly the changes which occur are irreversible. Often denaturation has a negative connotation，because it indicates loss of some properties. For example，many biologically active proteins lose their activity upon denaturation. In some instances，however，protein denaturation is desirable. For example，partially denatured proteins are more digestible and have better foaming and emulsifying properties than do native proteins. Thermal denaturation is also a prerequisite for heat-induced gelation of food proteins.

Vocabulary

actin 肌动蛋白

myosin 肌球蛋白

actomyosin 肌动球蛋白

nucleoprotein 核蛋白

amino acid 氨基酸

lipoprotein 脂蛋白

aroma 芳香，香味

polypeptide 多肽

burgeoning 迅速增长的

precursor 前体

casein 酪蛋白

protein 蛋白质

cellular 细胞的

regeneration 再生

curd 凝乳

repair 修复

dairy 乳制品

solubility 溶解度

denaturation 变性

textural 质构化，纹理性

emulsion 乳液，乳剂

tissue 组织

gel 凝胶，胶化

viscoelastic 黏弹性

micelle 胶束，胶囊

whipping 搅打…变稠

参考译文 蛋白质

蛋白质在生物系统中起核心作用，它对细胞的组成和身体功能至关重要，包括细胞再生和修复、组织维持和调节、激素和酶的产生、体液平衡和能量供应。此外，蛋白质是食物的重要组成部分，也是主要的营养物质之一，它有助于食品风味形成，是食品生产、加工和储存过程中热或酶反应时生成芳香物质和色素的前体物质，食品中的另一种组成成分——碳水化合物也参与了这种反应。蛋白质还可以通过构建稳定的凝胶、泡沫、乳液和纤维状结构，使食品具有一定物理特性。

组成和结构

蛋白质是非常复杂的，许多已被纯化和表征，其分子质量不等，一般在5000甚至超过100万u，它的组成元素一般有碳、氢、氧、氮和硫。α-氨基酸是蛋白质的基本结构单位，它由一个α-碳原子通过共价键与一个氢原子、一个氨基（—NH₂）、一个羧基（—COOH）和侧链R基组成，如下所示：

R基可以是氢原子或更复杂的基团，这样导致存在于植物和动物蛋白质中约23个或更多氨基酸的多样性。这里面有些氨基酸不能在人体内合成，但对维持人体生命活动至关重要，这些氨基酸必须从食品中获得，称之为必需氨基酸。另外一些氨基酸，可以在体内通过其他物质合成，我们的身体不必依赖食物的供给，这类氨基酸称为非必需氨基酸。

动物蛋白质含有大量的必需氨基酸和非必需氨基酸，具有更平衡的氨基酸谱。鱼类蛋白质中含有丰富的必需氨基酸。植物蛋白质所含必需氨基酸的含量通常较低，如赖氨酸和蛋氨酸。在所有蛋白质中，都含一些大量的非必需氨基酸如谷氨酸和天冬氨酸。

成千上万的蛋白质在结构和功能上的差异源于通过酰胺键连接的氨基酸序列的不同。从这方面看，通过改变氨基酸序列、氨基酸的类型和比例以及多肽的链长，可以合成出几十亿种具有特殊性质的蛋白质。

蛋白质可以根据其组成、结构、生物学功能或溶解特性进行分类。例如，简单蛋白质水解后仅含有氨基酸，如清蛋白、球蛋白、醇溶蛋白和谷蛋白；但结合蛋白质水解后还含有非氨基酸组分，如核蛋白和脂蛋白。

蛋白质来源

植物是蛋白质的主要来源，植物通过利用来自土壤中氮和水以及空气中的二氧化碳来合成蛋白质。大部分动物和鱼类则依赖植物来满足其蛋白质需求。

根据实际需求，食物蛋白质被定义为易消化、无毒、营养充分、具备功能性的食物中的蛋白质，并且容易获得。传统意义上，主要的食物蛋白质来源有奶类、肉类（包括鱼和家禽）、蛋类、谷类、豆类和油籽。在中国和其他一些亚洲国家，豆腐是食物蛋白的最大来源，新鲜、干制或油炸后调味均可食用。小麦蛋白也是一种食物蛋白，根据溶解性可分为白蛋白、球蛋白、醇溶蛋白和麦谷蛋白。

常见食物蛋白的来源见表4.1。

然而，由于世界人口的不断增长，需要开发非传统的蛋白源食物来满足未来人们对营养的需求。不过，这种新的蛋白源食物的适用性还取决于它们的成本，以及在加工和烹饪后满足正常蛋白质成分的功能。食物中蛋白质的功能与其结构和理化性质有关。如果要改善食物中蛋白质的功能，并且，这种新的或低成本的蛋白源食物能与传统蛋白食物竞争，那么，对蛋白质的物理、化学、营养和功能特性以及这些特性在加工过程中发生的变化有一个基本的了解是非常必须的。

蛋白质性质

人们对食品的偏好主要是基于其感官特性，如质地、风味、颜色和外观。食品的感官特性是食物中各种次要和主要成分之间复杂相互作用后的净效应。通常蛋白质对食品的感官特性有很大的影响。例如，烘焙食品的感官特性与面筋蛋白的黏弹性和成团性有关；肉制品的纹理性和多汁性在很大程度上依赖于肌蛋白（肌动蛋白、肌球蛋白、肌动球蛋白和多种水溶性肉蛋白）；乳制品的质构特性和凝乳性与酪蛋白胶束独特的胶体结构有关；一些糕点的结构和一些甜点的搅打特性取决于蛋清蛋白的性质。

从经验上看，蛋白质的各种功能性质可以看作是两种蛋白质分子特性的体现：流体动力学性质和蛋白质表面相关性质。蛋白质的一些功能性质如黏性（增稠）、凝胶化和组织化与蛋白质的流体动力学性质有关，这取决于蛋白质大小、形状和分子的灵活性。另一些功能性质如润湿性、分散性、溶解性、发泡性、乳化性以及和油脂风味的结合与蛋白质表面的化学和外表特性有关。

然而，当蛋白质所处的热、光、pH、工艺条件发生变化时，会发生结构变化，蛋白质这种结构变化称为变性。变性会导致蛋白质溶解度的变化，如果条件温和，这种变性是可逆的，但大多数变性是不可逆的。变性常常具有负面意义，因为这意味着一些性质的丧失。例如变性会导致很多蛋白质的生物活性丧失。不过，有些蛋白质的变性是必须的。例如，部分蛋白质变性后比天然蛋白质更易消化，具有更好的起泡性和乳化性。热变性也是食物蛋白通过热诱导凝胶化的先决条件。

Exercise

1.Answer questions

（1）What are proteins? What are their functions?

（2）List the different sources of protein in our dietary.

（3）List the different milk products used in our diets.

2.Translation

（1）Proteins are important constituents of food and one of major nutrients. They directly contribute to the flavor of food and are precursors for aroma compounds and colors formed during thermal or enzymatic reactions in production，processing and storage of food.

（2）Plants are the primary source of proteins as they synthesise protein by combining nitrogen and water from soil with the carbon dioxide from air. Most animals and fish depend on plants to provide them proteins.

（3）When proteins are exposed to heat，light or change in pH and other processing conditions，structural changes occur. These changes in structure of proteins are known as denaturation.