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Microorganisms no doubt outnumber other living entities on this planet and can be found existing actively or passively wherever living organisms occur. While the energy for on this planet is captured by green plants in the photosynthetic process, microorganisms are generally responsible for the final decomposition of the photosynthetic products. Animals play a minor role in the cycle.
Inasmuch as bacteria, yeasts and molds are to be found throughout the environment of man, it is to be anticipated that these microorganisms are in direct competition with other living entities for the energy for life. Whenever the conditions of nutrients and environment are favorable for microbial activity, it will be found.
Man must compete with all other living entities on earth. In order to retain food supplies for himself, he must interfere with natural processes. Through his study, and as a fruit of his curiosity, man has evolved a number of control systems. One is the preservation of food by controlling, yet encouraging, the growth of microorganisms. Under such a condition, man may employ microorganisms to create unfavorable conditions for other microbes, yet retain in the foodstuffs the nutrients desired.
While microorganisms were not identified as the important agents in food spoilage until a century ago, wine making, bread baking, cheese making and salting of foods have been practiced for more than four thousand years. For all those years mankind practiced food preservation using unknown, invisible, active, living organisms.
While food preservation system in general inhibits the growth of microorganisms, all such organisms are not detrimental. In fact some are commonly utilized in food preservation. The production of substantial amounts of acid by certain organisms creates unfavorable conditions for others.
To review terms for a moment, respiration is that process whereby carbohydrates are converted aerobically into carbon dioxide and water with the release of large amounts of energy. Fermentation is a process of anaerobic, or partially anaerobic, oxidation of carbohydrates. Putrefaction is the anaerobic degradation of proteinaceous materials.
Sodium chloride is useful in a fermentation process of foods by limiting the growth of putrefactive organisms and by inhibiting the growth of large numbers of other organisms. Yet some bacteria tolerate and grow in substantial amounts of salt in solution.
Fermentation of Carbohydrates
The word fermentation has undergone evolution itself. The term was emplyed to describe the bubbling or boiling condition seen in the production of wine. prior to the time that yeasts were discovered. However. after Pasteur's discovery, the word became used with microbial activity, and later with enzyme activity. Currently the term is used even to describe the evolution of carbon
dioxide gas during the action of living cells. Neither gas evolution nor the presence of living cells is essential to fermentations where no gas is liberated, and in fermentations accomplished solely with enz3'mes. s)
There is a clear difference between fermentation and putrefaction. Fermentation is a decomposition action on carbohydrate materials; putrefaction relates to the general action of microorganisms on proteinaceous materials. Fermentation processes usually do not evolve putrid odors. and carbon dioxide is usually produced. In putrefaction the evolved materials may contain carbon dioxide, but the characteristic odors are hydrogen sulfide and sulfur-containing protein decomposition products. A putrid fermentation is usually a contaminated fermentation. Putrid kraut or pickles result from microbial growths decomposing protein, rather than the normal fermentation of carbohydrates to produce acid. Industially Important Organisms In Food Preservation
There are three important characteristics microorganisms should have if they are to be useful in fermentation and pickling. (1) The microorganisms must be able to grow rapidly in a stuiable substrate and environment. and be easily cultivated in large quantity. (2) The organism must have the ability to maintain physiological constancy under the above conditions, and yield the essential enzymes easily and abundantly in order that the desired chemical changes can occur. (3) The environmental conditions required for maximum growth and reproduction should be comparatively simple.
The application of microorganisms to food preservation practices must be
such that a positive protection is available to control contamination.
The microorganisms used in fermentations are notable in that they produce
large amounts of enzymes. Bacteria, yeasts and molds, being single cells, contain the functional capacities for growth reproduction, digestion. assimilation and repairs in a cell, that higher forms of life have distributed to tissues.
Therefore. it is to be anticipated that single cell complete living entities(such as yeasts) have a higher enzyme productivity and fermentative capacity than found with other living creatures.
Enzymes are the active substances which control chemical reactions in fermentation. The microorganisms of each genus and species are actually a warehouse of enzymes, with its own special capacity to produce and secrete enzymes.
Man has yet to learn to synthesize them.
A dry gram of an organism endowed with high activity lactose fermenting enzymes is capable of breaking down 10,000 g of lactose per hour. This great chemical activity is associated with the single life-process requirements of the organisms, the ease with which they obtain energy for life, their great growth capacity and reproduction rate, and their great capacity for maintenance of the living entity. One generation may occur in a matter o{ minutes.
But there is a balance in effect. In living, the organisms consume energy.
The product of their actions is a substrate of lower energy than that native material upon which they were planted. However. the product of the activity in the instance of wine is one which man generally enjoys more than the native juice from which the wine was produced.
Order of Fermentation
Microorganisms have available carbohydrates, proteins, fats. minerals and minor nutrients in native food materials. It appears that microorganisms first attack carbohydrates, then proteins, then fats. There is an order of attack even with carbohydrates; first the sugars, then alcohols, then acids. Since the first requirement for microbial activity is energy, it appears that the most available forms, in order of preference, are the CH2, CH, CHOH, and COOH carbon linkages. Some linkages such as CN radicals are useless to microorganisms.
Types ,of Fermentations of Sugar
Microorganisms are used to ferment sugar by complete oxidation, partial oxidation, alcoholic fermentation , lactic acid fermentation, butyric fermentation and other minor fermentative actions.
(l) Bacteria and molds are able to break down sugar (glucose) to carbon dioxide and water. Few yeasts can accomplish this action.
(2) The most common fermentation is one in which a partial oxidation of sugar occurs. In this case, sugar may be converted to an acid. The acid finally may be oxidized to yield carbon dioxide and Water, if permitted to occur. For
example, some molds are used in the production of citric acid from sugar solutions.
(3) Yeasts are the most efficient converters of aldehydes to alcohols. Many species of bacteria, yeasts and molds are able to yield alcohol. The yeast,
Saccharomyces ellipsoideus. is of great industrial importance in alcoholic fermentatioas. The industrial yeasts yield alcohol in recoverable quantities. While other organisms are able to produce alcohol, it occurs in such mixtures of aldehydes, acids and esters that recovery is difficult.
The reaction from sugar to alcohol is many stepped.
(4) Lactic acid fermentation are of great importance in food preservation.
The sugar in foodstuff may be converted to lactic acid and other end products, and in such amounts that the environment is controlling over other organisms.
Lactic acid fermentation is efficient, and the fermenting organisms rapid in growth. Natural inoculations are such that in a suitable environment the. lactic acid bacteria will dominate, as in souring of milk.
(5) Butyric fermentations are less useful in food preservation than those noted previously. The organisms are anaerobic and impart undesirable flavors and odors to foods. The anaerobic organisms capable of infecting man causing disease are commonly butyric fermenters. Carbon dioxide. hydrogen.acetic acid and alcohols are some of the other fermentation products.
(6) In addition to the above there is a fermentation which involves much gas production. It is useful ;n food preservation. although gas production has disadvantages. Energy-wise it is less efficient to produce gases (carbon dioxide and hydrogen) which have little or no preserving power in concentrations found in comparison with lactic acid. Also, the important food spoilage organisms are capable of growing in such environments. In gassy fermentations sugar molecules are altered to form acids, alcohols and carbon dioxide. It is usually necessary to include some other controlling influence, such as adding sodium chloride to a substrate, with this form of fermentation.
(7) There are many fermentative actions possible in foods which are detrimental to the acceptability of treated foods. Generally the organisms capable of attacking higher carbohydrates such as cellulose, hemicelluloses, pectin, and starch will injure the texture, flavor and quality of treated foods.
Fermentation Controls
Foods are contaminated naturally with microorganisms and will spoil if untended. The type of action which will develop is dependent upon the conditions which are imposed. The most favorable to a given type of fermentation under one condition will be altered by slight changes in a controlling factor. Untended meat will naturally mold and putrefy. If brine or salt added, entirely different organisms will take over.
The pH Value of Food is a Controlling Factor-Most foods in native,fresh form which man consumes as food are acid. Vegetables range in pH value from 6.5 to 4.8. Fruits range from 4.5 down to 3.O. Animal flesh when killed is approximately neutral (7.2) but within two days the pH value will
be approximately 6.0. Milk has a pH value near 6.4. In as much as the two important fermentation in such foods are oxidative
and alcoholic, the growth of organisms will be controlled by the acidity of the medium. In fruits and fruit juices, yeasts and molds will quickly establish themselves. In meats yeasts are less active than bacteria. In milk, an acid fermentation is established in the matter of a few hours.
Source of Energy-Inasmuch as the immediate need of microorganisms is a source of energy, the soluble, readily available carbohydrates influence the microbial population that wiLI dominate. In milk the sugar is lactose; those organisms which quickly mount in numbers are the lactose formenting organisms.
Because suitable energy sources are generally available to microorganisms in man's foods, energy sources are not usually a limiting factor, with certain exceptions (such as milk).
Availability of Oxygen-The degree of anaerobiosis is a principal factor controlling fermentations. with yeasts. when large amounts of oxygen are present, yeast cell production is promoted. If alcchol production is desired, a very limited oxygen supply is required.
Molds are aerobes, and are controlled by the absence of oxygen, Bacterial populations which will dominate a substrate may be manipulated by their oxygen requirements and its availability. .
The end product of a fermentation can be controlled in part by the oxygen tension of the substrate, other factors being optimum.
Temperature Requirements-Each group of microorganisms has an optimum temperature for growth; the temperature of a substrate therefore exerts a positive control on their growth. To obtain the maximum performance during fermentation, the optimum temperature for the organisms must be created.
The temperature at which a food is held will determine within certain limits the nature of the organisms capable of either yielding the desired fermentation or spoilage, whichever the case may be.
The action of Sodium Chloride in Controlling Fermentations-Salts is one of the most important food adjuncts in food preservation. In drying it has been shown to have beneficial. In fermentations salt can exert a role in sorting the organisms permitted to grow.
微生物无疑在地球上数量超过其他的生命体，并且，凡是有生物存在的地方都能找到主动或被动地生活着的微生物。在地球上，一方面绿色植物在光合过程中捕获了为生存所需的能量，一方面微生物广泛地担负着对光合产物进行最终的分解。动物界在这个循环中扮演了次要的角色。
由于人类所处的环境到处可以找到细菌、酵母和霉菌，因而可以预料，这些微生物与其他生物体一道进行着为获取生存所需能量的直接竞争。无论何时营养条件和环境有利于微生物活动，就可以找到微生物。
人类也必须于地球上所有其他生物体进行竞争。为了保证自己的食物供给，人类必须干预自然过程。人类通过研究，作为好奇而得到的产物，已开发了一批控制自然过程的系统。其中之一就是通过控制（也包括促进）微生物的生长来保藏食物。在这样的情况下，人类可以利用一些微生物去创造不利于其他微生物的环境，而保留食物中必要的营养素。
尽管直到一个世纪前才认定微生物是食物腐败的重要因素，而酿造葡萄酒、烘烤面包、制作奶酪和腌制食品则已进行了4000年。在所有那些年代里，人类曾利用不知道的、看不见的活性生物从事于食品保藏的实践。
虽然食物保藏体系一般都抑制微生物生长，但不是所有的微生物都是有害的。事实上，有些微生物常常被用来保藏食物。某些微生物产生的大量的酸创造了不利于其他微生物的环境。
下面用片刻时间复习一下词汇。呼吸作用是碳水化合物在有氧条件下转化为二氧化碳和水，并放出大量能量的过程。发酵作用是碳水化合物缺氧氧化或部分缺氧氧化的过程。腐败只作用是蛋白性物质的厌氧降解作用。
氧化钠在食品发酵过程中非常有用，因为它限制了腐败微生物生长，并抑制大量其它微生物生长。然而，有些细菌能耐受高盐溶液并在其中生长。
碳水化合物发酵
发酵这个词本身经历了演变。在发现酵母以前，这个词被用来描述葡萄酒生产中出现的发泡和沸腾现象。而在巴斯德的发现之后，这个词便变成与微生物活动联系起来的词来使用，后来又与酶的活性联系起来。现今，这个词甚至被用来描述活细胞活动过程中二氧化碳气体的放出。但是，对于无气体释放的发酵和仅由酶来完成的发酵作用来说，气体的放出和活动细胞的存在都不是必要的。
发酵作用于腐败作用有明显区别。发酵作用是对碳水化合物的一种分解作用；腐败作用则涉及微生物对蛋白性物质的全面综合的作用。发酵过程通常不放出腐烂的气味，而且通常产生二氧化碳。在腐败过程中，放出的物质中可能含有二氧化碳，但其特征气味是硫化氢和含硫蛋白质的分解产物。腐败发酵作用通常即受污染的发酵作用。腐化的酸菜或泡菜是细菌生长分解蛋白质的结果，而不是碳水化合物正常发酵产酸所致。
食品保藏上重要的工业微生物
微生物如果要在发酵和腌渍上有用，它必须具备三个重要特性：（1） 这些微生物必须能在适合的底物和环境中生长并容易大量培养。（2） 这些微生物必须能够在上述条件下保持其生理稳定性，且易于产生大量所必须的酶，以期能出现所要求的化学变化。（3） 为使微生物的生长和繁殖最快所必须的环境条件应相对较简单。
微生物在食品保藏实践上的应用必须采取积极的防护措施，以控制污染。
用于发酵的微生物的显著特点是产生大量的酶。以单细胞存在的细菌、酵母和霉菌，其单个细胞中就具有生长、繁殖、消化、吸收和修复的功能，而在生命的高等形态中，这些功能分配给组织。因此可以预料，完全的单细胞生物体（例如酵母）具有比其它生物体更高的产酶和发酵能力。
酶是发酵作用中控制化学反应的活性物质。微生物的每个属、每个种实际上都是各种酶的贮存库，以其自身特定的能力产生和分泌各种酶。人类还有待于去学会合成各种酶。
干重1g具有高活性乳糖发酵酶的微生物每小时能分解10000g乳糖。这样强烈的化学活性与微生物的简单生命过程的需求有关，与它们生存所需能量的获得方便有关，与它们旺盛的生长能力和快速的繁殖速率有关，还与它们维持生命体的强大能力有关。数分钟时间就可以产生一代。
实际上也有一定的平衡。微生物在生命活动中要消耗能量。它们活动产生的产物于它们所赖以生长的天然物质相比是能量较低的物质。然而，以葡萄酒为例，微生物活动的产物正是通常比生产葡萄酒的天然果汁更加受人喜爱的东西。
发酵作用的顺序
微生物在天然食品材料中获取可利用的碳水化合物、蛋白质、脂肪、矿物质的微量营养素�？蠢�，微生物首先作用于碳水化合物，然后是蛋白质，最后是脂肪。即使是碳水化合物，也有一定的作用顺序；首先是糖类，其次是醇类，最后是酸类。由于微生物活动的第一需要是能量，所以看来最易利用的形式（按优先选择顺序）是CH2、CH、CHOH和COOH碳键。有些键（例如 CN 基团）对微生物是没有用的。
糖的发酵模式
微生物通过完全氧化、部分氧化、酒精发酵、乳酸发酵、丁酸和其他次要发酵的活动来发酵糖。
（1）细菌和霉菌能够将糖（葡萄糖）分解为二氧化碳和水。几乎没有酵母能够完成这一反应。
（2）最普通的发酵过程是糖发生部分氧化的发酵。在这种情况下，糖可以转化成酸。如果允许发生，酸最终也可以氧化成二氧化碳和水。例如，用某些霉菌从糖溶液生产柠檬酸。
（3）酵母是醛类变成醇类的最有效的转化者。细菌、酵母和霉菌的许多菌种能够产生酒精。葡萄酒酵母在酒精发酵中具有很大的工业重要性。这种工业用酵母产生可回收量的酒精。虽然其他微生物也能产生酒精，但是酒精中掺杂醛、酸和脂，因而难以回收。从糖到酒精的反应要经历许多步骤。
(4)乳酸发酵在食品方面非常重要。食物中的糖可被转化成乳酸和其他终产物，转化的量使得周围环境对别的微生物有控制作用。乳酸发酵的效率高，发酵菌的生产迅速。在自然接种的情况下，乳酸菌在合适的环境中将占主要地位，例如牛奶变酸的情形。
（5）丁酸发酵在食品保藏中不如前面提到的那些发酵那么重要。此发酵菌是厌氧菌，并会给食品带来令人不愉快的口味和气味。通常使人受感染生病的厌氧菌是丁酸发酵菌，二氧化碳、氢气、醋酸和醇是丁酸的另外一些产物。
（6）除以上所述外，还有一种涉及大量气体产生的发酵。这种发酵对食品保藏有用，尽管产气有不利的方面。就能量方面而言，这种发酵产气（二氧化碳和氢气）的效率很低，这些气体在与乳酸作比较时所遇到的浓度下，几乎没有或完全没有保藏效力。而且，重要的食品腐败微生物能够在这样的环境中生长。在产气发酵中，糖分子转化成酸、醇和二氧化碳。采用这种发酵形式，常常有必要引入一些别的有影响的控制措施，例如向底物添加氯化钠。
（7）在食品方面还可能有许多的发酵重要，它们对处理后的食品的接受性有害。一般来说，凡是有能力作用于诸如纤维素、半纤维素。果胶和淀粉之类高级碳水化合物的微生物会损害被处理食品的质构、风味和品质。
发酵控制方法
食品是天然受微生物污染的，如果不加注意，就会腐败。将要出现的微生物作用模式取决与外界施加的条件。对某条件下最为有利的给定发酵模式，也会因控制因素的微小变化而改变。没有照管好的肉会自然长霉并腐烂。如果加盐水或盐，完全不同的微生物就会占优势。食品pH值是控制因素——人类食用的许多天然新鲜食物是酸性的。蔬菜的pH值范围为6.5～ 4.6。水果的pH值为4.5～3.0。刚宰杀的动物的肉，其pH值接近中性（7.2），但两天之内pH值将接近6.0. 牛奶的pH接近6.4.
由于这类食品中的两种重要的发酵作用是氧化和醇化发酵，因而，微生物的生长将受介质酸度的控制。水果和果汁方面，酵母和霉菌将很快使自己立足。肉类方面，酵母的活力不如细菌。牛奶方面，在数小时的时间内就会出现酸发酵。
能量的来源——由于微生物的直接需要是能量来源，因而利用方便的可溶性碳水化合物影响着将要占支配地位的微生物的数目。牛奶中的糖是乳糖，因此在数目上很快增值的微生物就是乳糖发酵菌。由于微生物一般能在人类食物中获得合适的能量来源，所以除某些例外的情况（如牛乳），能量来源通常不是限制因素。
获得氧的难易程度——缺氧的程度是控制发酵的主要因素。对于酵母，当有大量氧气存在时，就促进酵母细胞的生产。如果希望产生酒精，供氧就要非常有限。
霉菌是需氧的，它受缺氧的控制�；实目刂啤；手姓贾鞯嫉匚坏母髦窒妇木植际芨骶男柩跚榭鲆约盎竦醚醯哪岩椎目刂啤�
如果其他因素均为最佳，那么发酵的最终产物可以部分地受基质的氧分压控制。
温度要求——每一类微生物都有一最适生产温度；因此基质的温度对微生物的生产起了积极控制的作用。为了在发酵过程中达到最大限度的操作效能，必须给微生物创造最适的温度条件。
食品存放的温度将有在一定限度内决定了微生物的种类和属性，这些微生物或者能产生所要求的发酵作用，或者能产生腐败作用，两者必居其一。
氯化钠在控制发酵方面的作用——盐类是食品防腐上最重要的食品添加剂之一。在干制方面，盐已显示出具有良好效果。在发酵方面，盐可以对允许生产的微生物起着筛选的作用。
