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.

    微生物無疑在地球上數(shù)量超過其他的生命體，并且，凡是有生物存在的地方都能找到主動或被動地生活著的微生物。在地球上，一方面綠色植物在光合過程中捕獲了為生存所需的能量，一方面微生物廣泛地擔(dān)負著對光合產(chǎn)物進行最終的分解。動物界在這個循環(huán)中扮演了次要的角色。

    由于人類所處的環(huán)境到處可以找到細菌、酵母和霉菌，因而可以預(yù)料，這些微生物與其他生物體一道進行著為獲取生存所需能量的直接競爭。無論何時營養(yǎng)條件和環(huán)境有利于微生物活動，就可以找到微生物。

    人類也必須于地球上所有其他生物體進行競爭。為了保證自己的食物供給，人類必須干預(yù)自然過程。人類通過研究，作為好奇而得到的產(chǎn)物，已開發(fā)了一批控制自然過程的系統(tǒng)。其中之一就是通過控制（也包括促進）微生物的生長來保藏食物。在這樣的情況下，人類可以利用一些微生物去創(chuàng)造不利于其他微生物的環(huán)境，而保留食物中必要的營養(yǎng)素。

    盡管直到一個世紀前才認定微生物是食物腐敗的重要因素，而釀造葡萄酒、烘烤面包、制作奶酪和腌制食品則已進行了4000年。在所有那些年代里，人類曾利用不知道的、看不見的活性生物從事于食品保藏的實踐。

    雖然食物保藏體系一般都抑制微生物生長，但不是所有的微生物都是有害的。事實上，有些微生物常常被用來保藏食物。某些微生物產(chǎn)生的大量的酸創(chuàng)造了不利于其他微生物的環(huán)境。

    下面用片刻時間復(fù)習(xí)一下詞匯。呼吸作用是碳水化合物在有氧條件下轉(zhuǎn)化為二氧化碳和水，并放出大量能量的過程。發(fā)酵作用是碳水化合物缺氧氧化或部分缺氧氧化的過程。腐敗只作用是蛋白性物質(zhì)的厭氧降解作用。

    氧化鈉在食品發(fā)酵過程中非常有用，因為它限制了腐敗微生物生長，并抑制大量其它微生物生長。然而，有些細菌能耐受高鹽溶液并在其中生長。

                            碳水化合物發(fā)酵

    發(fā)酵這個詞本身經(jīng)歷了演變。在發(fā)現(xiàn)酵母以前，這個詞被用來描述葡萄酒生產(chǎn)中出現(xiàn)的發(fā)泡和沸騰現(xiàn)象。而在巴斯德的發(fā)現(xiàn)之后，這個詞便變成與微生物活動聯(lián)系起來的詞來使用，后來又與酶的活性聯(lián)系起來�，F(xiàn)今，這個詞甚至被用來描述活細胞活動過程中二氧化碳氣體的放出。但是，對于無氣體釋放的發(fā)酵和僅由酶來完成的發(fā)酵作用來說，氣體的放出和活動細胞的存在都不是必要的。

    發(fā)酵作用于腐敗作用有明顯區(qū)別。發(fā)酵作用是對碳水化合物的一種分解作用；腐敗作用則涉及微生物對蛋白性物質(zhì)的全面綜合的作用。發(fā)酵過程通常不放出腐爛的氣味，而且通常產(chǎn)生二氧化碳。在腐敗過程中，放出的物質(zhì)中可能含有二氧化碳，但其特征氣味是硫化氫和含硫蛋白質(zhì)的分解產(chǎn)物。腐敗發(fā)酵作用通常即受污染的發(fā)酵作用。腐化的酸菜或泡菜是細菌生長分解蛋白質(zhì)的結(jié)果，而不是碳水化合物正常發(fā)酵產(chǎn)酸所致。
食品保藏上重要的工業(yè)微生物

    微生物如果要在發(fā)酵和腌漬上有用，它必須具備三個重要特性：（1） 這些微生物必須能在適合的底物和環(huán)境中生長并容易大量培養(yǎng)。（2） 這些微生物必須能夠在上述條件下保持其生理穩(wěn)定性，且易于產(chǎn)生大量所必須的酶，以期能出現(xiàn)所要求的化學(xué)變化。（3） 為使微生物的生長和繁殖最快所必須的環(huán)境條件應(yīng)相對較簡單。

    微生物在食品保藏實踐上的應(yīng)用必須采取積極的防護措施，以控制污染。

    用于發(fā)酵的微生物的顯著特點是產(chǎn)生大量的酶。以單細胞存在的細菌、酵母和霉菌，其單個細胞中就具有生長、繁殖、消化、吸收和修復(fù)的功能，而在生命的高等形態(tài)中，這些功能分配給組織。因此可以預(yù)料，完全的單細胞生物體（例如酵母）具有比其它生物體更高的產(chǎn)酶和發(fā)酵能力。

    酶是發(fā)酵作用中控制化學(xué)反應(yīng)的活性物質(zhì)。微生物的每個屬、每個種實際上都是各種酶的貯存庫，以其自身特定的能力產(chǎn)生和分泌各種酶。人類還有待于去學(xué)會合成各種酶。

    干重1g具有高活性乳糖發(fā)酵酶的微生物每小時能分解10000g乳糖。這樣強烈的化學(xué)活性與微生物的簡單生命過程的需求有關(guān)，與它們生存所需能量的獲得方便有關(guān)，與它們旺盛的生長能力和快速的繁殖速率有關(guān)，還與它們維持生命體的強大能力有關(guān)。數(shù)分鐘時間就可以產(chǎn)生一代。

    實際上也有一定的平衡。微生物在生命活動中要消耗能量。它們活動產(chǎn)生的產(chǎn)物于它們所賴以生長的天然物質(zhì)相比是能量較低的物質(zhì)。然而，以葡萄酒為例，微生物活動的產(chǎn)物正是通常比生產(chǎn)葡萄酒的天然果汁更加受人喜愛的東西。

                           發(fā)酵作用的順序

    微生物在天然食品材料中獲取可利用的碳水化合物、蛋白質(zhì)、脂肪、礦物質(zhì)的微量營養(yǎng)素。看來，微生物首先作用于碳水化合物，然后是蛋白質(zhì)，最后是脂肪。即使是碳水化合物，也有一定的作用順序；首先是糖類，其次是醇類，最后是酸類。由于微生物活動的第一需要是能量，所以看來最易利用的形式（按優(yōu)先選擇順序）是CH2、CH、CHOH和COOH碳鍵。有些鍵（例如 CN 基團）對微生物是沒有用的。

                            糖的發(fā)酵模式

   微生物通過完全氧化、部分氧化、酒精發(fā)酵、乳酸發(fā)酵、丁酸和其他次要發(fā)酵的活動來發(fā)酵糖。

   （1）細菌和霉菌能夠?qū)⑻牵ㄆ咸烟牵┓纸鉃槎�氧化碳和水。幾乎沒有酵母能夠完成這一反應(yīng)。

   （2）最普通的發(fā)酵過程是糖發(fā)生部分氧化的發(fā)酵。在這種情況下，糖可以轉(zhuǎn)化成酸。如果允許發(fā)生，酸最終也可以氧化成二氧化碳和水。例如，用某些霉菌從糖溶液生產(chǎn)檸檬酸。

   （3）酵母是醛類變成醇類的最有效的轉(zhuǎn)化者。細菌、酵母和霉菌的許多菌種能夠產(chǎn)生酒精。葡萄酒酵母在酒精發(fā)酵中具有很大的工業(yè)重要性。這種工業(yè)用酵母產(chǎn)生可回收量的酒精。雖然其他微生物也能產(chǎn)生酒精，但是酒精中摻雜醛、酸和脂，因而難以回收。從糖到酒精的反應(yīng)要經(jīng)歷許多步驟。

    (4)乳酸發(fā)酵在食品方面非常重要。食物中的糖可被轉(zhuǎn)化成乳酸和其他終產(chǎn)物，轉(zhuǎn)化的量使得周圍環(huán)境對別的微生物有控制作用。乳酸發(fā)酵的效率高，發(fā)酵菌的生產(chǎn)迅速。在自然接種的情況下，乳酸菌在合適的環(huán)境中將占主要地位，例如牛奶變酸的情形。

   （5）丁酸發(fā)酵在食品保藏中不如前面提到的那些發(fā)酵那么重要。此發(fā)酵菌是厭氧菌，并會給食品帶來令人不愉快的口味和氣味。通常使人受感染生病的厭氧菌是丁酸發(fā)酵菌，二氧化碳、氫氣、醋酸和醇是丁酸的另外一些產(chǎn)物。

   （6）除以上所述外，還有一種涉及大量氣體產(chǎn)生的發(fā)酵。這種發(fā)酵對食品保藏有用，盡管產(chǎn)氣有不利的方面。就能量方面而言，這種發(fā)酵產(chǎn)氣（二氧化碳和氫氣）的效率很低，這些氣體在與乳酸作比較時所遇到的濃度下，幾乎沒有或完全沒有保藏效力。而且，重要的食品腐敗微生物能夠在這樣的環(huán)境中生長。在產(chǎn)氣發(fā)酵中，糖分子轉(zhuǎn)化成酸、醇和二氧化碳。采用這種發(fā)酵形式，常常有必要引入一些別的有影響的控制措施，例如向底物添加氯化鈉。

   （7）在食品方面還可能有許多的發(fā)酵重要，它們對處理后的食品的接受性有害。一般來說，凡是有能力作用于諸如纖維素、半纖維素。果膠和淀粉之類高級碳水化合物的微生物會損害被處理食品的質(zhì)構(gòu)、風(fēng)味和品質(zhì)。

發(fā)酵控制方法

   食品是天然受微生物污染的，如果不加注意，就會腐敗。將要出現(xiàn)的微生物作用模式取決與外界施加的條件。對某條件下最為有利的給定發(fā)酵模式，也會因控制因素的微小變化而改變。沒有照管好的肉會自然長霉并腐爛。如果加鹽水或鹽，完全不同的微生物就會占優(yōu)勢。食品pH值是控制因素——人類食用的許多天然新鮮食物是酸性的。蔬菜的pH值范圍為6.5～ 4.6。水果的pH值為4.5～3.0。剛宰殺的動物的肉，其pH值接近中性（7.2），但兩天之內(nèi)pH值將接近6.0. 牛奶的pH接近6.4.

    由于這類食品中的兩種重要的發(fā)酵作用是氧化和醇化發(fā)酵，因而，微生物的生長將受介質(zhì)酸度的控制。水果和果汁方面，酵母和霉菌將很快使自己立足。肉類方面，酵母的活力不如細菌。牛奶方面，在數(shù)小時的時間內(nèi)就會出現(xiàn)酸發(fā)酵。

    能量的來源——由于微生物的直接需要是能量來源，因而利用方便的可溶性碳水化合物影響著將要占支配地位的微生物的數(shù)目。牛奶中的糖是乳糖，因此在數(shù)目上很快增值的微生物就是乳糖發(fā)酵菌。由于微生物一般能在人類食物中獲得合適的能量來源，所以除某些例外的情況（如牛乳），能量來源通常不是限制因素。

    獲得氧的難易程度——缺氧的程度是控制發(fā)酵的主要因素。對于酵母，當(dāng)有大量氧氣存在時，就促進酵母細胞的生產(chǎn)。如果希望產(chǎn)生酒精，供氧就要非常有限。

    霉菌是需氧的，它受缺氧的控制�；�質(zhì)的控制�；�質(zhì)中占主導(dǎo)地位的各種細菌的菌數(shù)分布受各菌的需氧情況以及獲得氧的難易的控制。

    如果其他因素均為最佳，那么發(fā)酵的最終產(chǎn)物可以部分地受基質(zhì)的氧分壓控制。

    溫度要求——每一類微生物都有一最適生產(chǎn)溫度；因此基質(zhì)的溫度對微生物的生產(chǎn)起了積極控制的作用。為了在發(fā)酵過程中達到最大限度的操作效能，必須給微生物創(chuàng)造最適的溫度條件。

    食品存放的溫度將有在一定限度內(nèi)決定了微生物的種類和屬性，這些微生物或者能產(chǎn)生所要求的發(fā)酵作用，或者能產(chǎn)生腐敗作用，兩者必居其一。

    氯化鈉在控制發(fā)酵方面的作用——鹽類是食品防腐上最重要的食品添加劑之一。在干制方面，鹽已顯示出具有良好效果。在發(fā)酵方面，鹽可以對允許生產(chǎn)的微生物起著篩選的作用。