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ʳƷרҵӢÓï LESSON 11 Food Preservation By Fermenttation

[ÈÕÆÚ£º2006-07-01]   [×ÖÌ壺´ó ÖРС]
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.

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Lesson 11

detrimental  É˺¦µÄ,Ë𺦵Ä

putrefaction  ¸¯°ÜµÄ,¸¯ÀÃ

chloride ÂÈ»¯Îï

putrefactive¸¯°ÜµÄ

putrid odor ¸¯°Ü³ôÆøζ

kraut Ëá×յIJË

warehouse »õ²Ö

lactose ÈéÌÇ

oxidation Ñõ»¯×÷ÓÃ

lactic ÈéÖ­µÄ

citric acid ÄûÃÊËá

inoculation ½ÓÖÖÒßÃç

anaerobic ÑáÑõµÄ

pectin ¹û½º

anaerobiosis ÑáÑõµÄ

  1)Under such a condition, many may employ microorganissms to creat unfavorable conditions for other microbes, yet retain in the foodstuffs the nutrients desired.

  ÕâÊÇÒ»¸ö¼òµ¥¾ä¡£may employ ºÍ retain ÊDz¢ÁÐνÓï¡£¾äÖÐfor other microbesÊÇ×÷¶¨ÓïÓõĽé´Ê¶ÌÓÐÞÊÎunfavorable conditions¡£to create¡­microbes¶¯´Ê²»¶¨Ê½×÷×´ÓïÐÞÊÎνÓïmay employ¡£yetÊÇÁ¬´Ê£¬in the food stuffsÊÇ×´Óthe outrients desiredÊDZöÓdesiredÊǹýÈ¥·Ö´Ê£¬×÷the nutrientsµÄºóÖö¨Óï¡£

     2)  While food preservation system in general inhibits the growth  of microorganisms£¬a11 such organisms are not detrimeotal£®

   While food¡­of microorganismsÊÇÈò½×´Óï´Ó¾ä£¬ÔÚ´Ëwhile£½although¡£a11 such orgaoisms are not detrimental ÊDz¿·Ö·ñ¶¨½á¹¹µÄ¾ä×Ó£¬Ï൱ÓÚnot  a11  such organisms are detrimental¡£

  3)  Neither  gas  evolution nor the  presence of 1ivioe cells is  essential  to fermentative action£¬however£¬as seen in 1actic acid fermentations  where  no gas is liberated£¬and in fermentations accomplished solely  with  enzymes£®

    ¾äÖУ¬as seen in¡­with enzymesÊÇ as is seen¡­with  enzymesµÄ·ÇÏÞÖÆÐÔ¶¨Óï´Ó¾äµÄÊ¡ÂÔ¡£ÆäÖÐasÊǹØϵ´ú´Ê£¬Ö¸Õû¸öÖ÷¾ä£¬ËüÔÚ´Ó¾äÖÐ×÷Ö÷ÓÒâΪ¡°Èç¡¢Ïó¡­¡­ÄÇÑù¡±¡£in lactic¡­1iberatedºÍin  fermentations¡­enzymesÊDz¢ÁеĽé´Ê¶ÌÓ×÷×´ÓÐÞÊδӾäνÓïis seen¡£where no gas isliberated  ÊÇÐÞÊÎ1actic acid fermeotationsµÄ¶¨Óï´Ó¾ä¡£

  4)  Bacteria£¬yeasts and mo1ds£¬being single cells£¬contain the  functiooal capacities  for  growth£¬reproduction£¬digestion, assimilation  and  repairs  in  a  cell£¬that higher forms of 1ife  have  distributed to tissues£®

    ¾ä×ÓµÄÖ÷ÓïÊÇBacteria¡¢yeastsºÍmo1ds¡£being single cellsÊǷִʶÌÓïÍ£×´ÓÐÞÊÎνÓïcontain¡£½é´Ê¶ÌÓïfo growth¡­repairs in a cellÊÇÐÞÊÎÖ÷¾ä±öÓïthe functional capacitiesµÄ¶¨Óï¡£¶ø½é´Ê¶ÌÓïin a cell×÷growth¡­repairsµÄ¶¨Óï¡£that higher¡­to tissues  ÊÇÐÞÊΠ growth£¬reproduction£¬digestion£¬assinlilation  and  repairsµÄ¶¨Óï´Ó¾ä£¬ÆäÖÐthatÔÚ´Ó¾äÖÐ×÷±öÓ¼´¿É½«´Ó¾ä±äÐÎΪhigher forms of life have distributed that to tissltes¡£

  5)  However£¬the  product of the activity in the instance of wine  is one which  man generally enjoys more than the nativc juice  from  which the wine was produced£®
    ¾ä×ÓµÄÖ÷ÓïÊÇthe product£¬½é´Ê¶ÌÓïin tke instance of wineÊÇ the activityµÄ¶¨ÓÓɽé´ÊofÓëÃû´Ê¶ÌÓïthe activity in the instance of wine¹¹³ÉµÄ½é´Ê¶ÌÓïÓÖÊÇthe productµÄ¶¨Óï¡£oneÊÇ×÷±íÓïµÄ´ú´Ê£¬ÆäºóÃæµÄwhich man¡­producedÊÇoneͬλÓï´Ó¾ä£¬¶øfrom which¡­producedÊÇÐÞÊÎthe native juiceµÄ¶¨Óï´Ó¾ä¡£

 6)  Whi1e other organisms are able to produce alcohol£¬it occurs in such  mixture of aldehydes£¬acids and esters that recovery is difficult.
    While other¡­alcoholÊÇÈò½×´Óï´Ó¾ä¡£itÊÇÖ÷¾äµÄÐÎʽÖ÷ÓÕæÕýµÄÖ÷ÓïÊÇthat recovery is difficultÕâÒ»Ö÷Óï´Ó¾ä¡£

  7)  The  most  favoreble to e given type of fermentation under one condition will be altered by slight changes in a controling factor£®
    The most favorableÊǾä×ÓµÄÖ÷Ó£½the most favorable type of action (type of actionÊÇÉϾäÌáµ½µÄ¸ÅÄî)¡£½é´Ê¶ÌÓïto a given type of fermention under one condition ÊÇ the most favorableµÄ¶¨Óï¡£under one conditionÓÖÊÇ a given type of fermentationµÄ¶¨Óï¡£

  8) The temperature at which a food is held will determine within certain 1imits the nature of the organisms capable of either yielding  the desired fermentation or spoilage£¬whichever the case may be£®
   Ö÷¾äµÄÖ÷ÓïÊÇThe tenlperature£¬at which a food is heldÊÇÐÞÊÎÖ÷ÓïµÄ¶¨Óï´Ó¾ä¡£Ö÷¾äµÄνÓïÊÇwill determine£¬±öÓïÊÇthe nature¡­spoilage£¬ÆäÖУ¬ÐÎÈݴʶÌÓïcapable of either yielding the desired fermentation or spoilage ÊÇthe organismsµÄ¶¨Óof the organisms¡­spoilageÊÇthe natureµÄ¶¨Ówhichever the case  may beÊÇ·ÇÏÞÖÆÐÔ¶¨Óï´Ó¾ä£¬ÐÞÊÎfermentationºÍspoilage£¬ÆäÖйØϵ´ú´ÊwhicheverÓÖÊÇÕâÒ»´Ó¾äÖеıíÓï¡£

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