食品專業(yè)英語 LESSON 9 Funoarental Principles Of Food Preserv

1、France in the late 1790s was at war and having difficulty feeding its people. Napoleon's fighting forces had a diet of putrid meat and other items of poor quality. The foods available couldn't be stored or transported except in a dry state. Recognizing an important problem prize was announce

2、d offering 12,OOO francs and fame to anyone inventing a useful method of food preservation.   Nicolas Appert, a French confectioner. working in a simple kitchen. observed that food heated in. sealed containers was preserved if the container was not reopened or the seal did not leak. He mod

3、estly called the process "the art of Appertizing". Appert received the award from Napoleon after spending ten years proving his discovery.  It should be appreciated that the cause of spoilage of food was unknown, The great scientists of the day were summoned to evaluate Apperts proces

4、s and offer explanations for its apparent success. The conclusion reached was that the process was successful because in some mysterious and magical fashion, air combined with food in a sealed container, preventing putrefaction. This was quite incorrect. Nevertheless, the canning process was discove

5、red and practiced for the next 5O years with some success, but in the darkness of ignorance.   Appert began work on his process in 1795. Peter Durand received patents in England in 1810 for glass and metal containers for packaging foods to be canned. The tin-plated metal containers were ca

6、lled "conisters" from which the term "can" is assumed to be derived. Early metal containers were bulky, crude and difficult to seal. By 1823 a can with a hole in the top was invented, allowing the food to be heated in boiling water baths with the hole covered with a loose lid. Th

7、e lid was soldered into place after the heat treatment. Hole-in-top cans are in use presently for canned evaporated milk, although the cans are sealed prior to heating. By 1824 Appert had developed schedules for proessing some 50 different canned foods. Meats and stews processed by Appert were

8、carried by Sir Edward Perry in 1824 in his search for a northwest passage to India. Several cans of food from this voyage were obtained from the National Maritime Museum in London in 1938 and opened. The food was found nontoxic for animals. Interestingly there were isolated from these canned product

9、s bacteria which had been dorman for at least 114 years. Given proper environment and substrate, they grow!  In the 1820s canning plants appeared in the United States in Boston andNew York. By 1830 sweet corn was being processed in Maine. By 1840 canneriesbegan appearing throughout the United S

10、tates.                         Temperature vs. Pressure   In 1851 Chevalier-Appert invented an autoclave which lessened the danger involved in the operation of steam pres

11、sure vessels. It was recognized that some foods could be processed for shorter times if higher temperatures were available. It was learned that the temperature of boiling water could be increased by adding salt. Demands for greater production in factories could be met if the cooking times for foods

12、could be reduced. For instance, the boiling water bath cooking of canned meats could be reduced from 6 hr to perhaps 1/2 hr by cooking the cans in a water-calcium chloride solution. Production could be increased thereby from some 2000 to 20,OOO cans per day. Losses due failure of containers were lar

13、ge. No pressure was applied to the cooking vessels. Commercial cans were unable to withstand the internal pressures developed by heating to 115.   The temperature at which water will boil is dependent upon the pressure. Using a pressure pressure it was possible to achieve temperatures in t

14、he vicinity of 115. However, these retorts were still dangerous to operate.     Spoilage of Food Caused by Microorganisms         In 1862 President Lincoln signed the Morrill Act, creating the land grant colleges (Purdue, Michigan, Massachu

15、setts, Illinois, etc. ). The great scientific debate in universities at that time was "spontaneous generation" of life. At this time Louis Pasteur, son of a well-decorated officer in Napoleon's army, became interested in the problems of the great wine and beer industries of France whic

16、h were threatened with ruin; their products were diseased and souring from "spontaneous generation" of life in bottles and kegs.   To the Academy of Sciences in France in 1864, Pasteur reported that be had found the cause of the disease of wine and beer to be a microscopic vegeta

17、tion. When given favorable conditions this vegetation grew and spoiled the products. However, boiled wine sealed from contamination in jars with even cotton plugs would not sour. In fact, it was possible to isolate this microscopic vegetation from the cotton plugs! It was this microscopic growth whi

18、ch spoiled foods, and it was neccessary for such organisms to gain entrance to heated foods if they were to spoil! Here was an explanation for the success of Appert more than half a century before. The concept of heat treating foods to inactivate pathogenic organisms is termed appropriately "pa

19、steurization" today.   It is interesting to note that magnifying lenses were used by Bacon in the late 1200s, but had never been focused on a drop of water until the 1600s by Leeuwenhoek. He had noted microscopic growth which he named "animalcules," but they were only a curi

20、osity in water to him. Two more centuries elapsed before this information was organized and synthesized into an explanation for "spontaneous generation" of life.   Appert had established that containers of food must be carefully sealed and heated. Cleanliness was important to his

21、 process, although he did not know that microorganisms were the agents of spoilage. Pasteur established several important principles. Most changes in wine depended on the development in it of microorganisms which were themselves the spirits of disease. Germs were brought by air, ingredients. machine

22、ry and even by people. Whenever wine contained no living organisms, the material remained undiseased.     Heat Resistance of Microorganisms Important in Canning   There are two important genera of bacteria which form spores. Both genera are rod forms, one (Bacillus) is

23、aerobic and the other (Clostridium) is anaerobic. When a rod is about to sporulate a tiny refractile granule appears in the cell. The granule enlarges, becomes glassy and transparent, and resists the penetration of various chemical substances. All of the protoplasm of the rod seems to condense into

24、the granule, or young spore. in a hard dehydrated, resistant state. The empty cell membrane of the bacterium may separate off, like the hull of a seed, leaving the spore as a free. round or oval body. Actually a spore is an end product of a series of enzymatic processes. There is no unanimity of opi

25、nion either of spore function in nature or of the factors concerned in spore formation.    Since no multiplication take place as a result of the vegetative cell-spore-vegetative cell cycle, few bacteriologists accept the concept of the spore as a cell set apart for reproduction. Inste

26、ad, various explanations of the biological nature and function of bacterial spores have been advanced. These include: the teleological interpretation of the spore as a resistant structure produced to enable the organism to survive an unfavorable environment; the idea that the spore is anormal restin

27、g state(a form of hibernation):the notion that spores are stages ina development cycle of certain organisms, or a provision for the rearrangement of nuclear material. It is interesting to note that the protein of the vegetative cell and the protein of the spore are antigenically different.  Spo

28、res appear to be formed by healthy cells facing starvation. Certain chemical agents (glutamic acid) may inhibit the development of spores. No doubt sporulation consists of a sequence of integrated biochemical reactions. The sequence can be interrupted at certain susceptible stages.   

29、  The literature on the subject of the heat resistance ofbacteria contains manycontradictions and discrepancies from the records of the earliest works to those ofthe present day. This lack of uniformity has been due in part to factors of unknown nature. Until the factors operative in the therma

30、l resistance of bacteria are understood, it will not be possible to control by other than empirical means the processes which require for their success the destruction of bacteria.  Heat may be applied in two ways for the destruction of bacteria. Oven heat may be considered as dry heat, used in

31、 the sterilization of glassware. Other materials are heated when moist or in the presence of moisture; this is commonly termed moist heat. Dry cells exhibit no life functions; their enzymes are not active. Cell protein does not coagulate in the absence of moisture.   The gradual increase i

32、n the death rate of bacteria exposed to dry heat isindicative of an oxidation process.   Whereas death by dry heat is reported as an oxidative process. death by moist heat is thought to be due to the coagulation of the protein in the cell. The order of death by moist heat is logarithmic in

33、 nature. The explanation of bacteria death as caused by the inactivation of bacterial enzymes cannot be correct. A suspension containing 99% dead cells has 80% of its catalase active. Since the order of death by moist heat is logarithmic in nature, death must be brought about by the destruction of a

34、 single molecule. This change is termed a lethal mutation. To a food technologist, death of a bacterium is described by its inability to reproduce. Heat inactivates or coagulates a single mechanism (gene?) preventing reproduction. The decreasing enzyme content of dead bacteria is the consequence of

35、inhibited growth and probably not the cause . Replacement of the enzyme molecules becomes impossible; the enzyme content slowly decreases.    Regardless of the explanation of death of bacterial spores. the logarithmicorder of this death permits the computation of death points, rates o

36、r times. independent of any explanation. The death rates or times permit the comparison of the heat resistance of one .species at different temperatures or of different species at the same temperatures. It is also possible to describe in quantitative terms the effect of environmental factors upon th

37、e heat resistance of the bacteria. Originally the standard method of establishing the heat tolerance of different species of bacteria was the thermal death point,i.e. , the lowest temperature at which the organism is killed in 10 min. This method cannot give comparable results unless conditions

38、 such as the age of the culture, the concentration of cells, the pH value of the medium, and the incubation temperature are standardized. Food technologists concerned with processing canned foods have adopted the thermal death time, keeping the temperature constant and varying the times of heating.

39、The thermal death time is the shortest time required at a given. temperature to kill the bacteria present.   It is necessary to know the time and temperature required to adequately sterilize canned foods. This procedure involves not only the destruction of spores by moist heat, but also th

40、e rate of heat penetration and heat conductivity of containers and their contents. The heat resistance of an organism is designated. by the c value(the number of minutes required to destroy the organism at 121) and the z value (the numbre of degree centigrade required for the thermal death time curv

41、e to traverse one logarithmic cycle). These two valuse establish and describe the thermal death time curve. and are a quantitative measure of the heat resistance of the spores over a range of temperatures.    It has been recognized that spores of different species, and of strains of t

42、hesame species, exhibit marked differences in heat resistance, but little or nothingis know in explanation. Some workers have believed that there might be a difference in heat resistance among the vegetative cells, which was transmitted to the spores. Comparing the beat resistance of vegetative cell

43、s and spores of a number of bacteria, considerable differences in the spore resistances are found among organisms. Differences in vegetative cell heat resistance is in some instances associated with high spore resistance. Other cultures of vegetative cells produce spores of low resistance. There is

44、evidently no significant relationship between the heat resistance of the vegetative cell and that of the spore produced therefrom. As noted previously, even the protein of the vegetative cell and   spore differ for a species.     Some researchers reason that the spores

45、of a strain are all of the same heatresistance. Others suspect that in a given spore suspension there are a predominant number of spores of relatively low heat resistance, a smaller number with greater heat resistance, and a still smaller number of very heat resistant spores. However, subcultures fr

46、om heat resistant selections do not yield survivors  of uniformly high heat resistance over the parent strain.   Factors Influencing The Heat Resistance of Spores    Concentration. The heat resistance of a suspension of bacterial spores is related to the number of organ

47、isms present. The greater the number of spores per milliliter, the higher resistance of the suspension.     Environment Factors. The resistance of bacterial spores is not a fixed property,but one which under ordinary conditions may tend to be relatively constant. The extent of ch

48、ange in resistance is determined largely by the physical and chemical forces which operate from outside the spore cell. Aside from purely theoretical interest, a better understanding of the cause of heat resistance of spores is of fundamental importance to the canning industry. There are relatively

49、few types of spore-forming organisms especially endowed with heat resistant properties, but these account for most of the spoilage potential in canning. Spore heredity. the environment in which grow, and a combination of these factors must play some part in the production of highly heat resistant sp

50、ores .   Different yields of spore crops can be determined in various media. This may be demonstrated by plate count or by direct microscopic count. There is little information indicating a relationship between the physiological factors influencing spore formation and the heat resistance o

51、f spores produced. The reaction (pH value) of the medium in which spores are produced has appearently little influence on their heat resistance.Continuous drying seems to enhance the resistance of spores, but this is irregular in effect. Freezing tends to weaken spores. The following data for an aer

52、obic spare-forming organism isolated from spoiled canned milk is noteworthy   (Curran 1935):                                Heat Resist

53、ance at 121     Spore Treatment                          Survival in Minutes     Wetted        

54、                                      5     Alternately wetted and dried    

55、0;          6     Dried                                   &#

56、160;             7     Frozen                                

57、0;              2   Spores formed and aged in soil are found to be more heat resistant than those formed and aged in broth or agar. Natural environmental conditions are evidently more conducive to the development of heat resistant

58、 spores than conditions prevailing in artificial cultures. The prolonged action of metabolic wastes from cells appears to decrease the heat resistance of spores.   Bacteria exposed to sublethal heat are more exacting in their nutrient and temperature requirements than undamaged bacteria. T

59、he composition of recovery media which organisms are placed after heating may have considerable effect on the apparent thermal destruction time of the organisms. Depending on the choice of media, heat treated bacteria may be found to be dead in one and alive in another.Thermophilic bacteria which fr

60、om spores in artificial media. produce sporesof comparable heat resistance to those formed on equipment and machinery in canning plants.Spores obtained from soil extractions and remixed with sterile soil are less heat resistant than those heated in the soil directly. The higher natural resistance of

61、 spores in soil may be due to some physico-chemical influence of the soil and not to any differences between the soil and cultured spores themselves.Anthrax spores remain viable and virulent in naturally contaminated water for as many as 18 years. while artificial cultures remain in this condition f

62、or perhaps 5 months. Soil organisms on corn may remain viable on naturally contaminated tissue for at least 7 years. while the artificially cultured die in 3 months. Artificial media apparently weakens cultures of organisms  If a culture is to be kept alive for a long period it is apparently de

63、sirable to have a medium which permits only a limited growth. limiting metabolic byproducts, than media which permit best growth. B. tuberculosis growing on a relatively poor medium may be kept viable for several years while growth on enriched media has viable organisms for only a few weeks. The pre

64、serving influence of natural environments may be a similar phenomena.第九課   食品罐藏原理     18世紀(jì)90年代末,法國處于戰(zhàn)爭時期,國民的食物供應(yīng)發(fā)生了困難.拿破侖部隊吃的是腐敗的肉和其他劣質(zhì)食物.這些可供利用的食物除干態(tài)的外,都不可能進行儲藏或運輸.認(rèn)識到這一嚴(yán)重問題之后,就宣告了一項獎金,將給予任何發(fā)明食物有效保藏方法的個人以12,000法郎和榮譽.    一位工作在簡陋廚房中的法國糖食師傅尼古拉·阿培爾發(fā)現(xiàn)：在密

65、封容器中加熱過的食物，如果不重新打開容器或密封不漏，它便被保存下來。他謙虛地把這種處理方法稱為“阿培爾技藝“。阿培爾在花了10年確認(rèn)他的發(fā)明之后，才從拿破侖那里拿到這項獎賞。    要知道，那時并不明白食品變質(zhì)的道理。于是召集了當(dāng)時的大科學(xué)家，對阿培爾的處理方法進行了評價，并對這種方法的明顯成功作出解釋。得到的結(jié)論是：這種處理方法之所以成功，原因是在密封的容器內(nèi)，空氣以某種神秘難測的方式于食物相結(jié)合，防止了食物的腐敗。這當(dāng)然不正確。盡管這樣，這種罐藏工藝終于被發(fā)現(xiàn)了，并經(jīng)過了那時以后50年的實踐，取得了一定程度的成功，但還是處在無知的黑暗之中。 

66、0;  阿培爾于1795年在他所提出的工藝方面開始工作。1810年彼特·杜蘭德在英國獲得用于包裝罐頭食物的玻璃容器和金屬容器專利。人們過去稱鍍錫鋼板的容器為“canister”(金屬罐）現(xiàn)在“can”這一用語被認(rèn)為是從canister派生出來的詞。早期的金屬容器笨重、粗陋且難以封口。到1823年，發(fā)明了一種頂上帶小孔的金屬罐，用不緊密的蓋子封住小孔，同時讓食物在沸水浴中加熱。熱處理之后，將蓋子就原位焊牢。這種孔蓋式金屬罐目前仍用于淡煉奶罐頭，只不過這種鐵罐是在加熱之前密封的。 到1824年，阿培爾已經(jīng)制定了加工約50多種不同罐頭食品的作業(yè)計劃。經(jīng)阿培爾處理的肉類和燉菜由埃德華·彼里爵士于1824年他探索通往印度西北航道是帶去。1838年，人們從倫敦的國立海事博物館得到了幾罐來自這次航行的罐頭食品，并將這些罐頭打開。