LESSON 9 FUNOAR4ENTAL PRINCIPLES OF FOOO

                        PRESERVATION BY CANNING

   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 announced 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 modestly 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 Appert’s process 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 discovered 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 called "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. The 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 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 products 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 and

New York. By 1830 sweet corn was being processed in Maine. By 1840 canneries

began appearing throughout the United States.

 

                       Temperature vs. Pressure

   In 1851 Chevalier-Appert invented an autoclave which lessened the danger involved in the operation of steam pressure 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 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 large. 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 the 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, Massachusetts, 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 which 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 vegetation. 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 which 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 "pasteurization" 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 curiosity 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 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. machinery 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 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 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 opinion 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. Instead, 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 a

normal resting state(a form of hibernation):the notion that spores are stages in

a 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.

  Spores 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.

     The literature on the subject of the heat resistance ofbacteria contains many

contradictions and discrepancies from the records of the earliest works to those of

the present day. This lack of uniformity has been due in part to factors of unknown nature. Until the factors operative in the thermal 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 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 in the death rate of bacteria exposed to dry heat is

indicative 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 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 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 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 logarithmic

order of this death permits the computation of death points, rates or 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 the 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 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. 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 the 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 curve 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 the

same species, exhibit marked differences in heat resistance, but little or nothing

is 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 cells 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 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 of a strain are all of the same heat

resistance. 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 from 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 organisms 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 change 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 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 spores .

   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 of 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 aerobic spare-forming organism isolated from spoiled canned milk is noteworthy

   (Curran 1935):

                                Heat Resistance at 121℃

     Spore Treatment                          Survival in Minutes

     Wetted                                     5

     Alternately wetted and dried               6

     Dried                                      7

     Frozen                                     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 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. The 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 from spores in artificial media. produce spores

of 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 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 for 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 desirable 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 preserving influence of natural environments may be a similar phenomena.