Sterilizing Canned Foods Principles Involved in Determining Proper Sterilizing Times and Temperatures G. Y.HALLMAN . ~ N DR. G. STEVEIVS, Research Department, Continental C a n Company, Inc., Chicago, 111. excessive strains on the can, The kinds of spoilage in canned foods and the defective soldering, i m p r o p e r able foods, the p r e s e r v a types and characteristics of organisms commonly handling of the cans, or cooltion is a c c o m p l i s h e d by causing spoilage are discussed' with methods f o r ing cans in highly contaminated hermetically sealing them in contheir diflerentiation. Some sources of contaminawater. U n p u b l i s h e d experitainers and destroying, by means lion are described. ments, by this laboratory, during of heat, the m i c r o o r g a n i s m s actual canning operations show capable of producing spoilage. The factors incolced in the determination o j the that cans, with seams made as The heat t r e a t m e n t of substerilizing times and temperatures f o r canned perfect as possible under comj e c t i n g the c a n n e d food t o a foods, such as the rate of heat penetration into the mercial conditions, show a much sterilizing t e m p e r a t u r e for a canned food product, thermal death times of larger p e r c e n t a g e of leakage definite time is known in the common spoilage organisms, various food factors, when cooled in highly contamicanning industry as the process. nated water than when cooled and the initial temperature of the canned product, The process sterilizes the food, in water containing a low bacand t h e h e r m e t i c a l l y sealed are discussed with methods for their ecaluation. terial count. A n -y o r g-a n i s m container prevents recontaminacapable of growth in the product tion. Thus, a food which has been sterilized in a hermetically sealed container is preserved may be responsible for the spoilage. Most frequently, nonindefinitely with respect to bacterial spoilage. However, heat-resistant bacteria such as cocci and nonspore-forming spoilage results when any organism, capable of growth in the bacilli are present. Leakage is caused by a condition of the food, is not destroyed by the process, or when spoilage organ- container and is not a result of insufficient sterilization. The other class of spoilage is caused by the survival of isms gain access to the food through leakage of containers spoilage organisms through the process. Underprocessing or subsequent to processing. Processing of canned foods is necessary for preserva- understerilization occurs in both acid foods and nonacid foods. tion because of their contamination with spoilage organisms It is caused by resistant organisms which have been studied from the soil and during their preparation for canning (9, 15). as groups rather than individual species. I n acid foods such a& fruits, berries, and tomatoes, the The prevention of spoilage is the primary factor in determining the proper process for a food. The type of organism microorganisms causing spoilage from underprocessing are and the degree of contamination determine the process capable of growth a t a pH of 4.5 or lower. As these foods are necessary for preservation. Commercial processes are based processed a t lower temperatures, little difference is found beon the heat treatment necessary to destroy the types of tween the flora of the understerilized and leaky cans. Yeasts organisms in the concentrations normally found in the product and nonspore-forming bacteria varying from coccoid bacilli under general commercial conditions. As the concentration to very long rods are found ( I S ) . I n some underprocessed of heat-resistant bacterial spores increases, the heat treatment cans there occur pure cultures of pleomorphic rods, some of must likewise be increased to destroy them. While it would which are very long (17 ) . The organisms causing spoilage in be ideal to formulate a heat treatment for each food that acid products have relatively low heat resistance and are would sterilize it when contaminated with maximum com- most commonly destroyed by a few minutes' heating a t 180" F. mercial concentration of heat-resistant spores, this is not (82.2"C.) ( I S ) . Spoilage in acid foods is usually accompanied practicable because of the deteriorating effect so severe a by swelling (16, 29), but recently the authors have had several process would have on the quality of the product. Instances instances of flat sour spoilage in tomatoes and tomato juice of excessive contamination are rare, and through the applica- in which the ends of the cans remained flat. tion of sanitary principles it may be reduced to normal. I n nonacid foods such as peas, corn, meats, and fish, spoilage The general causes for the understerilization of canned foods due to underprocessing is caused by very heat-resistant may be summarized as excessive contamination and insuffi- spore-forming bacilli. As these foods are processed a t higher cient process as to destroy normal contamination. temperatures than acid foods, all vegetative forms and the less However, to produce spoilage in a canned food, the spoilage heat-resistant spores are destroyed, and only the spores of organism must be capable of growth in the food under the great heat resistance are able to survive. There are only a conditions existing in the hermetically sealed can. Such few definite groups of bacteria capable of forming very heatfactors as the effect of the acidity of the food and the nutri- resistant spores which are able to germinate and cause spoilage ment in the food, together with the temperature and oxygen in canned foods. These groups of spoilage organisms found requirements of the organisms, are involved. Thus the in nonacid canned foods are known in the canning industry as: conditions in the can must be suitable for growth of the organ- putrefactive anaerobes; flat sours, including mesophilic ism before spoilage can result (10, BO, 21). flat sours, facultative thermophilic flat sours, and obligatethermophilic flat sours (3, 8, 11); thermophilic anaerobes; SPOILAGE and thermophilic anaerobes producing hydrogen sulfide (22). The bacterial spoilage of canned foods is divided into two When obligate thermophilic spoilage organisms, such as the classes (f3). The first involves the conditions of the con- obligate thermophilic flat sours, thermophilic anaerobes, and tainer whereby spoilage microorganisms gain access to the thermophilic anaerobes producing hydrogen sulfide, are not food after it is sterilized, and is known as leakage. This type destroyed by the process, spoilage may be prevented by coolof spoilage may be caused by defective closure of the can, ing the contents of the can below the minimum growth tem-
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perature of these organisms. Commercial practice has shown that immediate cooling of the contents of the can to 40" C. will prevent spoilage owing to these groups of organisms. Summarizing the spoilage of canned foods, leakage is found to be due to a condition of the container, and underprocessing is caused by nonheat-resistant organisms in acid foods, and in nonacid foods by the survival, through the process, of certain groups of bacteria producing very heat-resistant spores, To prevent spoilage due to underprocessing, under normal cannery conditions, a definite process for each canned food is required. Methods have been devised to determine this process.
I n the case of canned foods, heat penetration takes place by conduction or convection or both, depending upon the nature of the food. I n the case of pure water, which represents a product of the most rapid heat penetration, heating takes place by convection currents. Salt dissolved in water does not affect the heat penetration appreciably. A 50 per cent sugar solution has definitely slower heat penetration than water. Only 2 or 3 per cent of starch in water greatly impedes convection currents and reaches a maximum retardation between 6 and 10 per cent. Small amounts of gelatin, gums, and pectins also greatly retard the penetration of heat to the center of the container. When the liquid is able to circulate more or less freely, as with peas in brine or FACTORS INVOLVED IN DETERMINATION OF PROCESSES fruits in dilute sirup, rapid heat penetration takes place by A little more than a decade ago processes used in the convection currents, and the center of the container quickly canning industry had been developed largely as a result of reaches the sterilizing temperature employed. The heat experience which was expensive, as extensive spoilage fre- penetration in products of heavy consistency, such as cream quently occurred. I n recent years considerable knowledge style corn, is slow, and the center of the container does not has been added to the fundamental principles of processing reach the temperature of the sterilizing medium during the which have been applied in a practical way to commercial process. The fill or the amount of product placed in the canning. Methods have been developed for determining container influences the rate of heat penetration of products processes by correlating the rates of heat penetration of foods which can pack down in the container, such as crab meat, with the heat resistance or thermal death times of spoilage shrimp, and particularly the leafy product, spinach. Closely organisms. packed material of this nature impedes convection currents in the liquid surrounding the particles and affects the process THERMAL DEATHTIME necessary for sterilization. The thermal death time of an organism is the time required With a food of rapid heat penetration the centers of all to destroy completely a t a certain temperature a known sizes of containers are nearly a t the same temperature by the concentration of spores in a definite medium. Thermal time the retort has reached the processing temperature. death times are affected by the nature of the medium in which With a food of slow heat penetration in containers whose the spores are suspended, the sterilizing temperature, and the heights are greater than their diameters, the times necessary age, condition, and concentration of the spores. to heat the centers to a certain temperature are approximately The hydrogen-ion concentration of the medium greatly proportional to the squares of the radii (6). When the coninfluences the thermal death time of heat-resistant bacterial tainers are rotated during the process, the rate of heat penetraspores. At pH values of 4.5 or lower there is a marked de- tion is increased for those products, such as tomatoes and crease in heat resistance (1.2). Theheat resistance varies in spinach, wherein convection currents are impeded, but which different foods, The ratio of the thermal death time in a are sufficiently fluid to permit mixing. food to that in a neutral phosphate solution of pH 7.0 is known The following additional factors influence the sterilizing as the food factor. I n determining the food factor, the spores value of a process: initial temperatures of the food; time are suspended in a medium as nearly like that provided by the food itself as is possible to use. This is generally the taken by the retort to reach the process temperature, or coming-up time; and method of cooling. The lower the initial juice expressed from the food (14). Other conditions being the same, the thermal death time temperature, the less is the sterilizing value of a process. This decreases as the sterilizing temperature increases and, for decrease in sterilizing value is greater, the slower the penetraheat-resistant spores, is generally one hundred times longer a t tion of heat into the food. Heat enters the food during the coming-up time; hence, 212" F. (100OC.) than a t 250" F. (121.1OC.) (4). I n general, the thermal death time increases as the concentration of the this period has a greater sterilizing value, the more rapid spores increases ('7). Young moist spores, probably of the the rate of heat penetration. The slower the rate of cooling of the product, the greater is the sterilization during this first generation, appear to be the most heat resistant (1%). period of process. A product generally cools more rapidly HEATPENETRATION when the container is water-cooled than when it is air-cooled. Sterilization does not depend entirely on the maximum temperature attained by a food during the process but on the METHODSFOR DETERMINATION OF PROCESSES combination of time and temperature. For this reason heat Two methods by which processes may be calculated from penetration or the rate of flow of heat into a canned food is of great importance in determining processes for foods which are heat penetration and thermal death time data are: the general preserved by subjecting them to a certain temperature for a or graphical method (5) and the formula method (1, 2). A definite length of time. For successful preservation it is process calculated by these methods takes into account the necessary to know the temperature a t successive intervals sterilization obtained during all stages of the heating and of time during the process a t the slowest heating point in cooling operations and hence is precisely sufficient to sterilize the closed container. These temperatures are determined the food with reference to the spoilage organism which the usually by means of thermocouples specially designed for process is designed to destroy. The process is based on the this purpose. I n a well-Bled cylindrically shaped container area beneath the so-called lethality curve. The lethality the slowest heating point is on the vertical axis, generally curve is obtained by plotting on coordinate paper lethal rate values for various temperatures against the times required by midway between the ends of the container. The following factors influence the rate of heat penetration the center of the container to reach these temperatures. The and hence the process (6) : the nature of the food, the medium lethal rate value at a certain temperature is the reciprocal of surrounding the food, the size and shape of the container, the number of minutes required to destroy all spores of the organism at that temperature. and the type of process (agitating or still).
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The greatest value derived from the calculation of processes is in rapidly giving accurate knowledge of the following: sterilizing efficiency of a process; and equivalent process times for various sizes of containers and for various initial, retort, and cooling temperatures. I n general, much dependence can be placed on the calculated processes. However, owing to the variable properties shown by microorganisms, the calculated processes are usually checked by actually processing containers of food which have been inoculated with a definite concentration of spores of the spoilage organism. The inoculated cans are incubated a t the optimum temperature for growth of the test organism to determine if it has been destroyed. Thus, the calculated processes serve as guides for inoculation experiments or so-called experimental packs, which have been demonstrated to give satisfactory results in the commercial processing of canned foods. SUMMARY I n summarizing, the steps involved in the determination of the process for any canned food are: 1. The determination of the thermal death time of heatresistant spores of the spoilage organisms with which the food might become contaminated under commercial conditions. 2. The determination of the rate of heat penetration of the particular canned food in question. 3. The calculation of the theoretical process from the thermal death time and heat penetration data.
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4. The checking of the theoretical process by processing the canned food which has been inoculated with these heatresistant spores, and incubating to determine if these spores have been destroyed. LITERATURE CITED Ball, Katl. Research Council, Bull. 37 (1923). Ball, Univ. Calif., Pub. Public Health, 1, No. 2 (1928). Barlow, Thesis for M. S. Degree, Univ. Illinois, 1912. Bigelow, J. Infectious Diseases, 29, 528 (1921). Bigelow, Bohart, Richardson, and Ball, Natl. Canners Assoc., Bull. 16L (1920). Bigelow and Cathcart, Ibid., Bull. 17, (1921). Bigelow and Esty, J . Infectious Diseases, 27, 602 (1920). Cameron and Esty, Ibid., 39, 89 (1926). Cameron, Williams, and Thompson, Natl. Canners Assoc., Bull. 25L (1928). Cheyney, J . Med. Research, 40, 177 (1919). Donk, J. Bact., 5, 373 (1920). Esty and Meyer, J. I n f e c t i m s Diseases, 31, 650 (1922). Esty and Stevenson, Ibid., 36, 486 (1925). Esty and Williams, Ibid., 34, 516 (1924). James, J. Bact., 13,409 (1927). Mickle and Breed, N. Y.Agr. Expt. Sta., Tech. Bull. 110 (1925). Pederson, Ibid., Tech. Bull. 150 (1929). Pederson, J. Bact., 17, 30 (1929). Savage and Hunwicke, Food Investigation Board, London, Special Rept. 16 (1923). Savage, Hunwicke, and Calder, Ibid., Special Rept. 11 (1922). Weinzirl, J . Med. Research, 39,349 (1919). Werkman and Weaver, Iowa State Coll. J . Sn'., 2, 57 (1927). RECEIVED April 11, 1932.
A Physiological View of Freezing Preservation H. C. DIEHL,Frozen Pack Laboratory, Bureau of P l a n t Industry, Department of Agriculture, Seattle, Wash.
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H E phenomenon of ice formation in plant tissues has been observed by physiologists for many years, although presumably none of them, until recently, anticipated the practical use of freezing temperatures as a method of food preservation. The observations of these investigators dealt mainly with the effect of frost upon plants in the open or with the pathological aspects of freezing in horticultural products intended for human consumption. These investigations determined the nature and function of many physiological or physico-chemical principles operative in plant-tissue freezing, and t,he fundamental character of these studies does not seem to be radically changed by the distinct objectives of the past research and the present investigations centering on low-temperature preservation. Hence, physiologists who are interested in the problems of freezing preservation today may derive much information from the freezing experiments of the past century.
PREVIOUS INVESTIGATIONS Much consideration has been given in recent studies of the low-temperature behavior of horticultural products to the physical aspects of ice formation. The rate of ice formation and the size of the ice crystals formed have been singled out for particularly intensive examination, which has revealed some interesting and useful facts concerning these phases of freezing. Their physiological signscance has not been so thoroughly determined, however, and a number of hypotheses, which scientists some years back considered and then discarded as inadequate or not demonstrable, have reappeared. Among these is the belief that ice forms within the cells of plant tissues, and that changes in or injury to these tissues is caused by the crushing and piercing action of the ice crystals.
The critical consideration to which these hypotheses was subjected in research and deductive reasoning by Goppert ( 5 ) , Muller-Thurgau (2@, Sachs @ I , % ) , Nageli (23),Matruchot and Molliard (15), Prillieux (28), Wiegand (35,36),and Maximov (I?'), resulted essentially in the conclusion that ice formation in plant tissues was generally extracellular and exerted a desiccative effect upon the cell contents. According to this viewpoint, crystallization centers arose firfit in the relatively pure water present in the intercellular spaces, and their expansion occurred a t the expense of water withdrawn to a greater or less degree from the different cell constituents, depending upon the manner in which the water was combined with the latter. Some physiologists, especially Pfeffer (27) and Molisch (20), questioned the universality of the extracellular occurrence of the first ice crystals. Observations made in this laboratory lead the author to believe that a t very low temperatures, when the time element of heat transfer is greatly reduced, there may be essentially simultaneous ice formation within the cells (possibly eutectic in character) and in the intercellular spaces. It seems significant, however, that not even Pfeffer or Molisch sought to modify or alter the desiccation theory but accepted it in general, and that the opinions of those who sought to cast doubt upon it, like Mez (18) and his colleagues (1, 29, S 4 ) , who set up the critical temperature hypothesis, did not prevail. The factors upon which the desiccation of the cell contents during ice formation is predicated have been cataloged, to some extent. However, the mode and degree of their relationship is perhaps not sufficiently understood a t present to allow the formulation of a fully rounded theory. The interjection of an imbibition force or capillarity between the nas-
