Advances in Home Canning

by EDWARD W. TOEPFER and HOWARD REYNOLDS AMERICAN housewives canned more than 4 billion cans and jars of food in 1943, and nearly 3 1/2 billion quarts of food in 1944. The latter represented nearly one-half of the canned vegetables and two-thirds of the canned fruits that were available for civilian consumption that year. The market value of the canned foods exceeded a billion dollars a year. But much of it (45 million containers of the 4 billion put up in 1943) spoiled. The development of processes for home canning that will prevent such spoilage and give improved products is discussed here.

The process of preserving foods by canning was developed by Nicholas Appert, a French confectioner, less than 150 years ago. Neither Appert nor the scientists of his day knew why foods acquired the property of keeping after being heated in sealed containers. It was almost 50 years after Appert's discovery that Pasteur demonstrated that micro-organisms were the real cause of fermentation, putrefaction, and decay. With his discoveries, the ground work was prepared for developing canning processes on a scientific basis. It could then be shown that canned foods kept because heating destroyed spoilage micro-organisms present in the container and sealing prevented the entrance of others.

Appert used the boiling-water bath for heat treatments, which limited the heating temperature to 212 F. Often, however, the processes failed and the food spoiled. Eventually pressure canners were introduced to obtain the higher processing temperatures that seemed to be required for vegetables and meats. Processes or heat treatments were based on rule of thumb and experience. If foods spoiled, the process-time was increased until spoilage was eliminated or reduced. Such methods were used with reasonable satisfaction until about 1916. Between 1916 and 1922, a series of outbreaks of botulism that resulted in many fatalities in the United States focused attention upon shortcomings of the canning techniques in use. As a result, studies were undertaken to determine scientifically the times and temperatures required to destroy microorganisms causing spoilage of canned foods.

Cultures of Clostridium botulinum, the bacterium causing botulism, and other spoilage organisms were isolated and studied. It was found that many of these organisms would remain alive after heating for 5 to 6 hours or longer in boiling water. Bacteriologists also learned that bacteria subjected to lethal heat do not die instantly-both time and temperature are factors.

Since the object of the processing or heat treatment is to destroy all spoilage organisms within each container of food, information regarding the rate at which the temperature rises in the slowest heating part of the container was recognized as necessary. Studies of the penetration of heat, such as those reported by C. A. Magoon and C. W. Culpepper of the Department in 1921, were therefore undertaken. Foods were prepared and packed in cans or jars with thermometers or other temperature-measuring devices placed in the slowest heating spots. Data were obtained showing the temperature in the container at each instant during the process. Such heat-penetration data when combined with information on the time required to destroy spoilage bacteria at various temperatures provided the basis for computation of bacteriologically sound processes for canning.

W. D. Bigelow, G. S. Bohart, A. C. Richardson, and C. O. Ball in 1920 first solved the problem of applying the foregoing type of bacteriological and physical data to the calculation of thermal processes for canned foods. Later, Dr. Ball developed more flexible mathematical methods for thermal-process calculations. Further modifications and improvements were made by F. C. W. Olson and H. P. Stevens in 1939 and by O. T. Schultz and Mr. Olson in 1940.

Commercial processes for canning low-acid foods have been established largely by thermal-process calculations based on a reasonably adequate background of information, built up during years of research in laboratories of the can-manufacturing companies and the National Canners Association and in college and university laboratories aided by funds provided by the industry.

But the problems of home canning have not been solved so Successfully. Much of the spoilage in 1943 was undoubtedly due to under-sterilization that resulted from the use of inadequate processes. Water-bath processes for low-acid foods continue to be recommended by some distributors of information on home canning and are still widely used. But research in bacteriology has shown that they are not adequate to destroy resistant spoilage organisms that may be present.

Water-bath processes are successful when resistant organisms are absent, but fail in their presence. This fact has often been demonstrated to home canners by outbreaks of gross spoilage after canning by methods that had been used successfully in previous years. It is on that basis that all canning technologists agree that only properly developed steam pressure processes are adequate to guarantee nonspoilage of low-acid foods.

Because research on processes for home canning has been limited, pressure processes recommended for use by home canners generally have been derived from commercial ones, often by arbitrarily increasing the processing time to provide additional factors of safety. Since there are many differences in home and commercial canning equipment and techniques, the arbitrary adaptation of commercial processes to home conditions is questionable. Foods that are home-canned in glass containers have long cooling periods, which add to the sterilizing values of processes; slower heating times add further to the thermal value of equal processing times. Improper evaluation of these factors frequently has resulted in the use of processes for home canning greatly in excess of those required.

Products home-canned in the steam-pressure canner frequently are overcooked and unattractive in comparison with the commercial products. To improve the processes for steam-pressure canning, more facts were needed on the effect of heating and cooling times on the sterilizing values of processes, on the rates of heat penetration from which sterilizing values for different processes are calculated, and on the bacteriological conditions likely to exist during home canning procedures.

On the basis of existing bacteriological information about the numbers and kinds of bacteria that might be encountered and their thermal behavior, it is possible to make a proper evaluation of the effect of home equipment and techniques from heat-penetration data.

Problems, of wartime steam-pressure canners, jars, jar rings, and closures were added to the basic physical and bacteriological problems specifically related to home canning. In order to help meet urgent equipment problems, two studies were carried out by the Bureau of Human Nutrition and Home Economics.