A. Farmer's Bulletin 359

The earliest USDA publications with definite instructions for home canning was the Farmers' Bulletin 359 (Breazeale, 1909), issued in 1909 by the Bureau of Chemistry. "Domestic" canning was introduced as being economical and desirable, especially in farm kitchens. A discussion of decay, as caused by molds, yeasts and bacteria, was included along with an explanation that air must be excluded not for its own damaging properties but to exclude bacteria. It was explained that proper sterilization required heat, and that chemical preserving powders on the market were not recommended.

It was recognized that bacteria may be killed at the temperature of boiling water, but that spores retain vitality for long times at that temperature and will germinate upon cooling. The type of sterilizing heat process recommended was fractional sterilization - "the whole secret of canning" (Breazeale, 1909). The complete sterilization of a vegetable required that one heat the vegetable in the jar to the boiling point of water and maintain that temperature for one hour each of two or three successive days. The first day of boiling was to kill molds and almost all the bacteria, but not spores. The spores were thought to germinate upon cooling, and boiling the second and third days killed the new bacteria. If fractional sterilization were not practiced, about five hours of boiling on the first day was recommended.

Directions for canning a variety of vegetables were included in Farmers' Bulletin 359. The instructions consisted of how to select and prepare vegetables, how to fill jars, and how to sterilize jars. A few departures from boiling for one hour on three successive days were noted. Directions were included for the following vegetables: corn, string beans, eggplant, beets, okra or gumbo, summer squash, English peas, asparagus, cauliflower, carrots and parsnips, kohlrabi, lima beans, pumpkin or winter squash, succotash and a mixture called vegetable roast. Vegetables were packed either raw or cooked as for table use. Tomatoes were also included, but it was thought that they did not require sterilization after filling the jars with hot product.

B. Farmers' Bulletin 839

The next publication on home canning was issued by the "States Relation Service," which later became the Extension Service. This was Farmers' Bulletin 839, "Home Canning by the One-Period Cold-Pack Method," issued in 1917 (Benson, 1917). The described method had been adopted for home-canning club work in the northern and western states, and consisted of five steps: scalding or blanching (except berries and soft fruits), cold-dipping, packing, processing, and sealing. The application of heat for processing varied according to the character of the product and the kind of apparatus used.

Three basic processes were recommended for fruits, vegetables and meats: boiling water-bath (212F), water seal process (214F), and steam pressure process. Processes are summarized in a table with times for the scald or blanch and processing by boiling-water bath, water-seal, steam-pressure at 5 to 10 pounds and pressure cooker at 10 to 15 pounds pressure (sic, now referred to as psig, pounds per square inch gauge). No distinction is made between 5 to 10 psig and 10 to 15 psig; rather, one time is given for the range achieved in each type of canning vessel. Average time processes from that time period can be summarized as follows (Benson, 1917):

  A wider variety of foods was offered by this publication in comparison with the current publications. It was recommended, however, that one become familiar with the process on fruits and vegetables before attempting to can meats. The one-period cold-pack method was recommended for jams, jellies, and preserves, also; advantages cited were the elimination of expense and trouble in using paraffin and the decrease in molding and deterioration of the top parts of the packs. Screw-top containers were suggested.

C. Farmers' Bulletin 853

Great care was given to the mechanics of operating the canning equipment rather than the reasons or theory behind sterilization and spoilage in Farmers' Bulletin 839. However, one month later, Farmers' Bulletin 853, "Home Canning of Fruits and Vegetables," (Creswell and Powell, 1917) was also issued by the States Relation Service. This latter publication devoted a section to causes of spoilage (molds, yeast, bacteria and spores, and enzymes). A distinction was made between sterilization--the killing of all microorganisms--and processing--the heat treatment which kills vegetative cells but not spores.

Fractional sterilization of vegetables with three days of processing was recommended as the safest method with boiling-water canners. A continuous period for processing with a steam-pressure canner at 10 psig was recommended. The advantages of the steam-pressure canners were cited as saving time, labor, and fuel and eliminating the uncertainty of securing complete sterilization. "Stale" vegetables could be processed if the time were lengthened. Boiling-water bath processing only was recommended for fruits. Processing (30 minutes at simmering) was required for fruit preserves, optional for jams and marmalades, and not necessary for jellies. The first timetable for processing vegetables under steam pressure consisted of the following (Creswell and Powell, 1917):

  D. Home Canning Research Begins

The following excerpt is from a typewritten anonymous report about the history of home canning work in USDA (History, 1944):

"The Office of Home Economics of the States Relations Service began research in 1917 on home canning problems. Experimentation in "one period" and intermittent water bath processing with nine vegetables, involving some 1454 containers was carried on over a period of 3 years.

In 1919 steam pressure canning was included and the study of this method versus boiling water bath processing was continued over a period of seven years. Meat, poultry, and fish were canned as well as vegetables and fruit. Experiments were made, 1920-22, on the open kettle method of canning corn, with the addition of different amounts of salt, sugar and both hot and cold water. Several factors including the processing time were varied in a given experiment, so no conclusive results were obtained although "no spoilage" was reported in some instances.

Heat penetration studies were also carried on during this period by Minna C. Denton, who was one of the first investigators to give attention to this question as related to home canning foods. About 1918 she reported that it required 45 minutes for the temperature at the center of the jar of sliced carrots to reach 98C where it remained during the rest of this maximum temperature for 75 minutes. A summary showing comparison of rates of penetration through glass jars and tin cans was compiled by Miss Denton about this time with special comments relating to the factors affecting the rates of heat penetration in corn."

Both Farmers' Bulletins 839 and 853 were replaced in 1921 with "Home Canning of Fruits and Vegetables," Farmers' Bulletin 1211 (USDA, 1921). The results of bacteriology and heat penetration tests resulted in a very lengthy section of this publication being devoted to the "whys" of processing and reasons for spoilage. Notable points made for the first time in this type of publication included:

1. "Processing" as a canning term means the application of heat to insure preservation and secure quality, which may or may not result in "sterilization."
2. A partial vacuum within the container is desirable.
3. Heat sufficient to destroy bacteria which can grow in the absence of air and under the conditions of storage should be applied.
4. An argument against open-kettle canning was made, because of the possibility of contamination during the filling of jars.
5. Only fresh and "sound" produce was recommended for canning, due to increasing microbial loads and enzyme action in overripe, moldy, blemished or maturing produce.
6. A long section on inspection of canned goods before consumption was provided, including warnings about Bacillus botulinus (sic, now classified as Clostridium botulinum).

Apparently, in July of 1923, the Bureau of Home Economics was formed. The following is taken from anonymous file notes on the history of home canning work in the USDA (History, 1944):

"The first material on home canning published by the new Bureau was Miscellaneous Circular No. 24 in May 1924 on timetables for home canning of fruits and vegetables. This marked a departure from methods previously suggested. For the first time the emphasis was placed on steam pressure canning only for all vegetables except tomatoes, with water bath or pressure canner at 212F for fruits and tomatoes. Hot pack was also advised to decrease the heat penetration time. It was recommended that beets, carrots, mature lima beans, pumpkin and squash should not be canned since root vegetables could be stored satisfactorily and drying was possible for lima beans and okra. Timetables were given for both pint and quart glass jars and tin cans. The processing times and temperatures were apparently largely based on the experimental work of the Bureau and that of Magoon and Culpepper, and partly on the heat penetration work of the National Canners Association.

A brief circular No. 25, designed to prevent botulism and other poisoning, was brought out at the same time in cooperation with the Bureaus of Chemistry and Plant Industry on Directions for Examining All Canned Food Before Use.

Research on heat penetration in spinach was undertaken in 1924. As outlined the processing time was to be kept the same--90 minutes--and variations were to be made in retort temperatures, initial pack temperature, weights of pack, size of food container, and physical conditions of the vegetable as to age, wilt, and proportion of leaf to stalk.

Other studies undertaken include: the times and temperatures required for processing fruits and vegetables in glass containers and the influence of hot packing on the time required; the color change occurring in canned fruits and its cause; and summarizing the results of all meat canning previously done. Meats canned in the last few years were opened and scored. It was found that all meat processed in the water bath for 3 hours had spoiled, while that canned in glass and processed for 1 hour at 15 pounds pressure was good after 5 years storage. The effects on flavor of the canned product of canning with and without liquid and of searing before canning were judged by palatability tests. Little difference in flavor was reported for meat or chicken canned with or without liquid, but the material heated through more quickly when it was precooked with liquid and the container was filled with the boiling liquid. The exhaust period could then be eliminated.

A. Cut-and-Try Methods

The basis for heat process calculations in use today was founded with a concentrated effort begun in the early 1900s. Thermal processes, until about 1920, were based on individual experiences rather than scientific determination. The science of bacteriology was not yet developed enough to answer the needs of the canning industry and what was known about bacteria was not applied in a well-coordinated effort.

Ball (1927) summarized the theory of sterilization prior to this time as "cut-and-try" methods. When a process was to be determined, a limited number of cans were given the minimum process expected to sterilize the food. If the food in the cans did not spoil after storage, the process was adopted. If there was a considerable amount of spoilage, a more severe process was tried. This procedure was continued until spoilage did not occur.

The disadvantages of this method were also cited by Ball (1927). This procedure was costly for the canner and often resulted in heavy losses from spoilage years later. At that time, there were no explanations in bacteriology to explain why a process gave erratic results.

Prior to 1918, there was no established method for measuring rates of heat penetration into a can. There also was no accurate method to measure the factors such as consistency, initial temperature or can size. These factors influence the rate it takes for a product to attain the maximum temperature throughout. Early work consisted of measuring the highest temperature achieved with a maximum thermometer concealed in the can. It is not sufficient to reach a certain temperature; instead, the heat must be maintained long enough to exceed the death time requirement of spoilage organisms (Williams, 1940). To evaluate the sterilizing value of a given exposure to heat, it is necessary to know the food temperature at the coldest part of the container at all times throughout the process of heating and cooling jars. The adaptation and application of the thermocouple to measure temperature changes inside the container laid the foundation for process calculation methods which has led to the present state of the science of canning.

In some of the earliest heat penetration studies, maximum-reading thermometers were placed inside cans and jars, as cited by Alstrand and Ecklund (1952) and Clifcorn (1957) and described by Castle (1919) and Denton (1919). The problem with maximum thermometers was summarized by Clifcorn (1957):

"If a retort showed by thermometer test that a canner had got the heat up to 240F, for example, and kept it there for seventy minutes, that cooked the corn and killed the bacteria all through the can. The fact that it did not was one chief reason for studying heat penetration scientifically."

According to Clifcorn (1957), until 1920,

"The average canner did not know how to allow for the time consumed in reaching the maximum temperature and the time consumed in cooling off. The additional fact that the larger the container the more time consumed in carrying heat to the center of the can had not occurred to most canners. However, Dr. Bigelow stated: 'Even if we had such a curve for every product canned in every size of can, it would be meaningless unless we knew the heat which would kill each kind of bacteria which spoils canned foods'."

Although there are reports of bacteriological studies with canned foods as early as 1895 (Russell, 1896), it took another two decades for the landmark breakthroughs in heat processing studies of foods. H.L. Russell of the Wisconsin Agricultural experiment Station received an appeal from a "leading cannery" to investigate spoiled cans of peas which were "swelled" by gas and why some exploded and released unpleasant odors. The cause was traced to bacteria capable of fermenting sugar solutions with a "copious evolution of gas." Experiments were made by processing the peas under greater steam pressure, for, as Russell noted, "the factors of time and temperature in sterilization of any material have an inverse relation to each other" (Russell, 1896).

Prescott and Underwood of the Massachusetts Institute of Technology were responsible for years of work with the fundamental principles of bacteriology as applied to canning factories (Bitting, 1937; Clifcorn, 1957). An early paper entitled "Microorganisms and Sterilizing Processes in the Canning Industry" (Prescott and Underwood, 1897) detailed the examination of large numbers of spoiled cans of clams and lobster. The bacteria found were placed into descriptive classes and cultivated with artificial media. Inoculation experiments showed spoilage to be due to living bacteria in the cans.

Further studies were made by Prescott and Underwood to investigate the souring of canned sweet corn (Bitting, 1937). Descriptions of the types of spoilage bacteria in canned corn were quite detailed. These studies also included testing temperature differentials within retorts, and studying heat penetration in the center of cans. Barlow (Bitting, 1937) also investigated spoilage of corn stored at high temperatures and provided an early description of thermophilic bacteria. He also described several causes of flat sours and recognized that the flat sour may be caused by more than one organism.

With the advent of reliable heating and cooling curves for canned foods, bacteriologists began accumulating information on the heat resistance of food spoilage organisms in the early 1920s. There was a proliferation of literature on the thermal death-point of microorganisms and spores of C. botulinum (Bigelow and Esty, 1920; Weiss, 1921a; Esty and Meyer, 1922; Esty and Williams, 1924; Dickson et al, 1925). Bigelow (1921) demonstrated the logarithmic nature of thermal death time curves for spore-forming bacteria. Results of the bacteriological studies also showed that the heat resistance of microorganisms was affected by pH, age of the spores, and sodium chloride (Weiss, 1921a).

It has been previously mentioned that recommendations from USDA for home canning processes were modified in response to outbreaks of botulism after 1916. There were indications as early as 1917 (Dickson, 1917) that home-canning recommendations for vegetables were suspect. However, bacteriologists in the Bureau of Chemistry and States Relation Service of USDA denied this, apparently for almost 10 years (Tanner, 1934). It was not until Farmers' Bulletin No. 1471, "Home Canning of Fruits and Vegetables," (Stanley, 1926) was issued in 1926 that pressure canning was the only method recommended for low-acid vegetables.

Dr. E.C. Dickson of California conducted years of study with botulism cases (Dickson, 1917). In detailing the causes of outbreaks, he recognized that a large number of cases were due to eating spoiled home-canned fruits and vegetables. He tested the efficiency of the then recommended cold-pack procedure for peas, beans and corn with packs inoculated with spores of C. botulinum. The cold-pack method he used consisted of these steps: a five-minute steam blanch, dipping in cold water, packing in hot quart jars, filling jars with boiling water, adding one teaspoon of salt per quart, and processing in a water-bath for prescribed times (180 minutes for corn and 120 minutes for peas and beans). Administration of the liquid from the jars to guinea pigs resulted in death. Dickson made the claim, denied by USDA, that this cold-pack method of canning vegetables in the boiling water bath was not safe. Unfortunately, the term "cold pack" is still used erroneously by many home canners.

USDA argued that on the basis of statements in textbooks, the spores of C. botulinum were destroyed by heating for one hour at 175F (Tanner, 1934). However, another early report by Burke (1919), a colleague of Dickson in California, showed a much greater heat resistance of the spores of C. botulinum. Her experiments with 10 strains of C. botulinum involved heating suspensions of free spores at various temperatures and pressures for different periods of time. Conclusions from her studies include:

1. Spores of C. botulinum will survive boiling for 3½ hours or more; therefore, kettle canning cannot be relied on to sterilize material contaminated with spores of C. botulinum.
2. The one-period cold-pack process was not sufficient to kill spores of C. botulinum. Blanching in boiling water for 5 minutes did not destroy spores. Heating in boiling water for 5 hours or less would not sterilize food contaminated with C. botulinum.
3. Fractional sterilization on three successive days was of doubtful value because the initial exposure to 100C for 15 to 60 minutes delayed the germination of the spores until after the third sterilization period.
4. Pressure canning was the only method of sterilization considered safe; however, a pressure of 5, 10 or 15 psig for 10 minutes was not sufficient.

Weiss (1921a) showed resistance of C. botulinum spores to 100C for up to 5 hours and that the resistance was influenced by age of the spores, concentrations of sodium chloride, concentration of spores and pH. In a subsequent study (Weiss, 1921b) with 36 varieties of commercially canned foods, he demonstrated that the thermal death point of C. botulinum spores could be affected by temperature and time of exposure, syrup consistencies, food consistency, and acidity of the food.

Tanner and Dack (1922) showed that the effect of increasing temperature was a decrease in sterilization time for C. botulinum spores under dry heat conditions. Dickson et al (1925) continued their earlier work and demonstrated variations in heat resistance of these spores in different mediums. In particular, a thin layer of oil over the broth medium showed maximum survival times and greatest dormancies. Attention was directed once again to the need for applying this knowledge to canning of foods.

Studies on the heat resistance of bacterial spores, and in particular, spores of C. botulinum, continued (Viljoen, 1926; Sommer, 1930; Lang and Dean, 1934; Townsend et al, 1938; Reed et al, 1951). Despite the fact that the heat resistance discovered in the early 1900s was substantiated and many factors influencing the thermal death point uncovered, it took many years for USDA to incorporate this knowledge into its home canning publications. This will be further demonstrated in the review of publications.

C. Heat Penetration Tests

At the same time that these bacteriological investigations were being reported, others were gathering physical data on canning procedures, following the foundations laid by Prescott and Underwood (Bitting, 1937), Bitting and Bitting (1917) and others. Denton (1918) and Castle (1919) were among the first to measure heat penetration in food packs during home-canning procedures. Magoon and Culpepper (1921; 1922) made extensive investigations of temperature changes in containers during canning.

Thompson (1919) was one of the first to apply physical laws to calculating the lethal values of time-temperature curves for heat processing of cans. Bigelow et al (1920) studied these time-temperature relationships further and contributed useful formulas for calculating time-temperature curves for cans at any initial temperature, for any processing temperatures and for containers of varying sizes. Magoon and Culpepper (1922) took exception with both reports for implying that cooling curves were of the same form as heating curves. The latter researchers pointed out that the changing states of viscosity of the food will not be the same in cooling as in heating. Physical changes that occur during heating of a food were thought to also have an effect on heat transfer rates.

Magoon and Culpepper (1922) did demonstrate that cooling curves will differ between air and water cooling. Air cooling as used in home canning processes results in a continuation of heating and a slower fall in temperature. Therefore, the sterilizing effect is continued for a longer period.

Other findings of the studies by Magoon and Culpepper (1921; 1922), Thompson (1919), and Bigelow et al (1920) pointed out additional factors that influence heat penetration rates. Convection-heating foods were found to heat at faster rates than conduction-heating food products. Compactness of the pack, viscosity, and amount of free liquid all influence the rate of temperature change. Glass containers retard heat transfer in materials with free liquid, but apparently are not too significant in thick packs. Glass also cools faster in air than tin. Differences in container diameters will be of importance in packs of heavy consistency, but not in those with free liquid.

Salt and dilute sugar solutions, in the usual amounts used, have little effect on the rate of temperature change. Concentrated sugar solutions will retard heat transfer. Starch also has a marked retardation effect on temperature changes as do any materials of a viscous nature.

D. The Calculation of Processes and Sterilizing Values

Bigelow et al (1920) were the first to apply bacteriological and physical data to the calculation of thermal processes for canned foods. This method is usually referred to as the "general " or "graphical" method. The value of the method was that it could be used to calculate the exact lethality of a process. A disadvantage of the graphical method was that it could not be applied when initial temperatures, retort temperatures or container sizes varied from those in the heat penetration test.

Ball (1923) developed a formula method for process determination that was more flexible and relied less on empirical data. It could be adapted to all can sizes and retort temperatures. Mathematical equations are applied to heating and cooling curves of cans of food and thermal death-time curves of microorganisms. The important concepts introduced by Ball (1923, 1927) are the basis of modern methods for thermal process determinations.

The first of these concepts is "slope." The slope of the thermal death time curve is given as a Z value, which is the number of degrees Fahrenheit required for the curve to pass through one logarithmic cycle of time in minutes. F is the minutes required to destroy a given number of organisms at a given temperature.. The standard temperature was later taken to be 250F (Ball and Olson, 1955). The F value can compare the sterilizing values of different processes, but F values for microorganisms cannot be compared unless their Z values are the same. The number of minutes required to destroy a specified number of spores at 250F when Z = 18 is given by Fo. A Z value of 18 is usually assumed when thermal death time determinations have not made on the product, as a Z of 18 is attributed C. botulinum. The slope of the heating curve (straight line portion) is designated as fh. It is analogous to the rate of heat penetration. Improvements and simplifications of the methods for process calculations have been made, but the developments of that time period remain the basis for today's process determinations.

Olson and Stevens (1939) constructed nomograms for graphic calculation of thermal processes as a third major method to simply process calculations. When the heat penetration curve on semi-log paper is a straight line (not broken) and the Z value is 18F, nomograms provide a quick and simple procedure for process calculations. From heat penetration data obtained under one set of conditions, processes may be calculated for any initial temperature, any process temperature or any container size. This method has been widely used.

A. Farmers' Bulletin No. 1471

In May, 1926, Farmers' Bulletin No. 1471 (Stanley, 1926) was issued. For the first time, a distinction was made between acid and non-acid (sic, now classified as low-acid) foods in process methods. The results of bacteriological studies seem to have been incorporated into the recommendations. In particular, Stanley (1926, p. 3) mentions research with C. botulinum and stated:

"With nonacid vegetables and meats there is no assurance that they (bacteria causing botulism) are killed at the temperature of boiling water unless the material is heated for as long as 6 hours. The heating time may be decreased very much if a higher temperature is used. This is the reason for the recommendation that meats and nonacid vegetables be canned under pressure."

The timetables for canning fruits, tomatoes, pickled beets and pimentos offered cold packs and hot packs with equal process times in the boiling water canner. The timetable for nonacid vegetables offered process times at 240F, or 10 psig, only. Only hot packs were recommended for vegetables; reasons for this were to remove air, to shrink them, to facilitate packing, and to give an initial temperature near boiling.

Although the danger of contamination in open kettle canning was recognized, this method was given approval for fruits and tomatoes. In revisions of Farmers' Bulletin No. 1471 (Stanley, 1931; 1932; 1933), steamers and ovens were brought back as recommended methods of canning fruits, tomatoes and other acid products, even though heat penetration tests with these methods (Denton, 1918; Castle, 1919; Normington, 1919; Steinbarger, 1931; Haddock, 1933; Purdue University AES Report of the Director 51 (1930), reported by Tanner, 1934) demonstrated that the safety of these methods was questionable.

B. Research Conducted on Meats

According to a "History of Home Economics Work in the U.S. Department of Agriculture on Home Canning" (1944), no canning research was reported between 1927 and 1930. In May of 1930, the Bureau of Home Economics began compiling data on foods canned in the previous ten years. These foods had been stored two months to five years, with an average storage period of one year. Evidently a summary of the effect of temperatures and times of processing on the keeping quality of the canned foods was made in January, 1933. There were 4, 217 jars and cans of low-acid foods. Steam pressure canning resulted in only 12 percent spoilage, in comparison with 48 percent spoilage in water bath processed foods. Cited most often as spoiled were meats and fish, corn, lima beans, and peas. The following statement implies that the research procedures then in use were still not necessarily the most scientific: "Had bacteriological examinations been made on all these canned foods, additional spoilage might have been detected" (History, 1944).

During the next five years, processing times for beef were suggested by the National Canners Association and checked by the Bureau of Home Economics for palatability. Bacteriological examinations were made "in some instances." Control and inoculated packs of chicken were tested in 1933 for processing times of 50, 90 and 120 minutes at 240F. In 1934, mimeographed directions for canning beef and veal in community centers were issued as a result. Excessive spoilage resulted in revision of the processing requirements.

Increased processing times were recommended by the National Canners Association research staff for home canned meat. The Bureau of Home Economics compared the effects of the old and new processing times and pre-treatments on palatability of beef. Pre-treatment was determined to affect flavor more than processing time and "on the basis of these tests the times suggested by National Canners Association for heavier processing, were specified for beef, other meats and chicken in mimeographed directions for canning meat at home published in January 1935" (History, 1944). In 1937, 531 cans of meat canned in 1934 by these recommendations were opened by the Bureau and examined for spoilage. Less than 0.4 percent (two cans) showed spoilage "that might have resulted from processing" (History, 1944).

C. Farmers' Bulletin No. 1762

Recommendations for canning meats were once again incorporated in a publication with fruits and vegetables. In September 1936, Farmers' Bulletin No. 1762 (Stanley and Steinbarger, 1936) was issued. The processing times for meats adopted in 1935 were included in the timetables. Meat canning experiments did continue in 1937 and 1938, but the timetable remained the same through revisions of this publication to 1942 (Stanley et al, 1942).

The timetable recommended for meats (Stanley and Steinbarger, 1936) is reproduced below, evidently having been in effect from 1935 to 1942. According to current recommendations, based on more scientific process calculation methods, these earlier recommendations were, for the most part, in excess of the required processing. Note that 15 psig is recommended. The types of packs offered for meats were similar to those included in current publications.

As with meats, steam pressure canning was the only processing recommended for low-acid vegetables. In the absence of a pressure canner, instructions were to use some method of preservation other than canning. Some vegetables - greens, pumpkin, squash and cream-style corn - required a 250F (15 psig) processing temperature. All vegetable products required a hot-pack fill. Boiling water canning of fruits, tomatoes and other acid foods was recommended. Hot-packs were recommended for all products; raw-pack options were provided for selected fruits.

  Despite the studies in oven canning and steam canning cited earlier, Farmers' Bulletin No. 1762 and all its revisions continued to approve steam, oven and open kettle canning for fruits and tomatoes. Steam canning was approved for the same time schedules as in boiling water bath instructions. Oven canning was recommended for glass jars only, at oven temperatures of 250 - 275F. Processing recommendations were not clearly defined, yet oven canning was not declared as unsafe. The discussion of this method indicated that processing periods are "about half as long again as in the boiling-water bath...However, if the food is precooked and packed hot into the jars, the processing period in the oven may be shortened somewhat" (Stanley et al, 1942). Brief directions for the open kettle method were given for fruits and tomatoes canned in glass and users were cautioned as to the dangers of contamination.

D. Conservation Bulletin 28

In 1942, the U.S. Department of the Interior issued Conservation Bulletin 28 (Jarvis and Puncochar, 1942), "Home Canning of Fishery Products." It was strongly emphasized that "under no circumstances should any fishery product be canned unless a pressure canner is used. It is impossible to obtain a sufficient heat treatment or process, by any other means" (Jarvis and Puncochar, 1942). A lengthy discussion of canning principles explained the relation of microorganisms to canning, enzymes, and the limitations of sterilization.

The canning study for fish products involved 274 packs with 42 different products. Each pack consisted of at least 24 containers, some of which were inoculated and some which were controls. A canner of at least 30-quart capacity was required for processing. A variety of processes were described in detail, and those products not suitable for canning were listed at the end of the bulletin.

E. AWI Publications

One last series of home canning publications was issued by USDA prior to an intensive application of scientific process calculations to home canning research. In 1943, AWI- 61, "Canning Tomatoes" (USDA, 1943a) and AWI-41, "Wartime Canning of Fruits and Vegetables" (USDA, 1943c) superseded Farmers Bulletin No. 1762 (Stanley et al, 1942). These brief publications consisted mainly of directions for individual products, and contained only a brief explanation of canning principles. The boiling-water bath was the only method recommended for fruits and tomatoes; steam pressure canning was the only method recommended for low-acid vegetables. Because of the war, families were encouraged to share pressure canners.

Oven canning was described as less dependable than a water bath and not safe for vegetables. Open-kettle canning for fruits and vegetables was simply labeled risky and given a one-sentence warning about bacterial contamination during the filling of jars. Steam canning was not mentioned.

In 1944, AWI-93, "Home Canning of Fruits and Vegetables" (USDA, 1944a) replaced AWI-41 and AWI-61. Oven canning was labeled "dangerous" due to ineffective sterilization and serious accidents. Open-kettle canning was described as "wasteful" for fruits and tomatoes, "dangerous" for vegetables and was suggested only for preserves, pickles or other foods with enough sugar or vinegar to help prevent spoilage. The timetable for fruits offered boiling-water bath processes; for vegetables, pressure processes at 240F were recommended.

F. Further Contributions of Bacteriology

It has been previously stated that a landmark development as related to scientific heat process determinations was the discovery of a suitable apparatus to study heat penetration. Investigations of the technique of collecting and the significance of heat-penetration data were followed by bacteriological studies on the heat resistance of organisms and spores. Shortly after 1920, the concept of experimental packs inoculated with spores of known heat resistance was developed for the purpose of testing calculated processes (Cameron, 1938).

In order to run inoculated test packs, test organisms were selected, based on spoilage experience with the product under consideration or products of a similar nature. A collection of cultures with highly heat-resistant spores was developed by the National Canners' Association.

Experimentally inoculated packs with spores of the test organisms were processed for varying times. The method was eventually developed to the point where the range of process times incorporated 100 percent spoilage in the shortest cook and 0 percent spoilage in the two highest cooks. The range of times was decided upon by the calculated process. A minimum of 24 inoculated and 12 control cans was usually included for a test. The number of spores per can and the location of the inoculum were also standardized for the type of product. Test lots were usually incubated after processing, allowing sufficient storage time for maximum spoilage to occur (Williams, 1940).

Esty and Meyer (1922) determined the maximum heat resistance of C. botulinum spores in standard (neutral) phosphate solution at different temperatures. From their work, a sterilizing value of F250F equal to 3 minutes with a Z value of 18F was generally agreed upon for C. botulinum. Townsend et al (1938) indicated that it was difficult to produce spore crops of C. botulinum with such high resistance. It was also unsafe to bring C. botulinum organisms into a cannery. Therefore, considerable effort was made to find non-pathogenic organisms similar to C. botulinum in growth requirements and heat resistance which would readily produce spores. A putrefactive anaerobe, designated as No. 3679, was isolated from spoiled canned corn in 1927 by Cameron at the National Canners' Association laboratory (Townsend et al, 1938). The resistance of this organism was in excess of the maximum recorded for C. botulinum, met other requirements, and is the major reference organism in use today.

Townsend et al (1938) investigated thermal death times for C. botulinum in food systems, and compared thermal death time characteristics of Putrefactive Anaerobe No. 3679 with C. botulinum. They discovered that the Z value for P.A. No. 3679 was much higher than that for C. botulinum in foods. The Z values for C. botulinum were also found to be lower for food media than neutral phosphate. The sterilization factors of Esty and Meyer (1922) were corrected to F250F = 2.45 and Z = 17.6. However, the F and Z values of 2.78 and 18, respectively, derived from the Esty and Meyer (1922) data were more often used in canning process studies (Toepfer et al, 1946; Pflug and Odlaug, 1978).

Putrefactive Anaerobe No. 3679 spores were used to test for protection against spoilage which would threaten public health, or ensure protection from C. botulinum. Other thermophilic organisms, such as the anaerobe C. thermosaccharolyticum, strain No. 3814, or the flat-sour type B. stearothermophilus, strain No. 1518, were available as substitutes. These organisms had even greater heat resistance than P.A. No. 3679, and could be used to test for a process against thermophilic spoilage.

Resistance of spores to heat destruction was demonstrated to be related to the number of organisms present. The greater the number of spores per unit of volume, the higher the resistance of those spores. Bigelow and Esty (1920) studied the thermal death point of different cultures of thermophiles, not well identified in their research report, by varying the initial concentration of spores. They found that the larger the number of spores present in corn juice, the longer the time necessary to destroy them at three process temperatures.

Esty and Meyer (1922) studied the heat resistance of 109 strains of C. botulinum. The relationship of the concentration of spores to the heat resistance was determined on 15 different spore suspensions. Although the same strain showed variability in resistance under identical conditions, it was demonstrated that the resistance was greater for higher concentrations of spores. This characteristic continued to be demonstrated in later literature for spores of both C. botulinum and P.A. 3679. Table 6 demonstrates this concept in terms of the effect on F and Z values for P.A. No. 3679 in food substrates. These values were determined during a two-year study on heat penetration and processing values for the foods listed (Desrosier and Esselen, 1950, p. 14).

A load of 10,000 spores per No. 2 tin can became the standard for highly-resistant spores in studying commercial processing times for low-acid foods (Williams, 1940). This load was found to give good agreement between processes derived by inoculated packs and actual processes. It also represented a practical maximum load in commercial canning. No comparable quantity was determined by home-canned foods; therefore, spore suspensions in much of the research which forms the bases for home canning processes were standardized at 10,000 spores per milliliter (Tischer and Esselen, 1945).

A final important contribution to the development of the inoculated pack test method was made by Yesair and Cameron in 1936 (Williams, 1940; Townsend et al, 1956). They found that spores with different degrees of resistance could be separated from suspensions by centrifugation. This made it possible to secure and concentrate a spore suspension of desired resistance.

Bigelow and Cameron (1932) suggested groupings of food according to acidity because of spoilage relationships associated with this factor. All fruits and vegetables are acid to a degree, but Bigelow and Cameron (1932) originally devised three categories due to similarities and differences. Products with a pH of 6.0 - 7.0 were considered nonacid; those between 4.5 and 6.0 were called semiacid; and, pH 4.5 was the upper limit of an acid range. It had been learned that pathogenic spore-forming bacteria cease to be a factor in spoilage below pH 4.5.

In a subsequent publication, Cameron and Esty (1940) expanded the classifications of foods into subdivided acidity groups. Although these classifications were later dropped, pH 4.5 remained the upper limit for acid foods. Researchers devoted to the canning studies had determined repeatedly that the dividing line of acidity in foods which would allow or inhibit the growth of C. botulinum was pH 4.5. This was important in concluding that products of pH greater than 4.5 must be processed under pressure to ensure destruction of C. botulinum spores. Those acid foods of pH less than 4.5 could be safely processed in boiling water due to their inherent inhibitory conditions for spores of C. botulinum (Cameron and Esty, 1940).

The Food and Drug Administration currently uses pH as a basis for regulatory action in governing quality and safety of canned products. This action is in the form of Good Manufacturing Practice (GMP) Regulations, Title 21 Code of Federal Regulations (21 CFR). Part 114 of the GMP Regulations, 21 CFR, defines "low-acid foods" as any food, other than alcoholic beverages, with a finished equilibrium pH greater than 4.6. "Acid foods" are those with a natural pH of 4.6 or below. "Acidified foods" are defined as low-acid foods to which acid(s) or acid food(s) are added to result in a finished equilibrium pH of 4.6 or below (Food Processors Institute, 1979).

G. Heat Penetration and Processing Tests with Home Canning

The state of science applied to home canning through 1944 can be well-described by the following statement made to the Society of American Bacteriologists: "Considering the importance, but little work has been reported on the application of technical methods for the derivation of home canning processing times" (Esselen and Tischer, 1945).

The earliest studies on heat processing of home-canned foods were reported by Denton (1918), Skinner and Glasgow (1919)) and Normington (1919). Denton (1918) measured the maximum temperatures reached in jars of carrots processed in a boiling-water canner. Skinner and Glasgow (1919) conducted similar tests on asparagus canned in boiling water. Variations in blanching methods and packing brines were made and processing times were selected and tested by evidence of spoilage. Normington (1919) conducted extensive bacteriological examinations of home canned peas, using pressure, boiling water, and steam canning methods. She cited the previous use of thermocouples by other researchers and acknowledged the value of time-temperature curves in solving bacteriological problems of canning; however, she chose not to incorporate heat penetration data into her study of the causes of spoilage.

Sunderlin et al (1928a) cite a Canadian research report by MacFarlane in 1919, wherein process recommendations for hot water, intermittent sterilization and pressure canning were evidently decided on the basis of spoilage noted after storage. Biester et al (1921) looked at the effects of storage temperature on spoilage of vegetables canned by the boiling water method; no heat penetration data were taken. Edmondson et al (1922) and Levine (1923) checked spoilage after canning by the one period boiling water method; again, no heat penetration data were recorded.

Magoon and Culpepper (1921) measured heat penetration during processing and cooling with thermocouples. Although they did not use home-canning vessels, their results are among the first to demonstrate the increased sterilizing value of air cooling. In a study on the role of acidity in vegetable canning, Cruess et al (1925) did measure heat penetration with thermocouples. The value of this type of data in helping to interpret results on heat resistance was recognized in the design of these studies.

Redfield et al (1928) attempted to formulate more accurate time-tables for canning low-acid vegetables and meats in the home. Thermocouples were attached to glass jars and used to collect heat penetration data for different types of packs and initial temperatures. Unfortunately, center temperature was monitored only until it became constant for 15 minutes. The rest of the process was not monitored.

Sunderlin et al (1928a; 1928b) determined spoilage records for boiling-water canning of vegetables but failed to collect heat penetration data. On the basis of spoilage results, some boiling water processes were found "entirely satisfactory" for low-acid vegetables. These conclusions concerned keeping quality but the adequacy of the processes in terms of controlling C. botulinum was not investigated, despite the fact that reports on the heat resistance of C. botulinum spores had been published for at least a decade prior to this research.

Nelson and Berrigan (1939) and Nelson and Knowles (1940) studied heat penetration with mercury thermometers during heating and cooling of pressure-canned meat where pack styles were varied. These authors, as well as Cover et al (1943), all recognized the increased sterilizing effect of air-cooling on glass jars. Cover et al (1943) also questioned the necessity for the severity of the USDA processing schedules for meats. They used thermocouples to determine curves for exhausting, heating and cooling periods in several processing schedules for meats. Safe processing schedules were then calculated by the National Canners' Association from these data; inoculated packs using P.A. No. 3679 were used to confirm the schedules.

Esselen and Tischer (1945) conducted heat penetration tests for home canned products with thermocouples inserted into a number of vegetable packs. Tests were made in glass jars for boiling-water and pressure canners. Inoculated packs were used to check the accuracy of the theoretical processes calculated from heat-penetration tests. The results indicated that the boiling-water processes recommended by USDA would result in understerilization. The data also indicated that the recommendations for pressure canning were longer than theoretically calculated times. The researchers called for more experimental work before general recommendations were made.

H. USDA Makes Shift to Science

Through this period of time, recommendations to home canners were based mainly on industry's research. As described before, scientific methods of process calculations had been available to industry beginning about 1920. The necessity for pressure canning of low-acid foods was slowly adopted in home canning recommendations. However, it was customary to lengthen the recommendation considerably to insure a generous margin of safety to overcome the lack of precision in home methods (USDA, 1946c).

The Bureau of Human Nutrition and Home Economics, as it was now known, did find, after collecting heat penetration data and calculating theoretical processes, that their pressure process schedules could be shortened by 25 to 50 percent for some vegetables (USDA, 1946c). This was due in part to recognition and verification of the sterilizing value during cooling. For vegetables in glass pint jars, for example, the research showed that, on the average, 50 percent of the lethality occurs during the gradual cooling in the canner. The research also resulted in lengthening process times for a few vegetables in quart jars. The processing temperature for meats was reduced from 250F to 240F.

The majority of the current USDA process recommendations for low-acid foods are based on three years of research between 1944-46 which culminated in new publications. AWI-110, "Home Canning of Meat" (USDA, 1945b), was issued in 1945. AIS-64 (USDA, 1947e) and Home and Garden Bulletin No. 8 (USDA, 1947f), both entitled "Home Canning of Fruits and Vegetables," were released in 1947. A description of the research and the data were released in 1946 in Technical Bulletin No. 930 (Toepfer et al, 1946).

Research on acid foods by the BHNHE was started right after completion of low-acid process determinations (USDA, 1946c).