Critical Review of Home Preservation Literature and Current
Abstract | Table of Contents | List of Tables | Appendices | References
|Chapter IV Tables|
Effect of Depth of Water-Bath Upon Interior Temperature of Contents of Glass Jars
(boiled 60 minutes after water-bath began to boil)
A. History of Recommendations
The thermal processing of home canned foods by steam heating remains an issue of concern. Steam canners for fruits are being promoted by some manufacturers who claim this is energy-efficient equipment. Steam canning was recommended historically by the United States Department of Agriculture; however, research data from as early as the 1940s have shown the hazards in this method.
Farmers Bulletin 359, "Canning Vegetables in the Home," (Breazeale, 1909) issued in 1909 by the USDA Bureau of Chemistry, illustrates and describes the "steam cooker" used in home canning at that time. There were evidently several varieties of steam cookers in common use which had one or two doors and held a dozen or more quart jars. The following directions are given for all meats, fruits and vegetables (Brezeale, 1909):
This process, with the directions for clamping the jar lids deleted, was the recommendation of those who explained, "Therefore it is necessary in order to completely sterilize a vegetable, to heat it to the boiling point of water and keep it at that temperature for about one hour, upon two or three successive days, or else keep it at the temperature of boiling water for along period of time..." (Breazeale, 1909). The fractional sterilization over a period of three days was considered the whole secret of canning. Many years later heat penetration tests were applied to home canning methods and the inefficiencies of steam heating were realized.
Farmers Bulletin 839 (Benson, 1917), issued in 1917 by the States Relation Service, described five general types of canners for processing. Boiling-water vessels, either commercial or home-made, needed false bottoms or platforms to permit a free circulation of water around and under the containers. The water-seal outfit was also described for the first time. This consisted of a double-walled bath and cover which projected down into the water between the outer and inner walls, thus making two water jackets between the sterilizing vat and outer surface of the canner. A slight steam pressure was maintained since its free escape was prevented. The other two types of canners were for pressure processing.
Use of boiling-water baths and water-seal outfits required the following rules: support of the jars on a perforated platform, covering the tops of jars with water by at least 1 inch, counting the time as the water began to boil vigorously, and removal of the jars from the water when the processing time was completed. While the need for immersion in water was recognized, the reason given for immersion was so that liquid would not be lost from jars during the sterilization period. Immersion in boiling water was also recommended in Farmers' Bulletin 853, "Home Canning of Fruits and Vegetables as Taught to Canning Club Members in the Southern States" (Creswell, 1917), issued one month after Farmers' Bulletin 839.
The Office of Home Economics of the States Relation Service began research on home canning problems in 1917 (History, 1944). Other investigations, reported in 1918 (Denton) and 1919 (Castle) were recognized by the Office of Home Economics (History, 1944); however, when Farmers' Bulletin 1211, "Home Canning of Fruits and Vegetables," replaced Farmers' Bulletin 839 and 853 in 1921, steam bath canning was included, but with certain precautions.
Revisions and reprints of Farmers' Bulletin 1211 through 1923 continued to discuss steam bath canning as an alternative. It was recommended that the temperature of the steam should be maintained at the boiling point of water. Readers were warned that a steamer must have a tightly-fitted flanged cover (or equally efficient device) or the home canner should use a thermometer to count processing time at 212 F.
When Farmers' Bulletin No. 1471 (Stanley, 1926) was issued in 1926, the only types of canners recommended were boiling water and pressure canners. Boiling water directions called for the water to cover tops of the jars. However, the 1931 revision of Farmers' Bulletin No. 1471 again recommended steamers for processing fruits and a few other products. Explanations stated that non-pressurized steam surrounds the cans with the same temperature as in the boiling-water bath. Processing times were the same as for boiling water. The same recommendations were in the 1932 and 1933 revisions of Farmers' Bulletin No. 1471, as well as Farmers' Bulletin No. 1762 (Stanley, 1936). The latter publication includes the qualifying statements:
B. Review of Literature
Research does demonstrate steam canning is less effective than boiling water. Three very early studies, which apparently were not considered in USDA publications at the time, indicate the effects of low water levels in canning. As early as 1918, Denton (1918) concluded that when the water level drops to the rubber rings of the jars, a difference of 1-2 degrees C may be observed within the jars. She measured the temperature of the liquid in jars with a mercury thermometer. Although her trials were limited and the data were not published, her conclusions illustrated that low water levels reduced the sterilizing value and that there was a need for further experimentation.
The same apparatus with a mercury thermometer was used in research by Castle (1919) about the same time. Her purpose was to study the effect of the pack and depth of water bath upon the internal jar temperatures. Conclusions regarding water level in the canner agreed with those of Denton (1918). Complete immersion instead of a shallow bath maintained the internal temperatures at a higher level and for a longer period than did shallow baths. Table 16 demonstrates the results of Castle (1919).
Normington (1919) studied the heat-resistant organisms found in home-canned peas. While her intent was to describe the bacteriology of the product, her methods indicated some relationships between processing methods. According to "government" regulations, the peas were processed by steam, hot water bath, and autoclave at 15 psig. Only one lot was heated in steam (15 jars, pints and quarts), while larger samples were used in other treatments.
The steam-processed lot had 10 percent more spoilage than three of the five lots processed in boiling water for the same time period. While it is now know that peas should not be expected to be sterilized by either these procedures, the fact that spoilage ranged from 25 to 100 percent for these procedures was not incorporated into recommendations for many years.
|Size of Jar||Depth of
|Minutes contents of jar were held
at or above various
Discrepancies between boiling and steam canning were not explored further until much later.
Pflug and Nicholas (1962) conducted a study of heating rates in glass jars as affected by heating medium in the range of 165-225 F. Steam-air mixtures were found to be less efficient than boiling water or saturated steam environments. The efficiency of steam-air mixtures increased with increasing percentages of steam, and the slope of the heating curves decreased with increases in temperature. Therefore, it was concluded that when equal process times and temperatures were maintained in boiling water and steam-air processes, convection-heating food will receive a reduced sterilizing value in the steam-air medium.
These tests were not conducted in home-style canners; however, the differences in sterilizing values would indicate the need for such research. The steam-air mixtures would certainly simulate the situation in home steam canners which do not allow for complete exhaustion of air surrounding the jars would be difficult to monitor in home canners.
More recent studies have been conducted on low-water level steam canning with home canners. Harris and Davis (1976) compared product heating rates and heating efficiencies resulting from the use of different water levels in a pressure canner but operated at atmospheric pressure. Their method consisted of sealing the lid on a pressure canner with the vent open. Their hypothesis considered that "products will heat as well with a low level of boiling water as with a high level, provided the canning vessel is equipped wit a close fitting cover to hold saturated steam around and over the jars" (Harris and Davis, 1976). The study reached conclusions that high water levels in canning (i.e., complete immersion of jars) was wasteful of heat energy, limits the choice of equipment and is cumbersome for the operator. Using tomato juice as the canned product, low-water levels in the processing were found to result in heating the product as fast as water levels one inch over jar tops.
On the surface, this study implicates equal sterilizing values are possible with steam and regular boiling water canning. However, there are no qualifications given to the recommendations made, and it would appear that some are necessary. The canner was vented, while recently marketed steam canners could not be vented. The only product tested was tomato juice, which has different heating characteristics from solid packs and solids in liquids. The adequacy of the process was determined by heating periods of 20 minutes after adding product to canner. Initial temperature of the product was 120F, while standard fill recommendations are temperatures close to boiling. After a 20-minute heating period, which included recovery for the water bath, jars were removed, shaken and opened for internal temperature readings at the center of the jars. Standard procedures for measuring a sterilizing process call for the determination of heat penetration data by the use of thermocouples during heating. The adequacy of a complete processing schedule was not determined by the time periods used.
One other recent study also supported the use of low-water level (LWL) canning for tomato juice. Collins et al (1982) studied the sterilizing value of LWL baths in an attempt to provide a home canning alternative with lower energy costs. Tomato juice inoculated with B. coagulans was processed in LWL and HWL baths and stored up to 12 weeks at 27C. Microbiological examination showed <1 log aerobic mesophiles after storage, no mold and adequate pH values (4.11 to 4.14); therefore, it was concluded that the LWL was as adequate as HWL canning.
B. coagulans was an unacceptable choice for inoculating the samples, as this organism does not grow at pH < 4.3 (Rice and Pederson, 1954a). This study did use a LWL bath that approximates conditions in home steam canners more closely than Harris and Davis (1976). Temperatures were determined by thermocouples also. It is unfortunate that heat penetration data were not presented and that inoculated packs with suitable organism were not considered.
The origins for the application of oven heating to home canning seem rather vague; however, a wide distribution of directions for this procedure came from manufacturers of ranges, glass jars or other related equipment (Haddock, 1933; Tanner, 1934; 1935a; 1935b). USDA did recommend oven-processing in their consumer publications from 1931 through 1942 (Stanley, 1931; Stanley et al, 1942).
In 1931, the revision of Farmers' Bulletin No. 1471 by Stanley (1931) introduced a discussion of oven processing. Stanley states the recommended temperatures were 250F and 275F, free circulating of air around jars was important, and that juices may boil out of the jars. A shallow pan in the bottom of the oven under the jars could be used to catch these juices. It was recommended to begin timing the process as soon as jars were placed in the oven and to process jars 50 percent longer than for boiling water canning. Jars were to be removed from the oven at the end of the processing period. The same altitude corrections were to be made as for boiling water canning.
Several studies stressed the possibility of jar explosion as well as spoilage resulting from poor heat penetration. Studies at the Indiana Agricultural Experiment Station in 1930, as reported by Tanner (1934; 1935a; 1935b), showed that temperature inside jars do not rise above 212F. In addition, come-up time was very long: a jar of water with an initial temperature of 68F, processed in an oven at 275F, required 95 to 110 minutes to reach 212F. This meant two-thirds of the process time for some foods expired before maximum interior temperatures were reached. Higher initial temperature slightly reduced come-up time.
Steinbarger (1931) reported studies conducted in the USDA Bureau of Home Economics on heat penetration rates associated with oven canning. Rates varied with consistency, initial food temperature, oven temperature and jar size. Quart glass jars were processed in 250 and 275F ovens. The maximum temperature reached in these jars was 212F, except for some fruits with high sugar content which were a few degrees higher. Sealing jars to hold in pressure caused breakage, so jars could not be fully sealed before processing. The Bureau concluded that this method could be used for acid fruits and tomatoes. All foods required excessive come-up times (55-95 minutes), and higher initial temperatures slightly decreased come-up time. Similar results were reported by Tanner (1934; 1935a; 1935b), who also demonstrated that a five-hour process in an oven at 135C (275F) failed to destroy spores of C. botulinum.
Haddock (1933) also demonstrated that initial temperatures of jar contents affect come-up time and the maximum temperature of food was 212F. Haddock concluded that oven canning was influenced by many variables and should not be recommended. Variation between ovens was excessive, and come-up time from 113 to 212F for pint jars in different ovens under the same conditions ranged from 37½ to 120 locations within the same oven cavity. Fluctuations in oven temperature resulted in liquid loss from jars. Recommended processing schedules were also inadequate.
Heat penetration data and other parameters studied in the 1930s demonstrated that oven-processing was not safe. In publication AWI-93 dated 1944, USDA issued a warning against oven canning. The low interior jar temperature and possible physical accidents were cited as to why the method was dangerous. Although oven canning is still practiced by some and is occasionally recommended in print, research data from the 1930 decade was enough proof of its unacceptability.
Open kettle canning refers to the process of cooking foods directly in a kettle or pot, filling into sterilized jars and sealing with lids. The often cited advantages of this method were: foods heat evenly and quickly and it is more convenient. Disadvantages include pre-sterilization of empty jars, maximum obtainable temperature of 212F and the risk of contamination during filling and sealing of jars.
Open-kettle canning was recommended for tomatoes only in Farmers' Bulletin 359. The directions assumed that "everyone knew how to can tomatoes." Farmers Bulletin 853 recommended that jams and marmalades could be hot-packed in hot jars and sealed immediately; however, pints packed for market were to receive a 30-minute process in water at 188 F. Farmers' Bulletin 900, "Homemade Fruit Butters" (Close, 1917; 1922), recommended sealing jars of hot fruit butter with paraffin or steam sterilization.
Farmers' Bulletin No. 1471 discussed the danger of contamination in open-kettle canning. This danger of spoilage, however, could be partially avoided by sealing and inverting the jars while boiling hot. The method was considerable suitable only for fruits and tomatoes. The same recommendation was given for this method in Farmers' Bulletin No. 1762, beginning in 1936.
Farmers' Bulletin No. 1800 (Yeatman and Steinbarger, 1938; 1942; 1945) contained directions for homemade jellies, jams, and preserves. The explanation of preservation in this publication was that the combination of heat, sugar, and acid destroyed microorganisms during the cooking process. Storage of air-tight containers in a cool, dry environment was considered the key for preventing surface molds and yeast fermentation. Mold or fermentation was attributed to unsterilized jars, imperfect paraffin seals, and storage in a warm or moist atmosphere. It was believed moisture collected beneath paraffin, broke the seal and permitted the entrance of mold and yeast spores in the jellies, jams and preserves. Recommendations called for paraffin seals and no heat processing of the jars.
Research data relevant to open-kettle canning were not located in this literature search. However, it was possible to trace the demise of this method in USDA publications. Filed notes from a 1944 National USDA Conference on Home Food Preservation (Anon., 1944) indicated open-kettle canning was to be deleted from recommendations for tomatoes, fruits, vegetables, meat, poultry and fish. Disapproval was based on the danger of recontamination during filling of jars.
The method was still considered satisfactory for relishes, preserves, jams and jellies. Later that year, publication AWI-93 (USDA, 1944a) contained the following warning:
"For vegetables open-kettle canning is dangerous. They never become hot enough to destroy the bacteria.
"Use this method only for preserves, pickles, and other foods with enough sugar or vinegar to help keep them from spoiling."
Current recommendations for pickles and relishes remain the same (USDA, 1978) and a 5-minute process in boiling water is now recommended for sealed jars of jellies, jams, conserves, marmalades, and preserves for those residing in warm or humid climates. The use of paraffin is restricted as an option to jelly only.
A. Recent Study of Open-Kettle Canning
A 1975 USDA survey of home canning practices (Davis and Page, 1979) showed that 70 percent of households used open-kettle canning for some products. Forty-four percent incorrectly used open-kettle canning for fruits, 35 percent for tomatoes, 43 percent for tomato sauce, 14-26 percent, respectively, for low-acid vegetables and vegetable mixtures, and 57 percent for pickles. Two to 21 percent reported spoilage for products canned by this method. Eighty-five percent used open-kettle canning for jams and jellies and experienced one to two percent spoilage.
B. Mycotoxins Research
Recent research on the formation of mycotoxins associated with growth of molds on food surfaces adds emphasis to sealing jars of jams, jellies and all other fruit preserves with regular lids and processing 5 minutes in boiling water. Some molds capable of growing on jellies produce root-like filaments which penetrate into the product. Mycotoxins are metabolites of some mold species and are being investigated for mutagenic and carcinogenic characteristics.
Home-made apple juices and apple and rhubarb jams were examined by Lindroth and Niskanen (1978) for levels of patulin. Patulin is a fungal metabolite which has been found to be antibiotic, mutagenic and toxic to rats. Penicillium expansum has been shown to produce patulin in moldy fruits. Patulin was found in 40 percent of the home-made apple juice samples. Patulin was also found to have diffused to all levels of a jar of moldy apple jam, indicating that removal of the moldy layer would not reduce the toxin hazard.
The method of reducing hazards from mycotoxins which can be suggested at present is to process sealed jars in boiling water to prevent mold growth during storage.
A. Canner Construction and Size
Changes in pressure canning equipment during the past 25 years are numerous. Today's canners are of light-weight construction and quipped with newly designed lid closures, vent ports and weighted pressure regulators. These design changes permit more rapid heating, venting and cooling of the equipment and better regulation of pressures.
Earlier studies indicated 35 to 50 percent of the process lethality for foods in jars was associated with the cooling curve (Esselen and Tischer, 1945; Toepfer et al, 1946). As cooling time is reduced in new pressure canners, so may the total lethality be reduced. Accordingly, it's possible that the process schedule should be increased.
Taube and Sater (1947) found that process values for different weights and sizes of canners did vary. Although the differences in available systems at that time did not result in significant underprocessing, their findings indicated a need to re-evaluate lethalities associated with using standard process schedules in newer equipment system.
The success or safety of a pressure saucepan is a frequent concern in home canning. Taube and Sater (1948), household equipment specialists with the BHNHE, evaluated six types of pressure saucepans for canning vegetables. They found a longer process time was needed in saucepans compared to larger canners because the lethality normally associated with come-up and cooling times was greatly reduced. They recommended adding 20 minutes to established process schedules for pint jars in pressure canners. While no heat penetration data are available from their study, the authors used the same research methodology as contained in USDA TB 930.
Kraska and Kubista (1948) of the Kraska Food Laboratory also studied canning with a single pressure saucepan, and recommended compensatory cooling time of 30 to 45 minutes for vegetables. This time was approximately equal to the difference in come-up and cooling times between their saucepan and pressure canners. It was also concluded that venting one minute was unnecessary. Spores of P.A. No. l 3679 were used for inoculated packs. Without explanation, an additional 15 minutes was added to standard processes as an additional safety factor in their process recommendations. Since 1957, USDA has included a statement in their Home and Garden Bulletins to signal process times for pints in a pressure saucepan at 240F should be 20 minutes longer than for pressure canners.
Walsh and Bates (1978) stated that an independent study of heat penetration data promoted one manufacturer to delete the 20-minute extra process time for their pressure saucepans. They subsequently collected heat penetration data for vegetables processed in one light-weight pressure saucepan and two common household pressure canners. The F0 values for 20-minute additional processes in the saucepan were considered unnecessarily high for convection-heated foods, but not for foods heated by conduction. These authors concluded no venting and an additional 6 to 10 minutes is sufficient for convection-heated vegetables. No inoculated pack studies were employed, and no process recommendations were offered.
Nordison et al (1978) also collected and evaluated heat penetration data for canned vegetables, beef stew, beef and fish in several pressure canners and saucepans. In 4-qt. saucepans, an additional ten minutes was considered adequate for convection-heated products. Inconclusive results were obtained with foods exhibiting mixed or conduction heating.
Current issues with process schedules for modern pressure canners and saucepans cannot be resolved with existing data. It has been suggested that separate process recommendations for multiple models of canners and saucepans would only confuse the public and increase risks in home canning practices. Nevertheless, research data for all types of equipment should be collected and analyzed in order that the proper steps may be taken toward offering the most feasible solutions to these issues.
A common concern, therefore, in studies with pressure saucepans has been venting characteristics. Conflicting data and opinions also exist in regard to the venting (exhausting) practices required for standard canners. Esselen (1944) investigated venting characteristics of 11- and 25-quart size aluminum pressure canners. His report included a survey of 62 home-canning bulletins which offered a wide variety of instructions for venting pressure cookers. The tests made by Esselen determined the effects of venting procedures on interior temperatures at 10 and 15 psig with thermocouples at four locations within the canner. The data indicated a venting time of at least 10 minutes was necessary for the average-sized home pressure canner, while small-size canners required less time.
Since 1944, USDA has recommended that pressure canners be vented for 10 minutes prior to pressurization. A mixture of air and steam will create gauge pressure without the necessary corresponding high temperature (Esselen, 1944; Esselen and Fellers, 1950), and could result in understerilization.
Walsh and Bates (1978) reported that, based on independent studies of heat penetration data, one manufacturer eliminated the 7 to 10 minute exhaust recommendation for their weighted-gauge equipped canners. In tests with weighted-gauge models conducted by these authors, F0 values indicated significant differences in lethality between vented and unvented canners. However, the difference was attributed to additional come-up time at 100 C in weighted-gauge models and not inadequate venting. Weighted-gauge models appeared to be very accurate; i.e., there was no lag in the time between achievement of a continuous jiggle (used as an indication of processing temperature being maintained) and the corresponding process temperature in the headspace. Internal temperatures of unvented dial gauge canners were lower at the time when the pressure corresponded to 10 psig. Further, a three-minute exhaust period also resulted in adequate purging of air. It was suggested that an additional 3 to 4 minute process time at 10 psig could compensate for the exhaust period with self-venting, weighted-gauge models. These data indicate once again the need for comprehensive study of all canner styles as they relate to process requirements.
C. Canning at 15 psig
A few recommendations have been made over the years for home canning of meats at 250F (15 psig) (Cover et al, 1943). It was later found to be unnecessary, but the higher temperature was and is used successfully, especially with commercial products.
At present, 240F (10 psig) is the most commonly recommended temperature. More recent interest in conserving nutritional and textural characteristics in canned food has led to some research with higher pressure (temperature) and shorter time processes.
Use of 250F in home canning has been limited due to a lack of research at this temperature. Shorter process times would be required at 250F, as lethality accumulates about three times faster than at 240F (Zottola et al, 1978). These authors and Nordisen et al (1978) recently investigated heat penetration rates for low acid foods processed at 15 psig. Convection-heated foods required about one-third the time as specified for processes at 10 psig. Those foods heated by conducting or a mixed mechanism appeared to require equal time at 10 and 15 psig. It was concluded that each product be investigated before recommendations could be made.
A. Hot Packs
In the early 1900s, heating prior to filling food into jars was considered to have distinct advantages for both home and commercial canners (Magoon and Culpepper, 1924). In 1810, Appert (1920) first described preheating foods before bottling or filling food into jars. Preheating is the term that is currently used to describe the preparation of hot-pack products. Older references used terms such as scalding, steaming or blanching.
Many early theories existed to explain the benefits of preheating foods. One theory held that it started the flow of coloring matter in the product. Other writers said it assisted in cleansing the material, while some asserted that "acidity" of some vegetables and gumminess of others were removed. Shrinkage or reduction in bulk and increased flexibility, which facilitated filling of jars, were advantages noted by many sources. Tomatoes and fruits subjected to preheating peeled much easier. Other authors offered that it eliminated the exhaust period for canned vegetables (Magoon and Culpepper, 1924).
Early in the century, "cold dipping" food after preheating was a common practice. Hypothetically, it was thought cold shocking would make bacteria more susceptible to subsequent process temperatures. Other advantages to chilling - handling cooler produce and firmer pulp for filling - were also recognized. However, the synergistic value of preheating, chilling, and sterilization was not verified. It was accepted by the Society of American Bacteriologists in 1917 that death rates were the same for cold-shocked and control bacteria (Bruett, 1919), a conclusion reached by Bushnell in 1918. Later Bruett (1919) studied cold shocking of bacterial spores in peptone solution, and concluded that death rates of bacteria at high temperatures were unaffected. Bruett did not quantitate the removal of spores from food in this study.
Magoon and Culpepper (1924) conducted further study of other possible benefits associated with heating and cooling food prior to canning. Quantitative analyses of heating and cooling water, foods and sensory analyses were made. Shrinkage and nutritional losses were measured for spinach, peas, string beans, lima beans, sweet corn, tomatoes and sweet potatoes.
They concluded that in most cases, preheating reduced bulk due to loss of turgidity and expulsion of air. The reduction varied from a maximum of 50 to 60 percent for spinach to a minimum of 6 to 15 percent for peas. Shrinkage permitted a closer, more attractive pack; however, the danger of overfills was noted, especially with peas and lima beans which swell during processing. Expulsion of air also permitted higher vacuum in cans.
One to 30 percent of the total dry matter was lost from water-heated vegetables while steam heating reduced losses. Neither heating nor heating followed by chilling diluted natural pigments. Since air was eliminated from the tissue, the amount of oxidative discoloration was reduced in heat-packed foods. However, the flavor of unheated foods was superior inmost cases.
Some important chemical changes during precooking were observed. In most cases, protein was altered somewhat, starch was partially gelatinized, but sugars were altered very little. However, significant transformation of starch to sugar was noted in sweet potatoes. The degree of this change influenced consistency and sweetness of the canned product.
Preheating in fresh water did assist in cleansing raw foods; the opposite effect was noted when boiling water was used repeatedly. Chilling did not benefit quality, flavor or appearance of finished products. The only value of chilling was to facilitate peeling of produce. Preheating did not appear to reduce processing time. However, preheating was a suitable replacement treatment for "exhausting" if packed immediately.
Dubord and Esselen (1945) studied factors affecting potential spoilage bacteria on fresh vegetables. Washing and blanching reduced the bacterial load but blanching was more effective than washing. Inconclusive results were obtained as to the effect of holding vegetables a few days before canning on the numbers of spore-formers. The overall incidence of putrefactive anaerobes was low. This latter finding may partially explain why some people have success with boiling-water canning of vegetables.
B. Raw Packs
Interest in raw-packing of vegetables grew because of its convenience (Cilpin et al, 1951). Comparisons of hot and raw packs were first made with tomatoes (Gilpin et al, 1951). They concluded that tomatoes canned by the raw pack method were superior in odor, color, texture and flavor. Different preheating treatments had no significant effect on the palatability of hot packed tomatoes. In addition, inclusion of 250 mg of ascorbic acid per quart before processing did not improve either type of pack. Quart jars of hot-packed tomatoes were processed 10 minutes in a boiling water, while raw packs were processed 50 minutes. Explanations or sources for processing times are not given in the research report.
Dawson et al (1953) reported higher palatability ratings for raw-packed snapbeans than hot-packed beans precooked for 5 minutes. These products each received equal lethal processing treatments. They also concluded that raw-packed beans had a lower drained weight than hot-packed beans.
C. Exhausting, Venting and Vacuum
Esselen and Fellers (1950) presented a very thorough discussion of the development of vacuums and pressure during canning operations. When a home canning jar is sealed and heated, pressure develops within it, caused by the expansion and vapor pressure of the jar contents. The amount of pressure is influenced by the amount of headspace, the processing temperature and the temperature of the contents at the time the jar is sealed.
At a given temperature, internal pressure varies inversely with headspace volume and sealing temperature. When correctly used, home canning jar closures vent at relatively low pressures so that dangerously high pressures are not created within the jars. With the two-piece self-sealing metal lid closure recommended for use today, the closure vents even at low pressures. The older Mason jars and three-piece glass lids have dangerously high venting pressures and the closure could on be partially sealed during processing. These jars could alternatively be exhausted, then fully sealed and processed. Exhausting expels air. Metal cans are tightly sealed before processing and vent only slightly. Therefore, they also need to be exhausted until a food temperature of 170F is achieved before sealing.
Other advantages to exhausting were the same as those for hot packs. Expulsion of air from tissues helps prevent oxidative discoloration during storage, and induces desirable chemical changes in some cases. The higher initial temperature also enhances vacuum formation. In some cases, exhausting of cans could be eliminated by hot packing (Magoon and Culpepper, 1924).
Magoon and Culpepper (1922) discovered that can vacuum is not always proportional to the average temperature of the contents, but is determined mainly by headspace temperature. A short exhaust, therefore, could result in a high vacuum if sealed immediately. A long exhaust would be ineffective if the headspace is allowed to cool.
Vacuum is achieved in cans through removal of air prior to sealing either exhaustion or hot filling (Magoon and Culpepper, 1922; Peterson, 1949; Boyd and Bock, 1952). A good partial vacuum is of primary importance in food canning. The vacuum serves to inhibit the growth of spoilage microorganisms and to maintain an effective seal, preventing recontamination of the food after processing. In addition, vacuum prevents or retards oxidative changes in color, flavor and vitamin content of the product (Fellers et al, 1937; Esselen and Fellers, 1950).
Partial can vacuum also causes and maintains can ends in a concave appearance or swelling of can ends are indication of spoilage and loss of seal. A low oxygen content is desirable to minimize chemical changes in the product and to reduce internal corrosion in the can.
Vacuum is achieved in home canning jars by either hot-filling the container or heating the product in the jars or both. The heat causes internal gases and vapors to expand. These gases escape during processing by "venting." During cooling the residual gases and solids contract and a partial vacuum is formed within the container. The amount of the vacuum formed depends on the effectiveness of venting, and is enhanced by hot filling (Esselen and Fellers, 1950).
The type of closures and method of processing affect the venting characteristics of home canning jars. Loss of liquid from jars during processing led to research on venting and canner operations. Fellers et al (1937) reported on studies with home canning jars filled with bail closures. Fully sealing jars prior to processing was effective in preventing liquid loss and achieving high vacuums in these jars when pressure processed. Initial temperatures and headspace volumes had no effect on vacuum. This type of jar is no longer recommended for use in home canning.
Later, Esselen and Fellers (1948) conducted extensive tests on the effects of processing methods, initial temperatures and headspace on the venting characteristics of jars sealed with available closures. Closures requiring a partial seal before processing lost more liquid than two-piece metal self-sealing lids. Fluctuating process pressures or rapid cooling of the canner also increased liquid loss.
With boiling water canners, hot-packed jars contained less residual air than cold packs. The volume of residual air increased directly with headspace volume. The following recommendations were made to keep liquid loss at a minimum (Esselen and Fellers, 1950):
"For fruits and other acid foods, which are usually given a short process in a boiling water bath, a partial seal on the jars permits a more effective exhaust of residual air. If a full seal is used, a hot fill and small headspace (one quarter inch) are effective in reducing the amount of residual air left in the jars after processing. With such products a loss of liquid from partially sealed jars does not present a serious problem.
"On the other hand, with low acid foods which are processed in a pressure canner at 10 pounds steam pressure the use of a full seal to prevent an excessive loss of liquid will still provide satisfactory venting and permit most of the entrapped air to be exhausted from the jars during processing."