Heat Penetration Studies of Stewed Tomatoes in 6, 8, and 17 Quart Household Pressure Retorts

R.J. Pakola and E.L. Andress, Dept. of Foods and Nutrition, The University of Georgia, 208 Hoke Smith Annex, Athens, GA 30602-4356.

Paper 46B-7. Presented at the Institute of Food Technologists Annual Meeting Anaheim, CA, June 17, 2002.


Most current home pressure canning recommendations were developed using standard 17 to 21 quart pressure retorts. Today's marketplace offers a variety of smaller pressure-based cookers/canners. Retort size has the potential to affect come up and cooling rates, which contribute to the sterilizing value of a pressure process. As heating and cooling times are reduced so is total lethality. The objective of this study was to compare f(h) values, cumulative lethalities, and potential safe process times during pressure processing of stewed tomatoes in 6, 8, and 17 quart household pressure retorts. Stewed tomatoes were prepared and packed into 8 oz glass home canning jars. Jars were processed at 121.1°C for 25 minutes in 6, 8, and 17 quart pressure retorts after a 10 minute exhaust of the retort. Heat penetration data were collected at the cold spot, previously determined to be the geometric center of the jar. Continuous temperature profiles from 15 replications were collected using copper-constantan needle thermocouples connected to an electronic data logger. Potential process times and cumulative lethal rates were calculated for the destruction of Clostridium botulinum spores. The cumulative lethal rate did not reach the target F0 (3.0) during the process. The majority of the lethality was achieved during cooling. Processing in smaller retorts resulted in lower f(h) values (p<.001). Calculated process times from the data collected so far were found to be equivalent but due to the need to verify some data with additional work, a final conclusion about process schedules cannot be made at this time, Home canning in very small retorts should be avoided until safe process recommendations can be determined.


Currently, home canning in smaller pressure retorts is not recommended by USDA and the Cooperative Extension System (USDA, 1994). Existing recommendations were determined using standard size home canning pressure retorts that have a 17 or 21 quart capacity. Smaller pressure retorts are widely available in 4, 6, and 8 quart capacities. "Pressure canner" is a term usually associated with 16 quart and larger capacities. A recent survey by the National Center for Home Food Preservation indicates that people are using smaller pressure retorts to can food at home (Andress, 2001). Past research suggests that 4 quart retorts require a longer process time to achieve an equivalent lethality (Taube and Sater, 1948; Nordsiden and others, 1978). Thus home canning in smaller retorts may not produce a safe product.


The purpose of this study was to compare heating characteristics and potential process recommendations of a product in standard and smaller size pressure retorts.

Materials and Methods

Stewed tomatoes were selected for this study; the recipe used in this study is described in Figure 1. Half pint jars were filled with 200 + 10 grams stewed tomatoes for all experiments. Each retort load consisted of five jars. Jars were processed at 121.1°C for 25 minutes following a 10 minute exhaust (vent).

Heating profiles were collected in 6, 8, and 17 quart capacity aluminum pressure retorts using Ecklund needle type copper-constantan (type "T") thermocouples.

The cold spot, or slowest heating point in the jar, was determined from data taken at the center and half inch increments below the center by using 4 different lengths of needle probes. Data were recorded using the DAS-TC (Keithley Instruments, 1996) data acquisition system. The rate of heating, expressed as f(h), was determined and compared using the data analysis software (SAS Inc., 1999-2001).

For process calculations, heating profiles were collected at the cold spot in 15 jars of stewed tomatoes per retort size. Data were recorded using the E-Val™ Monitoring System (Ellab Inc., 2000). Data were imported into thermal processing software (TechniCAL Inc., 1998) to determine f(h) values, potential process times (Bb), and cumulative lethalities. A target lethality (F0) of 3.0 minutes was selected based on the choice of Clostridium botulinum as the microorganism of concern.

Figure 1. Stewed Tomatoes Recipe



Select firm, ripe Roma tomatoes. Wash and place tomatoes in boiling water until skins split. Dip in cold water and remove the skins and cores. Slice tomatoes into halves. Combine all ingredients in a large saucepan and cover. Heat to a boil and simmer for 10 minutes. Fill five half-pint canning jars, leaving ½ inch headspace. Remove air bubbles, adjust headspace if necessary, and wipe jar rims. Secure two-piece metal lids. Process jars in a pressure canner.

Results and Discussion


The f(h) value it the slope of the straight line portion of the heating curve. A larger f(h) value indicates a slower rate of heating. The slowest heating point of stewed tomatoes in half pint jars is the geometric center of the jar (Table 1).

Table 1. Determination of the Cold Spot of Stewed Tomatoes in Half Pint Jars
Thermocouple placement n Mean1 Standard deviation
center 15 32.5A 2.0
½" below center 15 31.5B 2.0
1" below center 13 29.0B 3.3
1½" below center 11 23.9B 4.7
1Means in the same column with different letters are significantly different (p<.05)


Jar temperature did not start to increase until the end of the 10-minute exhaust (Table 2). Mean jar temperatures achieved by the end of the 25-minute process were the same (p<.001) in the 6 and 8 quart retorts (Table 2).

Table 2. Mean Jar Temperatures (°C) of Stewed Tomatoes in Half Pint Jars (n=15)
  Retort size (quart)
  6 8 17
Initial 79.3 ± 5.81 80.5 ± 2.1 76.8 ± 3.2
Start of exhaust 76.8 ± 4.4 78.3 ± 2.3 74.1 ± 2.3
End of exhaust 79.8 ± 2.9 79.4 ± 2.1 76.0 ± 1.9
Start of process 84.8 ± 2.0A2 85.7 ± 1.0 A 81.4 ± 2.1B
End of process 113.7 ± 0.6A 113.7 ± 0.6A 112.2 ± 0.7B
At 36 min cooling 72.9 ± 5.2 86.0 ± 3.9 79.1 ± 6.0
1Mean ± standard deviation
2Values in the same row with different letters are significantly different (p<.001)


Retort temperatures across experiments were controlled (Table 3).

The f(h) values increased with retort size (Table 3). The jars processed in the smaller retorts had a faster rate of heating (p<.01). The difference between the f(h) values was not significant between the 6 and 8 quart retorts, probably because there is less difference in size between the 6 and 8 quart retorts as compared to the 8 quart and 17 quart retorts.

Target F0 (3.0) was not achieved in any retort by the end of the process time; at least 2/3 of the cumulative lethality was achieved during cooling (Table 3).

Even though the average calculated process time appears to follow the trend of f(h) values, the maximum achieved in each size retort does not. In practice, home canning process times are rounded off to the next highest 5-minute interval. Therefore, if these results were to be used for recommending a process time, based on the worst-case jar in each retort, the recommended process time for all three sizes would be 40 minutes (Table 3).

Table 3. Effect of Retort Size (n=15) on the Processing of Stewed Tomatoes
  Retort size (quart)
  6 8 17
Retort temperature (°C)
End of exhaust 101.4 ± 0.61 102.5 ± 0.4 102.7 ± 0.1
During process 121.4 ± 0.1 121.2 ± 0.1 121.1 ± 0.0
f(h) 30.6 ± 1.4A2 31.3 ± 1.5 A 33.1 ± 1.3sB
Cumulative F (min)
End of process 0.9 ± 0.2 0.9 ± 0.1 0.6 ± 0.1
End of cool in retort n/a3 4.8 ± 0.6 3.0 ± 0.5
Calculated process times (min)
Mean B(b) 33.6 ± 1.5 33.8 ± 0.9 36.0 ± 0.6
Maximum B(b) 37.3 35.9 37.0
Potential recommendation 404 40 40
1Mean ± standard deviation
2Values in the same row with different letters are significantly different (p<.01)
3Cooling data not available due to equipment failure
4Maximum B(b) rounded up to the next 5 minute increment


The target F0 was not achieved during the process. Most of the cumulative lethality was acquired during cooling.

Jars processed in smaller retorts (6 and 8 quart) had smaller f(h) values indicating a faster rate of heating during the process time than occurred in the 17 quart retort. This resulted in higher cumulative lethality for the smaller retorts, which was unexpected. These results are not consistent with past research on smaller retorts.

The 6 quart retort did not provide reliable cooling data. Conclusions cannot yet be drawn about the entire process in all three sizes of retorts.

In addition, the 17 quart retort had shorter come-up and cool down times than have been previously documented with older models (Toepfer et al., 1946). The 17 quart retort will hold a much larger volume than the 6 and 8 quart retorts; this study limited the number and size of jars to what was determined to be a full load in the smallest retort. Future work should explore the effect of varying the number and size of jars in the 17 quart retort.


  1. Andress, E. L. 2001. A national survey of current home canning practices in the US. National Center for Home Food Preservation, Department of Foods and Nutrition, The University of Georgia, Athens, GA. Unpublished data.

  2. Ellab Inc., 2000. E-Val Basic, V. 2.0. Software and Documentation, Ellab Inc., Arvada, CO.

  3. Keithley Instruments Inc., 1996. DAS-TC Data Acquisition System. Keithley Instruments Inc., Cleveland, OH.

  4. Nordsiden KL, Thompson DR, Wolf ID, Zottola EA. 1978. Home canning of food: effect of a higher process temperature (250 F) on the safety of low-acid foods. J Food Sci. 43:1734-1737.

  5. SAS Inc., 1999-2001. Statistical Analysis Software, v. 8.02. for Windows, SAS Institute Inc. Cary, NC.

  6. Taube, K. and Sater, V.E. 1948. Canning vegetables in the pressure saucepan. J Home Econ. 40:197-198.

  7. TechniCAL Inc., 1998. Carlsoft, V. 1.3.3. Thermal Processing Software, TechniCAL, Inc., New Orleans, LA.

  8. Toepfer, E.W., Reynolds, H., Gilpin, G. L., and Taube, K. 1946. Home canning processes for low-acid foods. USDA Technical Bulletin No. 930. Washington, DC: Bureau of Home Economics, U.S. Department of Agriculture.

  9. USDA. 1994. Complete guide to home canning. Agriculture Information Bulletin No.539. Washington, DC: CSREES-U.S. Department of Agriculture.


This material is based upon work supported by the Cooperative State Research, Education, and Extension Service, U.S. Department of Agriculture, under Agreement No. 00-51110-9762.

Document Use:

Permission is granted to reproduce these materials in whole or in part for educational purposes only (not for profit beyond the cost of reproduction) provided the authors and the University of Georgia receive acknowledgment and this notice is included:

Reprinted with permission of the University of Georgia. Pakola, R.J. and E.L. Andress. 2002. Heat penetration studies of stewed tomatoes in 6, 8, and 17 quart household pressure retorts. Athens, GA: The University of Georgia, Cooperative Extension Service.

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