Backgrounder: Heat Processing of Home-canned Foods

Why do individual home-canned foods have different heat processing times?

There are several factors that affect the way in which heat is distributed through the food in a jar during a home-canning process. It is this variation in heat penetration that determines the position of the “cold spot” (the slowest heating area) of the jar, which can be different for different jar sizes and shapes as well as different foods. The heating rate at the cold spot determines how long the process time needs to be. In the case of low-acid foods, this is to ensure that the food receives the heat necessary to kill Clostridium botulinum spores. Left alive inside a sealed jar of low-acid food at room temperature, the spores become bacterial cells that multiply and produce the toxin that causes botulism poisoning.

The time and temperature combinations at which C. botulinum, its spores and other bacteria are killed are established under certain conditions. However, the substrate (food) in which these bacteria are found is an important variable factor in the rate of destruction. The food factors that will influence the amount of heating needed to kill bacteria include: the consistency of the food; the pH (acidity); and, the presence of nutrients that are “protective” for bacteria (e.g., high protein and sugar levels). Other influences on the amount of heat delivered to the food in the jar are: the shape and size of the jar; the size, shape and texture of food pieces; the solid to liquid ratio; the temperature of the food at the beginning of the process; and, the temperature inside the canner. For example, heat penetration through a mass of liquid (faster) will be very different from heat penetration through puréed or mashed food (slower). This is apparent during stove-top cooking too, where different foods heat up differently based on their composition and consistency.

If the food is thick, puréed, or mashed; if there are large pieces of food in the jar; or, if the food is packed in too tightly, heat penetration can be slower than in more liquid or loosely packed foods. If a specific heat process is not calculated for each food and style of pack, the heating may not be adequate, and the food will be underprocessed.

How is the processing time for a food determined experimentally?

Heat penetration experiments, which are necessary for all low-acid foods and some acid foods, are carried out in a properly equipped laboratory. The food prepared by specific procedures is filled into jars and thermocouples (temperature measuring devices) are inserted through the lid, jar or can into the food in the jar. These are connected by wires to a monitor, and the temperature at the end of each thermocouple is recorded throughout the time the canner comes up to processing temperature, during a process at that temperature (e.g., boiling water or 240°F under pressure), and during at least some of the cooling period.

Determining the heat penetration curve is a two-step procedure. The first step is to put thermocouples in several areas of the jar to determine the “cold spot” (slowest-heating location) of the jar. Once that spot is located, more data is collected at the cold spot to have enough information to calculate the process time for this food under these specific conditions – i.e., in a particular jar type in this canner. The process time is the time needed to achieve a certain level of “lethality”, or killing of a number of target pathogens or spoilage organisms for that food.

In the case of low-acid foods, the processing time needs to ensure that the minimum temperature and time combination to destroy spores of C. botulinum is reached, so that the food will be safe when stored on the shelf. Microbiological confirmation tests then need to be done for low-acid foods to make sure that spores of C. botulinum will not survive the process time calculated from heat penetration data. In the case of acid foods, the target microorganisms will be different and ones determined to be the most heat resistant ones for that particular food that could make someone sick or spoil the food.

This process has to be done separately with each food, as well as any variation that alters pH, consistency, texture, distribution of solids and liquids, or other factors that result in a “new’ product”. Experimentally determining safe processing times for home-canned foods is thus a lengthy, expensive and time-consuming process, which explains why there are fewer home-canned processes available than many people would like. In short, there is no easy formula to work out processing times without experimentation and analysis that take into account how each food product heats in a particular canning situation.

Why do some foods have both hot and raw pack processing times, while others have one or the other?

The offering of hot and/or raw packs is usually based on quality issues with the finished product. However, USDA process recommendations have been developed over time by different laboratories and researchers. Sometimes it has been the choice of the researchers who developed the process recommendation to only use one method. Individual food characteristics can also lead to the need for specific preparation procedures. For example, in a hot pack process for a starchy food like potatoes, the food is precooked in water that is then discarded (some of the starch is drawn out into the water) and replaced by fresh boiling water when filling jars. If a raw pack process was chosen for the same product, the starch that now cooks out in the jar may later gelatinize and/or cause excessive cloudiness in the finished “raw pack”. This amount of starch in the jar also causes safety concerns during the canning processing, and makes it hard to detect any post-processing spoilage in the stored jar. As another example, many pickled products are hot packs because the pre-heating starts to acidify the food before it goes in the jar and results in a safer product.

Why are hot and raw pack processing times sometimes the same?

Hot pack and raw pack variations, if they are offered in USDA recommendations, have each been researched separately. This includes collecting heat penetration data and calculating an independent process time for each. So the process time is determined by the actual heating characteristics of the pack. Depending on preparation procedures and the type of process, the final result may be the same. Other times, it might be different. The temperature of the process (boiling water or pressure) and the length of the process needed can influence the differences between hot and rack pack rates of heating. Another consideration is that USDA home-canning processes are rounded off to the next higher 5-minute interval. If the hot and raw pack process times vary by less than 5 minutes, but in the same interval, the recommended process time will be the same. For example, if the hot pack is calculated as 11 minutes and the raw pack requires 14 minutes, they will both get rounded off to, and published as, a 15-minute process time.

Why should I not make up a processing time for a food that I wish to can?

Underprocessed low-acid foods run the risk of allowing survival of Clostridium botulinum and its spores, and consumption of these foods can lead to botulism, an often fatal disease, and one that involves expensive health-care costs and health complications for those that do survive. Again, there is no formula for converting a process time for one low-acid food to that for another food or jar size. Too many characteristics of the particular food and processing procedures can influence the rate of heating. If you are experimenting with untested recipes for pickled products or other acidified foods such as salsas and there is not enough acid to treat them as a boiling-water canned food, you may also end up with the same risk of botulism by underprocessing. Even if you have an acid food and do not process it long enough, food spoilage can result.

Why should I not purée or mash foods before canning them?

Packing food into a jar may seem easier or less wasteful of jar space with mashed or puréed food, but this style of pack greatly increases the product density and will have a very different heat penetration pattern than pieces of food in a liquid cover. Current USDA home-canning recommendations do not include mashed or puréed vegetables because there have not been the resources to do the amount of experimentation needed (e.g., to cover all variations in density that may result when a consumer mashes or purées food). So far, methods of preparation that are likely to result in more uniform results of heating patterns have been offered. The USDA process times are only intended for use with the preparation procedures that accompany them. Consumers put themselves at great risk for botulism if they choose to purée or mash vegetables and use the same processing time for a pack that is intended to be pieces of food in liquid.

Why should I not make additions/deletions of my own to the canning recipe? I want the canned food to taste exactly like one of my own recipes.

We all would like the convenience of great-tasting “one jar meals”. But, any additions or deletions made to an approved canning recipe would need a new process time calculated for it. It is not safe to change the recipe and use the same process time. One-dish meals often include thickening ingredients or are cooked down to a thicker consistency than expected for the process time for an individual ingredient. These situations are likely to result in hazardous foods. You may add your special ingredients after you open up the jar, when reheating or assembling the dish. Also keep in mind that after canning and storage, your special recipe may no longer taste exactly the same as when it is made fresh. Sometimes special recipes are best enjoyed as freshly made dishes.

What should I do if I desire to preserve one of my own recipes that does not match something with a recommended canning process?

We can't recommend making up your own canning process for your recipe. One option is to choose something close to yours that does have an approved procedure for home canning and can that product or recipe instead of yours. After canning, when you are ready to consume/reheat the food, add your special ingredients to adjust the recipe to your taste. Alternately, you may make your recipe and freeze it to preserve it, instead of canning.

Keep in mind that several products that we desire to have ‘home-canned’ are not available commercially, either. The commercial food manufacturing industry puts a lot of time and expense into research for their own safely canned products (they do not have a 'blanket processing' method or formula for adjustments, without collecting heat penetration data, either). Also, just because a canned food is made commercially and found on a store shelf does not mean a home canning process is available for the same or similar item. The heating characteristics under home preparation methods and canning procedures would have to be studied to come up with a home-canning process. The commercial canning industry also has more resources and methods at its disposal for controlling the consistency and maturity of raw ingredients going into a canned food. There will be more variability to take into account when researching a home-canning process to cover all the potential variables.



Elaine M. D’sa, Ph.D., former Research Coordinator, and Elizabeth L. Andress, Ph.D., Professor and Project Director, National Center for Home Food Preservation, University of Georgia, Athens, GA. December 22, 2005.  Revised September 23, 2016.

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.