Newsflash

Newsflash

Canning on Portable Burners

Can I can on portable gas or electric burners?

If you have a smooth cooktop and the manufacturer says not to can on it, then you might find yourself looking for an alternative. The alternatives for canning (if you have one of these no-canning recommended smooth cooktop ranges) are either to purchase and install a permanent set of electric coil or gas burners as a range top (without an oven) or to purchase a portable electric coil or gas burner. 

And that is where important decisions come in to play.  An installed range top requires the utilities to support it and can be quite expensive as a second range top. As for portable burners, they are not all alike and not all portable burners are appropriate for canning.  First of all, check the burner manufacturer's specifications and directions or contact their customer service department for more specific information about the appropriate use of a particular burner for canning. 

We cannot endorse a particular brand, but here are a few basic guidelines for you to keep in mind when selecting a portable burner for canning purposes: 

1. The burner must be level, sturdy, and secure. Look for enough height to allow air to flow under the burner, but not such that it will become unsteady with a full, heavy canner resting on it. One we have tested was about 4 inches high off the counter top, on short legs that allowed air circulation underneath but was plenty stable. 

2. Look for a burner diameter that is no more than 4 inches smaller than the diameter of your canner. In other words, the canner should not extend more than 2 inches from the burner on any side.  This is a common recommendation, but also make sure this is the recommendation for your canner brand. 

3. For electric burners, you want the wattage to be about equal to that of a typical household range large burner.  We have been successful bringing a boiling water canner to boiling with one that is 1500W/120V, but household range burners are more typically 1750W or higher and this kind of wattage may actually be a better choice if you can find it. We have not yet tried using a pressure canner on a portable electric burner. 

4. You want the burner to have housing that will hold up to the high heat under the canner for long heating periods, and not damage counter tops with reflected heat. We contacted a foodservice supply store to help us identify one like this; it cost us about $200. We used it successfully a few times to bring water to a boil, but have not used one repeatedly for canning. 

5. At least one pressure canner manufacturer advises not to can on any outdoor heat source. Your pressure canner can be damaged if the gas burner puts out too much heat. Higher BTU burners (over 12,000 BTUs) could also produce so much heat that the recommended come-up time for canning could be altered, potentially producing an unsafe final product. 

6. Again, check manufacturer’s directions and/or contact their customer service for more information about appropriate burners. When you are asking manufacturers about canning, specify whether you are asking about pressure canning (much more heat concentration) or boiling water canning. If the manufacturer's directions have been followed, and canning problems occur, then you must take it up with the manufacturer.  

February, 2024
National Center for Home Food Preservation

Newsflash

Pre-Sterilizing Jars before Canning

Do I need to pre-sterilize my jars for canning?

New canning jars out of the box are not sterile. Being in a box or covered in plastic wrap is not the same as a sterile environment. In addition to contamination by microorganisms that cannot be seen with our bare eyes, packaged jars may accumulate dust, small bits of debris, and even chips of glass in the case of breakage (which does happen sometimes in all the steps of transport from factory to store to home). 

Whether brand new or re-used many times over, you should always clean jars just prior to filling them when canning. Wash jars in a dishwasher or by hand, using detergent and rinsing well. Clean jars should then be kept warm prior to filling.  You can leave them in the closed dishwasher after the cycle, or use your canner as it is preheating, or create a separate water bath that will keep the jars both clean and warm.

Washing is also a good time to inspect jars for any cracks or chips, discarding or re-purposing those jars for non-canning uses if any imperfections are found. If you see scales or film from hard water left on your jars, then remove this by soaking jars for several hours in a solution containing 1 cup of vinegar (5% acidity) per gallon of water.

In order to actually sterilize jars, they need to be submerged in (covered by) boiling water for 10 minutes. When the process time for canning a food is 10 minutes or more (at 0-1,000 feet elevation), the jars will be sterilized DURING processing in the canner. Therefore, when process times are 10 minutes or more at this elevation, pre-sterilization of jars is not needed. It doesn't hurt your product to do it anyway, but it does require additional time and energy and is unnecessary. 

To pre-sterilize jars, place the cleaned jars right-side-up on a rack in a canner and fill the jars and canner with water to 1-inch above the tops of the jars. Bring the water to a boil and then boil for 10 minutes at elevations less than 1,000 feet elevation.  Add 1 additional minute for each additional 1,000 feet of elevation. When you are ready to fill the jars, remove the jars one at a time, carefully emptying the water from them back into the canner.  This will keep the hot water in the canner for processing filled jars.

Sometimes people choose to increase a 5-minute process time (at 0-1,000 feet elevation) for certain jams and jellies to 10 minutes so that they do not have to pre-sterilize the jars.  The extra process time is not harmful to most gels and spoilage should not be an issue as long as the filled jars get a full 10-minute treatment in boiling water.  (And remember your elevation adjustments to increase this process time as needed.)

In summary:

• Is a 5-minute process time enough to sterilize jars? No. If you are using a process time of only 5 minutes, such as for some jellied products, then you need to pre-sterilize jars before filling them (or increase the process time to the equivalent of 10 minutes at 0-1,000 ft elevation).

• If a process time is 10 minutes or more then will the jars be sterilized? Yes, if you are at 0-1,000 feet elevation, but be sure to wash and rinse them well, and keep warm, before filling them with food. If you are processing above 1,000 feet elevation, then you need to consider the elevation adjustments needed to sterilize jars so you use the equivalent to 10 minutes of boiling at 0-1,000 feet elevation.   

Revised August 2023
National Center for Home Food Preservation

Newsflash

Green Beans and Botulism

How can I can my green beans safely?

Easy to grow in a home garden and delicious year-round, green beans are a popular home-canned food. Just like with any other home-canned food, it is important to always use proper procedures and follow tested recommendations. Yet we have received many concerns about canning green beans this season, including confessions of improper processing. Unfortunately, in multiple situations we’ve had to recommend discarding entire batches due to under-processing, because there is a risk of botulism from under-processed green beans.  Botulism is a potentially deadly food poisoning.

To help you prevent waste, sickness, or worse, here are our responses to the most common questions about canning green beans:

Q: I want to can my green beans in a boiling water bath…is that ok? 

No. Green beans are a low-acid food and require the higher temperature from a pressure canning process for a pre-determined length of time in order to destroy the potentially deadly bacterial spores of Clostridium botulinum, unless they are adequately pickled. The ONLY processing we can support for non-pickled green beans is under pressure, using the directions and steps found on our website at Canning Green Beans. The correct procedures include the steps for managing the canning process found here: Using Pressure Canners.

Q: My neighbor gave me green beans that were canned using the oven method…is it safe for me to eat them? 

A: No. Using the oven method is NOT a recommended method of canning for green beans or any other food. It is dangerous because dry heat is slow to penetrate into jars (so recommended process times would not be enough), temperatures inside ovens vary (so a standard process time would be indeterminable), and no reliable, research-based safe process times have been developed for oven canning.  There are also stories that jars heated in a dry oven could explode or break more easily than with recommended canning procedures.

Q: Last month I force cooled the pressure canner with my last batch of green beans, but now I think that may not have been a good idea…what should I do?

A: We recommend that the jars of beans be discarded. This is the safest option when home-canned foods are suspected of being spoiled or improperly canned.  The cool-down time of a pressure canning process is calculated into the overall heat treatment required to destroy dangerous bacteria (Clostridium botulinum). So, if you do not let pressure canners cool down naturally and slowly to 0 pounds pressure, the jars did not receive a complete canning process.  These jars are therefore considered to be under-processed, which means it is not safe to store them at room temperature. If it had been less than 24 hours, you could have refrigerated the jars immediately and eaten them within one week or frozen the green beans for longer term storage.  Instructions for discarding suspect jars or detoxifying and cleaning unsealed spoiled jars are available on our website: Identifying and Handling Spoiled Canned Food .

Q: Last night I pressure canned my green beans using USDA recommendations, but this morning I noticed that 3 of the jars did not seal…can I re-process them?

A: Yes, if the jars received a proper pressure canning process but simply did not vacuum seal, then you can re-process them within 24 hours. Remove the unsealed lids and check the jars for nicks. Replace the jar if needed, and replace the lid with a new, properly prepared lid. Repeat the canning process, using the same processing time for this second process. Another option is to refrigerate the jars and eat the beans within a week, or to freeze the green beans for longer term storage. If more than 24 hours had passed, then we would recommend that you discard the beans.

There are cases of botulism from under-processed home-canned green beans and other vegetables. Here is just one short report that documents that this is hazard to be taken seriously.
Three Outbreaks of Foodborne Botulism Caused by Unsafe Home Canning of Vegetables—Ohio and Washington, 2008 and 2009 - Journal of Food Protection

Revised August 2020
National Center for Home Food Preservation

Newsflash

Canning Your Own Salsa Recipe

Can I can my own salsa recipe?

1. Salsas typically are mixtures of acid and low-acid ingredients; they are an example of an acidified food and appropriate for boiling water canning if the final pH of all components is less than 4.6.  If the mixture has less acidity, it would need to be treated as a low-acid canned food and require sufficient research to eliminate a botulism risk as a canned food.  If it is acid enough for boiling water canning, the actual proportions of ingredients and preparation method will help determine what the canning process time should be.  So there is no way to tell someone how to can a homemade salsa without having detailed knowledge of the recipe,  procedures used in preparation, and acidity and consistency of the final product.  The proportions of your tomatoes, peppers, herbs and other vegetables will greatly influence what the safe canning process should be.  

Summary, for all home canned foods:  For home canning recipes, the specific recipe, and usually the preparation method, will determine how the product (salsa, in this case) can be processed--whether in a boiling water canner (BWC) or a pressure canner (PC). A Boiling Water Canner can be used for acid and properly acidified foods, while a Pressure Canner is used for low-acid foods. Then, the process time in the canner will be dependent on the specific recipe and product characteristics. 

The Backgrounder on Heat Processing of Home-Canned Foods will explain some of the science behind the development of home-canning recipe, especially for low-acid foods or mixtures.
It explains why it is not always possible to home can foods like those that are commercially available/store-bought, or your own recipes. 

2. Our USDA and Cooperative Extension recipes and processes for home canning are all tried and tested, and processing times decided upon for the recipe as provided and tested.  We only recommend recipes and procedures we know to be safe, and encourage consumers to use tested, science-based home-canning recipes from reliable sources like our website or some equipment or home preserving ingredient manufacturers. Our recommended home canning recipes for salsa, as well as a discussion of how ingredients impact safety, are collected in the Sensational Salsas publication. We also have individual recipes with links to background information 

The most flexible procedure we offer for a home-canned salsa is our Choice Salsa.

3. Someone new to canning or who has not read general canning principles should start with those principles: General Canning Principles
and know how canners are meant to be used, whether for boiling water or pressure canned products: 

• Using boiling water canners
• Using pressure canners

4. At this time, we can only recommend tested recipes as safe for boiling water canning, and we ourselves do not offer a pressure-canning process for a low-acid salsa. There is a Mexican tomato sauce that is less acid and pressure canned only, but it is not a chunky salsa; it is more sauce-like. Mexican tomate sauce. 
The rest of our recipes noted as salsas have enough acid in them to make them safe from botulism when canned at boiling water temperatures only. 

5.  Your recipe could be frozen for long-term storage, but you will need to determine if you like the texture and flavor after freezing and thawing; there most likely will be changes in both texture and seasoning.  I would try a small batch the first time for freezing.  Many times herbs and spices are better tasting when added fresh after freezing and thawing, at serving time. 

6.  Please do not experiment with canning your own recipe that mixes low-acid vegetables together, even with “some” acid like vinegar or lime juice.  If done improperly, you put yourself at risk for botulism, a potentially fatal food poisoning.  This page has more on botulism and canned foods, as well as a section on the importance of food acidity and canning methods. 

7.  If you want to explore private testing of your recipe for canning, it most likely will require an investment through private companies.  You could contact your local Cooperative Extension office to see IF they have names of testing companies in your state, and/or if they could contact the Food Science Department at their state land-grant university to obtain help. 

8.  The University of Georgia sells a book of all types of home food preservation recommendations (not just canning), described at https://setp.uga.edu. 

Revised May 1, 2019
National Center for Home Food Preservation

Newsflash

Canning on Smooth Cooktops

Can I can on my smooth cooktop?

We have to say to follow manufacturer's advice because styles of smooth cooktops being manufactured differ in ways that influence suitability for canning. Some smooth cooktop manufacturers say do not can on them, while others who say it is okay still put stipulations on the diameter of the canner compared to the diameter of the burner. Boiling water or pressure canners may not be available that meet the maximum diameter pot they allow. There are several issues:

1. There can be damage to the cooktop from the excessive heat that reflects back down on the surface, especially if the canners used are too large of a diameter than is intended for the burner being used. The damage can range from discoloration of white tops to actual burner damage to cracking of the glass tops to fusion of the metal to the glass top. 

And by the way, even if a manufacturer says a burner/cooktop can be used for canning, people should also be aware the scratching can occur if the aluminum canner is slid or pulled across the cooktop. This often happens with large, heavy filled canners, so people need to be careful.

2. Many of these cooktops have automatic cut-offs on their burners when heat gets excessive. If that option is built in, and the burner under a canner shuts off during the process time, then the product will be underprocessed and cannot be salvaged as a canned food. The process time must be continuous at the intended temperature, or microorganisms may survive. Also, if the pressure drops quickly, most likely liquid and maybe even food will be lost from the jar (it will spill over from the area of higher pressure inside the jar to the lower pressure now in the canner around the jar).

3. Even if boiling water canning is approved by the manufacturer, it may be necessary to fashion your own canner out of a flat-bottomed stockpot with a bottom rack inserted. Many canners do not have flat enough bottoms to work well on a smooth cooktop to be able to maintain a full boil over the tops of the jars. The pot used as a canner must also be large enough to have lots of water boiling freely around the jars, and at least 1 inch over the tops of jars. If the canner is too small, then it starts boiling faster than expected and the total required heat the jars receive in the canner even before the process time begins can be too short.

4.  Some manufacturers of pressure canners do not recommend using them on a smooth cooktop. Follow the advice of your canner manufacturer.

Our recommendation, therefore, is to contact or consult information from the manufacturer of your smooth cooktop and your pressure canner, if interested in pressure canning, before making your decision to can (or not) on it. They are the recommended sources of this information and may also have up-to-date alternatives or suggestions for equipment that you can use. We also caution that you might have to be sure they understand how large your boiling water or pressure canner is, how long it must be heated at high heat, how long the hot canner may stay on the burner until it cools after the process time, and that the canner is made from aluminum (if it is). 

Reviewed February 27, 2018
National Center for Home Food Preservation

Newsflash

Canning in Pressure Cookers

What are the process times for canning in my pressure cooker?

USDA does not have recommended processes for canning in a small pressure cooker. The recommendation for using USDA pressure processes for low-acid foods is to use a canner that holds at least four (4) quart-size jars standing upright on the rack, with the lid in place.  The research for USDA pressure processes for vegetable and meat products was conducted in pressure canners that are most similar to today's 16-quart or larger pressure canners.

Pressure cookers have less metal, are smaller in diameter, and will use less water than pressure canners. The result is that the time it takes a canner to come up to processing pressure (that is, the come-up time) and the time it takes the canner to cool naturally down to 0 pounds pressure at the end of the process (known as the cool-down time) will be less than for the standard pressure canner. The come-up and cool-down times are part of the total processing heat that was used to establish USDA process times for low-acid foods. If the heat from the come-up and cool-down periods is reduced because these times are shortened, then the heat from the process time at pressure alone may not be enough to destroy targeted microorganisms for safety.  That is, the food may end up underprocessed.  Underprocessed low-acid canned foods are unsafe and can result in foodborne illness, including botulism poisoning, if consumed.

During earlier years of canning research, pressure saucepans were considered an alternative for home canning and it was thought that adding 10 minutes to the process times for standard canners would keep food safe. That proved not to be the case for a general, across-the-board recommendation, as there are several sizes of pressure saucepans and they were not all adequately tested.  In addition, the way heat transfers (penetrates) through food during the process is affected partly by the composition of the food and not all foods and styles of preparation were tested.  Later research published in journals has not resulted in an absolute recommendation either. Therefore, in the late 1980s the USDA published its recommendation to not use pressure saucepans (small cookers) for home canning. 

Some manufacturers may offer process directions for smaller pressure cookers.  Consumers using this equipment will need to discuss processing recommendations with those manufacturers; the USDA and National Center for Home Food Preservation recommendation is to not use them for canning with our processes.  

To be considered a pressure canner for USDA processes, the canner must be able to hold at least four quart-size jars, standing upright on the canner rack, with the lid in place.  It is also important to realize the canner should have a way to follow recommended venting procedures to remove air from inside the canner before it is pressurized, and to indicate that the canner remains at least at the target pressure throughout the entire process time. (Also see: Using Pressure Canners)

We cannot convert processes intended for use with regular pressure canners to ensure safety when canning in other types of equipment.

September 2015
National Center for Home Food Preservation

Newsflash

Canning in Electric Multi-Cookers

Should I can in my electric multi-cooker appliance?

Even if there are instructions for pressure canning in the manufacturer’s directions, we do not support the use of the USDA canning processes in the electric, multi-cooker appliances now containing "canning" or "steam canning" buttons on their front panels.  Our pressure process directions have not been developed for that type of appliance, and the canner being used does matter. Our recommendations were determined for stovetop pressure canners which hold four or more quart-size jars standing upright. 

We do not know if proper thermal process development work has been done in order to justify the canning advice that is distributed with these pressure multi-cooker appliances. What we do know is that our canning processes are not recommended for use in electric pressure multi-cookers at this time. 

Some of the major reasons we cannot recommend using electric multi-cookers for pressure canning:  

1.  Thermal process canning work relates the temperatures in the jars to the temperature inside the canner throughout the processing. No USDA thermal process work has been done with jars inside an electric pressure cooker, tracking the actual temperatures inside the jars throughout the process.  It is ultimately the temperature and heat distribution inside the jars that matters for the destruction of microorganism in the food product. The position of jars in the canner and flow of steam around them also impacts the temperature in the jars.  For example, there would be expected differences in jars piled together on their sides from those standing upright on the canner base.

2.  What matters is temperature, not pressure.  One manufacturer says its cooker reaches the pressure required for canning, but that alone does not prove the food in the jars is heated throughout at the same rate as in the canner used for process development. A manufacturer should do process development work to document temperatures throughout the unit at a given pressure and throughout the whole process time.  Just producing an interior pressure is not sufficient data for canning recommendations. For example, if air is mixed in the steam, the temperature is lower than the same pressure of pure steam.   That’s why a proper venting process is so important in pressure canning – to obtain a pure steam environment inside the canner. Also, one has to know how to make adjustments in pressure readings at higher elevations.  The same pressure and process time combination cannot be used at all elevations.

3.  In order to ensure the safety of the final product, the temperature in the canner must stay at minimum throughout the process time. Do power surges or drops with an electric canner cause the temperature to drop too low?  How will you the user know if that happens with your cooker?

4.  One of the big concerns is that the USDA low-acid pressure process times rely on a combination of heat from the time the canner is coming to pressure, during the actual process time, and then during the early stages of cooling the canner and jars.  Even after the heat is turned off under the canner, at the end of the recommended process time, the food remains at high enough temperatures for another period of time that can still contribute to killing of bacteria. This retained heat while the canner has to cool naturally to 0 pounds pressure before opening is used to advantage in calculating the total sterilizing value of the process to preserve some food quality. If anything is done to shorten the cooling period, including using a very small cooker, then the food could cool down more quickly, and be under-processed.  (That is why we recommend using only pressure cookers that hold four or more quart-size jars.) Bacteria are not killed in the food only during the process time; the time it takes the canner to come up to pressure, the process time, and the cool-down time all matter.  There is no way at this point in time to know exactly the percentage of contribution from cooling for each of the canning recommendations.

Please note: This statement about electric cookers does NOT include the Ball Automatic Home Canner for acid foods only, which is electric, but (1) is not a "multi-cooker", but a dedicated canner, (2) comes with its own instructions and pre-set canning options for specific food preparations, and (3) has had proper thermal process development done to support the recommendations with it. Jarden Home Brands also sells an electric boiling water canner, but it is not a pressurized appliance and for canning purposes operates similar to a traditional boiling water canner.  Directions from the manufacturer for this Ball canner, as well as for the Weck non-pressurized electric boiling water canners, should be followed to get them assembled and for managing temperature settings to achieve a boiling process.

For more information about canning in pressure cookers, please read NewsFlash: Canning in Pressure Cookers.

February 1, 2019
National Center for Home Food Preservation

Newsflash

Canning Homemade Soups

Can I can my favorite soup recipe at home?

Canning soup at home is an excellent way to preserve your vegetables with or without small portions of meats or seafood. The key to canning a safe, high quality soup is to follow directions provided by a reliable science-based source like USDA or partners in the Cooperative Extension System. 

Vegetable-based soups are usually mixtures of low-acid ingredients and they need to be pressure canned by a process that has been developed by research methods known to control for botulism food poisoning; we will not recommend any way to can vegetable or vegetable-meat soups in a boiling water canner.  Botulism is a potentially fatal foodborne disease.  Spores of the organism (Clostridium botulinum) that causes botulism can survive normal cooking temperatures and times.  The extra heat in pressure canning is needed to actually destroy the spores so when the closed jar sits at room temperature in storage, the spores will not grow out to cells that then produce the deadly botulinal toxin. The conditions in the sealed jar at room temperature are favorable for this organism to cause problems (moist, low in acid with a pH above 4.6 and very low in oxygen). 

There is only one version of pressure canning directions for home canned soups available from USDA and on this website.  Consumers should follow these directions exactly. If additional ingredients or thickening is desired, the soup should be canned as described and those variations should be made when the jar is opened for serving.

The USDA procedure is not an exact recipe; it allows you to have some choice of vegetables, dried beans or peas, meat, poultry, or seafood. It does NOT allow you to include noodles or other pasta, rice, flour, cream, milk or other thickening or dairy ingredients. 

If dried beans or peas are used, they must first be fully rehydrated (for each cup of dried beans or peas add 3 cups of water,  boil 2 minutes, remove from heat, soak 1 hour, heat to boiling, drain).   

Each vegetable should be selected, washed, prepared and cooked as you would for canning a ‘hot pack’ according to USDA or NCHFP directions. If there is not a separate canning recommendation for a vegetable, do not include it.  

Meats recommended for canning should be covered with water and cooked until tender, then cooled and the bones removed.  Next, all the prepared ingredients should be cooked together with hot water, broth or tomatoes, to boiling, and boiled for 5 minutes. Salt can be added to taste, if desired. Do not fully cook the soup before filling jars; the canning process completes the cooking at the same time it eliminates harmful microorganisms. 

A very important step in these procedures is that jars should only be filled halfway with the mixture of solids.  The rest of the jar is filled with the hot liquid leaving 1-inch headspace. 

Process the jars in a pressure canner according to instructions in the table relevant to your elevation, pressure canner type and jar size.

Slightly revised May 1, 2019
National Center for Home Food Preservation

Newsflash

Using Atmospheric Steam Canners

Can acid foods be processed in steam canners?

The University of Wisconsin, under the leadership of Dr. Barbara Ingham, has conducted research on appropriate use of atmospheric steam canners for home canning in collaboration with the National Center for Home Food Preservation (NCHFP).  Atmospheric steam canners are used for processing naturally acid or properly acidified foods with natural or equilibrated pH values of 4.6 or below. They are not pressurized vessels used for processing for low-acid foods. 

Sufficient studies and peer review have been completed that we are now able to say that as long as certain critical controls at various steps in the canning process are achieved, USDA and NCHFP process times for canning acid or properly acidified foods (pH of 4.6 or below) at home with properly research based recipes and procedures can be used. The research looked at temperature distribution in the steam environment surrounding the jars in a dome-style steam canner, heating patterns of several different food types during processing in the canner, and the contribution of standardized cooling procedures at the end of the process time.

Some of the key controls in addition to the acidity of the food product are knowing that the canner has had the air vented out of the steam before processing begins, and that the pure steam is at the temperature of boiling water at the start and during processing.  Jars must be preheated before filling with food and cooling prior to processing must be minimized.  Processing times must be adjusted for elevation, and must also be 45 minutes or less, including any elevation modification.  The processing time is limited by the amount of water the canner base will hold, and the canner cannot be opened to add water or for any reason at any time during the process.  Finally, cooling of jars must take place in still, ambient air without any forced, more rapid cooling. The slow cooling of processed jars is important to the overall food safety of the whole canning procedure.

Dr. Ingham provides further instructions and details about carrying out canning in an atmospheric steam canner using USDA acid food processing recommendations.

Please also see this update.

The results of this research were published in a peer-reviewed scientific journal in May 2015.
Willmore, P, Etzel, M, Andress, E. and Ingham, B. (2015). Home processing of acid foods in atmospheric steam and boiling water canners. Food Protection Trends, Vol 35, No. 3 (May-June), p.150–160.

August 24, 2020
National Center for Home Food Preservation

Newsflash

Acidifying Tomatoes When Canning

Why do I have to add acid when canning tomatoes in the pressure canner?

Tomatoes that are acidified for canning are done so to prevent botulism poisoning and other bacterial concerns by a combination of acid and heat; the control in vegetables, meat and other naturally low-acid foods is by heat alone.

The bacteria that cause botulism poisoning can grow and produce toxin in sealed jars of moist food at room temperature if the pH (measure of acidity) is above 4.6.  Vegetables, meat, fish, etc. are naturally fairly high above pH 4.6 (close to 6.0) and so pressure processes were developed for those to kill the heat-resistant spores of C. botulinum bacteria that are likely to be contaminating them.

Tomatoes also can have a natural pH above 4.6 (at least up to 4.8).  But rather than develop a pressure-only process as if they were all low-acid, since they are so close to 4.6, USDA decided instead to recommend a small amount of acid be added so they can be treated as a food with a pH less than 4.6 for home canning.  Therefore they are suitable for boiling water canning when the acid is added.  (The commercial industry often also adds citric acid to tomatoes to be able to give them a less severe heat treatment than would be needed for botulism and other bacterial controls.)

When you see the tomato product recommendations in USDA canning directions that offer both boiling water and pressure canning options, those pressure processes are still only the same amount of heat treatment as the boiling water option.  (Higher temperature=shorter process time.)  Those pressure processes are not the amount of heat and time that would be required for canning a low-acid food to control for botulism.  There has not been a properly researched process for pressure canning of low-acid tomatoes without added acid, so the available process times still require the addition of acid as if they are being processed in boiling water.

Another example of how an acid food has both a boiling water and pressure process available is canned peaches.  Peaches (in pint jars) can be canned for 20 minutes in boiling water or 10 minutes at 5 pounds pressure in weighted gauge canner.  That pressure process is not a botulism control either, just because it is pressure canning.  The two time-temperature combinations are the equivalent amount of heating with regard to killing bacteria.

There are some tomato products in the USDA canning procedures that only have a pressure process listed (for example, tomatoes with okra or zucchini, spaghetti sauces, Mexican tomato sauce, etc.).  If a pressure process is the only listed option, then it is the required processing method and we do not have a boiling water process option available. These products made according to the stated recipes and procedures are low-acid food mixtures.

September 4, 2013
National Center for Home Food Preservation