MSU Extension Food Preservation

What does “research-based” or “research-tested” really mean? Imagine researchers in white lab coats working over hot stoves, filling jar after jar with test recipe jams and salsas. It may sound funny, but canning recipes really are developed in lab-like situations set up like a home kitchen with researchers at the helm. The product needs to taste delicious but also be safe to eat.

When a new recipe is being developed, one of the main traits being studied is the pH of the product. pH is a measure of the acidity or alkalinity of a substance. Lemons, tomatoes and berries are considered acid foods and have a pH of less than 4.6 (the scale goes from 1 to 14, 7 being neutral). Foods like squash, beans and meat are low acid foods with a pH greater than 4.6.

The reason pH is important is because of the bacteria Clostridium botulinum, which is a common microorganism that produces a deadly toxin known as botulism. In high acid foods, the C. botulinum spores will not germinate and thus cannot produce the toxin. These foods can be canned in boiling water through a process called hot water bath canning.

Low acid foods, however, need a different type of processing due to the possibility of C. botulinum growth. The spores are destroyed at temperatures of 240 degrees Fahrenheit or above for specific periods of time. Since that temperature is above the boiling point (212 F at sea level), pressure canning is the method needed to reduce the likelihood of botulism toxin. Some foods like tomatoes are close to the borderline pH of 4.6, so acid is added to them (in the form of lemon juice or citric acid) so they can safely be canned in boiling water.

What does any of this have to do with using your own recipe or a recipe passed down from your great grandmother or a recipe you find online? Many people have created canning recipes and even published them in popular books throughout the world without ever measuring the pH of their product. Botulism can’t be seen, smelled or tasted, so the best way to prevent food poisoning is to only use recipes that have been rigorously tested by reliable sources and have a known pH.

In addition to pH, the processing time is extremely important to the safety of the finished product. Processing times for canning are based on the density of the product and how long it takes to heat that product to the proper temperature throughout the entire jar. Home canners most likely don’t have access to the high-tech equipment to perform those measurements and thus don’t know with any certainty that their own recipe has been thoroughly processed.

Michigan State University Extension recommends using one of the following research-based resources for home canning recipes:

• Michigan Fresh Fact Sheets
• National Center for Home Food Preservation
• So Easy to Preserve (6th edition)
• USDA Guide to Home Canning

Currently, MSU Extension is offering food preservation classes online. To see when upcoming courses will be available and to read informational articles, visit MSU Extension's Home Food Preservation. If you have a specific food preservation question, call the MSU Extension Food Safety Hotline at 877-643-9882.