By Sam Beattie
Food Safety Specialist
Iowa State University Extension

In the first article, Canning Has Changed from Grandma’s Day, we talked about the differences between preserving food products using either a pressure vessel or water bath. The relative acidity of the food is a deciding factor in the choice of which type of vessel to use. Acid foods such as fruits and some pickled products have enough acid to prevent the outgrowth of certain illness-causing bacteria, the most important being botulinum. This article will focus on preservation of fruits by low temperature and pressure canning.

When we think of jams and jellies, we often think of the grape jelly that accompanied peanut butter for PB and J sandwiches. Jelly, jams and preserves are all made from fresh fruit that has been processed to form a firm gel when cooled. The mechanism for the firm gel involves the plant material pectin, which is the glue that sticks plant cells together. It can be liberated from the plant fruit by extensive boiling of a combination of ripe and unripe fruit. However, not all fruit has enough pectin to form a firm gel nor do very ripe fruits, so we often use commercially available pectin from apples or citrus processing.

Normally pectin requires two important ingredients to set a firm gel: sugar and acid naturally present in the fruit. Sugar and acid cause the long pectin molecules to come together to form a net that holds the gel. Too little of either will cause the gel to be weak or worse – to not form at all.  Commercial pectin comes in several different forms including dry powder and liquid.

Both the acid and sugar content of jellies, jams and preserves are the reasons we can use a low temperature preservation technique such as boiling water baths.  Acidified foods such as pickles or fermented products such as sauerkraut also use acid as an important preservation component. Recipes that call for a specific type and amount of vinegar must be followed exactly. Furthermore, any other ingredient amount must not be altered according to the recipe.

Vinegar provides enough acid to acidify all ingredients sufficiently to use a water bath processing method. Food such as sauerkraut and some pickles develop acid as the cabbage or cucumbers ferment. Salt and vinegar that are added at the beginning of the fermentation help beneficial fermentation bacteria get to work and produce more acid. This acid will prevent the growth of botulinum bacteria.

The USDA and others have studied and developed many recipes that are proven to be safe. It is important that you preserve your bounty using these tested recipes. Some additional resources and recipes can be found at:  The National Center for Home Food Preservation – www.uga.edu/nchfp; Ball Blue Books – 1994 current year; “So easy to preserve,” Sixth edition, CES, UGA Press, available through Amazon.com.

This article is from the September 2008 issue of Acreage Living. Other articles in this month’s issue--
Overseeding Lawns
Use Garbage Disposals Sparingly

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