Top Food Preservation Questions

One of the first workshops I presented for UNH Cooperative Extension was on home food preservation in 1988. Since that time, I have answered hundreds of questions on home food preservation and presented many workshops. The following are some of the top questions asked and the answers.

Why do I have to pressure can low-acid vegetables such as green beans, carrots and beets? Can’t I just process them in a boiling water canner for several hours? The practice of not pressure canning low-acid vegetables can be deadly. Improperly home-canned green beans, carrots and potatoes have sickened several people and killed a few with botulism poisoning in the past few years. One such outbreak, which sickened 25 people and caused one death, was from improperly home canned potatoes made into potato salad for a church potluck.

The reason to pressure can is the difference in temperature between a boiling water canner and a pressure canner. Boiling water canners don’t go above 212° Fahrenheit. Low-acid vegetables and meat need to reach 240°F to destroy botulinum spores. The only way to achieve this temperature is by processing in a sealed pressure canner that reaches 240°F for the recommended processing time.

Why do I have to process the jars of food such as jams, pickles and tomatoes anyway? Since the late 1980’s, Cooperative Extension has been teaching that open-kettle canning is not safe. Open kettle canning is the process of heating food to boiling, pouring it into a canning jar, screwing on a lid, setting the jar on the counter and letting the heat of the jar “seal” the lid to the jar. This process is not safe because the jars don’t reach the right temperature for the correct amount of time to destroy spoilage organisms, yeasts and molds that can enter a jar while getting the product ready for processing nor do the jars form a good vacuum seal.

It can be dangerous to not process tomatoes and tomato products in a pressure or boiling water canner. The acid in tomatoes and tomato products may be low enough to allow bacterial growth. In addition to processing, tomatoes need to be acidified. Tomatoes that are acidified for canning are done so to prevent botulism poisoning and other bacterial concerns by a combination of acid and heat. To ensure the safety of whole, quartered, crushed or juice tomatoes acidify by adding 1 tablespoon of lemon juice or ¼ teaspoon of citric acid per pint. For quarts use 2 tablespoons of lemon juice or ½ teaspoon citric acid.

I have a delicious homemade salsa recipe I would like to preserve and give as gifts. Can you tell me how long to process it? The recommendation is to use current, tested recipes for all canned foods. The heat processing method required for a safe canned product is determined by the pH or acidity of the specific food being canned. Salsas are a combination of acid ingredients (tomatoes), and low acid ingredients (peppers, onions and garlic). The amount of ingredients and the addition of other acid ingredients, like vinegar or lemon juice, affect the pH of salsa. USDA uses special equipment to scientifically test each recipe to determine the processing method and time. Since your recipe has not been tested for canning and the scientific testing equipment is not available for home testing, processing methods and times cannot be recommended for your recipe.

The same consistent message about home canning has been given by UNH Cooperative Extension for a very long time -- follow the latest USDA guidelines and use tested recipes from reputable sources such as any Cooperative Extension office nationwide, USDA Complete Guide to Home Canning, National Center for Home Food Preservation, and the Ball® Blue Book.

For answers to your home food preservation, contact UNH Cooperative Extension’s Education Center and Information Line at 1-877-398-4769 or email The Center is open Monday – Friday from 9:00 am - 2:00 pm.


Where trade names such as Ball® are used, no discrimination is intended and no endorsement is implied by the University of New Hampshire Cooperative Extension.


Ann Hamilton
Food Safety Field Specialist
Full Extension Field Spec
Phone: (603) 447-3834
Office: Cooperative Extension, Taylor Hall, Durham, NH 03824