University of Vermont
Department of Plant and Soil Science

Summer News Article


Dr. Leonard Perry, Horticulture Professor
University of Vermont

This is the time of year gardens and fruit plants produce their bounty, with much of these available too at local farm stands and markets.  By storing some of this fresh produce now, you’ll be able to enjoy it in the coming months.  Whether you freeze or can fruits and vegetables, here are some rules, including some from North Dakota State University (, that you should follow to save these safely.
Terms you’ll see used include freezing, which is obvious.  Canning originally referred to storage in cans, but now primarily canning jars (that you can buy at most grocery, hardware, and home stores in summer) are used.  These are sealed either in a pressure canner, or hot water bath, depending on food.
If you have old cookbooks, including those classic church cookbooks, they often contain outdated and unsafe canning recipes.  In 1994, and then again in 2006, the USDA revised canning guidelines.  Make sure you’re following these, or current ones from companies such as Ball ( or Presto (
Follow recommended practices from the USDA, not the many you find online from bloggers.  Just because a practice hasn’t killed someone yet or made them sick, doesn’t mean it is safe or won’t affect you.
Just boiling jars of canned vegetables may not be enough to make the contents safe to consume.  The well-known food-borne illness botulism can grow in sealed cans or jars of low-acid foods, and grows in the absence of oxygen.  Even boiling at 212 degrees will not kill this organism, which is why pressure canners are needed to raise the temperature to 240 to 250 degrees, for 20 to 100 minutes.  The time needed will depend on the product, container, and other factors. 
So are pressure canners needed for all foods to be safe?  Not necessarily, as it is the food acidity that determines how to process them.  Low acid foods such as most vegetables, meats, poultry, seafood, soups, and mixtures of acidic and low-acid foods should be canned under pressure at higher temperatures. For high-acid food such as tomatoes, most fruits, jams, jellies and pickles, a boiling water bath can be used.  Even though pickles are quite acidic, they still need a hot-water bath at minimum to be safe.  Some molds, yeasts, and bacteria can live in acidic liquids, so need to be killed by heat.
So, if higher temperatures are needed, what about using an oven?  This, too, can be unsafe for low-acid foods, as are microwaves, and pressure cookers (the latter use less water than pressure canners, and less pressure).  Hot air is less effective at heating liquids thoroughly.  Boiling water or a pressure canner drives off oxygen, too, which helps keep foods from spoiling from other organisms than botulism bacteria.
Even though tomatoes are generally highly acidic, which prevents bacteria from growing, acid content can vary with variety and season.  To be safe, especially if using a water bath to can, add two tablespoons of bottled lemon juice per quart, or one-half teaspoon of citric acid.
If freezing vegetables, they should be “blanched”, or boiled briefly, prior to freezing to stop the enzymes that make vegetables keep ripening.  Even if they don’t spoil, unblanched vegetables may have undesirable flavors, textures, and color changes.  Just boil until they are barely cooked and still quite tender, then submerge in a pot of water and ice to cool quickly. Blanching examples are one and a half minutes for tender greens (not lettuce), two minutes for cut carrots, three minutes for green beans, and four minutes for corn kernels. While a few vegetables such as peppers were said to not need blanching in the past, the USDA now recommends this even for them.
When freezing, make sure to use containers or bags made, and labeled, for this purpose.  Regular plastic bags and containers, or ones such as held margarine or whipped cream, usually result in water loss and “freezer burn”.  This drying out is a quality, not a safety, issue.
When canning, make sure to use jars made, and labeled, for this purpose, such as the Mason-type jars with tight-fitting lids (not the old-fashioned ones with glass lids).  Using recycled jars, such as those from mayonnaise, can result in poor seals, contamination, or breakage.
In the past, you may have seen or received home-made jams and jellies sealed on top merely with paraffin wax.  This doesn’t provide a sufficient seal from contamination.  Instead, use two-piece self-sealing lids made for canning jars. Use fingertips to tighten the screw lids prior to canning, to ensure a good seal.  You can reuse the screw bands, but not the actual flat lids.
More details on how to preserve food safely, including the methods and USDA guidelines, can be found from the National Center for Home Food Preservation (  This site even has links to their book on this topic, free webinars, and a free online food preservation course.    

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