University of Vermont
Department of Plant and Soil Science
SAVING FOODS SAFELY
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
(www.ag.ndsu/pubs/preservation.html), that you should follow to save these
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often contain outdated and unsafe canning recipes.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
(www.freshpreserving.com) or Presto (www.gopresto.com).
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
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
(www.uga.edu/nchfp). This site even has links to their book on this
topic, free webinars, and a free online food preservation course.
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