How Pressure Canning Preserves Food

How food naturally spoils.
Once fresh fruits and vegetables have reached the peak of freshness (fully ripe), they begin to deteriorate/spoil and lose their peak (good) quality. This happens with all fresh produce but some perish at a quicker rate than others, especially those that have high water contents. Oxygen and enzymes that are found naturally in fresh fruits and vegetables start to break down the cells and create an environment where microorganisms (bacteria, mould and/or yeast) flourish. As the cells oxidise/react with the oxygen present, moisture leaves the cells. This results in spoiled fresh produce.
How canning preserves food.
To retain the good quality of fresh fruits and vegetables, we preserve them - either by refrigeration or freezing (slowing down or ceasing the rate of decay) or removing the moisture from the cells through the heating process of canning. The canning (bottling/preserving) process removes the oxygen from the cells, destroys the enzymes, kills the microorganisms present and also creates an environment where microorganisms cannot grow (i.e. high acid, low oxygen preserve). By processing your canned food in a boiling water canner or pressure canner, you are also creating a very strong vacuum seal that keeps the jar contents inside and keeps contaminants (i.e. microorganisms) outside. A stronger seal means a longer storage period and a much lower risk of contamination.
How to prevent spoilage in your home canned goods.
  • Use a safe, recommended recipe to ensure adequate levels of acidity in the jar contents
  • Use firm, ripe, fresh fruits and vegetables - not insect damaged, diseased or bruised pieces. Wash well before use to remove contaminants.
  • Peel certain types of fresh produce i.e. potatoes, carrots, onions - vegetables grown in soil naturally have a higher level of microorganisms on the outer skin
  • Remove the oxygen from the cells of fruits and vegetables by heating/pre-cooking before canning (also known as hot packing)
  • Use safe preserving jars or canning jars - discard old lids, use pre-boiled jars and jars suitable for pressure canning
  • Process for the correct time listed in the recipe for pressure canning (low acid foods). Do not adjust the time - too short a time period results in jar contents not heating through adequately; too long a period results in over processing
  • Store jars correctly in a cool, dark and dry place. Storing jars in a higher temperatures or in direct sunlight causes them to oxidise for example.

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