Home Canning Magazine


Food Preservation - Science

Food is preserved by treating it to destroy spoilage factors or to create conditions in which they cannot grow. Methods available to accomplish this are: drying, canning and freezing.

Temperature

Home canning involves heating jars of food to temperatures high enough to destroy microorganisms and enzymes that may cause spoilage. In home canning, this step is known as heat processing. Processing not only destroys spoilage organisms but also drives air out of the jars. As jars of food cool, the contents of the jar contracts. Because air has been forced out of the jar, a vacuum forms, pulling the lid down onto the mouth of the jar and resulting in the formation of an airtight vacuum seal. This airtight seal keeps both air and microorganisms from re-entering the jar.

Once spoilage microorganisms are destroyed and the airtight seal has formed, jars of food processed by the appropriate method for the appropriate time can be stored at room temperature indefinitely.

Time & Temperature

Each food has a specific density which affects the rate at which heat penetrates to the centre of the jar. This heat penetration factor together with the size of jar affect the total amount of time and heat that must be applied during processing.

As the jar volume increases and/or the food density increases, processing time must also increase to reach the critical temperature in the centre of the jar. As temperature increases over 100°C, processing time decreases.

For example, crushed tomatoes--with the addition of acid like lemon juice--can be processed at 100°C in a boiling water canner for 35 minutes. In a pressure canner, which reaches a temperature of 116°C at 10 lbs (68 kPa) of pressure, the required processing time decreases to 15 minutes.

Salt & Sugar

High concentrations of salt and sugar inhibit the growth of some spoilage microorganisms, however, some microorganisms are salt- or sugar-tolerant. Neither salt nor sugar alone are practical methods of food preservation. Salt or sugar concentrations high enough to inhibit spoilage would render the foods unpalatable. Salt and sugar resistant spoilage microorganisms must be destroyed by heat processing which also eliminates yeasts and molds that can cause surface spoilage.

pH - Acidity

Acidity is very important. The level of acidity determines the processing method. Acidity level is not the amount of acid present as a percentage, but the acid strength. This is measured on a pH scale. The pH scale uses seven as the neutral point: pH values below seven are acidic; those above seven are alkaline.

On a pH scale, the approximate growth limits of molds, yeast and bacteria are shown. The approximate acid strengths of some common foods are shown on the chart below.

High acid foods (pH values less than 4.6) can be safely processed in a boiling water canner. Low acid foods (pH values greater than 4.6) require processing in a pressure canner to destroy harmful bacteria.

Summary

Processing method and processing time are affected by:

  1. microorganisms present in the raw food
  2. acidity (pH) of the food
  3. the concentration of sugar and salt.

To ensure all spoilage factors are destroyed, follow recommended processing methods and times for each food as found in credible resources like the BERNARDIN Guide to Home Preserving. Remember to check the seal on jars before storing them in a cool, dark, dry place. Sealed lids curve downward.



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