The major method used for home preservation of food is temperature control. This includes canning with a pressure canner or a boiling water bath, blanching food before freezing, refrigerating food, and freezing it. Micro-organisms which cause disease and food spoilage are sensitive to environment temperature variations. By increasing the food’s temperature, micro-organisms are destroyed. When the temperature is decreased, their growth is inhibited.

Let’s look at the temperature scale illustration and see how temperature affects micro-organisms. To prevent growth of micro-organisms in food and subsequent microbial spoilage, food must be kept out of the temperature range that allows growth. This is most commonly achieved by refrigerating or freezing. Refrigeration slows down or stops microbial spoilage. Freezing stops it completely.

EFFECT OF TEMPERATURE ON MICRO-ORGANISMS

Degrees Fahrenheit
225 spore-forming bacteria killed
220-250 proper canning temperatures (low acid foods)
212 water boils; pressure canner needed to obtain temperatures above boiling
170-210 non-spore-forming bacteria, yeasts, and molds killed
120-140 some bacteria can grow
98.6 body temperature
70-80 room temperatures may fall in this range
60-120 DANGER ZONE
most bacteria, including food-poisoning types, grow rapidly over this temperature range
32 water freezes
30-60 cold-tolerant bacteria, yeasts and molds grow
0 no microbial growth

Enzymatic activity, while slowed down by freezing, is not stopped in many vegetables and these enzymes must be destroyed by blanching the vegetables before freezing to prevent enzymatic deterioration. Successful preservation by freezing must deactivate any enzymes that might be in the food as well as rapidly lower the food temperature to below freezing to stop microbial activity.

Freezer burn, a common problem with frozen foods, comes from improper packaging. Food moisture is lost in freezer burn, which results in undesirable flavor and texture changes. Freezer burn can be controlled by proper packaging, proper storing temperature, and avoiding long-term storage.

Refrigeration or storage above freezing, but below room temperature, preserves food for days and sometimes weeks. Refrigerated storage slows down activities of enzymes in the food and reduces metabolism of the contaminating micro-organisms.

Preservation of food by reduced temperature, refrigeration, or freezing is achieved because enzyme activity and microbial deterioration are slowed down or stopped.