The Whys of Food Preservation

by Edmund A. Zottola and Isabel D. Wolf[1]

The telephone rings in a county Extension office. A harried voice says, “My garden is growing more than my family can eat, what shall I do with it? How can I keep it from spoiling?”

How many times during the gardening season is this scenario repeated in an Extension office? Too often to count! The answers to these questions are readily available in the many bulletins, folders, and leaflets on food preservation available from county, State, and Federal Extension agencies. The publications tell how to preserve food safely and wholesomely, but do little else to explain why directions must be followed precisely. Let’s take a look at the whys.

To understand food preservation, first consider the sources. Home garden food comes from plants: sources of raw food are living, biological entities, continuing to metabolize after they are harvested. Plants also provide a source of food for micro-organisms which can grow on or in them, spoiling food before it can be eaten. The primary objective of food preservation is to prevent food spoilage by preserving food until it can be used by people.

Historically, food preservation and processing assured a food supply and prevented starvation. This is probably the major reason why food is processed today in many developing countries. In the United States, however, affluence and a plentiful food supply now influence the reasons for food preservation. Today, Americans live many miles from rural areas where food is produced. Consequently, food must be preserved to assure the nonfarm population an adequate supply.

Our people want a food supply that is safe, high in quality and appearance, adequate nutritionally, and reasonably priced. Many consumers try to obtain these food attributes by returning to the “old ways” of growing and preserving food themselves.

To understand food preservation, let’s look at five causes of food spoilage or deterioration (four are biological, the fifth physical or mechanical):

(1) The primary cause of food spoilage in the United States is microbiological. Micro-organisms are small living organisms such as yeast, molds, or bacteria. They are the chief causes of microbial spoilage.

Related to microbiological spoilage of food and also a concern in food preservation is microbiological food-borne disease. There are two types. Salmonellosis is an example of a food infection where food may not support growth of the micro-organisms but merely serves to transfer it from the source to the human host. In the second type, the micro-organism grows in the food and produces a poison or toxin which when eaten, causes illness symptoms. Staphylococcal food poisoning is the most common of the second type in the US.

Severity of the major types of food-borne disease in the United States varies from the finality of botulism to the mild discomforts of Clostridium perfringens food poisoning. Food preservation techniques, followed precisely, prevent food-borne disease.

(2) The second cause of food spoilage is vermin such as rodents, rats, mice and insects that attack the food and eat or contaminate it before humans can use it. These vermin ruin millions of pounds of food each year.

WHAT ELSE CAN WE DO WITH TOMATOES?!!

(3) Have you noticed how an apple, left at room temperature, eventually gets soft, wrinkles, and dries out? This spoilage is called senescence: an aging process caused by continued respiration of the apple, eventually making it useless as food. Other foods also spoil this way.

(4) Related to senescence is chemical deterioration of food. The development of rancid flavor in high fat-containing foods is a chemical reaction which brings about an undesirable change. Loss of color or bleaching and loss of vitamins, while food is stored, are chemical deteriorations that can be controlled with proper preservation methods.

Both senescence and chemical deterioration are conveyed by organic compounds called enzymes. These enzymes are produced by all living organisms and their function is to speed up or cause the metabolic reaction necessary for the organism’s continued existence. The enzymes will continue to act after the plant is harvested, and bring about deterioration of the food unless controlled or destroyed. Preservation methods have been developed to control or destroy these organic catalysts.

(5) The last cause of food spoilage concerns food handling. Physical or mechanical damage to the food causes bruising, crushing, cutting, and wilting or water loss. These mechanical defects, besides detracting from the food’s appearance, allow easier entry of micro-organisms, insects, and other vermin to cause spoilage and aging.

Food preservation processes have been developed to slow down, prevent, or stop completely these processes of food spoilage.

An inherent part of food preservation is the package containing the food before or after processing.

Packaging provides a convenient method of handling food, prevents contamination during and after processing, bars vermin infestation, supplies a container for storage, and is a necessary part of preservation. An example would be a mason jar with proper seal for pickling.

What are the major methods available for home preservation of food? How are they carried out? Why do they prevent spoilage, food-borne disease, and give desired attributes of safety, quality, appearance, nutrition, and economy? The economics of food preservation will be developed in a subsequent chapter. Let’s explore the following available methods for home preservation of food:

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.

Increasing the temperature of food to achieve preservation also results in destruction of the micro-organisms that produce spoilage and disease. Time and temperature regulate this preservation. Theoretically, since food will be stored at temperatures which will allow most microbes to grow, the ideal heat treatment needed to preserve the food would be one that completely sterilizes the food, that is, kills all attendant micro-organisms. To achieve complete sterilization, for example, every particle of food in a jar must reach the required temperature and be held there long enough to destroy all micro-organisms.