Basic Methods of Processing Food

John R. Matchett.

Enormous quantities of food are lost each year through attack by living organisms that are too small to be seen by the naked eye. The organisms, bacteria, yeasts, and molds, seem to be everywhere. They can use a great variety of materials for food if moisture is present. They can live and grow in a fairly wide range of temperatures.

In present-day technology we rely on low temperatures to preserve frozen foods and on low moisture content to protect dried foods. But we generally sterilize foods to be preserved some other way and pack them so that living organisms cannot get inside the package. The process is the modern adaptation of the discovery of the Frenchman Nicholas (or Nicolas) Appert less than 150 years ago. Appert learned that food placed in a bottle and then heated would keep if it were tightly stoppered while hot. That he should have achieved success with the crude containers of that day is astonishing, surrounded as he was by the world of microscopic enemies, which even today we have not learned to control completely.

Several interesting problems have been solved in the course of the development of preserving foods by heat treatment. Of these, the most basic ones have to do with the relative merits of various foods as media of growth for bacteria, yeasts, and mold, and the resistance of the organisms to destruction by heating. An important distinction may first be drawn on the basis of the acidity of the products being preserved by heat. Few of the bacteria dangerous to health grow in acid surroundings; all are easier to destroy by heat in an acid environment. Thus, canning of most fruits is relatively simple and can be accomplished by merely boiling in water.

The canning of vegetable and meat products is not so simple, because generally they lack the acidity that sensitizes bacteria, yeasts, and molds to destruction by heat. Then, too, meat and vegetables constitute a good material for the organisms to grow in.

Meat and vegetables then must be heated at temperatures higher than the boiling point of water. That is done by placing the food, sealed in the final container, in a steel tank strong enough to withstand steam pressures that will give temperatures of 240 to 250 F.

The most difficult problem in processing vegetables and other nonacid foods is the assured destruction of the bacterium known as Clostridium botulinum. Under favorable conditions it produces a virulent toxin. The toxin is quite easily destroyed by heat. The bacterium itself is harmless enough. It is the toxin that does the damage. Unfortunately, the bacterium has the faculty of retreating when it encounters adverse conditions into a dormant form a spore. These spores are resistant to heat. When conditions become favorable, they change back into the active (vegetative) form, ready to pour out toxin. The bacterium will not grow in the presence of air, nor will it grow in acid surroundings, hence it constitutes a health hazard only when it survives its competitors that cause souring. For this reason, food-processing schedules that destroy the spores with ample factors of safety have long since been adopted and well enforced.

In some ways the most interesting of the organisms that vex food processors are the thermophiles (Greek, therme, heat; philos, loving) , because they may survive the processing temperatures and grow rapidly while the canned goods are cooling, producing a condition in the can known as flat souring, that is, the can does not show swelling, but the contents have a sour taste. None of these bacteria produces disease.

These heat-loving bacteria, along with others like the brightly colored micro-organisms of hot springs, must live through some sort of biochemical system involving enzymes and other proteins much different from those that function at ordinary temperatures. Micro-organisms whose chemical activity sustains life at ordinary temperatures would be killed in an environment most favorable to growth of the thermophiles.

BUT MICROBIAL ACTIVITY in foods and beverages is not always undesirable. Organisms that produce undesirable changes in some foods are responsible for desirable changes in others. The fermentation industries ( pickle, cheese, and sauerkraut making and olive processing) and the farmer who ensiles his crops depend on the chemical changes brought about by yeasts and bacteria. Thus, fermented milks have been prepared for ages and were used by nomadic tribes that could not keep milk in its fresh condition. Through observation over the centuries and through research in more recent times, strains of micro-organisms best adapted to the intended use have been selected. These are used in pure culture in many instances; careful control is exercised to avoid contamination by ever-lurking enemies, which would ruin the food through the unwanted chemical reactions they may bring about or through interference with growth of the desired organism.

A demand for variety and savor in foodstuffs developed early in the course of Western civilization, and trade records indicate that herbs and spices constituted important items of trade. The long caravan route from India and the East witnessed an enormous traffic in spices, along with silk and dyes. Closing of the route in 1453 motivated the search for sea routes that led to the discovery of the New World. Thus. long ago man developed preferences for pleasant flavors and set for himself increasingly rigorous standards of quality that could be met only through sanitary processing of raw material well suited to a particular use.

Man learned to judge food products on the bases of color, flavor, and texture. He found that varieties of plant materials differed substantially with respect to each of these and that similar differences occurred among breeds of livestock. As knowledge of human and animal nutritional requirements accumulated, vitamin and mineral content of foods became of increasing concern. Selective breeding programs on animals and plants undertaken long since have resulted in the highly specialized products of agriculture today, each of which is designed for a specific purpose.

Such considerations provide the ground work for the second basic principle of food processing the selection of the raw material. It goes without saying that the material must be harvested at its optimum maturity and so handled after harvest that its color, flavor, texture, and nutritive value will be unimpaired when processing begins. Material that has deteriorated will not be made better by any manner of treatment. Protection against micro-organisms must be maintained with care.

It is equally necessary to avoid damage that can result from disturbing the enzyme systems in the raw products. That would cause several undesirable changes, such as darkening of cut or bruised fruits, development of off -flavors in certain vegetables, and destruction of vitamin C. Rapid handling and lowering of temperature are practical means of control.

In some instances, however, enzyme activity is necessary, if it can be controlled. Many fruits, for example, cannot be picked and transported when fully ripe. Therefore they are harvested "green" and permitted to ripen (a process brought about by enzymes in the fruit) under favorable conditions at the processing plant. Once the fruit is ripened, immediate processing is necessary to avoid damage by the very mechanism used for ripening. Even some meats frequently are made more tender by their own enzymes, controlled through the action of suitable temperatures.