Контрольная работа: The Food preservation

The rate at which food is frozen is also a factor, primarily because of aesthetic reasons. The more slowly food is frozen, the larger the ice crystals that are formed. Large ice crystals have the tendency to cause rupture of cells and the destruction of texture in meats, fish, vegetables, and fruits. In order to deal with this problem, the technique of quick-freezing has been developed. In quick-freezing, a food is cooled to or below its freezing point as quickly as possible. The product thus obtained, when thawed, tends to have a firm, more natural texture than is the case with most slow-frozen foods.

About a half dozen methods for the freezing of foods have been developed. One, described as the plate, or contact, freezing technique, was invented by the American inventor Charles Birdseye in 1929. In this method, food to be frozen is placed on a refrigerated plate and cooled to a temperature less than its freezing point. Or, the food may be placed between two parallel refrigerated plates and frozen.

Another technique for freezing foods is by immersion in very cold liquids. At one time, sodium chloride brine solutions were widely used for this purpose. A 10% brine solution, for example, has a freezing point of about 21ºF (-6ºC), well within the desired freezing range for many foods. More recently, liquid nitrogen has been used for immersion freezing. The temperature of liquid nitrogen is about -320ºF (-195ºC), so that foods immersed in this substance freeze very quickly.

As with most methods of food preservation, freezing works better with some foods than with others. Fish, meat, poultry, and citrus fruit juices (such as frozen orange juice concentrate) are among the foods most commonly preserved by this method.

Fermentation

Fermentation is a naturally occurring chemical reaction by which a natural food is converted into another form by pathogens. It is a process in which food «goes bad», but results in the formation of an edible product. Perhaps the best example of such a food is cheese. Fresh milk does not remain in edible condition for a very long period of time. Its pH is such that harmful pathogens begin to grow in it very rapidly. Early humans discovered, however, that the spoilage of milk can be controlled in such a way as to produce a new product, cheese.

Bread is another food product made by the process of fermentation. Flour, water, sugar, milk, and other raw materials are mixed together with yeasts and then baked. The addition of yeasts brings about the fermentation of sugar present in the mixture, resulting in the formation of a product that will remain edible much longer than will the original raw materials used in the bread-making process.

Thermal processes

The term «thermal» refers to processes involving heat. Heating food is an affective way of preserving it because the great majority of harmful pathogens are killed at temperatures close to the boiling point of water. In this respect, heating foods is a form of food preservation comparable to that of freezing but much superior to it in its effectiveness. A preliminary step in many other forms of food preservation, especially forms that make use of packaging, is to heat the foods to temperatures sufficiently high to destroy pathogens.

In many cases, foods are actually cooked prior to their being packaged and stored. In other cases, cooking is neither appropriate nor necessary. The most familiar example of the latter situation is pasteurization. During the 1860s, the French bacteriologist Louis Pasteur discovered that pathogens in foods can be destroyed by heating those foods to a certain minimum temperature. The process was particularly appealing for the preservation of milk since preserving milk by boiling is not a practical approach. Conventional methods of pasteurization called for the heating of milk to a temperature between 145 and 149ºF (63 and 65ºC) for a period of about 30 minutes, and then cooling it to room temperature. In a more recent revision of that process, milk can also be «flash-pasteurized» by raising its temperature to about 160ºF (71ºC) for a minimum of 15 seconds, with equally successful results. A process known as ultrahigh-pasteurization uses even higher temperatures of the order – of 194 to 266ºF (90 to 130ºC) – for periods of a second or more.

Packaging

One of the most common methods for preserving foods today is to enclose them in a sterile container. The term «canning» refers to this method although the specific container can be glass, plastic, or some other material as well as a metal can. The basic principle behind canning is that a food is sterilized, usually by heating, and then placed within an air-tight container. In the absence of air, no new pathogens can gain access to the sterilized food.

In most canning operations, the food to be packaged is first prepared in some way – cleaned, peeled, sliced, chopped, or treated in some other way – and then placed directly into the container. The container is then placed in hot water or some other environment where its temperature is raised above the boiling point of water for some period of time. This heating process achieves two goals at once. First, it kills the vast majority of pathogens that may be present in the container. Second, it forces out most of the air above the food in the container.

After heating has been completed, the top of the container is sealed. In home canning procedures, one way of sealing the (usually glass) container is to place a layer of melted paraffin directly on top of the food. As the paraffin cools, it forms a tight solid seal on top of the food. Instead of or in addition to the paraffin seal, the container is also sealed with a metal screw top containing a rubber gasket. The first glass jar designed for this type of home canning operation, the Mason jar, was patented in 1858.

The commercial packaging of foods frequently makes use of tin, aluminum, or other kinds of metallic cans. The technology for this kind of canning was first developed in the mid‑1800s, when individual workers hand-sealed cans after foods had been cooked within them. At this stage, a single worker could seldom produce more than 100 «canisters» of food a day. With the development of far more efficient canning machines in the late nineteenth century, mass production of canned foods became a reality.

As with home canning, the process of preserving foods in metal cans is very simple in concept. The foods are prepared and the empty cans sterilized. The prepared foods are then added to the sterile metal can, the filled can is heated to a sterilizing temperature, and the cans are then sealed by machine. Modern machines are capable of moving a minimum of 1,000 cans per minute through the sealing operation.

Chemical additives

The majority of food preservation operations used today also employ some kind of chemical additive to reduce spoilage. Of the many dozens of chemical additives available, all are designed either to kill or retard the growth of pathogens or to prevent or retard chemical reactions that result in the oxidation of foods.

Some familiar examples of the former class of food additives are sodium benzoate and benzoic acid; calcium, potassium, sodium sorbate, and sorbic acid; and sodium and potassium sulfite. Examples of the latter class of additives include calcium, sodium ascorbate, and ascorbic acid (vitamin C); butylated hydroxyanisole (BHA); and butylated hydroxytoluene (BHT); lecithin; and sodium and potassium sulfite and sulfur dioxide.

A special class of additives that reduce oxidation is known as the sequestrants. Sequestrants are compounds that «capture» metallic ions, such as those of copper, iron, and nickel, and remove them from contact with foods. The removal of these ions helps preserve foods because in their free state they increase the rate at which oxidation of foods takes place. Some examples of sequestrants used as food preservatives are ethylenediamine-tetraacetic acid (EDTA), citric acid, sorbitol, and tartaric acid.

Irradiation

The lethal effect of radiation on pathogens has been known for many years. Since the 1950s, research in the United States has been directed at the use of this technique for preserving certain kinds of food. The radiation used for food preservation is normally gamma radiation from radioactive isotopes or machine-generated x rays or electron beams. One of the first applications of radiation for food preservation was in the treatment of various kinds of herbs and spices, an application approved by the U.S. Food Administration (FFA) in 1983. In 1985, the FFA extended its approval to the use of radiation for the treatment of pork as a means of destroying the pathogens that cause trichinosis. Expects predict that the ease and efficiency of food preservation by means of radiation will develop considerably in the future.

That future is somewhat clouded, however, by fears expressed by some scientists and members of the general public about the dangers that irradiated foods may have for humans.

Сохранение продовольствия

Термин сохранение продовольствия, относится к любому из множества методов, которые препятствуют порче продуктов. Сохранение продовольствия включает в себя методы, такие как консервирование, соление, сушка и замораживание при температуре ниже нуля градусов, радиацию, пастеризацию, копчение, и добавление химических улучшителей. Сохранение продовольствия стало важным компонентом пищевой промышленности.

Научные принципы

Огромное количество примеров порчи продовольствия может быть вызвано одной из двух главных причин: (1) разъедание патогенами (болезнетворными микроорганизмами), такими как бактерии или плесень или (2) окисление, которое причиняет деструкцию эфирных биохимических составов и / или деструкцию животных и растительных клеток. Различные методы, которые были изобретены, для сохранения пищевых продуктов, разработаны для того, чтобы редуцировать или устранить первую или вторую (или обе) причины.

Например, простой и обычный способ сохранения продовольствия это нагревание при некоторой минимальной температуре. Этот процесс предотвращает или замедляет порчу, потому что высокие температуры уничтожают или инактивируют большинство видов патогенов. Добавление смесей, известных как Б.О.А. (бутилированный оксианизол) и Б.О.Т. (бутилированный окситолуол) к пищевым продуктам в других различных случаях так же предотвращает их порчу. Эти смеси известны своими антиоксидантными свойствами, предотвращающими химические реакции, которые вызывают окисление продуктов в результате их порчи.

Исторические методы сохранения продовольствия