Process of preservation by removal of water

Removal of water and the consequent lowering of water activity constitutes an important principle of food preservations. The removal of water is usually done by drying dehydration, or evaporation.

Although the term are sometimes interchangeably, drying normally refer to the sun drying, or air drying, evaporation refers to drying by the application of artificial heat but under natural draft, and dehydration implies drying by controlling application of application heat with mechanical circulation of air or under a vacuum.

Drying of food involves complete removal of water under controlled conditions in such a way that the food is not altered and results in minimum changes by the drying process. Dried foods contain moisture to the extend of 1-5 per cent and they have storage stability at room temperature of a year or longer.

The term dehydration refers generally to artificial drying under control conditions. However, in modern food processing the term does not refer to all processes that remove water from foods.

Evaporation is the removal by vaporization of part of the solvent form a solution or dispersion of essentially non-volatile solutes. Evaporation is distinguished from crystallization and drying by the fact that the final product is a concentrated dispersion or solution rather than precipitate solids.
Process of preservation by removal of water

Quick freezing of food

The two basic ways to achieve the freezing of foods are quick and slow freezing. The technical features of the quick freezing process are ultra rapid freezing at very low temperatures (-30 to -40 ° C) designed to bring the inside the product to a temperature of -18°C as quickly as possible. Rapidly frozen foods have a much longer storage life than foods stored in cold stores.

The products that are generally preserved by quick freezing are meat, fish, some vegetables such as peas, spinach, cauliflower, carrots, beans etc.

When the quick freezing process is used, these liquids solidify to form extremely fine crystals of ice, and the cellular structure is left intact, whereas, in the ordinary freezing process, where low temperatures are reaching more slowly, the texture of the product is altered.

In quick freezing, large quantity of food can be frozen in a short period of time.

Quick freezing is a method of increasing the shelf-life of perishable foods by subjecting them to conditions of temperature low enough to inhibit the oxidative, enzymatic and microbial changes, which are responsible for the changes in flavor and color of foods.

Quick freezing is ideal for producing good quality frozen foods. The food freezes immediately and keeps its shape and appearance.

Quick freezing is obtained by one of the following methods:
*Freezing Tunnel or Blast Freezer
*By Direct Contact with Metal Plate Surfaces maintained at Low Temperatures
*By immersion in (or by Spraying)  a Low Temperature Liquid
Quick freezing of food

Pretreatment before Process of Drying

Pretreatment before Process of Drying
The preservation of foods by drying is probably the oldest food preservation process practiced by humans. It is believed that many foods, especially grains and fruits of high sugar content, were preserved by primitive peoples by allowing them to dry in the sun. Spices and fish, cut into thin strips, were also preserved in this manner.

There are number of different methods of drying foods for preservation. The most important are sun drying, tunnel or cabinet drying, drum drying, spray drying and freeze drying.

Foods to be dried must be washed, and some peeled and cut. Others may be pre cooked. Cut fruits are subjected to darkening through enzymes action and must be either blanched or treated with salts or sulfur dioxide. However if treated with sulfur dioxide, the product must be so labeled, because of known allergenic reactions in some consumers. Certain vegetables may be pretreated in the same manner. Sulfuring may also be required to limit non enzymatic browning (the Maillard reactions). Browning refers to the development of brown color.

Various dried eggs products are also subject to browning and also susceptible to the development of off flavors. In this case, the reaction involves a combination of small amount of glucose, which is naturally present, with proteins. Because of this, dried eggs products, especially egg whites, may be treated with glucose oxidase and catalase. The glucose oxidase converts glucose to gluconic acid (which does not combine with amino groups) and hydrogen peroxide. The purpose of the catalase is to convert the undesirable peroxide to water and oxygen. The elimination of glucose may also be done by natural fermentation using microbes. However this process can be considered unsanitary and in order to avoid product spoilage and even food poisoning, it is necessary to hold the product at 130 degree F for several hours after drying.
Pretreatment before Process of Drying