Dehydration preservation is one of the oldest and most readily available methods for the long-term preservation of food products, especially fruits and fish, as well as meat and vegetables.
The preservative effect of dehydration is based on the cessation of the activity of microorganisms with a decrease in the moisture content in food products below 15%. Most microorganisms develop normally when the food contains at least 30% water. When canned with dehydration, microorganisms fall into a state of suspended animation, and when the product is moistened, they again gain the ability to develop.
In foods under the influence of drying, a number of structural and chemical changes occur, accompanied by a significant destruction of biological systems such as vitamins and enzymes. Dehydration canning can be done under atmospheric pressure and under vacuum.
Dehydration at atmospheric pressure
Dehydration at atmospheric pressure can be natural and artificial. Natural sun drying is an old way of preserving products used to date. Through natural sun drying, high-quality dry fruits (apricots, raisins) are harvested. A kind of natural drying is drying, by means of which roach and ram, fish and white fish are cooked.
Natural drying - the process is quite lengthy, and therefore dried products can be infected and general contamination. Solar drying is possible only in areas with a large number of sunny days. All this limits the industrial application of natural drying methods on a mass scale.
Artificial drying under atmospheric pressure can be jet, spray and film. The inkjet method is the simplest type of industrial drying. In this case, drying chambers of various systems are used, in which the products to be dried are exposed to a continuous stream of hot air heated in special heaters. Air movement and moisture removal are provided by a system of ventilation devices.
Drying of liquid products - milk, eggs, tomato juice - is carried out by spraying. These products are sprayed through a nozzle into a thin suspension (particle size 5-125 microns) in a special chamber with moving hot air (temperature 90-150 °). The suspension dries instantly and settles in powder form in special receivers.
Spray drying can be carried out in chambers with a rotating disk, to which heated milk is sent by a thin stream. A rapidly rotating disk, under the influence of centrifugal force, sprays the liquid into fine dust, which is dried by the incoming hot air. Despite the high temperature, due to the short duration of action of the spraying method, the dried product undergoes minor changes in its composition and is easily restored.
Known contact, film, drying method, in which drying is done by contact (application) of the dried product (milk) with the heated surface of the rotating drum and the subsequent removal of the dried product (film) using a special scraper (knife). This drying method is characterized by significant structural changes in the dried product, denaturation of its components and less recoverability during its hydration. So, the solubility of powdered milk obtained by the film method is 80-85%, while spray-dried milk is dissolved in the amount of 97-99%.
Vacuum drying is carried out under vacuum at a low temperature not exceeding 50 °. It has several advantages compared to atmospheric drying. In vacuum drying, the natural properties of the dried product are preserved to the greatest degree.
The dried product completely preserves vitamins and natural taste. So, as a result of drying eggs at atmospheric pressure, the destruction of vitamin Α reaches 30-50%, and during vacuum drying, these losses do not exceed 5-7%.
Freeze-drying (lyophilization) is the most modern and promising method of food preservation. This method provides the most perfect drying with maximum preservation of the natural, food, organoleptic and biological properties of the product. A feature of the method is that moisture is removed from frozen products directly from ice crystals, bypassing the liquid phase. This method of drying was called sublimation, or lyophilization.
For the first time, the freeze-drying method was used in 1935 to obtain dry therapeutic serums. Subsequently, this method began to receive dry serum and blood plasma, as well as biological products - endocrine, enzymatic and grafts of various tissues.
Subsequently, the method of freeze-drying was introduced into the production of dry food products. When freeze-drying, vitamins are well preserved, including such an unstable vitamin as ascorbic acid.
In modern sublimation units, the main part is the sublimator, which is a metal, cylindrical shape with spherical disks, a chamber into which dried food products are placed and create a deep vacuum. To condensate water vapor, special condensers are used - freezers cooled by compressor freon or ammonia refrigeration units. The units are equipped with rotary oil vacuum pumps with a gas ballast device. During operation of the installation, the tightness of the sublimator, condenser, all pipelines and parts included in the vacuum system is ensured.
In freeze-drying, three drying periods are distinguished. In the first period, after loading the product to be dried, a high vacuum is created in the sublimator, under the influence of which there is rapid evaporation of moisture from the products and the latter self-freeze. The temperature in the products decreases sharply, reaching -17 ° and below. Self-freezing takes place for 15-25 minutes at a speed of 0.5-1.5 ° per minute. By freezing, 15-18% of moisture is removed from the products.
The remaining amount of moisture (about 8.0%) is removed from the freeze-dried products in the second drying period, which begins when a stable temperature of about -15-20 ° is established in the products. Freeze-drying is carried out in the second period by heating the plates on which the dried products are located. In this case, products frozen in the first period do not thaw, and ice crystals in the product evaporate, bypassing the liquid phase. The duration of the second period depends on the nature of the dried product, its weight, moisture content and ranges from 10 to 20 hours
The third period is a thermal vacuum drying, during which the remaining absorption-bound moisture is removed from the product. In the process of thermal vacuum drying, the temperature of the dried products gradually rises to 45-50 ° at a pressure in the sublimator of 1.5-2.5 mm Hg. The duration of thermal vacuum drying is 3-4 hours. An important property of sublimated products is their easy reversibility, that is, recovery with the addition of water.
The most promising freeze-drying of food products using dielectric heating by high-frequency currents. Currently, food lyophilization is carried out at a number of canneries.
The process of sublimation and dehydration
In the manufacture of long-term food, Wise uses a unique and innovative processing method: our ready-to-eat first, second and meat dishes, as well as vegetables or fruits, can be either sublimated or dehydrated.
When processing, the most important thing is to correctly determine the type of food. Some products are better amenable to dehydration (dehydration) than sublimation, others - on the contrary, better tolerate the lyophilization process.
Thanks to an extensive system of scientific research and testing, we use either the process of sublimation or dehydrogenation, for each type of product separately, given its unique properties and features. Such a hybrid processing method allows you to provide the best taste, texture and nutritional value of the product.
Expensive ingredients, such as fruits and vegetables, retain their taste better through the process of sublimation. Other products intended for long-term storage, such as noodles or rice, are much tastier than dehydrated ones. Despite these differences, both processes provide a long shelf life.
Initially, "instant" products were intended for a poor buyer. As a rule, the manufacturing technology of such products is based on fairly cheap thermal drying: the product was heated to 100-120 degrees and moisture was evaporated from it. With this method, the cell structure is destroyed, the consistency of the product, its taste and aroma change, and nutritional value deteriorates. For example, vitamins remain only 20-30%. Such food is richly saturated with a variety of "chemistry". On the packages you can find the inscriptions: "with the taste of beef", "with the taste of chicken" and the like, but not "with chicken" or "with beef." Be sure to add various flavors, preservatives, binders.
Product Sublimation - a completely different method, and, accordingly, sublimated products are completely different. Sublimation (from lat. Sublimo - “I lift”) means the process of transition of a substance from solid to gaseous state, bypassing the liquid stage. In the production of freeze-dried foods, evaporation of moisture from a quick-frozen product bypasses the liquid phase. Sublimation technology includes two main stages: freezing and drying. During vacuum freeze-drying, moisture is removed from the product by evaporation of ice.
Product dehydration process
During the dehydration process, food is placed in a dry chamber where moisture is removed from the food by air blowing. All this happens at low temperatures, since food should in no case be heated so that the nutritional properties remain unchanged. This is a completely different process, which has nothing to do with thermal drying and looks more like sublimation.