STUDY OF ENERGY AND RESOURCE EFFICIENCY OF SUBLIMATION DRYING OF FRUITS AND VEGETABLES

ABSTRACT

This article addresses the practical importance of organizing drying in industry and analyzes the advantages and disadvantages of drying fruits and vegetables in a sublimated way compared to other methods. Also cited are the stages of implementation of the technological process and the norms of selection of raw materials. The result of the study shows that products dried in a sublimation style retain up to 98% of their vitamins in itself.

АННОТАЦИЯ

В данной статье рассматривается практическое значение организации промышленной сушки, а также анализируются преимущества и недостатки сублимационной сушки фруктов и овощей по сравнению с другими методами. Также приведены этапы реализации технологического процесса и критерии выбора сырья. Результат исследования показывает, что продукты, высушенные методом сублимации, сохраняют до 98% своих витаминов.

Keywords: fruit, vegetable, drying technology, sublimation , vacuum, economy, heat transfer.

Ключевые слова: фрукты, овощи, технология сушки, сублимация, вакуум, экономия, теплообмен.

Introduction. Drying is one of the most common methods of canning. Organization of the process of drying fruits and vegetables in the industry is of high importance. Firstly, the organization of high-quality drying of fruits and vegetables serves to increase the production profitability of farms specializing in horticulture and vegetable growing, as well as to increase their competitiveness and further economic development. Because organizing the drying of fruits and vegetables is one of the simplest, low-cost directions in the field of processing agricultural products. Secondly, dried fruits and vegetables are sold at high prices both in the domestic market and in the foreign market . Thirdly, by drying fruits and vegetables, depending on their characteristics, it is possible to store them for a longer period of time, and there is an opportunity to satisfy the needs of the population of our country for these products even during the off-season. Fourthly, in the territory of our republic, during the summer, many fruits fall and die. Organization of drying allows to prevent these products from spilling and dying. It can be seen that drying fruits and vegetables is a promising direction for farmers and farms [1; 2].

As an example of the advantages of the drying method of processing products, due to the fact that the dried products are several times smaller than their original mass, they can be easily transported by vehicles, and they can be stored for a long time without using freezers. Processing of agricultural products by the drying method creates an opportunity to provide the population of our country with these products at the same rate throughout the year. In this regard, the importance of drying fruits and vegetables in the sublimation method [2].

Research methodology. The article discusses the current state of the technology of sublimation drying of fruits and vegetables and analyzes the advantages and disadvantages compared to other drying methods. Scientific research works of Uzbek and foreign scientists in this field were studied, and specific tasks for creating an energy-saving vacuum sublimation drying device were determined.

Analysis and results. Sublimation is a process in which a substance changes from a solid state to a liquid state directly into a gas (or vapor) state [3].

The process of dehydrating materials under high vacuum while freezing is known as sublimation drying . Under these conditions, the moisture contained in the material is in the form of ice, and then this ice, without changing to a liquid state, directly turns into a gas (or vapor). turns into Sublimation drying is carried out at high vacuum, intermediate and low temperatures with a residual pressure of 133.3 ÷  3.3 Pa. In this method, the total energy and operating costs are much higher than other drying methods (except for dielectric drying), so this drying method is suitable for materials that cannot withstand high temperatures and whose biological properties must be preserved for a long time (meat, fruit, vegetables, medical and pharmaceutical products) are used for drying. In this process, most of the initial characteristics of the product, its shape, aroma, color, taste, and nutritional value are almost unchanged [4]. In the process of freeze-drying, the product is dried by evaporating moisture from the solid state of the product, excluding the liquid phase. At the same time, the molecular structure of the product changes slightly, the dried product is characterized by high porosity, and as a result of drying, the initial properties of the product are quickly restored. Currently, vacuum freeze-drying is widely used. It is used for drying not only expensive and special products, but also widely used food products. For example, Nestlé (Switzerland), the largest producer of baby food and fast food products, uses freezing technology for the production of food products (coffee, milk, cocoa, etc.) [6; 7].

According to Green Peace, freeze drying is the most environmentally friendly method among traditional drying methods. Because this process uses the safest heat sources for the environment (infrared rays, high-frequency currents, etc.) and energy (electric current) [8].

In order to carry out freeze-drying, it is important to strictly observe two factors:

1. The main part of the moisture content of the product should be in the form of ice, its total volume should not be less than 70 % of the product mass.

2. It is necessary to maintain the rules of freeze-drying with the pressure difference between the steam release above the surface of the product and the steam in the chamber.

The technological process of freeze-drying consists of several stages, which are carried out in a strict order:

1. Preparation of products in advance.

2. Freezing.

3. Direct drying.

4. Packaging of products.

The drying method, product storage period and method depend on the chemical composition of the product. It takes into account the protein, carbohydrate, fat, macro and microelements, and vitamins contained in the product. Products cannot be heated after drying, otherwise it may undergo denaturation. All products processed by the freezing method must be reliably isolated from contact with the environment . Otherwise, the fats and vitamins in it begin to oxidize. It is also important to remember the percentage of products that are quantitatively reduced under the influence of heat. Based on the described factors, the possible drying temperature is determined. In other words, the approximate amount of residual moisture is calculated. During the preparation of raw materials for freezing processing, a certain bacterial limit of food products is ensured.

Raw materials with certain characteristics must be selected for drying. Food preparation should also be of a high standard. Such a criterion allows the production of a dry product with the required quality indicators.

Criteria for choosing raw materials for freeze drying:

1. The product must taste good.

2. The number of initial components should be estimated. This criterion is necessary to calculate useful substances that remain after freezing and drying and during storage.

3. The size and shape of the product should match the technology used.

4. Lipid components should be oxidized in a minimal order.

5. Before freezing and drying raw materials, it is necessary to clean them from microorganisms.

Preliminary preparation of products for drying means heat treatment and cutting.

The quality of products depends on the speed of freezing. The faster this process occurs, the better biologically valuable elements are preserved. The freezing process can be carried out in a specially designed room and directly in the freezer itself. The last option involves the creation of a vacuum environment and partial evaporation of the liquid.

This method is much easier to implement compared to pre-freezing, but it cannot be used for all product categories. During vacuum freezing, some of the originals are lost. Raw meat and fish, as well as some fruits, juices and purées, are not vacuum-frozen. Dairy products, tea and juices are frozen in the form of porridge. Grinding them is done in the frozen state. For this purpose, the spray method is usually used, which is characterized by its high efficiency. During freezing, granules are formed, which are again divided into thin layers and dried. Freeze drying has its own technological features. During the initial freezing period in the chamber, it is necessary to prevent the product from defrosting before placing it in the drying section.

Products lose up to 70÷90 % liquid during freeze-drying in standard conditions. Further additional drying is carried out under temperature conditions. Permissible temperature limits at each stage are regulated by the technological flow of processing. The characteristics of the product and the drying cycle are the basis for maintaining a certain temperature. Different types of raw materials require specific temperature regimes. Usually, these values vary up to -15⁰ С÷45⁰ C degrees. Due to the high sugar content of fruit juices, they require values reduced to -25⁰ C. When freeze-drying meat and protein-rich products, the raw material is dried at a temperature of -16⁰÷-20⁰ C. About 50 % of moisture is lost during the freezing stage and 60% of the entire drying period is spent.

At the next stage, drying is carried out at a high temperature. At this stage, complete removal of moisture is carried out. In order to maintain the high quality of the product, it is important that these temperature indicators fully comply with the technological requirements. The drying period is also very important. The time allotted for each product is different. Drying time and heat requirements for it vary with the technological process and within certain limits. The useful properties of the product should be preserved as much as possible. The temperature range in the final drying stage is +45⁰÷ +85⁰ C. The period of the stage can be up to 40 % of the total processing period. 30 % of the initial amount of liquid is removed.

When consumed, they are digested and assimilated like a fresh product. Taste and smell properties are completely preserved. After processing, the product becomes porous, which leads to its better adsorption. In the initial stages of storage, products begin to actively absorb oxygen, as a result of which their rapid oxidation may occur. Along with the absorption of oxygen, moisture is also adsorbed. This also lowers the quality of the food product. All processed products must be pressed (densified) before packaging to avoid adsorption. In this case, it is necessary to exclude collision with the external environment. Such processing before packaging increases the efficiency of the subsequent transportation (delivery) of the product due to its smaller size.

Conclusion. In conclusion, it can be said that the total energy and operational costs of sublimation drying are higher than other drying methods (except dielectric drying). Therefore, this drying method is used for drying valuable substances, materials resistant to high temperatures and whose biological properties must be preserved for a long time (meat, fruit, vegetables, medical and pharmaceutical products). However, during this process, the product 98 % retains almost all its vitamins and its appearance does not change. It can be concluded from the data of this analysis that there is a need to create a new energy-saving vacuum-sublimation device in the construction of fruits and vegetables.

The results obtained from the ongoing scientific research work provide an opportunity to solve the following issues:

1) in economic terms - fruit and vegetable growing is one of the main incomes of the population in rural areas, making up the main share of the total agricultural products. It has a direct impact on the development of several branches of industry. Also, this branch occupies the main place in the country's agricultural exports;

2) in the social issue - the effectiveness of the sustainable development of fruit and vegetable farming has a direct impact not only on the living standards of the rural population, but also on increasing the social well-being of the entire population of the country.

As a result, the ultimate goal of these reforms is to provide food security in the country, and it is evident in the positive results of the economic reforms implemented in all processes of the development of the new Uzbekistan, and it reflects the increase in the standard of living of the population.

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