ANALYSIS OF HEAT TREATMENT OF FOOD PRODUCTS

ABSTRACT

Food processing is important. Heat treatment for them is especially relevant and important. In this article, the theoretical basis of heat treatment of food products is widely explained.

АННОТАЦИЯ

Обработка пищевых продуктов имеет важное значение. Термическая обработка для них особенно актуальна и важна. В данной статье широко изложены теоретические основы термической обработки пищевых продуктов.

Keywords: food products, heat treatment, pasteurization, heat exchanger.

Ключевые слова: продукты питания, термическая обработка, пастеризация, теплообменник.

There are two main types of heat treatment of food products, they are pasteurization and sterilization. Heat treatment of food products is carried out in order to reduce or destroy microbiological activity and enzymes, and as a result of physical and chemical changes, the product conforms to standard responses. In the manufacturing industry, food products are heat treated using various methods. [1]

The main purpose of heating is to reduce the activity of enzymes in this product, mainly in fruits and vegetables. Heating is not a process used for long-term storage of products, but is used in industry as a step before freezing, drying and canning.

The following are additional features of heating:

• prevention of microbes and pollution on the surface of the product;
• enables softening and packaging of products in their natural condition;
• in the release of intercellular air before canning and packaging;

In particular, heating is used in cases where freezing and dehydration do not give sufficient results in reducing enzyme activity. In the process of preservation of products, the activity of enzymes is sufficiently reduced.  Enzymes are proteins that are denatured at high temperature, and their activity decreases or stops under the influence of high temperature. Heating is carried out at a temperature of up to 100 ͦ C, under low atmospheric pressure [1]. Various constructions are used for heating processes. The advantages of this type of heat treatment are that the process is carried out quickly, there is a variety of heating, heat is well mixed with the product, and the process is carried out in a short time. Ordinary microwave ovens are also used in households to heat food products. The advantage of this type of heating equipment is that it can be heated continuously, but it has the disadvantage of being expensive. One of the heaters widely used in industry is steam heating. In the industry, such heaters are called steam heaters. With the help of this type of heaters, high efficiency is achieved due to the flat cross-section surface. Usually, steam heaters are made by adjusting the speed of rotation of the food product in a belt-like mesh or rotating cylinders. During heating using steam, the surface of the product heats up faster than the inner part, which can cause a change in the quality of the product. Therefore, it is necessary to pay great attention to the temperature and time during heating using steam.

Individual rapid heating is carried out in two stages. In the first stage, a thin layer of the product is sufficiently heated, and in the second stage, the inner part of the product is heated. Usually, in the second stage, the product is kept at a certain temperature for a while. At the end of the process, the product is cooled. We can see the individual rapid heating process in the following picture. In the 1st loading hopper, the product is sent to the 2nd heating hopper, then the product is held in the 3rd holding hopper. At the last stage, it is cooled in the 4th cooling chamber. [2]

Figure 1. Individual rapid heating scheme

1-loading hopper; 2-heating chamber; 3-holding chamber; 4- cooling chamber

As an example, in a single rapid heating device, a 10 mm thick cut carrot product is heated in a heating hopper for 25 seconds, held for 50 seconds. The total duration of the procedure is 4 minutes. When heating small fruits and vegetables, 0.5-1 kg of steam is used for 6-7 kg of product.

Heating with hot water. This type of heating method is heated to a temperature of 70-100 ͦ C using heated water. Using this method, the product is heated in slowly rotating drums. The rotation speed of the drum is fast or slow, depending on the type of product and the heating time. Heating devices of this type are also made in 3 stages. But in this type of process there is no holding phase. In the first stage, a partial product is heated as a pre-heating process. In the second stage, the main process is carried out, that is, the product is heated to the maximum specific temperature. The last stage is the cooling stage.

Pasteurization. The process of pasteurization mainly involves heating liquid, flow able products to 100 ͦ C the process of heating to temperature is considered to be the process of pasteurization, which is the exact control point (CCP) of the international food safety (HACCP) system. That is, pasteurization is carried out in order to ensure that the product is safe. The main goal of the pasteurization process is to destroy the activity of enzymes in food, to neutralize harmful bacteria and disease-causing microorganisms. There are 4 main methods of pasteurization [3]. In most cases, after the pasteurization process, the shelf life of products can be extended from several days to several months. The shelf life of the product depends on the pH environment of the product. Activity of microorganisms in the product increases in a low acidity environment (pH<4.5), so the acidity environment (pH>4.5-5.0) is normalized as a result of pasteurization. The duration of the pasteurization process and the temperature level are determined depending on the physical and chemical properties of the product. Below you can see the duration of pasteurization of milk depending on the temperature.

The above temperature and the duration of the process are considered sufficient to destroy disease-causing microorganisms in milk. As we mentioned above, there are several different methods of pasteurization due to the ratio of pasteurization to temperature and time.

Batch (pot) pasteurization. This type of pasteurization is also called low-temperature long-term pasteurization. Using this method, the product is based on pasteurizing the product in a pot by heating the outer wall surface of a simple pot-shaped device. Batch pasteurization is carried out for 30 minutes to a temperature of 63-71 ͦ C. Batch pasteurization is not widely used nowadays. We can see the general view of batch pasteurization through the following picture.

Figure 2. Batch (pot) pasteurization

Short time pasteurization at high temperature. In this type of pasteurization, milk is pasteurized at a temperature of 72 ͦ C for 15 seconds.

The following components are required for the above processes:

• Plate heat exchangers
• Holding pipe (at certain temperature)
• Storage tanks (for raw and processed products)
• Balance tank (to adjust the heat exchanger)
• Management and monitoring system (to see the results)

Sterilization. (UHT-ultra high temperature) sterilization is carried out at a high temperature for a short time at 135 ͦ C for 1-4 seconds The sterilization process is the main critical point in industrial and food production. During sterilization, all living organisms, including spore-forming bacteria, are completely killed. There are two different methods of sterilizing food, the first is sterilization using heating, and the second is sterilization without heat. In the food industry, liquid and liquid products are sterilized using heat. During sterilization, all microorganisms are destroyed, but vitamins are preserved.

Theory of heat dissipation. Heat transfer between two bodies is called heat dissipation. Heat flow always goes from a hot body to a cold body. Heat transfer occurs rapidly when the temperature difference is large. As the temperature difference decreases during the heat transfer process, the heat transfer process slows down and the process stops when the temperature equalizes.

Thermal conductivity can pass in 3 ways:

• conductive
• convective
• radiation

Figure 3. Types of heat transfer

• Conductive heat transfer is observed when heat is transferred to each other through solid bodies.
• Convective heat transfer occurs only in gases and liquids.
• Heat conduction through radiation occurs with the help of radiation rays [4].

Theoretical foundations of heat transfer. Convective and conductive heat transfer are used in the processes of heat treatment of dairy products. Heat conduction is carried out by two principles, i.e. directly and indirectly.

Direct heating. In direct heating, heating is mixed with the product.

• to heat water. Water vapor is sprayed directly into the water and convection and conduction take place.
• cheese products are heated directly using hot water or its steam.

Indirect heating. In most cases, indirect heating is used in heat treatment processes for dairy products. In indirect heating, a heat-conducting surface or object is placed between the heater and the product. During heating, the heat flow always moves from the hot surface to the cold surface. Figure 4 shows the flow of heat [5].

Figure 4. The movement of heat flow

Let's assume that hot water is the heating medium and milk is the moving product from the cold side. We use a plate heat exchanger as an intermediate surface. First, the heating agent, i.e. hot water, heats the surface of the plate, and then the plate uses its own heat to heat the cold milk. In this way, according to the laws of heat conduction, cold milk is heated using hot water. There is a boundary layer on each plate of the plate heat exchanger, and the velocity of the liquid becomes almost 0 when it meets the boundary layer. The speed of the fluid flow increases as it moves away from the boundary layer and reaches the highest speed in the center of the channel.

Similarly, the speed of hot water flow has the highest value in the center of the channel. The hot water that is closer to the intermediate body, i.e. the plate, heats the milk more and cools down faster than the other part of the channel. In the boundary layer, heat is transferred in a convective and conductive manner. When heating the surface of the plate wall from the boundary layer, conductive heat transfer takes place. The phenomenon of heat transfer of liquids in the center of the channel occurs through convection.

Heat exchanger. During the pasteurization of liquid food products, various heat treatment devices are used for their heat treatment. Heat exchangers use an indirect method of heat transfer. In general, heat exchangers consist of two channels. That is, a channel for the flow of heat and a channel for the product. The flow of hot water (red) moves in one channel and the flow of milk to be heated (blue) in another channel is in contact with each other. Heat is transferred through heat surfaces. The temperature of the incoming hot water flow enters in the state t_i2 and leaves it in the state t_o2 after transferring its own heat. As a product, milk has a pre-heating temperature of t_i1 at the entry point and exits in a heated state of t_o1. We can see the temperature change in Figure 5 [5].

Figure 5. Temperature changes in the heat exchanger

References:

1. https://safefood360.com/free-resources/whitepapers/preview/thermal-processing-of-food/
2. Lee, F.A. The Blanching Process. In Advances in Food Research; Academic Press: NY, 1958;
3. P.K. Mandal., A.K. Biswas. ANIMAL PRODUCTS TECHNOLOGY; Stadium press India.
4. John H. Lienhard. A heat transfers textbook-fifth edition; USA Massachusetts. Phlogiston press -2019
5. Gosta Byland. Tetra pack Dairy processing HANDBOOK. Sweden. Lund. Tetra pack processing Systems publishing 1995