EXPERIMENTAL STUDY OF FREEZE DRYING OF STRAWBERRY FRUITS

ABSTRACT

The purpose of this work is to study the process of freeze drying of strawberry fruits grown in Uzbekistan. Experimental studies were conducted in the laboratory of Islam Karimov Tashkent State Technical University. In the course of the study, the optimal parameters of freeze drying of strawberries using the FD-04H unit were determined. The process included three main stages: initial freezing at -40 °C, sublimation at heating to 50 °C in vacuum (pressure 10-30 Pa) and final drying at 0 °C. From the results obtained, it follows that the initial product moisture content of 90% decreased exponentially, reaching 5% at the final drying stage.

АННОТАЦИЯ

Целью данной работы является изучение процесса сублимационной сушки плодов земляники, выращенных в условиях Узбекистана. Экспериментальные исследования проводились в лаборатории Ташкентского государственного технического университета имени Ислама Каримова. В ходе исследования были определены оптимальные параметры сублимационной сушки клубники с использованием установки FD-04H. Процесс включал три основные стадии: начальное замораживание при температуре -40 °С, сублимацию при нагревании до 50 °С в вакууме (давление 10-30 Па) и окончательную сушку при 0 °С. Из полученных результатов следует, что начальная влажность продукта 90% снижалась по экспоненте, достигая 5% на финальной стадии сушки.

Keywords: freeze drying, temperature, humidity, drying time, nutritional value.

Ключевые слова: сублимационная сушка, температура, влажность, время сушки, питательная ценность.

Introduction. Strawberries are one of the most sought-after fruits in the world market due to their high nutritional value and the presence of bioactive compounds important for human health. Studies show that strawberries are rich in polyphenols, anthocyanins and phenolic acids, which have powerful antioxidant properties and protect body cells from oxidative stress. These compounds also have anti-inflammatory effects, reduce the risk of cardiovascular diseases and slow down the aging process of cells [1].

Strawberries are a natural source of micronutrients and vitamins such as vitamin C, copper, potassium, phosphorus, magnesium and manganese, which play an important role in strengthening bones, improving blood circulation and supporting normal nervous system function. In addition, strawberries are rich in dietary fibre and vitamin B6, which help improve digestion and boost metabolism. The presence of omega-3 fatty acids has a positive effect on heart and vascular health (Fig. 1) [2].

Figure 1. Strawberry fruit and its structure

High nutritional value and a rich composition of biologically active substances make strawberries a popular product of functional nutrition. In particular, it is widely used in the production of natural dietary products, sports nutrition and pharmaceuticals. To preserve the maximum amount of useful substances and increase the shelf life of strawberries, various drying methods are used, in particular freeze drying. This method allows preserving the physicochemical properties of strawberries, turning them into a concentrated product with a high content of bioactive components [3].

Thus, strawberries are not only a popular fruit with a pleasant taste, but also a product with high nutritional value, which makes them in demand in the food, nutritional and pharmaceutical industries, contributing to the promotion of human health [4].

Materials and Methods.  For experimental studies we used freeze drying equipment model FD-04H, which is available in the laboratory “Processes and apparatuses of processing of agricultural and food products” of Tashkent state technical university (Fig. 2) [5].

Figure 2. Experimental sublimation equipment model FD-04H

Sublimation drying consists of three main steps:

1. Freezing stage. In the first stage, the product is cooled to a temperature of -35 °C to -40 °C. During this period, the process of moisture crystallisation begins, during which ice nuclei are formed and then crystal growth occurs. As a result, the water inside the product is completely frozen.

2. Primary drying stage. In this stage, most of the moisture is removed by sublimation of the ice. To do this, the drying chamber is depressurised and the temperature is gradually increased. This process removes about 90 per cent of the moisture. One of the most important parameters is the pressure in the chamber. The lower the pressure, the more efficient the sublimation process. During the experiment, the pressure was maintained in the range of 10-30 Pa.

3. Secondary drying stage. Secondary drying aims at removing the bound moisture that remains after primary drying. The process is carried out at near room temperature and low pressure. After secondary drying is completed, the final moisture content of the product is reduced to 5 %.

The optimum freeze drying parameters chosen in this study are as follows:

- freezing temperature: -35 °C to -40 °C;

- drying temperature: 35 °C to 50 °C;

- vacuum chamber pressure: 10-30 Pa;

- process duration: 10-48 hours.

Scientific methods applied:

1. Temperature control: regulation system to maintain a stable temperature regime;

2. Vacuum control: use of infrared sensors to monitor pressure (10-30 Pa);

3. Gravimetric moisture analysis: determination of residual moisture in the product after each step;

4. Differential thermal analysis: observation of temperature and moisture content changes over time.

Results and discussion. The presented drying curve illustrates the nature of the moisture content change of the material during the freeze drying process, which consists of three main stages: freezing, primary and secondary drying (Fig. 3).

During the freezing stage (0-1 hour), the material is cooled to -40 °C with little change in moisture content, which remains at 90 per cent. This stage is necessary for the moisture to change to a solid state (ice), which is a prerequisite for successful sublimation.

Primary drying (1-7 hours) is accompanied by intensive moisture removal due to sublimation of ice at reduced pressure (30 Pa) and temperature of 50 °C. During this period, there is a dramatic reduction in moisture content, from 90% to about 15%. This is the most energy-intensive stage of the process.

Secondary drying (7-10 hours) removes residual, bound moisture. The rate of moisture reduction decreases and by the end of the process it reaches the required level of 5%.

Figure 3. Curve of sublimation drying of strawberries on the FD-04H equipment

Conclusion. Freeze drying is a three-step process involving freezing at -40 °C, primary drying at 50 °C and 30 Pa, and secondary drying. During 10 hours of drying, the moisture content of the product is reduced from 90 % to 5 %, with the main moisture removal (up to ~15 %) occurring during the 6 hours of primary drying due to ice sublimation. In secondary drying, the bound moisture is removed within 3 hours. This temperature-vacuum treatment ensures the preservation of the porous structure and biologically active components. The method is particularly effective for thermolabile substances and is widely used in the pharmaceutical, biotechnology and food industries.

References:

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