DEPENDENCE OF AIR HUMIDITY AND TEMPERATURE DURING APRICOTS DRYING

ABSTRACT

This article discusses the issues of drying apricot fruits, as well as the infusion of temperature and air humidity during the drying process. Humidity of dried apricot ranges from 8% to 25%, which ensures their long-term storage. Drying not only lengthens the shelf life, but also ensures a high preservation of the quality and nutrients of the raw materials. In addition, during drying, the weight of the product is reduced by 75-80%, which significantly reduces the cost of transportation.

АННОТАЦИЯ

В данной статье рассмотрены вопросы сушки плодов абрикоса, а тек же вливание температуры и влажности воздуха в процессе сушки. Влажность сушенных плодов абрикоса должно составляет 8 - 25%, что обеспечивает длительное их хранение. Сушка не только увеличивает сроки хранения, но и обеспечивает высокую сохраняемость качества и питательных веществ абрикоса. Кроме этого, при сушке масса продукта уменьшается на 75-80%, что значительно сокращает затраты при транспортировке.

Keywords: apricot, compressor, pressure release, relative humidity, dew point temperature

Ключевые слова: абрикос, компрессор, сброс давления, относительная влажность, температура точки росы.

Introduction. Preservation of fruits and vegetables by drying is one of the oldest types of canning. Humidity of dried fruits and vegetables ranges from 8 to 25%, which ensures their long-term storage. Drying not only lengthens the shelf life, but also ensures a high preservation of the quality and nutrients of the raw materials. In addition, during drying, the weight of the product has reduced by 75-80%, which significantly reduces the cost of transportation. There are various drying methods, but a necessary requirement for all methods is to provide a high quality dried product at the lowest production cost.

In the Republic of Uzbekistan, agricultural production is increasing every year, for example, in January-September 2021, 2070.7 thousand tons of fruits and berries, 1313.7 thousand tons of grapes were harvested [1].  However, at present, only 20% of these products are industrially processed. Therefore, the development and implementation of new drying methods that provide high quality of the final product at low cost is relevant.

In this paper, the problem of rational drying of fruits (using the example of apricot) has considered using methods of influencing the product with high pressure with its abrupt release: analysis of the dependence of pressure and drying temperature in the zone of instantaneous pressure release.

Analytical review of literature. The scientific and technical literature presents an analysis of the combined thermo mechanical method of drying materials: heating a wet material under pressure, followed by its discharge. Thus, in [2], a review of studies on this topic has given, where it said that dehydration due to the thermal energy accumulated by the material first used in the process of drying by “relieving” pressure in the works of V.V. Yagov and others. The physical basis of this method is to maximize the effect of the intense molar vapor transfer that occurs after preheating the wet material under pressure and then rapidly reducing it. The value of the initial pressure determines the depth of heat treatment of the material. In other works, positive experimental results have obtained on the application of this method for drying wood [3] and dispersed materials (zeolite, lignin, and peat) [4]. Thus, in [4], the use of a combined drying method for zeolite using microwave heating made it possible to reduce the drying time by 1.5 times compared to the convective method at the same temperature regime. In [5], a method for instant drying of fruits was implemented, including preheating the fruits, depressurizing the drying hopper, repeating the stage, followed by vacuum drying, until the mass moisture content of fruits and vegetables reaches 10–25%. However, existing pressure relief drying technologies cannot be used for our local raw materials. Therefore, the determination of rational regime parameters for drying with pressure release, designed for local raw materials, is relevant.

The aim of this work is to study the dependence of pressure and temperature during the process of drying apricot halves using instantaneous pressure release.

In accordance with the purpose of the study, the following tasks were set:

• study of the effect of exposure and instantaneous pressure release of air as a drying agent during processing;
• study of the dependence of temperature and pressure drying with instantaneous pressure relief.

Material and methods of performed research. The object of the study was a local variety of apricot (Subhoni). The fruits of this variety contain up to 25% sugar (sucrose, fructose and glucose). Malic and citric acids mainly represent organic acids. Rich in mineral salts, microelements, organic acids, pectin substances, various vitamins: A, C, B1, B2, PP. The initial humidity is 80-85%.  Apricot is a dried fruit crop and is widely used in drying [6]. Apricots (dried apricot fruit with a stone), kaisu (a dried whole apricot fruit without a stone), dried apricots (half apricot slices) are prepared from the fruits. For our case, dried apricots were obtained with a final moisture content of 18-20%. Before drying, the apricots were washed, cut into two halves with separation of the stones, and the material was processed in a citric acid solution.

To study the combined drying method using pressure relief and infrared (IR) heating, a laboratory setup was created (Fig. 1) [7,12].

Figure 1. Scheme of the experimental laboratory device:

1 - compressor; 2 - automatic data collection system using a personal computer; 3 – sealed chamber for thermo mechanical processing of product samples; 4 - IR emitters; 5 – window for feeding a pallet with product samples; 6-pallet with product; 7 - receiver

The basis of the installation is a sealed special chamber in which the studied material samples had placed. The pallet for the material is a container with a mesh bottom (mesh size 6x6 mm, wire diameter 0.6 mm), a sample of apricot halves with a layer thickness of 8-10 mm was placed in the pallet. The pallet has placed on corner supports. IR generators have placed above the pallet to heat the material. The chamber has a quartz glass window. The chamber is equipped with a compressed air supply system with automatic maintenance of the set pressure. To carry out the removal of moisture by the method of pressure relief, the unit has a special valve and receiver. The installation allows heating the sample under pressure up to 0.8 MPa and releasing the pressure. The sample has heated using sources of infrared radiation with a power of 1 kW each. The plant is equipped with a system for monitoring and automatically collecting data on indicators of the heating process and pressure relief. The main indicators that allow you to control and manage the process are temperature, moisture content of the material and pressure in the chamber. The raw material is first processed in a sealed chamber with pulsed infrared radiation and pressure with instantaneous pressure release, and then subjected to conventional convective drying in a drying chamber.

The moisture content of the material was determined according to the State Standard 33977-2016 (Interstate standard. Fruit and vegetable processing products. Methods for determining the total solids content. Fruit and vegetable products. Methods for determination of total solid content).

Research results and discussion. A number of theoretical and experimental studies have carried out to solve the set tasks. The physical basis of the proposed fruit drying method is the concept, according to which, by increasing the pressure and heat treatment, it is possible to change the temperature of the material, which, in turn, creates the possibility of maximizing the effect of intense molar vapor transfer, which ensures the transition of moisture to a free state. Because of this, the boundary between bound and free moisture shifts to the region of lower moisture content when the pressure has released. Now of pressure release, a rapid movement of moisture occurs throughout the entire volume of the product that a pressure drop has created between the center and the surface of the processed material, which contributes to the formation of a moisture flow directed towards the surface of the particle in the form of steam. On its way, the vapor-air mixture entrains liquid droplets and pushes towards the surface [7, 8]. It is of great importance to take into account the parameters of the air in the method of preliminary treatment of the product during the drying process that we offer. Also, air and product temperature play a big role in the processing of apricots under the influence of pressure. After pressurization in the working chamber, the product and air temperature must be controlled to prevent condensation of water vapor in the air. The researches were carried out at different air temperatures above 25⁰C. Saturated vapor pressure is determined in the temperature range from 0 to 100⁰C using Antoine's formula:

                                          (1)

The relative humidity of moist air is denoted by x (kg/kg) or (g/kg). Air humidity has determined by this ratio:

                                            (2)

Partial pressure of water vapor:

                                                          (3)

The dew point temperature in the working chamber is determined by the following formula:

                                                 (4)

Dew point temperature is controlled by formula (4).

Figure 2. Graph of dependence of dew point temperature on relative humidity and temperature

Conclusion. After releasing the pressure in the device, it is not possible to apply pressure again, because after releasing the pressure, the temperature in the chamber decreases and the temperature in the chamber decreases due to water vapor moving from the surface of the product. When the pressure is applied to the working chamber, the water vapor in the air changes to the state of dew due to the decrease in temperature, and the product being dried is moistened. The product is under pressure and the pressure drop affects the structural part of the product. Swelling occurs on the surface of the product. As the internal energy of the product increases, swelling occurs as a result of the movement of water vapor in the pore environment to the external environment.

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