MASS MOISTURE EXCHANGE CHARACTERISTICS OF FRUIT CROPS

ABSTRACT

In this article, the authors experimentally determined rational drying modes for removing moisture from melons and fruit crops - melons "Ak-gulyaba", "Shakar palak", apricots - "Subkhany", "Isfarak", subjected to drying. Experiments were also conducted in the Feutron climate chamber and desorption isotherms of melons and fruit crops were obtained depending on the influencing factors at temperature (t=20-60 ⁰С), relative air humidity (10-70%). The forms of moisture bonding with the product were studied.

АННОТАЦИЯ

В данной статье авторами экспериментальным путем определены рациональные режимы сушки при удалении влаги из бахчевых и плодовых культур – дыни “Ак-гулябы”, “Шакар палак”, абрикоса – “Субханы”, “Исфарак”, подвергаемых сушке. Также проведены эксперименты в климатической камере Feutron и получены изотермы десорбции бахчевых и плодовых культур в зависимости от влияющих факторов при температуре (t=20-60 ⁰С), относительной влажности воздуха (10-70%). Изучены формы связи влаги с продуктом.

Keywords: variety, drying, temperature, desorption, isotherms, moisture, melons and fruit crops, duration, moisture content.

Ключевые слова: сорт, сушка, температура, десорбция, изотермы, влага, бахчевые и плодовые культуры, продолжительность, влагосодержание.

Introduction. One of the priority areas of the economy and providing the country with foreign exchange earnings is agriculture. More than 60% of the entire population of Uzbekistan lives in rural areas, and each household has land plots where fruits are grown, which in the future will be used as raw materials for the production of dried fruits, as well as their subsequent export. It is known that the hot Central Asian climate causes increased accumulation of sugar in fruits, thereby improving their taste.

A significant portion of fruit crops subjected to drying are wet colloidal-capillary-porous systems. Sorption and desorption isotherms were used to determine rational drying modes when removing moisture [1].

The nature of the sorption and desorption isotherms depends on the type of moisture bond with the material. In the hygroscopic region, the moisture bond depends on the structure and properties of the materials.

During the drying process, with changes in drying conditions, environment and temperature, the equilibrium conditions are violated. At the same time, the moisture content gradient at the phase boundary causes the transfer of moisture from inside the fruit to the surface and its evaporation into the environment.

The pattern of moisture transfer during the drying process of fruits is determined by the nature of the interaction of water molecules with their structure. The movement of moisture in fruits is due to the presence of a gradient of transfer potential in them. Consequently, the transfer potential covers the phenomena of heat transfer and moisture movement, and the magnitude of the transfer is linked to the energy of the bond between moisture and the material.

It must be emphasized that any transfer of liquid and vapor in a wet material occurs from a higher to a lower potential.

Materials and methods. The mass-exchange characteristics of fruit and vegetable crops determine the intensity of moisture transfer, and the thermophysical characteristics determine the intensity of heat transfer.

Thus, heat transfer is characterized by a temperature gradient , heat flow  and the coefficient of thermal diffusivity  and the transfer of mass (moisture) is determined by the gradient of the potential of the substance (moisture content)  mass flow (moisture)  mass transfer coefficient (diffusion of moisture .

In the works of a number of scientists [2,3] the hygroscopic properties of various types of fruit and vegetable crops have been examined.

However, at present, the introduction of modern non-traditional methods of processing and drying fruit and vegetable crops into production to obtain new products requires research and mathematical description of the hygroscopic properties of fruits and vegetables. It should be noted that fruit and vegetable crops grown in Central Asia differ in properties and structure from those grown in other regions. In this regard, when exposed to non-traditional processing methods, processes occur that lead to changes in the cellular structure of fruit crops. Therefore, in order to obtain high-quality dried products, as well as for their long-term storage, it is necessary to study the mass-moisture exchange characteristics and hygroscopic properties of the latter [1,4,5,6].

In order to determine the equilibrium moisture content of fruit crops: melon varieties “Ak-gulyaba”, “Shakar palak”, apricot - “Subkhany”, “Isfarak” - experiments were conducted in the “Feutron” climate chamber.

The samples are in an air flow of a certain humidity. The specified concentration of water vapor is provided by sulfuric acid of different concentrations. After equilibrium is reached, the samples are weighed and their moisture content is determined. Equilibrium is considered achieved if the mass of the sample does not change by 0,0005 g. All isotherm points are taken for the same sample. Air humidity was monitored and recorded within the range of 10 to 70%, and the ambient temperature of 20-60 ⁰C in the chamber was regulated automatically. The chamber temperature measurement error was 10 ⁰C. The dew point temperature was maintained at a constant accuracy of 0,5 ⁰C.

Results and discussions. The results of the conducted studies of desorption isotherms of melon and apricot fruits, obtained at different temperatures (t=20-60 ⁰С), relative air humidity (10-70%), are shown in Fig. 1 and 2.

Judging by the graphs of desorption isotherms, the fruits have an “S” shape typical for colloidal - capillary-porous materials.

At present, there is no fully developed theory describing sorption and desorption isotherms, therefore it is impossible to provide an analytical solution to the dependence of the equilibrium moisture content on the relative humidity of the air for colloidal - capillary-porous bodies. In this regard, the equations of sorption and desorption isotherms obtained by mathematical processing of experimental data are of considerable interest.

G.K. Filonenko proposed a mathematical description of sorption and desorption isotherms, which are described by the equations:

;                                (1)

where a and b - are coefficients characterizing the type of material;

K and B - are coefficients determining the dependence of the equilibrium moisture content on the type of material and the ambient temperature in the corresponding sections of the isotherms;

 и - equilibrium parameters at the points of conjugation of the rectilinear and curvilinear sections of the isotherms.

Based on the methodology of G.K. Filonenko and A.I. Chuprin and using the MATLAB program for the mathematical description of the process reflecting the relationship between the equilibrium moisture content of fruits, relative air humidity and temperature, an equation for the desorption isotherm was obtained:

.                                   (2)

The coefficients m and n are determined for each type of product, for example for an apricot fruit , .

In real drying of the material, moisture is not removed by monomolecular adsorption. Therefore, in our study, the isotherms in the transition region from monomolecular to polymolecular adsorption were not studied.

a)

b)

Figure 1. Desorption isotherms of melon fruits depending on the processing method at t= 20 ⁰C:

a) 1 - dried product treated with IR rays; 2 - dried product treated with IR rays and pre-treated with sugar syrup; b) 1 - dried product without IR treatment; 2 - dried product without IR treatment and pre-treated with sugar syrup.

In our studies, the middle section of the curves (Fig. 1a) gives an idea of ​​the samples of melon fruits (treated with IR exposure before drying, curve 2) in the range from 5 to 14%; and for the products treated with IR exposure and pre-soaked in sugar syrup before drying (curve 1) in the range from 5 to 24%, melon fruits were also studied (Fig. 1b), not treated with IR exposure before drying (curve 2) in the range from 5 to 16,5% and samples not treated with IR exposure, but pre-soaked in sugar syrup before drying (curve 1) in the range from 5 to 22% and evidence of the presence of moisture of a polymolecular adsorption nature was obtained.

Also, studies were conducted on samples pre-treated with the additive (Fig. 2a). From the curves it is evident that at relative humidity = 70% with an increase in temperature, the equilibrium humidity decreases from 31.5% (curve 1) to 28% (curve 2).

The section of the desorption isotherm curves with the convexity toward the relative humidity axis characterizes capillary-bound and osmotic moisture. As can be seen from the curves in Fig. 2b, at a relative humidity of 50%, with an increase in temperature from 20 to 60 ⁰C, the equilibrium moisture content Wp of apricot fruits decreases from 17,5 to 11.4%.

a)

b)

Figure 2. Desorption isotherms of apricot fruits depending on the processing method:

a) Pre-treated with sugar syrup, 1-t=25 ⁰C, 2-t=60 ⁰C; b) 1.2 at t=25 ⁰C, 3.4 at t=60⁰C. 1.3- not treated; 2.4 – pre-treated with IR exposure.

Depending on the method of processing apricot fruits, the desorption isotherms have a characteristic appearance. Fig. 2b shows the desorption isotherms of apricot fruits processed with IR rays (curves 2,4) and not processed (curves 1,3). The results show that at 20 ⁰C in the samples (curve 1) the equilibrium moisture content is 26.1%, and in the samples (curve 2) 22,8%. At a temperature of 60 ⁰C, the equilibrium moisture content is 18,5% (curve 3), and in the samples 16,7% (curve 4).

Thus, as a result of the research, equilibrium moisture content (Table 1) of fruit crops was determined.

Table 1.

Equilibrium moisture content of fruit crops

The table shows that for peaches and most other products, the equilibrium moisture content decreases with increasing temperature.

Conclusion. A number of studies have shown that isotherm curves do not have clear boundaries of transition from one state to another. Equilibrium moisture content, final moisture content of fruits at different temperatures and relative air humidity, and forms of moisture connection with the product have been studied with different processing methods.

References:

1. Ginzburg A.S., Savina I.M. Mass and moisture exchange characteristics of food products.-M.: Light and food industry. 1982.-280 p.
2. Lykov A.V., Auerman L.Ya. Theory of drying colloidal – capillary – porous materials of the food industry -M.: Pishchepromizdat. 1976.-287 p.
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