Analysis of the influence of heating temperature on the convective drying process

SRSTI 65.35.03                                

Convective drying is an advanced method of food preservation [1]. The best possible results of the drying stage are achieved by carrying out numerous experiments. Therefore, it is necessary to select the preferred characteristics of drying regimes on a scientific basis. The result will be high-quality finished products. It is important to demonstrate the sensory and physicochemical properties of the dried product [2].

Through the method of convective drying, a diverse range of fruit raw materials can be preserved. Since fruits are seasonal products, producing dried fruits ensures their availability to the population year-round, thereby enhancing the nutritional structure of diets. Dried fruits can be sold as standalone products or used as food температура, плоды, массовая доля влаги, additives in various products to enrich them with vitamins and trace elements.

The purpose of the study: selection of temperature regimes of convective drying of fruits.

Objects of research. The objects of the study were fruits such as apricots, prune plums, apples.

Materials and research methods

Experimental studies were carried out in the testing regional laboratory of engineering profile "Structural and biochemical materials" of the M. Auezov South Kazakhstan University (Shymkent, Tauke Khan str. 5), InnovTechProduct LLP, Shymkent. The experiments were conducted during the spring and summer months.

Generally accepted standard methods of raw material research were used to implement the tasks. The raw materials were sourced from the southern region of Kazakhstan.

The range of analyzed convective drying modes was selected in such a way as to ensure the preservation of useful elements in fruits [5]. The experiments were carried out on a laboratory convective drying unit at heating temperatures from 40 to 80°C in increments of 5 degrees. The fruits were dried on pallets in one layer. The control parameters of drying were the drying heating temperature (0 °C), the mass fraction of the moisture of the fruit (%) [6]. Regulatory documents were used for comparative characteristics of the products obtained.

Results and discussion

For the experiments in question, we selected apricots. The apricots were dried in one layer, previously divided into two halves. At the stage of apricot cleaning, the seeds were removed. The halves of apricots were laid out on sheets of convective drying with the pulp facing up. The apricot fruits did not touch each other. The specimens were subjected to convective drying using a blowing fan. The set drying time at a temperature of 50 degrees is 18-24 hours. At a temperature of 65 degrees, the drying time was 18 hours. Apricot drying was stopped when the mass fraction of dried fruit moisture reached 10-20%.

Below we present a graph of the decrease in the mass fraction of apricot moisture during convective drying (Fig.1).

Figure 1. Influence of heating temperature during convective drying of apricot on its mass fraction of moisture

The graph shows that the dynamics of the decrease in the mass fraction of apricot moisture is an uneven process. The value decreased at a drying temperature of 40 °C from 28% to 10% at a drying heating temperature of up to 80 °C. The optimal results were obtained with the values of the mass fraction of dried apricot moisture of 1820% at a drying heating temperature of 55-65 °C.

Next, we aimed to establish the influence of the heating temperature during the drying of plums on the reduction of their mass fraction of moisture. Drying of the prunes was stopped when the mass fraction of moisture in the dried fruit reached 20-25%. The plums were dried in a single layer, having been previously divided into two halves. At the stage of plum cleaning, the seeds were removed. The plum halves were laid out on sheets of convective drying with the pulp facing up. The plum fruits did not touch each other. They were thoroughly blown by a convective drying fan. The set drying time at a temperature from 4550 °C to 70 °C for the finished product is 16 hours.

We will present a graph illustrating the decrease in the mass fraction of moisture in prunes during convective drying, depending on the drying heating temperature (Fig. 2).

Figure 2. The influence of the heating temperature during convective drying of the plum on its mass fraction of moisture

The research diagram demonstrates the following effective characteristics. The normative values of the mass fraction of moisture of prunes at the drying stage in the region of 20-25% were noted at a heating temperature of 65-70 °C [5].

The subsequent experiments were conducted on apples. The initial study examined the heating periods and their impact on the mechanical properties of apple slices. The findings of the study indicate a need for energysaving measures to facilitate rapid drying. The product obtained exhibited preserved properties of high quality in comparison with conventional drying [6].

The next object of the experiments was also apples [7]. We made semi-finished products from apples with a thickness of 1 mm, 4 mm, 5 mm, and 6 mm. The prepared semi-finished products of fresh apples were placed on a convective drying mesh pallet in one layer. At the end of convective drying, the following results were observed, which are shown in Table 1.

Table 1. Results of convective drying of apples

Mode	Thickness of the semifinished product 1 mm		Thickness of the semifinished product 4 mm		Thickness of the semifinished product 5 mm		Thickness of the semifinished product 6 mm
	Product Readiness	Mois-ture, %	Product Readiness	Moisture, %	Product Readiness	Moisture, %	Product Readiness	Moisture, %
1	Ready	20	Ready	20	Not ready, drying	30	Not ready, drying	35
2	Ready	20	Ready	20	Not ready, drying	30	Not ready, drying	35

As indicated in the table, the thickness of the semi-finished product significantly influences the degree of drying of the finished product [8]. When the thickness of fresh apples is 1 mm, apple chips with a visually appealing appearance and pleasant taste are obtained, and the moisture content of the finished product conforms to standard values. We observed that with thicknesses ranging from 1 mm to 4 mm and under modes 1 and 2, we consistently produce dried fruits with physical-chemical properties meeting standard specifications. However, when the thickness of the semi-finished product increases to 5 or 6 mm, inadequate drying of the finished product occurs, resulting in an excess of moisture content above normative levels. Both drying time and temperature significantly influence the production outcomes.

We will consider a graph illustrating the decrease in the mass fraction of moisture in apples during convective drying, depending on the drying heating temperature (Fig. 3).

Figure 3. The influence of the heating temperature during convective drying of apples on its mass fraction of moisture

Based on the conducted experiments, the duration of drying and the yield of finished fruit products during convective drying were established (Table 2).

Table 2. Duration of drying and yield of finished fruit products during convective drying

Name dried fruits	Drying time in hours	Output of finished products
Dried apricots	18-24	13-18
Prunes	16-18	19-20
Apples	2	18-20

The chemical composition of dried fruits post-convective drying is detailed in Table 3 [9,10]. The work investigated the effect of convective drying of feijoa on the physicochemical characteristics, as well as drying parameters (energy consumption and drying speed). Feijoa fruit pieces were dried at temperatures of 50, 60, and 70°C, air speed of 0.5 and 1 m/s, and thickness of 0.003 m and 0.005 m. Optimal conditions were a temperature of 50.83 °C, air speed of 1 m/s, and thickness of 0.003 m. The results show that convective drying is a suitable and cost-effective method for obtaining functional ingredients while maintaining biocompounds and biological activity [11].

Table 3. Chemical composition of dried fruits during convective drying

Name of indicators	Meaning
Dried apricots
Moisture, %	13-18
Proteins, g	4,0-4,8
Fats, g	0,085-0,1
Carbohydrates, g	44-48
Prunes
Moisture, %	20-25
Proteins, g	0,96
Fats, g	0,16
Carbohydrates, g	28,08
Apples
Moisture, %	20
Proteins, g	2,3
Fats, g	0,1
Carbohydrates, g	83

Practical research yielded the duration of       ranging from 40°C to 80°C, with increments of 5

convective drying for all objects at temperatures       degrees (Table 4).

Table 4. Duration of convective drying of fruits at temperature selection, h

Type of fruit	Drying temperature, °С
	40	45	50	55	60	65	70	75	80
Apricot	36	32	24	22	20	18	16	14	12
Plum	26	24	22	20	18	16	14	12	10
Apple	3,5	3,0	2,5	2,0	1,5	1,0	0,5	0,2	0,1

The whole process of convective drying can be represented in three stages. During the first stage, the temperature of the product decreases by several degrees, and the relative mass decreases by 2-5%. At this phase, the moisture of the macro capillaries of the product is removed. After 10 minutes, when the temperature in the chamber reaches the required level, the temperature and the rate of dehydration of the product begin to increase, and the second stage of dehydration begins [12].

Convective drying is characterized by the presence of a second stage of drying. During this period, there is an increase in the drying rate. With numerous fruit varieties globally, each fruit possesses unique characteristics. After conducting multiple experiments, it was observed that an increase in heating temperature correlates with the highest rate of variation in the relative mass of the final object [13].

The obtained research results are as follows. The time of the highest rate of moisture removal is influenced not only by temperature but also by the structure and size of the fruits themselves. During the experiments, large parameters and the thickness of the fruit influenced a decrease in the intensity of moisture evaporation from the center of the product to its outer layers. Therefore, there is an increase in time, which is so important to establish the highest rate of moisture removal.

During the second stage, microcapillary moisture and osmotic moisture evaporate during drying [14].

During the third stage, there is a specific reduction in the drying rate, during which mono- and polymolecular adsorption moisture is removed from the material. This period is characterized by the highest binding energy. To evaluate sensory properties, an approach based on indicators such as color, taste, smell, and consistency was employed. The proposed indicators were rated on a scale of 5 points [15]. Thus, the total points ranged around 20 points. Table 6 presents the results of the sensory analysis.

Table 5. Results of organoleptic evaluation of dry fruits when selecting the temperature of convective drying

Type of fruit	Indicator	Drying temperature, °С
		40	45	50	55	60	65	70	75	80
Apricot	Taste	4	4	45	5	5	5	2	2	0
	Color	4	4	4	5	5	5	2	2	1
	Smell	4	4	4	5	5	5	3	3	0
	Consistency	2	2	3	5	5	5	2	2	1
	Total	14	14	16	20	20	20	9	9	2
Plum	Taste	4	4	5	5	4	3	2	2	1
	Color	4	4	5	5	3	3	1	1	1
	Smell	4	4	5	5	3	2	2	2	0
	Consistency	4	4	5	5	3	2	4	2	2
	Total	16	16	20	20	15	10	9	7	4
Apple	Taste	3	3	4	4	5	5	3	2	8
	Color	3	4	4	5	5	5	3	3	2
	Smell	4	4	4	4	5	5	3	1	0
	Consistency	3	3	3	4	5	5	3	1	2
	Total	13	14	15	17	20	20	12	7	4

The optimal technological characteristics of sensory properties in apricots (from 16 to 20 points) were achieved under the following conditions: the heating temperature range of 55-65 ° C. When the temperature rises to 80 °C the scores of organoleptic properties are reduced to 2 points.

The maximum scores of sensory characteristics were obtained at the heating temperature of 50-55 °C for drying apples (Table 5). Indicators corresponding to the standard for prunes were observed at a temperature of 60-65 °C.

Conclusion

The proposed method was used to study the effect of heating temperature on the efficiency of the fruit drying process during convection. Taking into account the optimal ratio between the dehydration time and the degree of preservation of important fruit components, the following values of the heating temperature during drying should be recommended for the production of fruit processing: for apples - 50-55 ° C, apricots - 5565 °C, prunes 60-65 ° C.

Funding

This research was supported by the Ministry of Science and Higher Education of the Republic of Kazakhstan, grant AP19678142 (2023-2025).