Plum is a high-yielding stone fruit plant, the origin of which is the result of a natural hybridization of cherry plum and blackthorn, which historically occurred in the Caucasian and Asia Minor regions. Plum cultivation continued in the Mediterranean and Central Asia. From Mediterranean Italy, plum has penetrated European countries and has firmly taken the position of one of the most popular fruit trees in the world of fruit growing (Doroshenko, Ryazanova and Maksimtsov, 2015; Eremin et al., 2000; Popova et al., 2020). The scientific interest in studying the medicinal properties of plums is explained by the high content of phenols in its fruits, mainly anthocyanins, which are natural antioxidants (Prichko, 2009; Kim, Jeong and Lee, 2003; Gościnna et al., 2021). Many world scientists have substantiated the connection between the use of plums and the improvement of cognitive function (in particular, a positive effect on memory and related processes) (Igwe and Charlton, 2016; Turaeva, 2017). In addition to the aforementioned and antioxidant properties, the interrelation of the inclusion of plum fruits in the diet and the reduction of risk factors for cardiovascular diseases is emphasized.The antiinflammatory, analgesic, antibacterial and anti-cancer properties of home plum gum have been analyzed in dozens of studies (Islam et al., 2017; Milskaya, 1998). Several scientific works emphasize the following: the chemical composition of plum fruits allows the use of this fruit as a source of raw materials for the production of candied fruits and dried fruits. As a rule, fresh fruits and vegetables have a short shelf life, since various microbes and enzymes affect them and quickly spoil. Therefore, fresh fruits and vegetables must be eaten immediately, and the surplus must be skillfully preserved for the autumn-winter and spring periods. In this regard, the development and implementation of modern processing technologies for perishable raw materials in the form of fruits, fruits, and vegetables with full preservation of the nutritional and taste qualities of the final product is relevant in the modern world (Prichko, Machneva and Droficheva, 2020; Amin et al., 2019, Svanes and Johnsen, 2019). Candied fruit – fruit processing products made by single or multiple cooking in sugar syrup, dried to a mass fraction of dry matter of at least 80%, sprinkled with sugar, powdered sugar, or glazed (GOST 28322, 2014). American scientists in the course of a retrospective study found that dried fruits are no less beneficial for health than fresh ones, and in winter they can be successfully replaced (Sullivan et al., 2020). The analysis made it possible to conclude the relevance of research in the field of processing plum fruits into dried fruits and candied fruits. In this regard, it is of interest to study the influence of varietal characteristics on the quality indicators of candied fruits.

The taste and technological qualities of plums are largely determined by their chemical composition (Eremin et al., 2009). On the example of 7 plum varieties investigated in the laboratory for storage and processing of fruits and berries of the FSBSI NCFSCHVW, technical indicators of fruits are presented (Table 1).

The studied varieties vary significantly in the content of dry substances, sugars, acids, vitamins, and polyphenolic substances (Table 2).

There is a wide range of variation in the acid content in plum fruits 0.72 – 1.31%. The varieties such as Blue Moon, Big Stanley, and August Delish stand out for their high acidity. The Blue Moon variety (sugar-acid index 8.4) has the sourest taste.

The plum culture is characterized by a low content of vitamin C (a natural antioxidant), on average 5 – 6 mg.100g^-1. The varieties Blue Moon and Grossa di Felisio with an increased level of vitamin C accumulation 7.0 – 7.5 mg.100g^-1 were identified. In terms of the amount of vitamin P, plum varieties differ 2 times: Big Stanley fruits accumulate up to 40 mg.100g^-1, and Stanley varieties more than 70 mg.100g^-1. The total content of polyphenolic substances varies slightly, taking into account the variety, from 166.4 mg.100g^-1 (Big Stanley variety) to 189.0 mg.100g^-1 (Grossa de Felisio variety), which also form the antioxidant activity of fruits (Sheikh, Saini and Sharma, 2021; Demidov, Popov and Savelyev, 2015; Qin et al., 2020; Li et al., 2021; Makarova and Eremeeva, 2015). Large varietal differences can be traced in the content of anthocyanins: 60.2 mg.100g^-1 for the Blue Moon variety, 32.9 mg.100g^-1 for the Big Stanley variety (Figure 1).

The composition of free amino acids is presented in detail in two studied varieties: Stanley and President. In the fruits of the Stanley variety, 12 amino acids were identified, which is 71.3 mg.100g^-1, including arginine 2.1, phenylalanine 0.1, histidine 0.37, leucine 0.18, methionine 2.6, valine 0.18, proline 19.0, threonine 42.5, tryptophan 2.8, serine 0.42, alanine 0.96 , glycine 0.02 mg.100g^-1.

It has been established that the amino acid composition of the fruit of the President variety is represented by 13 amino acids, including five essential ones (methionine, phenylalanine, leucine, valine, and threonine). Among the essential amino acids, methionine was found in the maximum amount (3.8 mg.100g^-1), the content of phenylalanine is 0.27 mg.100g^-1, leucine – 0.24 mg.100g^-1, valine – 0.45 mg.100g^-1. The amount of threonine is less than 0.1 mg.100g^-1. Among the nonessential amino acids, the maximum amount contains arginine (0.62 mg.100g^-1), proline (2.4 mg.100g^-1), and α-alanine.

The aroma-forming substances of the fruits of the President variety are represented by 3 aldehydes (acetaldehyde, furfural, 5-methyl furfural), the total amount of which is 2.2 mg.100g^-1. In addition to them, 2 saturated aliphatic carboxylic acids were noted – propionic and acetic in the amount of 8.2 mg.100g^-1, as well as a small amount of aromatic and aliphatic alcohols and esters.

In the new varieties August Delish, Big Stanley, Grossa di Felisio, President in 2020, a complex of macronutrients was investigated, which makes it possible to establish that plum is a valuable raw material for the production of canned food enriched with minerals. The fruits contain a significant amount of potassium, sodium, and, which is very important for the processed raw materials, calcium, which can maintain the structure of the fruit tissue during processing (Table 3).

The data given in the tables allow us to conclude that each variety is characterized by its own set of biologically active substances and is of interest to food processing enterprises. According to the results of evaluating the technical indicators of plum fruits for 2019 – 2020, the most interesting for the production of candied fruits is the variety: President, Grossa de Felisio, August Delish, the fruits of which were processed in comparison with the control, which has wide production plantings in the territory of the Russian Federation (variety Stanley) (Kaufmane et al., 2007; Prichko and Droficheva, 2019).

Candied fruits are complex drying objects, where it is necessary to take into account such indicators as the initial and final moisture content, chemical composition, structural and mechanical changes, and mass transfer characteristics. Candied fruit drying is a process that, in essence, is not only thermophysical but also technological (Meretukova, 2012). As a technological process, drying of fruits and other moist materials is a combination of heat and mass transfer processes, accompanied by biochemical, technological, and structural-mechanical changes. When dried in candied fruits, not only the native properties of the product should be preserved, but also be significantly improved. The main feature of the process of sorption of raw materials is that the mechanical, thermophysical, diffusion properties of the raw materials change significantly upon saturation, which has a significant effect on all subsequent technological operations (Deiana et al., 2019; Gonzalez-Cebrino et al., 2013; Gonzalez et al., 2016).

The fruits, after pitting and blanching, were kept in sugar syrup three times for 4 to 6 hours. For the production of candied fruits from plum fruits, a high initial concentration of syrup is undesirable, because this increases the stickiness of the fruit, which significantly reduces the attractiveness of the appearance of the finished product (Alekseeva et al. 2018; Cerri et al., 2019; Reichel et al., 2017; Richmond, Bowyer and Vuong, 2019; Sommano et al., 2013). The change in the concentration of the syrup in the first period of exposure between the fruit and the syrup is achieved mainly due to their dehydration and due to this dilution of the concentration of the syrup. After 4 hours of exposure, the concentration of the syrup decreases to 45 – 50% and insignificant phase changes are already taking place in the system. The third filling is carried out with part of the released syrup, brought to 60% concentration. During this period, the phenomenon of dehydration of fruits was also noted with a simultaneous, but already less saturation of them with syrup.

At this technological stage of candied fruit production, the following varieties proved to be unsuitable for this type of processing of plum fruits: Blue moon and August Delish. The fruits of the Blue Moon variety did not hold their shape after the first pouring with syrup, and the fruits of the August Delish variety lost a significant amount of coloring pigment and acquired a pale appearance.

Optimization of drying processes in a drying oven with IR rays has been carried out under conditions that prevent sugar caramelization, protein denaturation, which worsen the presentation and quality of candied fruits (Nunes et al., 2009a; Nunes et al., 2009b). Plum fruits were dried in 2 stages: in the first two hours at a temperature of 40 – 45 °C, then the temperature was increased to 60 – 65 °C for 4 – 5 hours. If necessary, especially for large fruits, the raw material was dried at 40 °C to a moisture content of 20 – 23%. During the drying process, the raw materials were periodically turned over and the temperature in the drying oven was controlled. The drying time is determined by the intensity of the technological process, the type and size of raw materials, and on average is 28 – 36 hours for plum fruits (Cabrera-Bañegil et al., 2020; Prichko, 2002; Pisano et al., 2019; Ionica et al., 2013; Michalska et al., 2016). The drying process in an oven with infrared rays led to a decrease in polyphenolic substances by 10%, and vitamins from 13% (P) to 29% (C) in finished candied fruits (Table 4).

The content of macroelements in candied fruits concerning the original fresh raw materials decreased by 10 – 13%, according to the accumulation of potassium and sodium, the following varieties were distinguished: President, Stanley, and Big Stanley (Table 5).

When processing experimental data for a significance level of p = 0.05, the following statistical criteria were used: Cochran's criterion – to assess the homogenity of the variance; Student's test – to assess the significance of the calculated coefficients; Fisher's criterion – to assess the adequacy of the obtained regression equations (Moscattello et al., 2019; Michalska et al., 2016; Xiang et al., 2020).

To assess the quality of plum fruits and to highlight the best ones in terms of a set of signs, a regression, correlation analysis of technical, biochemical indicators was carried out (Reichel et al., 2017; Prichko, Machneva and Droficheva 2020). A model of the relationship between the quality of fruits (Y-sensory assessment) and complex chemical indicators has been obtained. The regression equation for plum varieties looks like this at (R = 0.914):

Y = 8.2-0.3X₁ - 0.004X₂ + 0.11X₃ - 0.16X₄ + 0.66X₅ + 0.7X₆ + 0.001X₇ - 0.0002X₈;

Where:

-X₁ – is the shape index; X₂ – average weight, X₃ – dry matter; X₄ – acidity, X₅ – sugar; X₆ – vitamin C, X₇ – vitamin P, X₈ – anthocyanins.

Good agreement between the experimental and calculated data indicates the adequacy of the found regression equations.

Organoleptic evaluation of candied plum fruits of the variety: Stanley, President, Grossa di Felisio, August Delish was carried out (Figure 2).

The candied fruits of the plum variety Grossa di Felisio were distinguished by their pronounced sour tastes, due to the high acidity of fresh fruits.

Based on the organoleptic assessment carried out, it can be concluded that there is a prospect of using such varieties as Stanley and President for the production of candied fruits, which have a pleasant taste and appearance, with a high tasting score – 4.9 points. The technical characteristics of the quality of plum and candied fruits of the following varieties have been established: President and Stanley (Figure 3).

Taking into account the peculiarities of the chemical composition of candied fruits from the fruits of plum varieties: Stanley and the President, a comparative mathematical calculation of the energy value of the finished product was made (Table 6).

The varieties that carry high valuable technical and biochemical characteristics have been identified. According to the content of biologically active substances of the 7 studied varieties, the following are distinguished: Grossa di Felisio, Blue moon, Stanley. The most valuable varieties for processing in terms of accumulation of total polyphenols, anthocyanins, vitamin C, P are Grossa di Felisio and Blue moon. According to the content of minerals (potassium, sodium, calcium), capable of maintaining the structure of fruit tissue during processing, the President variety stands out. Theoretically and experimentally confirmed the possibility of forming the quality of candied fruit based on the complex influence of the factors of the technological process, taking into account their variation within the specified limits and characteristics of varieties. According to the technical indicators that appear during processing, the fruits of the plum variety proved to be unsuitable for the production of candied fruits: variety Blue moon after the first filling with syrup, the structure of the fruit tissue changed and the finished candied fruits did not look attractive to the consumer.

According to the results of the organoleptic evaluation, candied fruits from plum fruits of the Grossa di Felisio variety were distinguished by pronounced sour tastes, due to the high acidity of fresh fruits, for the production of which it was recommended to increase the sugar consumption rate. Candied fruits from plum fruits of the August Delish variety had a pale, slightly colored appearance with a slightly harmonious taste.

Based on the studies carried out, experimental data were obtained on the influence of the varietal characteristics of plum fruits on the technological indicators of the quality of candied fruits and the yield of the finished product was determined. The results of experimental biochemical and technological analyzes of fruits at individual technological stages of production made it possible to judge the influence of varietal characteristics on the quality indicators of the finished product. For the industrial production of candied fruits, plum varieties with high biochemical and technical indicators are recommended: Stanley, Big Stanley, President, Kabardinskaya early, Grossa di Felisio.