The content of organic acids and carotenoids in Sorbus fruits

Interest in biologically active substances as food additives is increasing every year due to the greater impact on the human body compared to substances of chemical origin, poor digestibility of the latter and the occurrence of numerous side effects. Therefore, the relevance of studying new species and varieties of plants containing biologically active substances important for the body remains the main task of pharmacognosy.

Mountain ash (Sorbus aucuparia L.) from the Rose family (Rosaceae) is one of the most common plants growing in our country, as it is not whimsical and frost-resistant. Mountain ash is the only pharmacopoeial species of the genus, it grows in the European and Siberian parts of Russia, in the mountainous regions of the Caucasus and Crimea. Medicinal plant raw materials are fruits. In medicine, rowan fruits are used as a multivitamin, diuretic, tonic. Pharmacologically active substances reduce cholesterol, increase the strength of blood vessels, increase immunity, and also reduce oxidative processes in the body. The fruits are round, apple-shaped with a remaining cup of orange or red color. Rowan fruits are standardized according to the content of organic acids, they also contain carotene, cryptoxanthin, tannins, vitamins, flavonoids, pectin substances, triterpene compounds (ursolic acid, oleanolic acid), essential substances, etc. Rowan fruits are used as medicines in the form of syrups, decoctions and vitamin collections ^[1]. Rowan fruits can be used as cost-effective natural antioxidants instead of synthetic ones. Mountain ash hybrids have excellent taste characteristics and have larger fruits compared to ordinary mountain ash ^[2].

The aim of the work was to determine the quantitative content of the sum of organic acids and carotenoids in the fruits of mountain ash and its varieties.

The objects of the study were the ordinary River and 4 varieties ‘Burka’, ‘Garnet’, ‘Scarlet Large’, ‘Titan’ from the collection of the Botanical Garden-Institute of the Volga State Technological University (Yoshkar-Ola, Republic of Mari El). The fruits of these varieties are large, especially in the "Scarlet large" variety. By origin, the varieties are hybrids of the common River: ‘Garnet’ – with hawthorn, ‘Burka’ – with sorbonia, ‘Scarlet Large’ – with Moravian mountain ash, ‘Titan’ – with pear and red–leaved apple tree ^{[3, 4]}. ‘Scarlet large’ is a variety of selection of the All-Russian Research Institute of Fruit Breeding cultures. It was obtained from pollination of the flowers of R. with an ordinary mixture of pollen of Moravian mountain ash and pear. The fruits are spherical, scarlet-red, weighing 2-3 g. The flesh is juicy, with bitterness. ‘Burka' is a variety of selection by I.V. Michurin. It comes from the crossing of R. vulgaris with aronia chernoplodnaya. The fruits are spherical, red-brown, weighing 0.6–1.0 g. The flesh is tart, without bitterness. ‘Garnet’ is a variety of selection by I.V. Michurin. It was obtained from pollination of the flowers of R. with ordinary blood-red hawthorn pollen. The fruits are spherical, faceted, burgundy, weighing 1.0–1.6 g. The flesh is yellow, tart, without bitterness. ‘Titan' is a variety of selection by I.V. Michurin. It comes, perhaps, from the ‘Burka’ variety. The fruits are spherical, slightly faceted, burgundy, with a waxy coating, weighing 1.0–1.6 g. The flesh is yellow, tart, without bitterness. Morphologically, it is very similar to the ‘Garnet’ variety ^[5].

The research was conducted in 2019. The fruits were harvested in the second decade of September from the illuminated side of the plants and dried in an electric dryer for vegetables and fruits "Breeze". The moisture content in the fruits was determined by weight according to the OFS.1.5.3.0007.15 "Determination of the moisture content of medicinal plant raw materials and medicinal products". The study on the content of the sum of carotenoids in terms of beta-carotene was carried out by the spectrophotometric method. Hexane was used as a comparison solution. To determine the amount of organic acids in terms of malic acid in rowan fruits, the titrimetric method according to FS was used.2.5.0093.18. 0.1 M sodium hydroxide solution was used as a titrant ^[6]. The obtained data were identified using the Statistica 6.0 program. The reliability of the difference was assessed by the Student's distribution at a confidence level of 95%.

According to the requirements of the general pharmacopoeia article "Determination of moisture in medicinal plant raw materials and medicinal plant preparations", the moisture content of rowan fruits should not exceed 18% ^[6]. The results of studies on humidity are shown in Figure 1. The moisture content in the fruits of the studied mountain ash varied from 0.49 to 0.60%, which is within the permissible value. According to these values, no statistically significant difference was found between the varieties.

Fig. 1 Moisture content of rowan fruits, %

Organic acids are biologically active substances found in plants in the form of salts, esters or in free form. In plants, they have a bacteriostatic effect, and also maintain an acid-base balance ^[1]. Free organic acids predominate in fruits ^{[1, 7]}. The main source of common organic acids in rowan fruits is malic acid ^[2]. In addition, rowan fruits contain ascorbic, tartaric, citric, sorbic, succinic acids. The qualitative composition of organic acids in fruits undergoes significant changes as they form and mature, the most diverse composition is noted during the period of biological maturity ^[5]. In the human body, acids are involved in metabolism and maintenance of acid-base balance of blood and other physiological fluids, suppress fermentation processes in the intestine and promote normal digestion ^[7].

The content of organic acids in terms of malic acid in rowan fruits is shown in Figure 2. The sum of organic acids in terms of malic acid according to the FS.2.5.0093.18 should be at least 3.2% ^[6]. According to our data, the content of the sum of organic acids in the fruits of R. vulgaris is 3.25%. The largest amount of organic acids was found in the fruits of rowan varieties ‘Titan’ and ‘Burka’ (3.93% and 3.92%, respectively). A large amount of organic acids (3.71%) was also found in the fruits of the Pomegranate variety, but less than in the above-listed varieties. The lowest content of the studied compounds was found in the fruits of the "Scarlet Large" variety (3.06%). According to this indicator, there was no statistically significant difference between the varieties ‘Titan’, ‘Burka’ and ‘Garnet’. Also, there was no significant difference between common mountain ash and the ‘Scarlet Large’ variety.

Fig. 2 The content of organic acids in terms of malic acid in rowan fruits, %

The quantitative content of organic acids in plants is influenced by factors such as species and varietal specificity, harvest time, geographical location of the harvest site, degree of fruit maturity, temperature, etc. ^[7]. According to E.V. Sergunova and co-authors ^[8], in Moscow, the content of the sum of organic acids in the fruits of R. vulgaris was 3.35%, which is comparable with our results. In the Southern Federal District, the value of this indicator was 3.63%, which is slightly more than in our study ^[9]. In the fruits of R. vulgaris in Dagestan populations, the amount of organic acids was 5.9% ^[7]. According to V.Y. Zhilkina and co-authors, the amount of organic acids in the fruits of industrial samples of the common River was 3.78% ^[10]. In the urban environment of Orenburg, the amount of organic acids in the fruits of this type of mountain ash was 1.87–3.76%, and in the Botanical Garden of OSU – 3.3%, which correlates with the data we obtained ^[11]. A study by R.G. Abdullina et al. ^[12] in Ufa found that the content of the sum of organic acids in the fruits of R. vulgaris is 4.52%, the ‘Pomegranate’ variety is 5.22%, which is also higher than in our studies. It is known that hot and dry weather during the ripening of rowan fruits inhibits the biosynthesis of organic acids ^[13]. Perhaps the low content of these compounds in our study is due to weather conditions. In the work of V.N. Mezhensky ^[5], the content of organic acids in the fruits of Michurinsky hybrids is 0.8–2.4%, which is lower than the values obtained in our study.

The main component of carotenoids in mountain ash is beta-carotene ^{[2, 14]}, it accounts for 50-85% of all carotenoids ^[5]. The use of foods rich in carotenoids reduces the level of cardiovascular, cancer, cataracts, as well as diseases associated with a decrease in immunity. Beta-carotene pigment is an antioxidant, binds and removes radicals, strengthens the immune system, reduces the risk of infection with bacterial and infectious diseases, softens the effect of harmful environment on human health, in addition, strengthens the nervous system and promotes stability in stressful situations ^[15]. Beta-carotene is a precursor of vitamin A. The content of beta-carotene in rowan fruits in the conditions of the Botanical Garden-Institute of PSTU is shown in Figure 3.

Fig. 3 Carotenoid content per beta-carotene in rowan fruits, mg%

The greatest amount of beta-carotene is contained in the fruits of the variety ‘Titan’ (5.60 mg%) and ‘Scarlet Large’ (5.03 mg%). In the fruits of the Burka variety, its content was 4.81 mg%. The smallest amount of this compound was found in the fruits of mountain ash (3.25 mg%) and the ‘Pomegranate’ variety (3.09 mg%).

Figure – General view of rowan varieties: a) ‘Titanium', b) ‘Scarlet Large’

According to A.A. Bogachev et al. ^[16], the content of beta-carotene in the fruits of mountain ash cultivated in the Penza, Tambov, Moscow regions and the Republic of Mordovia varied from 6 to 8 mg%. Studies by S.A. Streltsina and co-authors ^[4] established the content of beta-carotene in the fruits of varietal mountain ash in the conditions of the north–western zone of Russia: ‘Burka’ – 1.77 mg%, ‘Scarlet Large’ – 1.82 mg%, ‘Titanium’ - 2.22 mg%, mountain ash - 2.74 mg%, which is significantly less than the values we received. In Zymone research etc. ^[2] in the conditions of Lithuania, the highest content of beta-carotene was characterized by ‘Large Scarlet’ (445 mcg/g) and ‘Garnet’ (361 mcg/g), and ‘Burka’ and ‘Titanium’ accumulated a smaller amount of this compound (150 and 100 mcg/g, respectively). The higher beta-carotene values indicated in this study are probably due to differences in the methodology, as well as milder climatic conditions of the region. Mikulic-Petkovsek and co-authors ^[17] in the conditions of Slovenia indicate a high amount of total carotenoids in the Burka variety (85.1 mg/kg). In the botanical garden of Ufa, R.G. Abdullina and co-authors ^[18] found that the fruits of rowan cultivars are characterized by a higher content of carotenoids compared to R. vulgaris, which is partially consistent with the results we obtained: the fruits of most varieties exceed the fruits of R. vulgaris in terms of the content of the compounds studied. The authors of Kampuss etc. in Latvia ^[19] identified the variety ‘Pomegranate’ with the highest content of carotenoids in fruits (13.04 mg%), the variety ‘Scarlet Large’ contained 9.24 mg% of the studied compounds in fruits, R. ordinary – 9.52 mg%. In studies by Bronislavas et al. in Lithuania ^[20], the highest content of carotenoids was noted in the variety ‘Scarlet Large’. In addition, these authors found that during the ripening of rowan fruits, the amount of carotenoids increased and was maximum in mid-September, while the content of organic acids was highest in mid-August and subsequently decreased.

Thus, the greatest amount of the sum of organic acids was found in the fruits of the varieties "Titan" and "Burka", and carotenoids – in the fruits of the varieties "Titan" and "Scarlet large". The content of the studied biologically active substances in the varieties is higher than in the pharmacopoeial form of R. ordinary. However, in the fruits of this mountain ash growing in the Botanical Garden-Institute of the State Technical University, the content of the sum of organic acids and carotenoids is lower than that of mountain ash in other regions. The fruits of the "Titan’ variety are the most promising in use as sources of biologically active compounds.