Characteristics of cultivars considered in this study are given in the Material and Method section. Eighteen cultivars and three local genotypes were tested in the study. The evaluated genotypes differed in fruit size, skin and juice color and in the time of fruit ripening.
Weights of fruit were from 2.7–3.2 g (“Marasca”) to 5.5-5.9 g (“Grosenkirch”), skin color was from light red (“Dradem”, ‘Malinówka”, “Meteor”, Montmorency” and “Wróble”) to almost black (“Demesova”) and juice color from colorless in “Montmorency” to very dark red in “Demesova”. Harvest maturity was described as:
In our study antioxidant capacity measured by determining substances reacting with Folin–Ciocalteu reagent, reducing power FRAP and DPPH radical scavenging activity was performed and the chromatographic profiles of sugars, organic acids and phenolic compounds such as anthocyanins, phenolic acids, flavonols and flavan 3-ols were established.
2.1. Sugars
Sugar content is a very important attribute in sensory evaluation, and it is decisive for the acceptance of the sour cherry fruit taste. Generally, the higher the sugar content in the fruit, the less sour the taste. Free sugars were determined by high-performance liquid chromatography (HPLC) and presented in
Table 1. Total sugars (TS) were from 8.62 g/100 g FW in the “Turgieniewka” cultivar to 15.38 g/100 g FW in the “Lucyna” cultivar (
Table 1). It was similar to sugar content in other sour fruit, for example, cornelian cherry, in which total sugars in the fruits were 4.1–16.4% [
14,
15] and less than in sweet cherries, where 7.7–26.5% of total sugars was determined [
16,
17,
18]. Sour cherries can contain from 6.0 g/100 g FW to 16.33 g/100 g FW of carbohydrates [
19,
20]. As in many other fruits, in carbohydrate composition, glucose and fructose were dominant and represented 54.8–78.7% of all sugars. Glucose was found from 2.81 g/100 g FW in “Wieluń 17” to 5.68 g/100 g FW in “Lucyna”, and it was the dominant sugar in most cultivars. Fructose was dominant only in “Meteor” (4.88 g/100 g FW), “Montmorency” (4.22 g/100 g FW) and “Wieluń 17” (3.68 g/100 g FW). Fructose content was determined from 2.74 g/100 g FW (“Granatnaja”) to 4.88 g/100 g FW (“Meteor”). In Hungarian cultivars, Papp et al. [
12] determined 6.06-9.08 g/100 g FW of glucose and 3.54-4.91 g/100 g FW of fructose. Sorbitol content varied from 1.93 g/100 g FW (“Nana” and “Turgieniewka” to 3.12 g/100 g FW (“Meteor”). Saccharose was determined in concentration from 1.03 g/100 g FW (“Nana”) to 1.41 g/100 g FW (“Demesova”), but not in all cultivars. In “Grosenkirch”, “Marasca”, “Montmorency”, “Paraszt Meggy” and “Turgieniewka” cultivars saccharose was not detected. Xylose (1.30–1.36 g/100 g FW) was determined only in three cultivars, i.e., “Granatnaja”, “Sokówka Nowotomyska” and “Wróble”, as was galactose (1.39–1.53 g/100 g FW), detected only in “Lucyna”, “Meteor” and “Mołodiożnaja”.
2.2. Organic Acids
Organic acids are fundamental to the typical sour cherry taste, which distinguishes it from sweet cherries. Total acid (TA) content (
Table 2) was between 2300.5 mg/100 g FW and 1294.4 mg/100 g FW. The cultivars “Paraszt Meggy”, “Suda Hardy”, “Wróble” and “Krasnaja Płodorodnaja” contained more than 2200 mg/100 g FW. “Malinówka”, “Marasca” and “Montmorency” contained less than 1600 mg/100 g FW of total organic acids. The organic acid content in sour cherry is similar to or less than in cornelian cherry (
Cornus mas L.), in which 1.7–4.6% organic acid content was determined [
14,
15], and in Japanese quince, which contains 3.5–4.5% organic acids [
21]. Sweet cherries contain 0.6–3.7% organic acids [
18].
In the sour cherry genotypes studied, five organic acids were identified and quantified: malic, oxalic, malonic, shikimic and fumaric. The main organic acid in sour cherry was malic acid, whose content ranged from 1027.4 mg/100 g FW to 1976.2 mg/100 g FW and accounted for 78.2–88.3% of total organic acids. The second acid according to content was malonic acid (11.2–21.3% in TA) and its content ranged from 231.5 mg/100 g FW in the “Turgieniewka” cultivar to 406.5 mg/100 g FW in the “Mołodiożnaja” cultivar. Oxalic, shikimic and fumaric acids were determined in much lower concentrations; these acids together accounted for 0.4–1.4% of total sour cherry organic acids. The profile and content of organic acids are confirmed by other studies [
12,
13,
17,
18].
An important index for estimation of the organoleptic properties and consumer”s acceptability of sour cherry fruit is the Total Sugar/Titratable Acidity (TS/TA) ratio [
2,
6,
22]. For the investigated sour cherry the TS/TA ratio varied from 4.17 for the “Turgieniewka” cultivar to 7.95 for the “Montmorency” cultivar. Research performed by Wojdyło et al. [
6], Najafzadeh et al. [
8] and Grafe and Schuster [
13] showed that the TS/TA ratio for sour cherries was from 3.4 to 19.5.
For other fruits, which are sweeter than tart cherries, the sweetness/sourness TS/TA ratio is higher. For Idared apples the TS/TA ratio was 19.7–28.5 [
23], for apricots 8.4–9.2 [
24], but for pomegranates and cornelian cherry it was 2.3–7.0 [
14,
25]. This acceptability index in sweet cherries can be above the value of 25 [
18].
2.3. Phenolic Compounds
Phenolic compound content in sour cherry fruit was investigated by many authors [
2,
6,
7,
18,
26,
27,
28,
29,
30,
31,
32,
33]. In our study anthocyanins, phenolic acids, flavonols and flavanols concentration were quantified (
Table 3).
Fruit color was from light red to almost black and depended on the anthocyanin content, which was from 17.97 mg/100 g FW in “Dradem” to 131.28 mg/100 g FW in “Wieluń 17” (
Figure 1). In addition to the above-mentioned cultivars, “Demesova”, “Sokówka Nowotomyska” and “Grosenkirch” contained more than 120 mg/100 g FW. “Montmorency”, “Wróble” and “Nana” were the cultivars with the content of anthocyanins lower than 50 mg/100 g FW. According to data reported by other authors anthocyanin content in sour cherries was from 21.0 to 285.0 mg/100 g FW [
7], from 11.3 to 93.5 mg/100 g FW [
12], from 65.1 to 82.4 mg/100 g FW [
18], from 45.0 to 109.0 mg/100 g FW [
22] and from 2.7 to 28.0 mg/100 g FW [
29]. Sweet cherry fruit can contain from 1.2 to 900.0 mg/100 g FW [
16,
17,
30]. Other red stone fruits, such as cornelian cherry, contain, depending on the cultivar, 29.2–341.2 mg anthocyanins in 100 g FW [
15,
31].
The main anthocyanins are: cyanidin 3
O-(2′glucosyl) rutinoside, and cyanidin 3
O-rutinoside; on average, these anthocyanins accounted for 62.8% and 27.0% of all anthocyanins (
Supplementary Materials Table S1).
In most cultivars, cyanidin 3-
O-(2′glucosyl) rutinoside was dominant, but it was absent in the “Demesova” and “Granatnaja” cultivars, in which cyanidin 3
O-rutinoside was the main anthocyanin and represented over 93% of total anthocyanins. Cyanidin 3
O-sophoroside, cyanidin 3
O-glucoside, and cyanidin 3
O-sambubioside 5-rutinoside were on average 5.2%, 1.8%, and 1.3%, respectively. In “Demesova”, there was no cyanidin 3
O-sambubioside 5-rutinoside but cyanidin 3
O-glucoside comprised 5.5% of all anthocyanins (in other cultivars cyanidin 3
O-glucoside accounted for 0.6–3.3% of total anthocyanins). Differentiation of the sour cherry anthocyanin profile was reported previously by Simunic et.al. [
29], Blando et al. [
32], Homoki et al. [
33] and Filimon et al. [
34].
The second group of phenolic compounds present in sour cherries is flavonols. Flavonols, similarly to anthocyanins, strongly influence the antioxidant activity and play an important role in the health benefits of sour cherries. Total flavonol content was between 10.41 mg/100 g FW and 25.08 mg/100 g FW (
Figure 2). In the examined cherries the highest concentration, above 20 mg/100 g FW, was determined in “Suda Hardy”, “Sokówka Nowotomyska”, “Marasca” and “Wieluń 17” cultivars, and the lowest, under 14 mg/100 g FW, in “Wróble”, “Zagoriewskaja”, “Nana”, “Krasnaja Płodorodnaja”, and “Mołodiożnaja” cultivars. According to many authors [
18,
35], quercetin, kaempferol and isorhamnetin rutinosides and glucosides are dominant in different proportions in sour cherry cultivars (
Supplementary Material Table S2). According to other studies, in sour cherries, flavonols are at the level of 2.6–8.3 mg/100 g FW [
2,
27,
35].
The next important group of phenolic compounds present in sour cherries is phenolic acids, which were determined at the level from 16.56 mg/100 g FW in “Turgieniewka” to 76.25 mg/100 g FW in “Grosenkirch”, and up to 126.99 mg/100 g FW in the “Meteor” cultivar (
Figure 3).
Neochlorogenic, chlorogenic and
p-coumaric acids were dominant in sour cherry fruit (
Supplementary Material Table S3). A review on sour cherry fruits [
2] reported 42.4 mg of phenolic acids per 100 g FW, but Wojdyło et al. [
6] found a phenolic acid content from 39 to 263 mg/100 g FW (259–1200 mg/100 g DW). In the cultivars “Dradem”, “Lucyna” and “Turgieniewka” Wojdyło et al. [
6] determined 218 mg/100 g FW, 118 mg/100 g FW and 168 mg/100 g FW of phenolic acids respectively, and in our research, these three cultivars contained 44.00 mg/100 g FW, 30.94 mg/100 g FW, and 16.56 mg/100 g FW respectively. In sweet cherries, 3.4–123.2 mg/100 g FW of phenolic acids was determined [
2,
16,
27] and in cornelian cherry, from 10.9 to 37.5 [
15].
Flavan 3-ols were not quantified in all cultivars (
Figure 4), and were determined at a level from 8.7 mg/100 g FW (cv. “Mołodiożnaja”) to 63.1 mg/100 g FW (cv. “Wieluń 17”). Monomers and dimers were mainly detected (
Supplementary Material Table S4), similar to the research of Karaaslan et al., Wojdyło et al. and others [
6,
20,
36].
The total phenolic compounds by HPLC were from 96.56 mg/100 g FW in the “Nana” cultivar to 268.98 mg/100 g FW in the “Wieluń 17” cultivar. Other researchers determined 120.0–312.0 mg of phenolic compounds in 100 g of sour cherry fruit [
27,
35,
37]. Due to the numerous cultivars and varieties, the composition and content of sour cherry constituents vary widely, and their level depends mainly on the cultivar, maturity, agronomic factors and climatic conditions [
6,
20,
22].
2.5. Principal Component Analysis
The Principal Component Analysis (PCA) was applied to data to explain the relationships between sour cherry cultivars. The two first components explained 64.8% of the variation (
Figure 5). The first component (45.4%) was associated with antioxidant activity (FCRS, DPPH, FRAP), anthocyanin (ANT) and flavonol (FL) content and maturity time (MT), while the second component (accounting for 19.4% of total variance) was mainly associated with phenolic acids (PA), flavan 3-ols (F3), organic acids (TA) and TS/TA ratio.
The high content of phenolic compounds was correlated with antioxidant capacity and with the time of maturity. Cultivars whose harvest maturity was medium to very late (“Wieluń 17”, “Grosenkirch”, “Suda Hardy”, “Marasca”, “Sokówka Nowotomyska”) contained more flavonols and anthocyanins and were characterized by higher antioxidant activity. In the second cluster were cultivars with low phenolic compound and antioxidant capacity and high content of organic acids (“Granatnaja”, “Krasnaja Płodorodnaja”, “Demesova”, “Paraszt Meggy” and “Turgieniewka”), whose harvest maturity was classified as early or early to medium.
The results show that Polish genotypes such as “Wieluń 17”, “Sokówka Nowotomyska”, “Sokówka nr 9”, “Sokówka nr 6”, and “Lucyna” are characterized by a high content of bioactive compounds and also perform very well in terms of other characteristics, compared, for example, to the very popular and commonly studied Montmorency variety.
Our research indicates large biodiversity of the varieties grown in Poland, which is consistent with the objectives of sustainable agriculture.