Changes in Biologically Active Compounds During the Ripening Period in Selected Hungarian-Bred Sour Cherry Varieties (Prunus cerasus L.)

Abstract

The sour cherry (Prunus cerasus L.) is an important fruit species in Eastern Europe due to its multiple uses. The aim of this study was to evaluate the ripening-stage-dependent compositional changes in some Hungarian-bred sour cherry varieties (“amarelle” type ‘Korai pipacs’, “morello” types ‘Érdi bőtermő’ and ‘Újfehértói fürtös’), with a special view on biologically active compounds (anthocyanins, polyphenols, vitamin C, melatonin), organic acids, sugars, and antioxidant characteristics. The measured soluble solid content varied within a narrower range than reported in the literature, whereas the total acid content and soluble solid content were consistent with previous data. As the analyses were based on samples from a single year (2022), potential year-to-year variability should be considered when interpreting the results. The “morello” type varieties reached a higher amount of different sugar compounds than the “amarelle” type variety. Among the examined organic acid compounds, malic acid was detected in the highest quantity (176.75 to 669.44 mg 100 mL⁻¹). The vitamin C data (5.74 to 13.46 mg 100 mL⁻¹) had similarity to the literature data. The “amarelle” type ‘Korai pipacs’ reached the highest antioxidant content (131.11 mM AS L⁻¹) in the third picking time. Among the naturally occurring pigments the “morello” type, straining sour cherries reached a higher amount (113.71 µg mL⁻¹ in ‘Érdi bőtermő’ and 59.4 µg mL⁻¹ in ‘Újfehértói fürtös’ of cyanidin glucosides), than the observed “amarelle” type (23.42 µg mL⁻¹ in ‘Korai pipacs’ of cyanidin glucosides). Melatonin was detected in all examined varieties (1.56 to 13.25 ng mL⁻¹).

1. Introduction

The annual global sour cherry, syn. tart cherry, (Prunus cerasus L.) production is about 1.5 million tons and shows a slightly increasing trend [1]. This stone fruit species is typical for the Eastern European countries, including the Russian Federation, Poland, Ukraine, Serbia, Hungary, Romania, Czech Republic, Bosnia and Herzegovina, Belarus, Croatia, and Latvia, with rich genetic resources. This area provides almost half of the global production [1].

The sour cherry is used for fresh consumption and processed products in Eastern Europe [2]. Currently, ‘Érdi bőtermő’ (syn.: ‘Danube’) and ‘Újfehértói fürtös’ (syn: ‘Ungarische Traubige’, ‘Balaton’) together with some local-bred varieties dominate the global table sour cherry market [2]. Apart from fresh consumption, the sour cherry is an important raw material for drinks, dairy, food preservation, sweet, baking, and confectionery, as well as oenology and distilling industries [3]. Apart from Eastern European countries, this crop is also grown in Turkey, Iran, Germany, Italy, Denmark, Norway, Canada, and the United States of America. In these countries, the sour cherry is used mainly for food industrial purposes (concentrate, jelly, jam, marmalade, pies, stewed fruit, syrup, dried fruits) [4,5,6,7,8,9,10,11]. Novel sour cherry products include vinegar [9], concentrate from the powder [11], and sour cherry beer [12].

Fruit taste, which depends on the SSC/TTA ratio of the genotypes, varies by genetic background [13,14,15,16], rootstocks [17], and maturity stages of the fruits [17]. In some countries, the fresh consumption of sour cherries is not preferred [18,19] due to the unilaterally acid character of the fruits. The dwarf and semi-dwarf rootstocks induced the best values of fruit characteristics and phenolic compounds among mid-Serbian climate conditions [14]. In practice, processing factories decide when they are ready to accept the fruit. This timing does not always coincide with the optimal stage of ripeness.

There are three types of sour cherries: the “morello” type has dark red flesh and strongly staining juice, the “marasca” type has medium to dark red flesh with slightly tart flavour, and the “amarelle” type has white or yellowish flesh and non-staining juice. In Hungary, the “morello” and “amarelle” type sour cherries can be found in the assortment; there is no “marasca” type among the native population or among the cultivated varieties in the production.

This stone fruit species has an important role in human nutrition, as its biologically active antioxidant compounds play an important role in the prevention of numerous diseases. Among its biologically active compounds, the sour cherry contains polyphenols, inside this group flavonoids, and within this sub-group anthocyanins, as well as melatonin and vitamin C in a large quantity. Anthocyanins are naturally occurring pigments responsible for red to blue colouration in plants. In sour cherry, these naturally occurring pigments determine the characteristic fruit colour, which ranges from light red to deep burgundy depending on the cultivar and ripening stage.

In sour cherry, the colouring pigments are responsible for fruit colouration ranging from light red to deep burgundy. The main colouring components of the sour cherry fruit are cyanidin glycosides, but pelargonidin, peonidin, petunidin, and malvidin have also been detected [20,21]. Consumers are becoming more mindful, with an increasing demand for natural food ingredients, including naturally occurring pigments [22]. The anthocyanidin compounds of sour cherries are suitable for replacing synthetic pigments in food applications [5].

Several clinical studies have confirmed the positive effects of anthocyanins on the human body [23,24,25,26,27,28,29,30]. In addition to the anthocyanin content of the sour cherry fruits, there is a keen interest in its melatonin (N-acetyl-5-methoxytryptamine) content. The average melatonin content in Hungarian-bred sour cherry cultivars is 2.319 µg g⁻¹ [30,31,32].

The antioxidant profile of fruits varies widely among species [32,33], and within a species it can differ significantly among cultivars [20,33,34,35] as well as due to site-specific environmental conditions [36].

Similarly, the ascorbic acid content shows considerable fluctuation, influenced by cultivar characteristics, orchard management system, local growing conditions, the physiological stage of ripening, and year-to-year climatic variation [37]. These factors collectively contribute to pronounced qualitative and quantitative differences in both antioxidant composition and vitamin C levels.

The aim of this study was to evaluate the ripening-stage-dependent compositional changes in some Hungarian-bred sour cherry (Prunus cerasus L.) varieties, with a special view on biologically active compounds (anthocyanins, polyphenols, vitamin C, melatonin), organic acids, and sugars. However, this study contains only one-year data from 2022; its novelty value lies in examining “amarelle” type sour cherry alongside “morello” types.

2. Results

Pronounced cultivar-dependent differences were observed in soluble solids content, acidity, and their ratio. The lowest SSC was produced by ‘Korai pipacs’ and the highest was detected in ‘Újfehértói fürtös’ during this research. All observed varieties showed an increasing trend in the accumulation of this compound. ‘Újfehértói fürtös’ in all picking periods differed significantly from the others, due to its high SSC values.

The lowest acid values were produced by ‘Érdi bőtermő’, and the highest values were reached in ‘Újfehértói fürtös’. In the case of all observed varieties, the acid concentration started to increase until the second picking time, and after that it started to decrease. In the third picking time, ‘Korai pipacs’ and ‘Érdi bőtermő’ reached similar values, but ‘Újfehértói fürtös’ differenced significantly with its higher TTA values.

The lowest SSC/TTA ratio was in ‘Korai pipacs’ during the first and second picking times, but ‘Újfehértói fürtös’ reached the lowest rate in the third picking period. The highest value was detected in the first picking time in ‘Újfehértói fürtös’, and in Érdi bőtermő’ in the second and third picking times. In the case of ‘Korai pipacs’ and ‘Újfehértói fürtös’, this value started to decrease until the second picking time, and after that, it increased strongly. ‘Érdi bőtermő’ showed an increasing trend during the research period. In the case of ‘Korai pipacs’ and ‘Érdi bőtermő’, there were significant differences in the second picking time (Table 1).

Table 1.

Changes of soluble solid content (SSC °Brix), total titratable acid content (TTA g 100 g⁻¹), and soluble solid content/titratable acid content of ‘Korai pipacs’ (KP), ‘Érdi bőtermő’ (EB), and ‘Újfehértói fürtös’ (ÚF) during the three different ripening stages (Érd, Hungary, in 2022, SD_ssc = 1.09, SD_TA = 1.25, SD_SSC/TA = 2.22).

	SSC	TTA	SSC/TTA
KP 1st	11.37 ± 0.6 Aa	1.58 ± 0.09 ABb	7.20 ± 0.49 ABb
KP 2nd	12.80 ± 0.12 Aab	1.98 ± 0.05 Bc	6.46 ± 0.17 Aa
KP 3rd	14.77 ± 0.23 ABb	1.39 ± 0.05 Aa	10.63 ± 0.46 Cc
ÉB 1st	11.60 ± 0.18 Aa	1.53 ± 0.17 ABb	7.58 ± 0.72 ABa
ÉB 2nd	14.36 ± 0.17 ABb	1.62 ± 0.04 ABb	8.86 ± 0.33 Bb
ÉB 3rd	15.67 ± 0.51 Bb	1.33 ± 0.01 Aa	11.78 ± 1.02 Cc
ÚF 1st	18.36 ± 0.18 Ca	2.28 ± 0.05 Bab	8.05 ± 0.14 Ba
ÚF 2nd	18.71 ± 0.18 Ca	2.35 ± 0.04 Bb	7.96 ± 0.16 ABa
ÚF 3rd	20.61 ± 0.32 Db	2.22 ± 0.03 Ba	9.28 ± 0.29 BCb

Capital letters denote statistically significant differences (p < 0.05) in observed compounds among cultivars at various stages of ripeness. The lower-case letters indicate the significant differences (p < 0.05) in observed compounds within each variety during ripening.

The lowest glucose concentration was produced by ‘Korai pipacs’ in first two picking times. In the first two picking times, ‘Újfehértói fürtös’ reached the highest value, while in the third sample collection period, ‘Érdi bőtermő’ had the highest value. All cultivars showed a consistent increase in glucose concentration until the second picking time, after which levels decreased. In the second picking time, ‘Újfehértói fürtös’ reached a high glucose value, which significantly differed from the value reached by ‘Korai pipacs’ (Table 2).

Table 2.

Changes of important sugar compounds (mg mL⁻¹), detected by HPLC, of ‘Korai pipacs’ (KP), ‘Érdi bőtermő’ (EB), and ‘Újfehértói fürtös’ (ÚF) during the three different ripening stages (Érd, Hungary, in 2022).

	Glucose	Fructose	Sucrose	Sorbitol	Mannitol
			mg mL−1
KP 1st	57.84 ± 2.23 Aa	45.77 ± 1.93 Aa	0.48 ± 0.01 Aa	2.73 ± 0.13 ABb	0.05 ± 0.01 ABb
KP 2nd	77.29 ± 3.16 Bb	64.49 ± 3.01 Bb	0.65 ± 0.03 Bb	4.61 ± 0.19 Cc	0.26 ± 0.05 Dc
KP 3rd	9.72 ± 0.42 Cc	10.30 ± 0.48 Ac	0.87 ± 0.04 BCc	0.16 ± 0.007 Aa	0.02 ± 0.002 Aa
EB 1st	76.32 ± 3.11 Bb	58.04 ± 2.34 Ba	0.99 ± 0.04 Cb	3.78 ± 0.14 Bb	0.04 ± 0.01 Aba
EB 2nd	82.59 ± 3.98 Bc	64.96 ± 3.21 BCb	1.17 ± 0.05 Cc	4.72 ± 0.21 Cc	0.09 ± 0.02 Bb
EB 3rd	66.63 ± 3.02 Aba	51.48 ± 2.22 Aa	0.78 ± 0.03 Ba	2.17 ± 0.09 ABa	0.11 ± 0.02 Bb
ÚF 1st	79.46 ± 3.45 Bb	62.67 ± 3.02 Bb	1.02 ± 0.05 Ca	5.65 ± 0.18 CDb	0.16 ± 0.04 Cb
ÚF 2nd	101.60 ± 4.87 Dc	76.97 ± 3.44 Cc	3.43 ± 0.12 Db	8.98 ± 0.37 Dc	0.18 ± 0.03 Cc
ÚF 3rd	53.84 ± 2.31 Aa	35.01 ± 1.58 Aa	1.03 ± 0.04 Ca	1.69 ± 0.07 Aa	0.05 ± 0.01 Aba

Capital letters denote statistically significant differences (p < 0.05) in observed compounds among cultivars at various stages of ripeness. The lower-case letters indicate the significant differences (p < 0.05) in observed compounds within each variety during ripening.

Accumulation of fructose had a similar trend as the glucose. The lowest amount of fructose was produced by ‘Korai pipacs’ in all picking times. In the first two picking times ‘Újfehértói fürtös’ reached the highest value, while in the third picking time ‘Érdi bőtermő’ did. In the second picking time, ‘Újfehértói fürtös’ reached an extremely high fructose value, which differed significantly from all values reached for varieties during every picking time (Table 2).

Sucrose quantity was the lowest in the first two picking times in ‘Korai pipacs’, and in ‘Érdi bőtermő’ in the third picking time. In the case of ‘Korai pipacs’, the accumulation of sucrose had an increasing trend, but the remaining two varieties showed an increasing trend until the second picking time, and then concentration started to decrease. ‘Érdi bőtermő’ and ‘Újfehértói fürtös’ had a significant difference in the first picking time compared to ‘Korai pipacs’. ‘Újfehértói fürtös’ differed from the other two observed varieties in the second picking time (Table 2).

The lowest sorbitol concentration was produced by ‘Korai pipacs’ and in the third picking time the sorbitol concentration was under the limit of the HPLC. The highest values were detected in ‘Újfehértói fürtös’ during the research period. The trend of the sorbitol concentration was similar in each case, as the concentration started to increase until the second picking time, and after this point it decreased. ‘Újfehértói fürtös’, collected in the second picking time, differed significantly (Table 2).

For mannitol, the lowest amounts were produced by ‘Érdi bőtermő’ in the first two picking periods, and by ‘Korai pipacs’ in the third. The highest amounts were reached by ‘Újfehértói fürtös’) in the first, by ‘Korai pipacs’ (0.26 mg mL⁻¹) in the second, and by ‘Érdi bőtermő’ in the third picking period. ‘Érdi bőtermő’ had an increasing trend during the mannitol accumulation, ‘Korai pipacs’ and ‘Újfehértói fürtös’ had an increasing trend until the second picking time, and after that, the concentration started to decrease. The involved “amarelle” type ‘Korai pipacs’ produced the outstanding high mannitol concentration, in the second picking time, which differed significantly from the others (Table 2).

The ascorbic acid (Vitamin C) content was the lowest in ‘Korai pipacs’ and the highest in ‘Újfehértói fürtös’ during all sample collection periods. This compound content showed an increasing trend in all three observed varieties. In the first two picking periods ‘Érdi bőtermő’ and ‘Újfehértói fürtös’ had a significant difference from ‘Korai pipacs’, in the third picking period only ‘Újfehértói fürtös’ differed significantly from the other two varieties (Table 3).

Table 3.

Changes of important acid compounds (mg 100 mL⁻¹), detected by HPLC, of ‘Korai pipacs’ (KP), ‘Érdi bőtermő’ (EB), and ‘Újfehértói fürtös’ (ÚF) during the three different ripening stages (Érd, Hungary, in 2022).

	Vitamin C	Malic Acid	Oxalic Acid	Succinic Acid	Fumaric Acid
			mg 100 mL−1
KP 1st	5.74 ± 0.18 Aa	176.75 ± 7.36 Aa	7.27 ± 0.26 Aa	0.86 ± 0.03 Aa	0.21 ± 0.01 ABb
KP 2nd	5.85 ± 0.21 Aa	189.89 ± 8.21 Aa	14.63 ± 0.63 Bb	1.09 ± 0.04 AB	0.22 ± 0.01 ABb
KP 3rd	8.89 ± 0.38 Bb	194.44 ± 8.62 Aab	20.96 ± 0.94 Cc	1.16 ± 0.03 AB	0.12 ± 0.01 Aa
EB 1st	8.29 ± 0.40 Ba	384.00 ± 15.48 Bb	16.71 ± 0.44 Bb	0.97 ± 0.03 Aab	0.24 ± 0.01 ABab
EB 2nd	8.46 ± 0.32 Ba	363.79 ± 17.32 Bb	23.11 ± 1.10 Cc	1.25 ± 0.05 AB	0.27 ± 0.01 Bb
EB 3rd	9.92 ± 0.46 BCb	178.22 ± 6.89 Aa	7.87 ± 0.26 Aa	0.77 ± 0.02 Aa	0.21 ± 0.00 ABa
ÚF 1st	8.86 ± 0.34 Ba	669.44 ± 22.48 Cb	36.34 ± 1.24 Db	1.77 ± 0.07 Ba	0.85 ± 0.02 Dc
ÚF 2nd	9.55 ± 0.40 BCa	630.68 ± 36.41 Cb	59.17 ± 2.11 Ec	2.51 ± 0.11 Cb	0.41 ± 0.02 Cb
ÚF 3rd	13.46 ± 0.53 Db	216. ± 7.84 Aa	9.08 ± 0.36 Aa	3.24 ± 0.14 Dc	0.23 ± 0.01 ABa

Capital letters denote statistically significant differences (p < 0.05) in observed compounds among cultivars at various stages of ripeness. The lower-case letters indicate the significant differences (p < 0.05) in observed compounds within each variety during ripening.

The malic acid content was the lowest in ‘Korai pipacs’ and the highest in ‘Újfehértói fürtös’ during all sample collection periods. Malic acid content in ‘Érdi bőtermő’ and ‘Újfehértói fürtös’ showed a decreasing trend, but there was a slightly increasing trend in ‘Korai pipacs’. In the first two sample collection periods, all three varieties differed significantly from each other and in the third period there was no significant difference between the examined varieties (Table 3).

It was interesting that the oxalic acid content changed during the trial. In the first two sample collection periods, the lowest values were produced by ‘Korai pipacs’, and the highest values were by ‘Újfehértói fürtös’. In the third sample collection period the lowest value was produced by ‘Érdi bőtermő’ and the highest by ‘Korai pipacs’. In the case of ‘Korai pipacs’, the oxalic acid concentration showed an increasing trend, but for the other two observed sour cherry varieties, the concentration of this compound increased until the second sample picking, and decreased after that. The values of ‘Újfehértói fürtös’ had a significant difference in the first two sample picking periods compared to other varieties (Table 3).

For succinic acid, ‘Korai pipacs’ and ‘Érdi bőtermő’ samples reached the lowest value in the first and second picking times, and ‘Érdi bőtermő’ showed the lowest concentration in the third. The accumulation of succinic acid showed an increasing trend in ‘Korai pipacs’ and ‘Újfehértói fürtös’, whereas in ‘Érdi bőtermő’ the concentration increased up to the second harvesting time and subsequently decreased at the full-ripening stage (Table 3).

The fumaric acid was the lowest in ‘Korai pipacs’, and the highest in ‘Újfehértói fürtös’ in all picking times. The concentration of this compound started to increase slightly until the second picking time in the first two varieties, and it decreased at the full-ripening period. On the contrary, ‘Újfehértói fürtös’ had a decreasing trend. Only the first two picking times of ‘Újfehértói fürtös’ had a significant difference (Table 3).

The lowest cyanidin concentration was detected in ‘Korai pipacs’ during the whole sample collecting period, and the highest was measured in ‘Érdi bőtermő’. Cyanidin remained stable in ‘Korai pipacs’, but showed an increasing trend in ‘Érdi bőtermő’ and ‘Újfehértói fürtös’. ‘Érdi bőtermő’ collected in the second and third picking times had a significant difference from the other samples (Table 4).

Table 4.

Changes of compounds with antioxidant-properties (cyanidin, pelargonidin, delphinidin in µg/mL, and melatonin in ng/mL) of ‘Korai pipacs’ (KP), ‘Érdi bőtermő’ (EB), and ‘Újfehértói fürtös’ (ÚF) during the three different ripening stages (Érd, Hungary, in 2022).

	Cyanidin	Pelargonidin	Delphinidin	Melatonin
	µg/mL	µg/mL	µg/mL	ng/mL
KP 1st	23.42 ± 1.01 Aa	66.26 ± 2.86 Aa	2.53 ± 0.10 Aa	13.25 ± 0.55 Dc
KP 2nd	23.69 ± 1.11 Aa	77.33 ± 3.89 Ab	3.12 ± 0.12 ABb	10.68 ± 0.43 Cb
KP 3rd	23.42 ± 0.98 Aa	80.65 ± 2.11 Ab	4.20 ± 0.19 Bc	3.59 ± 0.10 ABa
EB 1st	42.05 ± 1.97 Ba	117.57 ± 5.24 Ba	3.54 ± 0.16 ABa	9.38 ± 0.38 BCb
EB 2nd	92.30 ± 3.36 Cb	158.78 ± 7.13 Cc	4.41 ± 0.20 Bb	11.65 ± 0.47 Cc
EB 3rd	113.71 ± 4.71 Dc	147.27 ± 7.01 Cbc	4.68 ± 0.23 BCb	8.61 ± 0.33 BCa
ÚF 1st	34.72 ± 1.06 Aba	117.61 ± 4.38 Ba	6.45 ± 0.31 Ca	7.73 ± 0.31 Bb
ÚF 2nd	49.77 ± 1.94 Bb	131.76 ± 6.13 BCb	7.53 ± 0.34 CDb	7.32 ± 0.29 Bb
ÚF 3rd	59.40 ± 2.06 BCc	159.86 ± 7.25 Cc	7.97 ± 0.37 Db	1.56 ± 0.02 Aa

Capital letters denote statistically significant differences (p < 0.05) in observed compounds among cultivars at various stages of ripeness. The lower-case letters indicate the significant differences (p < 0.05) in observed compounds within each variety during ripening.

‘Korai pipacs’ had the lowest pelargonidin concentration. In the first picking time, the highest pelargonidin concentrations were measured in the fruits of ‘Érdi bőtermő’ and ‘Újfehértói fürtös’. At the second picking time, the fruits of ‘Érdi bőtermő’ had the highest pelargonidin content, while at the third picking time the highest values were measured in ‘Érdi bőtermő’ and ‘Újfehértói fürtös’. Accumulation of this compound showed an increasing trend in ‘Korai pipacs’ and ‘Újfehértói fürtös’. For ‘Érdi bőtermő’, the amount of pelargonidin started to increase until the second picking time and decreased at the full ripen period. All ‘Érdi bőtermő’ and ‘Újfehértói fürtös’ samples had significant differences from ‘Korai pipacs’ (Table 4).

The lowest delphinidin concentration was detected in ‘Korai pipacs’, and the highest was measured in ‘Újfehértói fürtös’. In all varieties, accumulation of this compound had an increasing trend. ‘Újfehértói fürtös’ had significant differences in all picking times from each other (Table 4).

The melatonin concentration was very low across all varieties observed. The lowest amount of melatonin was by ‘Újfehértói fürtös’ and ‘Korai pipacs’ in the third picking times. The highest amount of melatonin was detected in the first and second picking time in ‘Korai pipacs’, and in ‘Érdi bőtermő’ in the second and third picking time. All observed varieties had different trends; ‘Korai pipacs’ and ‘Újfehértói fürtös’ had a decreasing trend, while ‘Érdi bőtermő’ produced an increasing trend until the second picking time, and then started to decrease (Table 4).

The lowest value of polyphenol content (TPC) was observed by ‘Korai pipacs’, and the highest was reached by ‘Újfehértói fürtös’ in all picking times. For all observed cultivars, the accumulation of polyphenols showed a slightly increasing trend. ‘Érdi bőtermő’ picked in the second and third picking times differed significantly from other varieties, as examined in this trial (Table 5).

Table 5.

Changes of total polyphenol (TPC) (mg GS 100 g⁻¹) and ferric reducing/antioxidant power (FRAP) (mM AS L⁻¹) of ‘Korai pipacs’ (KP), ‘Érdi bőtermő’ (EB), and ‘Újfehértói fürtös’ (ÚF) during the three different ripening stages (Érd, Hungary, in 2022).

	TPC	FRAP
	mg GS 100 g−1	mM AS L−1
KP 1st	153.70 ± 2.75 Aa	39.51 ± 3.09 Ba
KP 2nd	210.70 ± 4.14 Aab	42.29 ± 2.17 Ba
KP 3rd	233.28 ± 5.21 Bb	131.11 ± 6.67 Db
EB 1st	528.12 ± 18.19 Ca	32.00 ± 7.74 ABa
EB 2nd	538.32 ± 16.11 Cb	54.19 ± 2.49 Cb
EB 3rd	560.20 ± 27.87 Cb	56.37 ± 3.04 Cb
ÚF 1st	953.81 ± 15.50 Da	23.05 ± 3.17 Aa
ÚF 2nd	946.97 ± 16.69 Da	49.47 ± 2.01 Bb
ÚF 3rd	1006.70 ± 33.45 Eb	50.17 ± 4.81 Cb

Capital letters denote statistically significant differences (p < 0.05) in observed compounds among cultivars at various stages of ripeness. The lower-case letters indicate the significant differences (p < 0.05) in observed compounds within each variety during ripening.

The lowest FRAP values were detected in ‘Újfehértói fürtös’ in the first and the third picking times, while in the second picking time ‘Korai pipacs’ reached the lowest value. The highest values were reached by ‘Korai pipacs’ in the first and third picking times. In the case of second picking time, ‘Érdi bőtermő’ had the highest value. In all studied varieties, the FRAP values had an increasing trend. ‘Érdi bőtermő’ values picked in the second and third picking times and ‘Korai pipacs’ values in the third picking time had significant differences (Table 5).

3. Discussion

These results are comparable to those reported in previous studies, however, the differences observed likely stem from the fact that we monitored the entire ripening process, whereas earlier research typically assessed SSC only at the optimal ripening stage. Based on our results, SSC values of all examined varieties showed increasing tendencies during the whole ripening period. This trend was similar to results of Aslantaş et al. (2016) [18], who measured the ‘Kütahya’ Turkish-bred sour cherry variety under Turkish climate conditions, where they collected results five times during the whole ripening period, as well as results of Pedisić et al. (2007) [36], who collected from “marasca” type sour cherry (Prunus cerasus var. Marasca) under Croatian climate conditions. Grafe and Schuster (2014) [38] examined 78 sour cherry accessions, among them eight Hungarian-bred varieties, collected from the ex situ gene bank collection (Dresden, Germany). They stated that the Hungarian-bred genotypes reached higher SSC values compared to the other observed genotypes. Wang et al. (2021) [39] had similar conclusions (13.1–21.8 °Brix) after examining 21 sour cherry genotypes, among them seven Hungarian-bred, in a Chinese orchard at the optimal ripening time.

The Hungarian-bred sour cherry varieties, measured in this trial, produced higher SSC values (11.37–20.61 °Brix) compared to “amarelle” type ‘Montmorency’ (13.74 °Brix) grown at the Michigan State University’s Clarksville Horticulture Experiment Station (Clarksville, MI, USA) [40]. However, our SSC values were lower compared to “morello” type genotypes (17.0–26.5 °Brix) due to the different genetic background and climatic conditions [41].

TTA values of the observed Hungarian-bred varieties were similar to the literature data; however, we must note that this parameter showed considerable variation both among the cultivars and across the different ripening stages (Table 1). The TTA values showed decreasing trends during the full ripening period in our trial, as published by Aslantaş et al. (2016) [18]. Those data were influenced by the climatic conditions [41].

The SSC and the TTA values together determine flavour of the fruits, for consumers’ preference and processing possibilities. Our results confirmed that there were differences in the varieties and in SSC/TTA rate by ripening period. Wojdyło et al. (2014) [42] examined 33 sour cherry varieties, and detected big differences in SSC/TTA rate (5.7–15.3). The acidic character of ‘Újfehértói fürtös’ is caused by a higher SSC and high acid content. However, the sweet flavour of ‘Érdi bőtermő’ is caused by relative high SSC and lower acid content. Acidic flavour of ‘Korai pipacs’ can be matched with lower SSC (11.37–14.77 °Brix), and high acid content (1.58–1.39%), such as the German-bred ‘Schattenmorelle’ (13.8 °Brix, 1.8%) [20].

The character of sour cherries’ flavour is influenced by sugar and acid fractions. Based on our results, the main component of the sour cherries was glucose followed by fructose, sucrose, and sorbitol in smaller quantities, as well as mannitol on the limit of the detectability. This is similar to published data [43,44,45,46,47]. Changes in the glucose and fructose contents could be described using a saturation model, and in the case of sucrose content with an exponential model during the ripening [45]. According to Sokół-Łętowska et al. (2020) [48], the main component of “amarelle” type ‘Montmorency’ was fructose, and it did not contain sucrose, but in the “morello” and “marasca” type sour cherries the main component was glucose. On the contrary, the “amarelle” type, Hungarian-bred ‘Korai pipacs’ contained slightly more glucose than fructose, as well as a significant sucrose content which increased for the third picking time (fully ripened). The carbohydrates were present in different quantities in the varieties and influenced the ripening status.

In the examined sour cherry genotypes, five organic acids were detected being malic acid, ascorbic acid, oxalic acid, succinic acid, and fumaric acid, with the main component being malic acid, which showed similarity to the literature [45,47]. The oxalic and fumaric acids could be detected in much smaller quantities in the examined fruits. Contrary to previously published results, succinic acid could be measured in the Hungarian-bred sour cherry varieties in a notable quantity. Among the varieties, both the organic acid content and the vitamin C concentration showed considerable differences, which were strongly influenced by the ripening status. As sour cherry is an excellent source of vitamin C, the increase in its content during ripening was particularly notable. It increased during the maturity period, but according to results of Aslantaş et al. (2016) [18], this compound showed a decreasing trend like the other acid components. Wojdyło et al. (2014) [42] examined vitamin C content of 33 sour cherry varieties, and Bonerz et al. (2007) [20] sought to detect the vitamin C content of four sour cherry varieties; both research groups observed significant differences in vitamin C content of the genotypes. Wojdyło et al. (2014) [42] detected 18.59 mg 100 mL⁻¹ vitamin C content in ‘Érdi bőtermő’ under Polish climatic conditions. This concentration is approximately double the value measured in our study. Papp et al. (2010) [43] measured in small-scale a smaller vitamin C content, compared to our values. Genetically coded characters of the varieties can be modified by fruit sites and other growing technology elements.

Burkhardt et al. (2001) [44] first reported that the fruits of sour cherry contained melatonin in large amounts (quantity). A similar amount of this compound was detected in ‘Montmorency’ (13.46 ng g⁻¹), and was similar to the value detected in the frame of this research in “amarelle” type ‘Korai pipacs’, although a significant amount of melatonin was detected in ‘Újfehértói fürtös’ (2.06 ng g⁻¹) as well. However, no significant difference was observed in the literature data between quantity of melatonin and the ripening status of the fruits. In this research, there was not any significant difference in concentration of melatonin and the fruit ripeness at the first and second picking times, while for the third picking time its concentration decreased notably.

The antioxidant content of sour cherries can be characterized by total phenol and FRAP values, as well as the anthocyanidin profile [42,47]. Our results indicated some differences in the quantity of antioxidants compounds, which is influenced by its ripening status. Literature results have confirmed the polyphenol content of sour cherry varieties on a wide scale [20,33,34,42,47]. According to results of Khoo et al. (2011) [34], 34 hybrids, derived from the cross of Danish-bred ‘Stevnsbaer Brigitte’ and ‘Érdi bőtermő’, contained the highest polyphenol and anthocyanidin contents (754 mg GS 100 g⁻¹, 272 mg 100 g⁻¹), followed by ‘Érdi bőtermő’ (222 mg GS 100⁻¹, 98 mg 100 g⁻¹) under Danish climatic conditions. ‘Érdi bőtermő’ fruits in Hungarian climatic conditions had more than twice the polyphenol content (528.12–560.20 mg GS 100 g⁻¹) than Khoo et al. (2011) [34]. Papp et al. (2010) [43] detected a remarkably high polyphenol content (4440 mg GS 100 g⁻¹) in ‘Pipacs 1’ and other “amarelle” type sour cherry in Hungarian climatic conditions. Pedisić et al. (2007) [36] examined polyphenol and anthocyanidin content of the “marasca” type sour cherry genotypes, collected from two different Croatian fruit sites (Zadar, Split). This research group observed significant differences in the antioxidant content of the examined varieties at different ripening stages and also reported a continuous increase in antioxidant levels during maturation. These variant results confirm that synthesis of antioxidant compounds depends on the variety, ripening status, and the climatic effects.

Kirakosyan et al. (2009) [3] reported that the Hungarian-bred sour cherry varieties had a higher anthocyanin content compared to the sour cherry varieties derived from western Europe and the United States of America. Previous researches have stated that the main component of the total anthocyanin content was cyanidin (80–90%) [3,20,47]. Based on our results, the main naturally occurring anthocyanidin components detected in the Hungarian-bred sour cherry varieties were cyanidin and pelargonidin, while delphinidin appeared in smaller amounts. A notably high cyanidin concentration was measured in ‘Érdi bőtermő’, whereas the “amarelle”-type ‘Korai pipacs’ showed significantly lower levels of this compound. Pelargonidin was also detected in substantial amounts in ‘Érdi bőtermő’ and ‘Újfehértói fürtös’, whereas the “amarelle”-type ‘Korai pipacs’ contained considerably lower concentrations, similar to its cyanidin content. According to our findings, the levels of the examined anthocyanidin components increased continuously during the ripening period. It is likely that the non-“morello”-type “amarelle” cultivars (e.g., ‘Montmorency’, ‘Korai pipacs’), which originate from a different genetic centre, differ from the “morello”-type varieties native to the Carpathian Basin [48,49,50,51,52,53].

The internal quality characteristics of the Hungarian sour cherry cultivars, examined in 2022, clearly confirmed the decisive role of genetic background and the ripening process. Monitoring the entire ripening period highlighted the dynamic changes in sugar, acid, vitamin, and antioxidant components, which in several respects differed from earlier results that had examined only the optimal stage of ripeness. The differences among cultivars, as well as the variations observed during ripening, indicate that the quality parameters of sour cherries are the result of complex genetic and environmental interactions. This knowledge provides an important basis for cultivation technology decisions and for the selection of cultivars intended for processing purposes.

4. Materials and Methods

The experimental orchard was established at the Experimental Farm of Fruit Research Centre of Hungarian University of Agriculture and Life Science (in Érd, Central Hungary; 47°20′35.61″ N and 18°51′35.75″ E, 120 m above sea level), in the spring of 2004; all trees were grafted on selected seedlings called Prunus mahaleb seedling rootstock ‘C. 500’, which is the dominant rootstock for sweet and sour cherry orchards in Hungary [54]. The trees were planted in a randomly block design (based on randomly block design) to tree-distance 6 m × 4 m (24 m² tree⁻¹, eq. tree density: 416 trees ha⁻¹, 10 replications/variety) between the rows, and trained to spindle canopy. The experimental orchard was not irrigated.

The trial was planted on loamy soil with high lime (pH = 7.5, total lime content in the top 120 cm layer 5%) and humus content (0.8–1.5%). Considering the Arany-type cohesion index (KA) the KA = 49 refers to medium compactness.

In 2022, the average annual temperature and the spring months were colder, the average temperature during the vegetative season was 1.2 °C hotter in that year compared to the previous ten years, and the precipitation was extreme low, 18% less yearly precipitation compared to the previous decade. The distribution of rain was not ideal for the sour cherry trees, as 71% of the total rainfall was during the vegetative season. This rate was 67% during the same period of the past ten years. The increased average temperature and less precipitation caused a long-term drought period especially during the vegetative season, therefore this year is called “year of historic drought”. The detailed meteorological conditions of the site are presented in Table 6.

Table 6.

Meteorological data during the sample collection.

Parameters	2022 Year of Examination	2012–2021
average yearly temperature	11.7 °C	13.7 °C
average yearly temperature during the growing season (between April and September)	18.5 °C	17.3 °C
number of frosty nights during spring (between March and May)	36	19.2
average yearly luminous flux	1021 L/m2/day	1016 L/m2/day
annual average of sunshine hours	2070 sunny hours	2056 sunny hours
yearly precipitation	443.6 mm	535.5 mm
precipitation during the vegetative season (between April and September)	315.2 mm	362.5 mm

4.1. Plant Material

Three sour cherry state approved varieties were tested in this trial. ‘Korai pipacs’ (henceforth KP) is an “amarelle” type sour cherry variety, derived from the cross between ‘Pándy’ and ‘Császár meggy’. This variety is self-fertile; its ripening time is at the beginning of the 2nd week of June. The fruits are medium or large with 21 to 22 mm in diameter, and 4 to 5 g fruit weight. Its fruit skin is carmine, but its flesh is white or light yellow, and its juice is pinkish, non-staining. The flavour is sourish–sweet, and this variety has a fine aroma. ‘Érdi bőtermő’ (henceforth EB) is a “morello” type sour cherry derived from the cross of ‘Pándy’ and ‘Nagy angol’. This self-fertile variety dominates the Hungarian sour cherry production; its ripening time is at the end of the 2nd week of June. The fruits are medium to large with 22 to 23 mm in diameter, and 5 to 6 g fruit weight. Its fruit skin is dark carmine, but its flesh is dark red, and its juice is dark red, medium staining. The flavour is harmonic sourish–sweet. The control for this trial was ‘Újfehértói fürtös’ (henceforth ÚF), which is derived from the selection from the local population in a small town called Újfehértó. This “morello” type, self-fertile variety is the second most important in the Hungarian sour cherry production based on its growing area. Its ripening time is at the beginning of 1st week of July, about 10 days before ‘Schattenmorelle’. The fruits are medium to large with 22 to 24 mm in diameter, and 5 to 6 g fruit weight. The fruit skin is shiny dark red, but its flesh is sanguine, and its juice is dark red, slightly staining. Its flavour is harmonic sourish–sweet, and it has high sugar and acid content [2,55].

4.2. Sample Collection and Preparation

Fruit samples (5000 g per cultivar and sampling date) were collected at three stages throughout the ripening period (Table 7): pink fruit (BBCH 81), light red fruit (BBCH 85), and fully ripened fruit (BBCH 89). The fruits were collected from the four cardinal points of the canopy, taking into consideration the typical appearance of the ripening stages, described in Table 7, to have homogeneous samples and mixed. The fruits were picked individually and selectively to decrease the variability, because all fruits did not ripe at the same time; there is a 1 to 2 day difference between them, depending their position on the trees. At the last stage, ‘Korai pipacs’ reached a carmine colouration, while ‘Érdi bőtermő’ and ‘Újfehértói fürtös’ developed dark carmine to dark red hues. The first sampling came before the optimal ripening stage, the second represented the optimal harvest time, and the third corresponded to the overripe stage.

Table 7.

Sample collection times by variety [56].

	BBCH 81	BBCH 85	BBCH 89
Korai pipacs (KP)	2 June 2022	8 June 2022	15 June 2022
Érdi bőtermő (EB)	2 June 2022	8 June 2022	15 June 2022
Újfehértói fürtös (ÚF)	14 June 2022	21 June 2022	28 June 2022

After each sampling event, the fruits were immediately transported to the preparation laboratory. The stems and seeds were removed, and the fruit material was divided into five equal portions. The fruit flesh was then homogenized at room temperature using a household hand blender. All five replicates were frozen at −28 °C until further analysis.

For the measurements, 5 g of each sample was placed in a Falcon tube and diluted with water to 25 mL using an Edmund Bühler SM 30 control shaker (Edmund Bühler GmbH, Bodelshausen, Germany) (200 rpm/min). The samples were then centrifuged at 4 °C in a Hettich Mikro 22R ultracentrifuge (Andreas Hettich GmbH & Co. KG, Tuttlingen, Germany) (15,000 rpm/min for 5 min). The supernatant of centrifuged extractant was filtered on a 0.45 µm MILLEX^®-HV Syringe Driven Filter Unit (MilliporeSigma, 400 Summit Drive, Burlington, Billerica, MA, USA) (SLHV 013 NL, PVDF Durapore), purchased from Millipore Co. (Bedford, MA, USA). The samples thus obtained were injected into the HPLC system or used to the spectrophotometric measure. Five replicates were done in all cases.

4.3. Determination of Soluble Solid Content and Total Titratable Acid Content

The soluble solid content (SSC) of the homogeneous, filtered fruit juice was determined in °Brix (g 100 g⁻¹) according to the regulation Codex Alimentarius (1995) [57] using a refractometer (ATAGO Palette PR-10, Atago Co., Ltd., Tokyo, Japan). Total titratable acidity (TTA) was determined with 0.1 N NaOH solution based on MSZ EN Nr 12147:1998 Hungarian Standard [58]. The total titratable acidity (TTA) was given in malic acid equivalents (m/m%). Five replicates were done in all cases. The SSC/TTA ratio is a value calculated from the ratio of the total soluble solids to the titratable acid content.

4.4. HPLC Analyses

HPLC equipment: The WATERS (Milford, MA 01757, USA) HPLC system had the following components: 2487 UV/VIS Dual λ absorbance detector for organic acids, 2475 multi λ fluorescence detector for melatonin, 2414 RI refractive index detector for sugar determinations, a 1525 Binary HPLC pump, a column thermostat, a 717plus autosampler (set to 5 °C), and an in-line degasser, controlled using EMPOWERTM2 software (Empower™ 2 Feature Release 2 (Waters Corporation, Milford, MA, USA)).

4.4.1. Determination of Sugar Components

Chromatographic separation was performed on a Sugar Pak I column (300 mm × 6.5 mm) tempered to 90 °C. The linear mobile phase consisted of water containing 50 mg L⁻¹ Ca-EDTA, flow rate 0.5 mL min⁻¹. Detection was carried out using a WAT 2414 RI detector, the cell temperature set to 40 °C. Calibration was based on external standards of glucose, fructose, sucrose, sorbitol, and mannitol. This aqueous-based approach is consistent with previously published methods for sugar analysis in fruits [59,60].

4.4.2. Determination of Organic Acid Components

Organic acids were separated on a Shodex RSpak KC-811 column (Showa Denko, Tokyo, Japan) equipped with a KG-G guard column (Showa Denko), both tempered to 40 °C. The mobile phase was water containing 0.1% phosphoric acid, linear flow rate set 1 mL min⁻¹. Detection was performed at 220 nm using a WAT 2487 UV/VIS dual λ absorbance detector (Waters CO). Calibration curves were prepared from aqueous standard solutions of malic, succinic, fumaric, and oxalic acids [61,62,63].

Vitamin C was quantified on an Atlantis dC18 column (Waters Corporation) (4.6 mm × 150 mm, 5 µm) using an isocratic elution of methanol and 0.1% phosphoric acid (15:85, v/v) at 1 mL min⁻¹. Detection was performed at 254 nm with WAT 2487 UV/VIS dual absorbance detector. Calibration was based on serial dilutions of an ascorbic acid standard solution. Aqueous extraction of vitamin C is also documented in the literature under controlled conditions [61,62].

4.4.3. Determination of Anthocyanidin Components

Anthocyanidins were separated on a Symmetry C18 column (Waters Corporation) (4.6 mm × 150 mm, 5 µm) tempered to 40 °C. The mobile phase consisted of water, methanol, and acetonitrile containing 2.5% acetic acid (35:5:10, v/v/v); linear eluent flow was 1 mL min⁻¹. Detection was carried out at 530 nm with WAT 2417 UV/VIS dual absorbance detector. Quantification was based on external standards of cyanidin, pelargonidin, and delphinidin. The use of aqueous or aqueous–organic extraction for phenolic compounds is supported by several studies [59,60,64].

4.4.4. Determination of Melatonin

Melatonin was analysed on an Atlantis dC18 column (4.6 mm × 150 mm, 5 µm) using an isocratic mobile phase 1 mL min⁻¹ containing 0.1% formic acid in water and methanol (60:40, v/v). Detection was carried out with WAT2475 Multi λ Fluorescence detector (Waters Corporation) set λ_ex = 280 nm and λ_em = 348 nm. Calibration was based on serial dilutions of a melatonin as a standard solution. The applicability of aqueous or aqueous–buffer extraction for melatonin in plant tissues has been demonstrated previously [65,66].

4.4.5. Determination of Some Antioxidant Parameters with Spectrophotometer

Polyphenol content was determined in the presence of Folin–Ciocalteu’s reagent and the absorbance was measured at λ = 765 nm in a UV/VIS spectrophotometer (Hitachi U-2800, Tokyo, Japan), and compared to a calibration curve which was prepared with known amounts of gallic acid, according to the method of Singleton and Rossi (1981) [67]. The results were calculated as mg gallic acid equivalent (GAE) per 100 g.

The ferric reducing/antioxidant power (FRAP) assay was carried out according to Benzie and Strain (1996) [68]. The FRAP assay is based on the reduction of the Fe³⁺-2,4,6-tripyridyl-S-triazine complex to the ferrous form (Fe²⁺) and the intensity of the reaction is monitored by measuring the change of absorption at 593 nm.

4.5. Statistical Analysis

The statistical analysis was performed using IBM SPSS 27 program (SPSS Inc., Chicago, IL, USA). Two-way analysis of variance (ANOVA) was applied to evaluate the mean differences between factors and to separate homogeneous groups. When significant differences were found, Tukey’s Honestly Significant Difference (HSD) post hoc test was used for pairwise comparisons. Differences were considered statistically significant at the p < 0.05 level (n = 5).

Capital letters denote statistically significant differences (p < 0.05) in observed compounds among cultivars at various stages of ripeness. The lower-case letters indicate the significant differences (p < 0.05) in observed compounds within each variety during ripening. Sampling was performed at three harvesting times corresponding to BBCH stages 81, 85, and 89. Two characters with the same letter size indicate that there is no statistically significant difference between the two groups.

5. Conclusions

The present study demonstrated that the examined Hungarian-bred sour cherry cultivars differ markedly in their antioxidant profiles and anthocyanin composition across the ripening stages. ‘Korai pipacs’ showed the highest melatonin content and lower sugar levels, while ‘Érdi bőtermő’ and ‘Újfehértói fürtös’ exhibited elevated antioxidant capacity and favourable SSC/TTA ratios.

The anthocyanin composition, dominated by cyanidin glycosides, underscores the importance of these cultivars as natural sources of naturally occurring pigments.

1. Fruits of ‘Korai pipacs’ are suitable for usage in functional foods. The main advantage of this cultivar is its higher melatonin content compared to the other varieties examined in this study. Furthermore, due to its lower sugar content, ‘Korai pipacs’ may be more suitable for individuals who need to limit their sugar intake.

2. ‘Érdi bőtermő’ and ‘Újfehértói fürtös’ are suitable for both fresh consumption and food industrial purposes due to their outstanding antioxidant content, favourable sugar compounds, and SSC/TTA rate (concentrate).

3. This study presents fruits of the ‘Érdi bőtermő’ and ‘Újfehértói fürtös’ cultivars as sources of naturally occurring pigments. Further investigations are required to characterize the alterations in anthocyanidin compounds during processing.

Abbreviations

The following abbreviations are used in this manuscript:

BBCH	Biologische Bundesanstalt, Bundessortenamt and CHemical industry
EB	Érdi bőtermő
FRAP	ferric reducing/antioxidant
FW	fresh weight
GAE	gallic acid equivalent
HPLC	High-performance liquid chromatography
KP	Korai pipacs
SSC	soluble solid content
ÚF	Újfehértói fürtös
TPC	total polyphenol content
TTA	total titratable acidity

Author Contributions

Conceptualization, G.F., G.S. and G.B.; methodology, G.F. and G.V.; validation, G.F., G.V., G.S. and G.B.; formal analysis, G.F., M.G., S.M., V.K., E.M.-G., L.K., G.V. and G.B.; resources, G.F. and G.B.; data curation, G.F., G.V. and G.B.; writing—original draft preparation, G.F., M.G., S.M., V.K., E.M.-G., L.K., G.V. and G.B.; writing—review and editing, G.F. and G.B.; visualization, G.F. and G.B.; supervision, G.F. and G.B. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Funding Statement

This research received no external funding.