Introduction

Wild edible fruits including rowanberry, rose hip, cornelian cherry, barberry, myrtle, elderberry etc. have played an important role in supplementing staple foods by supplying vitamins, minerals, and trace elements in order to obtain a balanced diet for healthier life (Vijayan et al. 2008; Ercisli et al. 2008; Serce et al. 2010; Fazenda et al. 2019). Their interest as a source of nutraceuticals has been highlighted in recent studies (Leonti 2012; Guney et al. 2019).

The wild edible fruit species have been part of the human diet and they can be an interesting genetic resource for the development of new food products (Tardio et al. 2006; Butiuc et al. 2019). Wild edible fruits may have great potential as a source of unusual colours and flavours, bioactive compounds and as sources of dietary supplements or functional foods (Salvatore et al. 2005). In addition due to continuous seed propagation of wild edible fruit species, there is a great variability for most of the morphological and biochemical characteristics of wild non-cultivated fruits that could be important for breeding activities to obtain healthier fruits.

Sorbus is a large genus including nearly 250 species and among Sorbus species, rowanberry (Sorbus aucuparia) and service tree (Sorbus domestica) are more common for fruit production. Sorbus aucuparia found between 800–2500 m and Sorbus domestica found between 100–1300 m in Turkey and have been using as ornamental plants in botanical gardens and park areas. In rural areas and forests, fruits of particularly rowanberry (Sorbus aucuparia) and service tree (Sorbus domestica) are used as food or food ingredients (Gil-Izquierdo and Mellenthin 2001). Also, alcoholic drinks, for example, wine, beer, and spirits, such as brandy or vodka, were made from or flavored with rowanberry juice or their fruits (Mikulic-Petkovsek et al. 2017) mostly because of the biochemical compounds that help to clear and preserve alcoholic drinks, adding flavor, astringency, bitterness, and extra sugars. In Turkey, rowanberry fruits have been consumed due to their nutritive and medicinal properties. It is surmised that it reduces diabetic symptoms, the reason of which these fruits have been traditionally used as an antidiabetic agent (Termentzi et al. 2008).

In literature, there was limited information about morphological and biochemical diversity among rowanberry fruit. Thus, in this study, we aimed to determine and compare some important morphological and biochemical features of Sorbus aucuparia genotypes, naturally growing in Coruh valley in Turkey due to their potential application in functional foods.

Materials and Methods

Plant Material

Commercially ripe fruit of Sorbus aucuparia L. is naturally grown in Coruh valley located in the Northeastern part of Turkey were sampled. During commercial harvest period, red color Sorbus aucuparia L. fruit was collected in 2017 from 12 pre-selected genotypes that show higher yield, pest and diseases free and more attractive bigger fruit.

Morphological Characteristics

The external fruit color of 40 fruit from each genotype was determined with a Minolta Chroma Meter CE-400, recording L∗, a∗, and b∗ color coordinates on the berry surface. Color parameters a∗ and b∗ extend from −60 to 60; a∗ negative is for green and a∗ positive is for red and b∗ negative is for blue and b∗ positive is for yellow. The data were expressed as h° (hue angle) calculated as tan−1 (b∗/a∗) in 0 to 360° (0° = red, 90° = yellow, 180° = green, and 270° = blue). Minolta a* and b* values were used to compute values for chroma as well.

Biochemical and Bioactive Composition

Sample Preparation and Extraction

For the analyses of the organic acids, specific sugars, total phenolic contents, and total antioxidant capacity analyses, the harvested fruit was immediately frozen and stored at −20 ºC until further analysis. During the analysis, the fruits were taken from refrigerator and thawed to 24–25 ºC. A laboratory blender was used to homogenise the fruit samples (100 g lots of fruits per genotypes) and a single extraction procedure taking 3 g aliquots transferred inside tubes and extracted for 1 h with 20 mL buffer including acetone, water (deionized), and acetic acid (70:29.5:0.5 v/v) was carried out (Singleton and Rossi 1965).

Extraction of Sugars and Organic Acids

Five grams of samples slurries were mixed with deionized water or metaphosphoric acid (2.5%) for the analysis of individual sugar and organic acid, respectively. The obtained homogenates were centrifuged at 10,000 rpm for 10 min. The samples were filtered into HPLC vials using 0.45 μm PTFE membrane filter for analysis. All HPLC solvents were sonicated. All samples and corresponding standard injection were repeated three times and the mean values were calculated.

Chromatographic Conditions

The Perkin Elmer HPLC system controlled by Totalchrom navigator software (version 6.2.1), consists of a pump and UV detector was used for analysis of the samples. Organic acids separation and determination were performed as on the method reported by Shui and Leong (2002). The sugars were determined using the method of Bartolome et al. (1995) with help of HPLC with refractive index (RI) detector. The separation was carried out on SGE SS Exsil amino column (250 × 4.6 mm ID). The isocratic elution was performed using acetonitrile (80%) and deionized water (20%) with a flow rate of 0.9 mL/min. The column was operated at 30 °C and the sample injection volume was at 20 μL. Quantification of organic acids and sugars were performed against the reference standards.

pH and Vitamin C

Flesh parts of fruit samples were used to assess pH and vitamin C. Vitamin C was determined with the reflectometer set of Merck Co (Merck RQflex). pH of fruit was determined by a pH-meter.

Total Phenolic Content

The total phenolic content (TPC) of the samples was evaluated using the method of Singleton and Rossi (1965). In this procedure, each extract (1 mL) was mixed with Folin-Ciocalteu’s reagent and water 1:1:20 (v/v). The samples were incubated for 8 min. Then sodium carbonate (10 mL) having a concentration of 7% (w/v) was added. After incubation for 2 h, the absorbance at 750 nm was measured. The total phenolic content was calculated against the reference standard calibration curve of Gallic acid. The TPC was expressed as mg of gallic acid equivalents (GAE) per 100 g of sample (fresh weight (FW)).

Ferric Reducing Antioxidant Power Assay

FRAP (Ferric reducing antioxidant power) assay was used for antioxidant capacity analysis. For this purpose, acetonic fruit extract (50 µL), FRAP reagent (2.95 mL), acetate buffer (0.1 mol/L), TPTZ (10 mmol/L), and ferric chloride of 20 mmol/L (10:1:1 v/v/v) were used. The values of samples absorbance were compared with those of the reference standard calibration curves in the range of 10–100 µmol/L of Trolox was used to determine FRAP values of samples. The FRAP was expressed as mM per 100 g of Trolox equivalent on the basis of the fresh weight of the fruit (Benzie and Strain 1996).

Statistical Analysis

The experiments were performed in quintuplicate. For analysis of variance, the obtained data were used for means calculation. Duncan multiple range tests were performed at the significant level of p < 0.05.

Results and Discussion

Tree growth habit and fruit external color indices (L, chroma and hue) were shown in Table 1. As indicated Table 1, there were statistical significant differences among rowanberry genotypes. Most of the genotypes (eight genotypes) had vase growth habit and rest of the 4 genotypes had upright growth habit (Table 1). Fruit external color indices as L, chroma and hue values were found between 30.12–41.04; 27.07–32.81 and 30.44–44.06, respectively (Table 1). Mikulic-Petkovsek et al. (2017) reported L, chroma and hue values in 2 rowanberry cultivars between 33–45; 28–30 and 33–43 indicating some similarities with our results. The term sensory quality of fruits is mainly related to their visual appearance (e.g. colour, size), and can be affected by many factors including genotype. Thus the most important factor affecting red color development in fruits was genetic background. Some cultivars or genotypes lack the ability to synthesize large quantities of anthocyanin. The other factors are light and temperature, tree nutrition, crop load and stress. Among consumers highly colored fruits are desired. Thus it is important for fruit cultivars that are genetically programmed to produce highly colored fruits (Ozturk et al. 2009).

Table 1 Growth habit, and color indices (L, Chroma and Hue) of rowanberry genotypes
Full size table

Sugar contents of rowanberry genotypes are given in Table 2. Glucose and fructose are the predominant sugars for all rowanberry genotypes. Sorbitol was also found high level in rowanberry fruits after glucose and fructose. In the experimental material, the respective sugars were estimated overall as follows: Glucose (30.15–43.28 g/kg FW), fructose (22.40–34.12 g/kg FW), sorbitol (16.24–21.13 g/kg FW) and sucrose (0.62–2.04 g/kg FW) (Table 2). Similar results have been reported by Mikulic-Petkovsek et al. (2017) for rowanberry cultivars grown in Czech Republic, with glucose, fructose, sorbitol and sucrose content were between 33.29 and 47.68%; 20.62 and 38.50%; 26.83 and 27.80% and 0.45 and 1.67%, respectively. Sugars are important compounds for quality evaluation of rowanberry fruits, and in terms of sugar composition, fructose and glucose are the predominant sugars for most rowanberry cultivars (Mikulic-Petkovsek et al. 2017). Results suggesting that the Sorbus fruits has high sugar content and could be advantage for processing and also genotype/cultivar is one of the most important factors that have a direct impact on sugar content of rowanberry. The result also suggesting that content of sorbitol in Sorbus genotypes was very high in comparison with other fruit species including different berry species (Mikulic-Petkovsek et al. 2012).

Table 2 Specific sugars of rowanberry genotypes (g/kg FW)
Full size table

Malic acid was the dominant organic acid in fruits of rowanberry genotypes with high percentage (19.41–25.38 g/kg FW) and followed by citric acid (1.94–6.23 g/kg FW) and tartaric acid (0.78–1.34 g/kg FW), respectively (Table 3). There were statistically significant differences on citric and malic acid considering all genotypes. However tartaric acid not significantly differed among rowanberry genotypes (Table 3). Mikulic-Petkovsek et al. (2017) found that almost 60–88% of total organic acids in Sorbus fruits were Malic acid. They also point out that content of citric and tartaric acid together represented 7–39% of total analyzed acids. The result suggesting that rowanberry fruits could be used in purees and jams if it is necessary to slightly increase the acidity of the products. A large number of different organic acids are present in the fleshy parts of all fruits, but the contents of these can vary greatly both between fruits of different species and their cultivars. The presence of organic acids in the fleshy parts of fruits affects both their palatability and their utilization in fruit products. The organic acids, Malic, Citric, Oxalic, and Tartaric are very abundant in some fruits (Walker and Famiani 2018).

Table 3 Organic acids in rowanberry fruits (g/kg FW)
Full size table

pH, vitamin C (ascorbic acid), total phenolic and total antioxidant capacity of the rowanberry genotypes are given in Table 4. pH and ascorbic acid content of genotypes were between 3.41–3.84 and 28.4–38.2 mg/100 g FW (Table 4). Previously pH and vitamin C content of rowanberry were determined as 3.68 and 35 mg/100 g (Baltacioglu 2006). The results suggesting that rowanberry berries had average vitamin C content. Piir and Niiberg (2003), who indicated the range of vitamin C content in rowanberry fruits of 12–86 mg/100 g FW and Kampuss et al. (2009) reported 10–51 mg/100 g in rowanberry fruits.

Table 4 PH, acidity and vitamin C values of rowanberry genotypes
Full size table

The rowanberry genotypes show wide variation of total phenolic content. Total phenolic content varied from 161 mg GAE per 100 g to 204 mg GAE per 100 g. Previously, total phenolic content was found between 134–220 mg GAE per 100 g fresh samples between Rowanberries (Baltacioglu 2006; Mikulic-Petkovsek et al. 2017). The result indicates total polyphenol richness of rowanberries and high polyphenol content of rowanberries comparable with high polyphenol included fruits such as elderberry, blackberry, and raspberry.

Total antioxidant capacity (FRAP values) of genotypes are shown in Table 4 and results indicated statistically significant differences (p < 0.05) among genotypes in the total antioxidant capacities. Total antioxidant capacity was the lowest as 2.93 mM TE per 100 g FW and was the highest as 5.68 mM TE per 100 g. Mikulic-Petkovsek et al. (2017) reported total antioxidant capacity of rowan cultivars between 3.4–4.9 mM TE/100 g fresh weight bases. These differences may be caused by factors such as sample type used, the species differences, geographical area, and the degree of ripening, climate conditions and experimental conditions. It has been reported that Sorbus fruits could contribute to the health. When compared with other fruit species, Sorbus fruits have higher or similar antioxidative activity like plums, which have 2.91–5.87 mM trolox/100 g (Kaulmann et al. 2014).

Conclusion

In conclusion, it seems that there are many benefits to consuming rowanberry fruits daily. We know from the healthy plate example, that rowanberry will not supply us with everything our body needs but it does make up a good portion of it. Hopefully this provided us with plenty of information that will encourage not just eating more rowanberry fruits but living a healthy lifestyle and eating a healthy diet that includes rowanberries.