Introduction

The sweet chestnut (Castanea sativa Mill.) is one of Europe’s most important tree species for various purposes. Southern European countries are currently one of the largest chestnut producers [1,2,3]. In 2020, the production area of sweet chestnuts in orchards in Slovakia was 8 ha [2]. In comparison to walnuts, sweet chestnuts have a higher moisture content of around 50%; they also have a higher fiber content (4–10%) and nearly 30% of starch, which makes up the majority of the dry matter (50–80%) [4,5,6,7]. The high content of carbohydrates is mainly represented by sucrose and fructose [5]. Some publications also report trace amounts of glucose and maltose. The carbohydrate content is strongly influenced by origin and genotype [8]. One of the significant parameters for assessing the quality of chestnuts is the sucrose content [9]. Despite the low content of proteins (2–7%) and fats (2–5%), chestnuts are a source of free amino acids, including essential ones (Leu, Lys, Val, Phe, and Thr). The low-fat content helps lower blood cholesterol levels [7]. In addition, chestnuts contain a significant amount of minerals (potassium, calcium, and magnesium) [10]. They are considered a high-quality source of bioavailable minerals [11]. Regarding microelements, Mn, Fe, Zn, and Cu have the largest proportions [1, 8]. Sweet chestnuts also contain significant concentrations of healthy compounds, including vitamins and antioxidants [3, 12]. In addition, they are a source of other antioxidants, such as carotenoids and phenolic compounds [5, 13]. The authors’ team was inspired to study and analyze sweet chestnuts by the unfavorable situation regarding chestnut preservation in Slovakia, on the one hand, and the many positive effects of chestnuts on human health, on the other. The aim of this pilot study was to analyze sweet chestnuts from different locations where their cultivation has a tradition of several hundred years, and to assess the influence of the given location on the content of monosaccharides (glucose and fructose), disaccharides (maltose and sucrose), and selected minerals (K, Na, Ca, Mg, and P). Similarly, there has been an increase in awareness and interest in these nuts and in the recovery of chestnut trees in Slovakia.

Materials and methods

Study areas and samples

The oldest center of chestnut cultivation in Slovakia is the Malokarpatsko–Dolnopovažská area. The second important center is the area near Jelenec (area of Inovecko–Tríbečské hory), and the third important one is the area of Modrý Kameň (Štiavnicko–Krupinské hory). The village of Krná is considered to be the oldest locality in the area of the Slovak Ore Mountains [2]. In most cases, the owners of chestnut trees could not name the varieties based on the very old age of the trees (10–300 years). Only varieties from the Lefantovce area are known: Lefantovce sampling point (2) – Migoule variety; Lefantovce sampling point (3) – Judia variety. Individual sampling areas (Štiavnicko-Krupinská area – area 1; Malokarpatsko–Dolnopovažská area – area 2; Inovecko–Tríbečské hory – area 3; Slovak Ore Mountains – area 4) are shown in Fig. 1. The climate in growing areas (Table 1) represents worse conditions for the growth of chestnut, but this tree has adapted through long-term acclimatization. Some sampling points are part of the forest (Rovňany, Jelenec), but most are in an orchard without any caring activities (plant sprays).

Fig. 1
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Map of the studied area and sampling points

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Table 1 Climatic conditions in growing areasa
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Each sampling point represents 1–3 trees depending on distance and abundance. At each sampling point, 1.5 kg of chestnuts were collected (average sample). For the determination of carbohydrates and mineral substances, 500 g were taken from an average sample (1.5 kg). Chestnut samples were collected during the seasonal harvest (September and October). Sweet chestnut samples were stored at − 18 °C for one month after harvesting; modified before analysis—shelled, homogenized, and dried (40 °C). Samples were analyzed for mineral substances and measured for carbohydrate content using the method by Orsák et al. [14] with the following modifications.

Sample preparation for the determination of carbohydrates

The crude lipidic fraction was removed from dried and finely chopped sweet chestnuts. About 2 g of a dried chestnut sample (Memmert UF 110, Germany) was weighed into a 50 mL Erlenmeyer flask. Further, 25 mL of 80% ethanol was added and introduced into an ultrasound bath for 20 min at 60 °C. The resulting suspension with Carrez’s reagents was centrifuged at 6000 rpm for 5 min. The supernatant was concentrated under a vacuum (Rotavapor R-200; Büchi Labortechnik, AG, Flawil, Switzerland) until total ethanol removal and then diluted in deionized water to a final volume of 1 mL. The sample was subjected to HPLC-RI analysis. All samples were analyzed in three replicates.

Analysis of carbohydrates by HPLC-RI

The Waters 2695 High-Pressure Liquid Chromatograph (Sciencix, Burnsville, USA) with Refractometric Detector 2414 (Sciencix, Burnsville, USA) was used to determine the amount of saccharides (d-glucose, d-fructose, sucrose, and maltose) in chestnuts. The mobile phase flow rate was 1.25 mL/min, the chromatographic column temperature was 35 °C, and the detector temperature was 40 °C. Isocratic elution mode was used with the mobile phase composed of 75% acetonitrile and 25% deionized water. The Luna® 5 m NH2 100 column (250 4.6 mm, Phenomenex Inc., Lane Cove, Australia) was used for the separation, and the injection volume was 10 μL at a time analysis of 12.5 min. The carbohydrate content is expressed as g/100 DW (dry weight).

Sample preparation and analysis of mineral substances

The contents of mineral substances (K, Na, P, Mg, and Ca) in the chestnuts (1 g of sample + 10 mL nitric acid + 5 mL perchloric acid) were determined after their mineralization using a closed microwave digestion system (Mars X-Press 5). Before determining potassium, sodium, magnesium, and calcium contents, the sample was diluted (2 mL of sample + redistilled water). To determine phosphorus, the sample was diluted as follows: 1 mL sample + 8 mL solution (C6H8O6, H2SO4, (NH4)2MoO4, and C4H4KO7Sb,0 × 5H2O). The measurement was carried out on VARIAN AA 240FS; phosphorus on a Shimadzu UV-1800 spectrophotometer. All measured elements’ concentrations are expressed as mg/kg of DW.

Statistical analyses

Descriptive statistical analysis (Microsoft Excel, Redmond, WA, USA) and analysis of variance (One-Way ANOVA multi-range tests, method: 95.0 percent LSD) were used to evaluate the results.

Results and discussion

Carbohydrates

Carbohydrate contents determined in chestnut samples from four different areas of Slovakia are shown in Tables 2, 3, 4 and 5.

Table 2 Carbohydrate content (g/100 g DW) in sweet chestnuts (area of Štiavnicko–Krupinské hory)
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Table 3 Carbohydrate content (g/100 g DW) in sweet chestnuts (Malokarpatsko–Dolnopovažská area)
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Table 4 Carbohydrate content (g/100 g DW) in sweet chestnuts (area of Inovecko–Tríbečské hory)
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Table 5 Carbohydrate content (g/100 g DW) in sweet chestnuts (area of Slovak Ore Mountains)
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The nutritional composition of fresh chestnuts is variable depending on the variety and the growing area. The lowest content of glucose (0.178 g/100 DW), fructose (0.271 g/100 g DW), and maltose (0.171 g/100 g DW) was determined in chestnuts from Lefantovce (area of Inovecko–Tríbečské hory). Chestnuts with the highest carbohydrate content (glucose 3.152, fructose 3.170, and maltose 1.584 g/100 g DW) are from the sampling point Svätý Jur (Malokarpatsko–Dolnopovažská area). There are statistically significant differences in the content of glucose, fructose, maltose, and sucrose in chestnuts between individual sampling points. Unprovable differences represent less than 8%; the least evident differences are for sucrose (Tables 2, 3, 4 and 5).

Several authors [15,16,17] state a glucose/fructose ratio (Glc/Fru) close to one. In our samples, this ratio ranges from 0.40 to 1.33. A higher fructose content compared to glucose was found in samples Modrý Kameň (4) (Glc/Fru = 0.46), Krná (1) (Glc/Fru = 0.44), and Rovňany (1) (Glc/Fru = 0.40). These chestnuts appear to be suitable components of functional foods as this ratio affects the level of glucose in the blood (Glycemic Index (GI, %) of glucose = 100, GI (%) of fructose = 23). A higher GI diet is associated with a greater risk of type 2 diabetes [18]. The contents of glucose, fructose, and maltose were higher in the analyzed samples compared to the analyses of other authors. In the case of glucose and fructose, this can be explained as a result of hydrolysis. Since the samples were not analyzed immediately after collection but after processing and storage. As stated by several authors, the glucose content ranged from an undetected amount to 6.63 mg/100 g DW; fructose was between 0.025 and 5.18 mg/100 g DW; and maltose was between 0.08 and 0.63 mg/100 g DW. A higher maltose content (0.93–2.63 mg/100 g DW) was determined in a total of 17 genotypes by Cristofori et al. [19]. Comparable values are reported by other authors [6, 8, 20].

Sucrose is the main carbohydrate in sweet chestnuts and one of the most important parameters for assessing the commercial quality of chestnuts [16]. The lowest sucrose contents were determined in chestnuts from sampling points Rovňany (9.355 g/100 g DW) and Jelenec (9.713 and 9.863 g/100 g DW) and the highest in chestnuts from sampling points Močenok (29.46 g/100 g DW) and Lefantovce (29.89 g/100 g DW). In studies by other authors, sucrose contents are between 2.034 and 15.494 g/100 g DW in six French sweet chestnut varieties planted in Romania [7], 4.03–23.30 g/100 g DW in four varieties from Portugal [21], 58.8–125.8 g/kg DW, determined in eight varieties from Missouri and California, the USA [22], 6.82–17.40 g/100 g DW in seventeen local varieties and two foreign chestnut hybrids grown in the Marmara region in Turkey [23], overall 73.9 ± 20.1 g/kg DW determined in 21 varieties from Tenerife, Spain [16], 106–185 g/kg DW in chestnuts from Switzerland [24], 10.94–22.07% DW, determined in three Italian sweet chestnut during two consecutive years [11], 15.0–16.2 g/100 g DW in cultivars Longal and Judia (Bragança, northeastern Portugal) [17], and 29.7 ± 1.2 g Suc/100 g DW in samples of “Marrone di Roccadaspide” chestnuts, obtained from local farms in the Campania region, Italy [20].

Mineral substances

The content of five mineral substances determined in sweet chestnuts in all four studied areas ranges from 377 (Závada) to 789 (Lefantovce (1)) mg K/100 g DW, 0.65 (Modrý Kameň (6)) – 6.90 (Rovňany (2))) mg Na/100 g DW, 26.9 (Závada) – 103 (Doľany) mg Ca/100 g DW, 58.7 (Závada) – 101 (Svätý Jur (4)) mg Mg/100 g DW, and 96.5 (Závada) – 179 (Svätý Jur (3)) mg P/100 g DW. Apart from Na, the lowest content of mineral substances was found in sweet chestnuts from the Slovak Ore Mountains. The highest content of Ca, Mg, and P was in sweet chestnuts from the Inovecko—Tríbečské hory area (Figs. 2, 3, 4 and 5).

Fig. 2
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Mineral content (mg/100 DW) in sweet chestnuts (area of Štiavnicko–Krupinské hory)

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Fig. 3
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Mineral content (mg/100 DW) in sweet chestnuts (Malokarpatsko–Dolnopovažská area)

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Fig. 4
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Mineral content (mg/100 DW) in sweet chestnuts (area of Inovecko–Tríbečské hory)

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Fig. 5
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Mineral content (mg/100 DW) in sweet chestnuts (area of Slovak Ore Mountains)

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The average values are comparable with the results by Borges et al. [1]. On the other hand, Ciucure et al. [10] stated significantly higher contents of K and Na, lower contents of Ca, and comparable contents of Mg; de Vasconcelos et al. [25] determined comparable K contents, higher Na contents, and lower Ca. The effect of area on the content of mineral substances was less significant than the content of carbohydrates. In the case of Na content, there were statistically significant differences (P value < 0.05) in only 5.2% of the compared samples. In the content of K (Ca, Mg, and P), there were statistically significant differences (P value < 0.05) in 14.7 (23.4, 36.8, and 58.4%) of the compared samples. The K content was 65.5–73.9% of the mineral content (K, Na, Ca, Mg, and P). Sweet chestnuts contain essential mineral substances for human health, including enzyme activation and various functions [26]. The ratio of the mineral components (Na:K) is important for proper nutrition. It is the low Na:K ratio that makes chestnuts interesting. Low Na and high K diets act synergistically to reduce blood pressure and the prevalence of arterial hypertension [27]. We determined the Na:K in individual chestnut samples ranging from 0.0009 (Modrý Kameň (6)) to 0.0100 (Rovňany (1)). The climatic conditions of all the studied growing areas were similar (altitude 196–272 m. n. m.; temperature 23–24 °C; amount of rain 6–8 mm). Statistical differences between the content of carbohydrates and mineral substances between sampling points are therefore mainly influenced by the variety and age of trees.

Conclusion

Based on the obtained results, it is not possible to clearly state the effect of area on the content of carbohydrates in sweet chestnuts. Chestnuts from growing area 4 (Malokarpatsko–Dolnopovažská area) were characterized by a higher content of glucose and fructose, but the lowest content of maltose. Overall, the lowest content of glucose and fructose was in chestnuts from sampling point Lefantovce (growing area 3). The lowest sucrose content was determined in chestnuts from growing area 2 (Slovak Ore Mountains) and the highest from growing area 3 (area of Inovecko–Tríbečské hory). Mineral substances are significant not only because of their total number but also because of the ratio in which they are represented. The highest value of the Na: K ratio (0.01) was in chestnuts from sampling point Rovňany, which is considered optimal for maintaining cardiovascular health. Since the climatic conditions of all the studied growing areas were similar, the content of carbohydrates and mineral substances is mainly influenced by variety and age of trees. The results obtained from the analysis of sweet chestnuts from important chestnut areas in Slovakia confirm the appropriate representation of the monitored indicators of their quality. In the following research, it is necessary to focus on a further, more complex evaluation of chestnuts from these areas or to expand the collection of samples from less-known areas.