Abstract
Poland is one of the largest producers of hazelnuts in Europe; however, information about the contents of biologically active compounds in hazelnuts grown in Poland is limited. The aim of this study was to analyze and compare the concentrations of phenolic acids, flavonols, and tocopherols in raw and roasted hazelnut kernels and their skin, in addition to determining the effect of the cultivar and year of cultivation. The experiment was carried out in 2016–2018, and the nuts of six hazelnut cultivars were used for research purposes. Among the detected phenolic acids, p-coumaric acid was the most abundant in roasted and unroasted kernels, whereas the apigenin content represented 43%–45% of the total flavonols. The harvest year had a significant effect on the polyphenol content of hazelnuts, whereas roasting had a minor impact. The hazelnut skin contained 168- to 378- fold higher contents of total phenolic compounds than the raw and roasted nuts. Gallic acid was the most abundant phenolic acid in hazelnut skin, contributing 95.0% of the total polyphenols on average. α-Tocopherol was the most abundant tocopherol in the hazelnut cultivars, followed by β + ƴ-tocopherol and δ-tocopherol in hazelnut cultivars. The statistical analysis showed that roasting and year of cultivation did not affect the tocopherol contents of the investigated samples.
Highlights
Hazelnuts are among the most popular nuts worldwide, mainly cultivated along the coasts of the Black Sea regions of Turkey, as well as in other countries in Europe
To the best of our knowledge apigenin was detected for the first time in hazelnuts, in which it was found to be the predominant flavonol in both roasted and unroasted kernels
The six investigated cultivars showed a similar profiles of phenolic compounds and no single hazelnut cultivar could be regarded as the best source relative to the others
Summary
Hazelnuts are among the most popular nuts worldwide, mainly cultivated along the coasts of the Black Sea regions of Turkey, as well as in other countries in Europe. Poland is one of the largest producers of hazelnuts in Europe, with production of approximately 5300 tons per year and yield production 1.4 t ha−1 [1]. In Poland the cultivation of only large-fruited, so-called table cultivars, with large, oval nuts has been developed, almost all of which are sold as a raw product for fresh consumption [2]. Hazelnut has been reported to have the highest α-tocopherol content [7]. Α-Tocopherol, which is the tocopherol present at the highest level, exhibits vitamin E activity and is an antioxidant exerting its activity by scavenging free radicals [8,9,10,11], and play an important role in the prevention of lipid oxidation, extending shelf-life, and protecting sensory attributes [12] Hazelnut has been reported to have the highest α-tocopherol content [7]. α-Tocopherol, which is the tocopherol present at the highest level, exhibits vitamin E activity and is an antioxidant exerting its activity by scavenging free radicals [8,9,10,11], and play an important role in the prevention of lipid oxidation, extending shelf-life, and protecting sensory attributes [12]
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