Polyphenols contribute to the taste, stability and health benefits of nut consumption. Almonds and hazelnuts are rich dietary sources of polyphenols. The objective of this work was to characterize the polyphenols of almonds and hazelnut, focusing on tannin content. Almonds contained a wide variety of polyphenols including tannins, phenolic acids and aldehydes, flavonoids, lignans, and stilbenes. Almond polyphenol content varied by cultivar, location, harvest year, and processing. Proanthocyanidins were the most abundant polyphenol class in almond, accounting for approximately a third of its polyphenol content. Following solvent extraction, up to an additional 21% of total almond proanthocyanidins were liberated by alkaline hydrolysis, depending on cultivar. In contrast, acid hydrolysis was not an effective method to recover bound almond proanthocyanidins. Almond also contained a significant portion of ellagitannins and gallotannins, 55 and 27 mg per 100 g almond respectively. Hazelnut was an abundant source of proanthocyanidin, and processing significantly affected polyphenol content. Roasted hazelnut without skin had 65 mg proanthocyanidin per 100 mg hazelnut, whereas raw hazelnuts with intact skin had 1610 mg proanthocyanidin per 100 mg hazelnut. Raw hazelnut proanthocyanidins were more highly polymerized relative to roasted hazelnut proanthocyanidins. Both raw and roasted hazelnut contained a significant portion of alkaline-hydrolysable proanthocyanidins, liberated mainly as monomers and dimers. Alkaline-liberated proanthocyanidins were 90% and 49% of total proanthocyanidin for roasted and raw hazelnut. Hazelnut skin alone contained 37 mg proanthocyanidin per gram, with 20% being hydrolysable. In conclusion, alkaline-hydrolysable proanthocyanins constitute a significant portion of almond and hazelnut proanthocyanidins. Nut type, cultivar, and processing affect proanthocyanidin content and the proportion of proanthocyanidins recovered following alkaline hydrolysis.

Almond and hazelnut are rich in tannins, with varying proportions of bound proanthocyanidin

253rd National Meeting (2017)

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