Elsevier

Food Chemistry

Volume 151, 15 May 2014, Pages 15-20
Food Chemistry

Preparation and characterisation of peanut seed skin polyphenols

https://doi.org/10.1016/j.foodchem.2013.11.072Get rights and content

Highlights

  • Polyphenols from peanut seed skin were separated using α-amylase inhibition as a guide.

  • The polyphenols consist of oligomeric proanthocyanidins with gallic acid.

  • The polyphenols strongly inhibited α-amylase activity.

  • The polyphenols suppressed an increase in blood glucose levels and area under the curve.

Abstract

Using α-amylase inhibition as a separation guide, polyphenolic compounds from peanut seed skin were prepared. During preparation, specific α-amylase inhibitory activities were increased about 4-fold. High-resolution MALDI-TOF mass spectra showed that the structure of this sample was a series of polyflavan-3-ols, up to 15-mer, composed of catechin/epicatechin units together with several afzelechin/epiafzelechin units and gallocatechin/epigallocatechin units. The observed precious mass values suggest that the polymers consist of both interflavanoid C–C linkages (A-type) and interflavanoid ether linkages (B-type). Oral administration of the polyphenol fraction to rats fed corn starch significantly suppressed an increase in blood glucose levels in a dose dependent manner. Administration of the polyphenol fraction to rats fed maltose or sucrose delayed the increase in blood glucose levels. These results suggest peanut seed skin contains polyphenols with strong α-amylase inhibitory activity, which retard absorption of carbohydrates and mainly function through inhibition of α-amylase.

Introduction

The peanut (Arachis hypogaea L.) is not in fact a nut, but a species of the legume family which originated in South America. It is an annual herbaceous plant which grows 30–50 cm in height and is cultivated around the world in tropical, subtropical and warm temperate regions. Peanuts are comprised of kernels, skins and hulls. Peanut skins and hulls are generated in large quantities as waste products during industrial seed skin-free kernel processing. The phenolic compounds in peanuts are mainly located in the skins and hulls (Yen, Chang, & Duh, 2005) and although the weight of the skin is small in relation to the total nut, it contains a large ratio of the total polyphenols present in the nut. Nepote, Grosso, and Guzman (2005) reported that the content of phenolic compounds in peanut skin was 115–149 mg/g dry skin, depending on the solvent used. Yu, Ahmedna, and Goktepe (2005) reported that the total phenolics measured about 90–125 mg/g dry skin. The phenolic compounds are not essential for the survival of the plant and it is suggested that these compounds may protect the plant against environmental stresses. They are known to be useful in formulating nutritional or medicinal supplements for the treatment of several diseases. Some investigators reported that peanut seed skin polyphenols have an antioxidant capacity and hypolipidemic effects (Bansode et al., 2012, Lou et al., 2004, Tamura et al., 2012). One of the important activities of polyphenols is the inhibition of digestive enzymes, especially carbohydrate-hydrolysing enzymes such as α-amylase and α-glucosidase. Xiao, Ni, Kai, and Chen (2013) reviewed reports on the structure–activity relationship of polyphenols inhibiting α-amylase. We have investigated the inhibitory effects of nut polyphenols against α-amylase. In vitro studies have shown that many nut polyphenols inhibited carbohydrate-hydrolysing enzymes, including polyphenols from chestnut (Tsujita et al., 2011), Japanese house chestnut (Ogawa et al., 2008), almond (Tsujita, Shintani, & Sato, 2013), evergreen mangrove seeds (Gowri, Tiwari, Ali, & Rao, 2007) and berry (McDougall and Stewart, 2005). In this paper, we focused on the separation and characterisation of polyphenols from peanut seed skin. We also focussed on the effects of the separated materials on α-amylase activity and carbohydrate absorption.

Section snippets

Materials

Roasted peanut (Arachis hypogaea) seed skin was obtained from Chuon Co., Ltd. (Matsuyama, Japan). α-Amylases from porcine pancreas, pancreatic lipase (Type VI-S) from porcine pancreas, intestinal acetone powder from rat, Folin–Ciocalteu reagent, (+)-catechin and 2,2-diphenyl-1-picrylhydrazyl (DPPH) were obtained from Sigma–Aldrich Japan (Tokyo, Japan). Cyanidin chloride was obtained from ChromaDex (Irvine, CA). Trolox was obtained from Calbiochem (Darmstadt, Germany). Sephadex LH-20 was

Results and discussion

Roasted peanut seed skin was extracted with water and aqueous organic solvent (70% aqueous methanol, ethanol, acetone or acetonitrile). We found that aqueous acetone was the most effective solvent, producing the highest freeze-dried weight, total polyphenol content and α-amylase inhibitors activity (the specific α-amylase inhibitory activities of aqueous methanol, ethanol, acetone and acetonitrile were 28.5, 22.4, 32.4 and 24.9 U/mg dry weight, respectively). α-Amylase inhibitors were not

Conclusion

Peanut seed skin polyphenols consist of oligomeric proanthocyanidins composed of catechin/epicatechin units together with several afzelechin/epiafzelechin and gallocatechin/epigallocatechin units. They exert an anti-diabetic effect by inhibiting α-amylase, retarding carbohydrate absorption and reducing postprandial hyperglycemia. Therefore, they are strong candidates as an agent that inhibits carbohydrate absorption.

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