Antioxidant and deodorizing activities of phenolic components in chestnut inner shell extracts

Highlights

• Chestnut inner shell is the potential source for natural antioxidant and deodorant.

• Aqueous ethanol was most effective in extracting the phenolic compounds with the highest DPPH radical scavenging activity.

• Dilute alkaline solution was preferred for the extraction of the tannin and flavonoids.

• The contents of tannins and flavonoids were positively correlated to deodorizing activities against trans-2-nonenal and methyl mercaptan.

Abstract

Phenolic compounds were extracted from chestnut inner shell using aqueous alcohols and alkaline solutions (50% ethanol, 50% methanol, 1% NaOH, and 2% NaOH) at different temperatures (25–90 °C). The phenolic composition, antioxidant activity, and deodorizing activity of the extracts were evaluated for comparison. Total phenolic content and antioxidant activity were increased as the extraction temperature increased. However, the phenolic composition and antioxidant activity were significantly different among the extracts prepared under different conditions. Aqueous ethanol (50%, v/w) was most effective in extracting the total phenolics, resulting in the highest DPPH radical scavenging activity. The alkaline solutions appeared more effective in extracting specific phenolics, such as tannins and flavonoids, than aqueous alcohols. Moreover, the alkaline extracts exhibited the higher activities in deodorizing trans-2-nonenal and methyl mercaptan than the alcoholic extracts. The deodorizing activity of chestnut inner shell extracts (CISE) were positively correlated to the residual contents of tannins and flavonoids (R² = 0.85–0.94, and 0.81–0.93, respectively).

Introduction

Various bio-wastes from the forest and agricultural industries have been of interest because of their high levels of residual nutraceutical components. The re-use of the wastes as value-added materials is, not only cost-saving, but also environmentally beneficial. Chestnut (Castanea crenata) is a beech family plant natively grown in the northern hemisphere regions. The nuts contain hard hull and inner thin skin, both of which are about 10% of the whole weight of chestnuts (Hwang et al., 2001), and typically removed to obtain the edible flesh. Those hull and inner skin obtained in the peeling process are used mainly as fuel (Vázquez et al., 2012). According to Hwang et al. (2001), tannin content in the inner and outer shells were significantly different depending on chestnut varieties, which were in the ranges of 7.83–71.42% and 0.31–2.04%, respectively.

The phenolic compounds and antioxidant activity of those by-products from chestnut have been examined in recent years (Vázquez et al., 2008, Vázquez et al., 2009, Vázquez et al., 2012). Vázquez et al. (2008) reported that the total phenolic content in the chestnut (C. sativa) shell extracts was in a range between 26.2 and 59.7 g GAE/100 g extract, which was significantly higher than that in the eucalyptus bark extracts (11.9–22.3 g GAE/100 g extract). In addition, a positive correlation between the total phenolic contents and antioxidant activity was observed. Another study by the same research group (Vázquez et al., 2012) was carried out to investigate the influence of extraction conditions on the antioxidant activity of chestnut bur extracts. According to the results from a response surface methodology, total phenolic content in the chestnut bur extract reached the maximum when aqueous methanol and ethanol solutions were used: 36.32 g and 26.11 g GAE/100 g extract, respectively.

Likewise, total content and composition of the phenolics in extracts depend on the solvents and extraction conditions (Dai and Mumper, 2010). Among the organic solvents, methanol in mixture with water has been generally more efficient than acetone for the extraction of low molecular weight polyphenols, whereas aqueous acetone is better for the extraction of high molecular weight flavonoids (Prior et al., 2001, Guyot et al., 2001, Labarbe et al., 1999). Typically, mild heating helps the extraction of phenolics (Spigno et al., 2007), but excessive heating over 70 °C was reported to induce the degradation of certain phenolic compounds including anthocyanin (Havlikova and Mikova, 1985). Therefore, both the solvents and physical conditions for the extraction of phenolic compounds should be carefully considered for the extraction efficiency and stability of phenolic compounds.

Many studies (Yasuda and Arakawa, 1995, Cabrera et al., 2006, Lodhia et al., 2008) revealed that polyphenols and their derivatives exhibited deodorizing activity. For instance, (−)-epigallocatechin gallate (EGCg), the main phenolic constituent in green tea, showed a substantial deodorizing activity against methyl mercaptan (CH₃SH), which is a major odor component in human halitosis (Tonzetich and Richer, 1964). It may be attributed to the addition of a methylthiol group to the ortho-quinone generated by atmospheric oxygen (Yasuda and Arakawa, 1995).

It is well-known that tannins from green tea or persimmon are effective in removing body odor, detected mostly for the elderly people (Yasuda and Arakawa, 1995, Lodhia et al., 2008), and soaps containing the tannins are also commercialized by several companies in USA and Japan. Personal body odor is affected by various environmental and internal factors, such as gender, age and eating habits (Yamazaki et al., 2010). Among the odor substances, trans-2-nonenal is known to be generated with aging, and characterized as the key component of the body odor in the elderly people (Haze et al., 2001).

Chestnut inner shell contains a large quantity of polyphenols, especially condensed tannins (Hwang et al., 2001). No study has been reported on the deodorizing activity of the chestnut inner shell. In this study, phenolic extracts were prepared from chestnut inner shell with different solvents and different conditions, and antioxidant and deodorizing activities of the extracts were investigated. Not only the total phenolic content (TPC), but also the condensed tannin content (CTC) and total flavonoid content (TFC) of the extracts were compared. For analysis of deodorizing activity, both trans-2-nonenal and methyl mercaptan were tested as odor substances. Additionally, the correlation between the phenolic composition and deodorizing activity of the chestnut inner shell extract (CISE) was examined.