Analysis of the differences in bitter and astringent substances in the pellicle of different walnut varieties

Abstract:【Objective】The pellicle is the key part that causes the bitter and astringent taste of walnut kernels. In order to find out the main bitter and astringent substances in walnut pellicle and find variet-ies with light bitter and astringent taste, we compared the differences in bitter and astringent substances in walnut pellicle among different walnut varieties.【Methods】Sensory evaluation was conducted on the taste quality of fresh walnut kernels and pellicle in the early stage, and the water extract of pellicle was  tested using electronic tongue. In this study, fruits of five walnut varieties (‘Zhonglin 3’,‘Jing 861’,‘Xiangling’,‘Bokexiang’and‘Nonghe 1’) were selected as the test materials at mature stage. The to-tal tannin content was determined by the Folin colorimetric method, and the content of bitter and astrin-gent substances such as phenolic acids, flavonoids and alkaloids were determined by HPLC. The ob-tained data were processed using SPSS 23.0 software for correlation analysis and principal component analysis was conducted using SAS 8.0 software.【Results】The total tannin content was highest in‘Zhonglin 3’and lowest in‘Nonghe 1’. The tannin content in‘Zhonglin 3’was 2.62 times that in ‘Nonghe 1’. The content of different phenolic acids was in a descending order of gallic acid > caffeic  acid > chlorogenic acid > ferulic acid > vanillic acid > ellagic acid > p-coumaric acid > syringic acid, and total phenolic acid in‘Zhonglin 3’was significantly higher than the other varieties (p < 0.05). The content of rutin in‘Xiangling’,‘Zhonglin 3’and‘Jing 861’was significantly higher than in the other varieties (p < 0.05); catechins from high to low were catechin, epigallocatechin, epigallocatechin gal-late, gallocatechin, gallocatechin gallate, epicatechin and epicatechin gallate.‘Zhonglin 3’and‘Jing 861’had a significantly higher content of catechins than the other varieties (p < 0.05), and the total amount of catechins and ester catechins in‘Nonghe 1’was the lowest. As for alkaloids, theobromine was not detected in any varieties, and the content of caffeine was significantly higher than that of the-ophylline in all the varieties. The caffeine content in‘Bokexiang’was significantly higher than in the other varieties (p < 0.05). The content of theophylline in‘Zhonglin 3’was significantly higher than that in the other varieties (p < 0.05), and the content of caffeine and theophylline in‘Nonghe 1’was the lowest. Correlation analysis showed that all bitter and astringent substances were positively correlated with bitterness and astringency, and there was a significant positive correlation between gallocatechin and bitterness, and coumaric acid, epicatechin, and epicatechin gallate were significantly positively cor-related with bitterness. Total tannins, epicatechins and astringency are significantly positively correlat-ed. These indicated that all the bitter and astringent substances tested in this study contributed to bitterness and astringency. Among them, gallocatechin, coumaric acid, epicatechin and epicatechin gallate contributed more to bitterness, while total tannin and epicatechin contributed more to astringency. The result of principal component analysis showed that the cumulative contribution rate of the two principal components was 83.08%, which contained 83.08% of the bitterness and astringency index. The first principal component could be interpreted as the“total bitterness”factor. All bitter and astringent sub- stances contributed to the first principal component, among them, the contribution of epicatechin, total tannins, chlorogenic acid, ellagic acid, ferulic acid, epicatechin gallate, epigallocatechin gallate and the- ophylline were greater. The second principal component could be interpreted as a“bitter structure”fac- tor, which included coumaric acid, gallocatechin, epigallocatechin, and caffeine. According to the score of the first principal component, the taste could be divided into two categories: lighter and heavier bitterness and astringency.‘Nonghe 1’and‘Bokexiang’had lighter bitterness, and‘Zhonglin 3’,‘Jing 861’and‘Xiangling’had heaver bitterness and astringency. According to the score of the second prin-cipal component, the varieties with astringent taste greater than bitter taste were‘Nonghe 1’,‘Xian- gling’and‘Jing 861’, and the varieties with bitter taste greater than astringent taste were‘Bokexiang’ and‘Zhonglin 3’. From the results of the principal component analysis, it could be seen that the total tannin, chlorogenic acid, ellagic acid, epicatechin, gallocatechin, epigallocatechin gallate, caffeine and other substances basically contain bitterness and astringency information of walnut pellicle.【Conclusion】Total tannin, chlorogenic acid, epicatechin, gallocatechin, epigallocatechin gallate and caffeine are the main substances that affect the bitter taste of walnut pellicle. In terms of taste intensity,‘Zhonglin 3’is the highest,‘Nonghe 1’the lowest. This provides inspiration for future research on the taste quali- ty of walnut pellicle and the selection of walnut varieties with lower bitterness and astringency.‘Nonghe 1’has the lightest bitterness and astringency, so it can be used as an important material for walnut breeding and can be promoted to meet the needs of consumers.