Elucidation of the potential mechanism of tannase in removing the astringency of hickory nuts and its effect on flavor profile utilizing wide-targeted metabolomics, E-nose, and HS-SPME-GC–MS

Abstract

Hickory (Carya cathayensis Sarg.) is a unique economic tree species in China, and its nuts are rich in nutrition. However, its seed coat harbors abundant astringent compounds including phenolic compounds, flavonoids, tannins, and others, necessitating their removal prior to consumption. Traditional high-temperature boiling, while effective in reducing astringency, incurs nutrient loss, environmental pollution, and elevates processing costs. Consequently, there is a pressing need to improve de-astringency techniques for hickory nuts. Using widely-targeted metabolomics, electronic nose analysis and other techniques, the effects of microbial tannase on the astringency, nutrient composition and flavor of hickory nuts were explored. Herein, it was found that the astringency, total phenol and condensed tannins of hickory nut were significantly reduced after the treatment with microbial tannase. Widely-targeted metabolomics analysis further unveiled that most of the phenols, flavones and tannins containing gallic ester linkages in hickory were degraded upon microbial tannase treatment, suggesting that the primary mechanism by which microbial tannase exerts its de-astringent effects may lie in the targeted hydrolysis of galloyl-containing compounds. Notably, the tannase-treated hickory kernels demonstrated a notable retention of nutrients such as soluble proteins and total oils compared to boiled hickory kernels, highlighting its gentle processing nature. In terms of aroma, the tannase-treated hickory kernels were more similar to the non-treated, owing to the preservation of a higher content of pyrazines, pyrans, furans, esters, and alcohols associated with fruity, floral, butterscotch, and sweetness of hickory nuts. These results suggest that microbial tannase can effectively reduce the astringency and preserve the volatile compounds of hickory nut, providing a theoretical basis for developing a novel and green de-astringent technology for the hickory nut processing industry.