Biochemistry of Tea Fermentation: Conversion of Amino Acids to Black Tea Aroma Constituents

Abstract

SUMMARY: 14C‐amino acids were added to fresh tea‐leaf homogenate undergoing conversion to black tea. After conversion (30 min, 25°C), the volatile compounds present in the headspace over the reaction mixture were collected and analyzed by gas chromatography. Results showed that leucine, isoleucine, valine and phenylalanine were partially converted to the aldehydes expected from a Strecker degradation. These aldehydes are constituents of black tea aroma. Further, drying of the fermented mixture caused an additional amount of the aldehydes to be formed. In contrast, no detectable volatile compounds were formed from aspartic acid, glutamic acid, glutamine, arginine, threonine. serine or theanine under the same conditions. Production of aldehydes from amino acids was shown to be dependent on the enzymic conversion process: Tea leaf which had been inactivated by steam treatment was not effective in causing formation of volatile aldehydes from the amino acids. Identical results were obtained in a model tea fermentation system composed of a crude soluble enzymes extract from tea leaves, purified epigallocatechin gallate and 14C‐amino acids. Ascorbic acid was found to inhibit formation of aldehydes from amino acids in this model tea fermentation system; dehydroascorbic acid by itself was found to be effective in causing formation of volatile aldehydes from amino acids.

Impact of light irradiation on black tea quality during withering

“…Additionally, amino acids could be converted to aldehydes by decarboxylation and oxidative deamination. For example, glycine can be transformed into formaldehyde, alanine into acetaldehyde, valine into isobutyraldehyde, leucine into isovaleraldehyde, and so on (Co and Sanderson 1970). Furthermore, under high temperature conditions, amino acids, sugars and catechins can also produce furan, pyrrole, pyrazine and phenol compounds as well as their corresponding methyl, ethyl, hydroxide radical, acetyl derivatives, acetic acid and so on (Robinson and Owuor 1992).…”

Tea aroma

“…The amino acid levels, however, decrease during fermentation (Roberts and Wood, 1951;Bhatia and Deb, 1965) and this is accompanied by the production of aldehydes (Co and Sanderson, 1970;Saijo and Takeo, 1970a). Valine, leucine, isoleucine (Co and Sanderson, 1970) and phenylalanine (Co and Sanderson, 1970;Saijo and Takeo, 1970b) are thus converted to 2-methylpropanal, 2-methylbutanal, pentanal and phenylacetaldehyde respectively.…”

“…Valine, leucine, isoleucine (Co and Sanderson, 1970) and phenylalanine (Co and Sanderson, 1970;Saijo and Takeo, 1970b) are thus converted to 2-methylpropanal, 2-methylbutanal, pentanal and phenylacetaldehyde respectively. These reactions are catalysed by polyphenol oxidase or peroxidase (Saijo and Takeo, 1970a,b;Srivastava, 1986) in the presence of oxygen and catechins.…”

Fermentation: The Key Step in the Processing of Black Tea

“…These volatile compounds include essential oils and amino acids. Amino acids combine with orthoquinone, which is an oxidized form of catechin, and play the most important role in determining the aroma of black tea (Co and Sanderson, 1970;Mahanta and Hazarika, 1985;Bhattacharyya et al, 2007). In the Indian tea industry, the fermentation process is judged using two defined smell peaks: the first nose and second nose.…”