Research On Identification And Application Of Green Tea Kokumi In Tea Infusion

Green tea is a widely consumed green tea in China,which has a good reputation for its strong sense of kokumi,causing a taste of thickness,mouthfulness and continuity.Kokumi is recognized as the sixth taste following the five basic tastes(sweetness,saltiness,sourness,bitterness,and umami).In the tea sensory evaluation system,“thickness” and “richness” are mainly related to the high content of water leachate,while “kokumi” has receptors like the other five basic tastes and can present a complex taste at a lower concentration.However,the research on tea flavor were mainly focused on bitterness,sweetness and umami taste,which were responsible for a very limited kokumi-enhancing effect.The composition of non-volatile kokumi-tasting compounds needs to be clarified.In this work,composition of kokumi compounds and their quantitative analysis in tea infusion were investigated.Ultra-high-performance liquid chromatograph–tandem mass spectrometry(UPLC–MS)and high-performance liquid chromatograph(HPLC)combined with a series of purification methods and sensoryguided analysis were used to screen out the key kokumi compounds in green tea infusion.The sensory reconstitution technique was applied to investigate the taste properties of the key kokumi substances and their contribution to the quality of te,which provided a theoretical basis for the study of the mechanism of tea quality.And a design of a tea flavor thickening additive was devised to provide basic data support for the development of tea thickening flavor additives.The main research results are as follows:(1)The chemical substances that provide the taste of kokumi in tea infusion were mainly amino acids and peptides.In this study,the tea infusion was separated and purified by ultrafiltration and gel filtration,and the results showed that the kokumi components were mainly present in the fractions with molecular weight less than 3000 Da.The gel chromatography was used to further purify and enrich the kokumi substances.The fractions with different kokumi intensity were invested for amino acids,tea polyphenols,proteins,peptides and other major flavor components of tea,and the results showed that the contents of peptides and amino acids were significantly different.Fractions with higher kokumi intensity obtained more peptides and amino acids.(2)The key kokumi amino acids and peptides in tea infusion were theanine,glutathione and ?-Glu-Gln.For the six tea samples with significantly different sense of thickness,two peptides were identified and quantified by UPLC-MS,and 21 amino acids were determined by HPLC.The key kokumi amino acids and kokumi peptides were theanine,glutathione and ?-Glu-Gln,as revealed by principal component analysis and partial least squares analysis.(3)Theanine,glutathione,and ?-Glu-Gln were able to significantly increase the intensity of richness and reduce negative flavors such as bitterness and astringency in the tea infusion through sensory interaction.Analysis of the sensory taste attributes of the three key kokumi substances showed that theanine mainly provided umami,and glutathione and ?-Glu-Gln had some bitterness and astringency.Theanine,glutathione and ?-Glu-Gln did not possess or only possessed a very low intensity of kokumi by themselves,but showed a strong kokumi-enhancing effect when they were present in solution together.The sensory interaction of kokumi-tasting substances created the sense of kokumi in tea infusion.(4)Flavor recombination experiments showed that the intensity score of the model tea infusion increased significantly after the addition of the kokumi substances.The kokumi intensity score of the model tea infusion was 0.51,and the kokumi intensity score of the standard tea infusion prepared by the national standard method was 4.00;after the addition of GSH and ?-Glu-Gln,the kokumi intensity score increased significantly,rising to 3.02 and 3.76,respectively,which was close to the kokumi intensity of the standard tea infusion.At the same time,the addition of the kokumi substances retained the tea flavor of the model tea infusion without any negative flavor.The flavor recombination experiments demonstrated that the kokumi-tasting amino acids and kokumi-tasting peptides could improve the kokumi taste of the tea infusion while preserving the tea flavor.(5)A natural-source kokumi additive with tea flavor was designed in this work.A sensory additive with tea flavor was designed using theanine,glutathione,pectin and protein,taking into account the cost and market preference.Applying the response surface methodology,the optimal flavor ratios for the kokumi enhancer with tea flavor were 48 ?g/m L for theanine,18 ?g/m L for GSH,30 ?g/m L for pectin and 19 ?g/m L for protein.This data provided basic research for the development of the kokumi flavor additive for tea.