Choice of standard sample in determination of free amino acids content in tea

The stipulation by the national standard that both glutamic acid and theanine can be used as the standard sample in determination of free amino acids in tea is blamed for the difference of this index. By paired-comparison t-test statistics, data of glutamate and theanine absorbances, amino acid contents by glutamic and theanine standards and in literatures were paired one to one, and the variance from the average difference of each sample were analyzed. The result showed that glutamic acid absorbance was very significantly higher than that of theanine, and the difference between the two increased with the increased the amount of the amino acids. Free amino acids content in all samples was lower by glutamic acid standard than by theanine standard, this difference reached significant level. Based on the portion and molecular weight of each amino acid, the average molecular weight of the mixture of the amino acids in tea was calculated as 147¹50u, close to the molecular weight 147.1u of glutamic acid but far from the weight 174.2u of theanine. Furthermore, the lower absorbance of glutamic acid made the measurement more insensive. Therefore, it is suggested that the national standard for determination of free amino acids in tea retain glutamic acid and cancel theanine as standard sample.