茶中游离氨基酸测定标样的选择

茶中游离氨基酸测定中,国家标准没有对标准样品谷氨酸和茶氨酸加以区别,造成这一指标测定结果的混乱。运用统计学成对比较t检验方法,将2种标样的2个吸光度、按2种标样算出的样品的2种氨基酸含量以及文献中2种标样下游离氨基酸的2种含量分别一一配对,进行样本差数方差分析,结果表明,谷氨酸的吸光度高于茶氨酸,达到极显著水平,且差异随着氨基酸量的增加而增大;样品游离氨基酸含量谷氨酸标样测定结果普遍低于茶氨酸测定结果,差异达到极显著或显著水平。根据茶中各种氨基酸各自的含量和分子量,用加权法计算出混合氨基酸的相对平均分子量为147~150u,与谷氨酸分子量147.1u接近,偏离茶氨酸分子量174.2u;此外,谷氨酸吸光值高,测定更加灵敏。为此,认为茶中游离氨基酸测定的国家标准应该保留谷氨酸做标样,取消茶氨酸作标样。

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 147~150u,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.