How to pick and process tea to increase amino acid content?

Amino acids are important flavor compounds in tea, and they undergo various enzymatic or non enzymatic reactions during tea processing to be converted into important components of tea aroma and pigments.

At present, 26 types of amino acids have been found in tea, including 20 protein derived amino acids and 6 non protein derived amino acids. Among them, the content of non protein derived amino acid – theanine can account for more than 50% of the total amino acid content, followed by glutamic acid, arginine, serine, and aspartic acid.

This issue mainly reviews the research progress on the effects of tea germplasm resources, environmental conditions, and different processing technologies on the synthesis, metabolism, and transformation of tea amino acids, providing an effective research basis for accurately improving the amino acid content and comprehensive quality of tea in the later stage.

Factors affecting amino acids before harvesting

1.The impact of tea germplasm resources on the synthesis, metabolism, and transformation of amino acids in tea leaves

The content of amino acids, especially theanine, varies among different tea tree varieties. The theanine content in yellowing or whitening varieties is higher than that in green leaf varieties, and its content increases with the position of one, two, and three leaves. The theanine content in tender shoots is the lowest, opposite to that in green leaf varieties.

It is recommended to consider picking slightly more mature buds and leaves, such as one bud with three leaves or one bud with four leaves, when processing and utilizing yellowing or whitening tea tree resources, in order to achieve the best utilization effect. In addition, the content of theanine in stem tissue is higher than that in leaf tissue, and different tea products can be developed according to different parts to fully utilize resources.

2. The influence of production environment on the synthesis, metabolism, and transformation of amino acids in tea leaves

The amino acid content in tea harvested in spring is significantly higher than that in summer tea. The reason is that strong sunlight and high temperatures in summer can lead to lower nitrogen metabolism and stronger ability to hydrolyze and convert theanine in tea trees, resulting in lower amino acid content in tender shoots in summer. Therefore, reducing light intensity through shading measures can promote the accumulation of free amino acids and improve tea quality. In addition, tea gardens at higher altitudes have a large temperature difference between day and night, frequent clouds and mist, and strong ultraviolet radiation, which are beneficial for enhancing nitrogen metabolism and accumulating theanine in tea trees. The amino acid content of the tea produced is generally higher. An environment with acidic soil pH (4.5-5.5), abundant organic matter, and suitable nitrogen phosphorus potassium ratio can also significantly improve the efficiency of theanine synthesis.

Factors affecting amino acids after harvesting

1.The influence of processing technology and storage on the synthesis, metabolism, and transformation of amino acids in tea leaves

The changes in amino acid content in tea during processing are mainly influenced by two factors. On the one hand, under the influence of humidity and heat, some small molecule proteins or peptides undergo local hydrolysis and pyrolysis, leading to the accumulation of amino acids; On the other hand, amino acids are oxidized, hydrolyzed, converted, and combined with sugars and polyphenols to form color, aroma, and taste substances, resulting in a decrease.

(1) Spread and wither

During the spreading and withering stages, protein hydrolysis forms free amino acids, so the total amount of amino acids generally increases. However, the content of theanine decreases due to dehydration activating the expression of theanine hydrolase, which hydrolyzes theanine. If anaerobic treatment is carried out on fresh tea leaves in the early stage of this stage, the active substance gamma aminobutyric acid will significantly increase due to the activation of glutamate decarboxylase and the increase in gene expression, which will also promote the accumulation of alanine and histidine.

(2) Fermentation stage

Fermentation is mainly divided into polyphenol oxidase fermentation and microbial fermentation. During the fermentation process of tea, especially black tea, the cells are damaged after rolling and twisting, and the phenolic substances in the cells are oxidized by polyphenol oxidase. The resulting quinone compounds have the opportunity to undergo reversible or irreversible chemical reactions with amino acids, such as substitution reactions with functional groups of proteins or amino acids such as amides, amines, thiols, indoles, and imidazoles, producing Schiff bases, N-quinone derivatives, S-quinone derivatives, and some aldehyde compounds produced by Strecker degradation. And many of these aldehydes play an important role in presenting the aroma of black tea. In addition, during the fermentation process, amino acids can also bind with catechins to form colored compounds, which are related to the formation of black tea soup color. Therefore, the content of amino acids in black tea is a dynamic change between amino acid formation and degradation transformation. Due to the long fermentation time of Pu’er tea, the amino acid content, especially theanine content, generally decreases significantly after fermentation

(3) Closing and drying

During the withering and drying stages, high temperature plays a dominant role, and amino acids often undergo oxidative polymerization with neighboring quinones or Maillard reactions with carbonyl compounds (such as reducing sugars). In addition, amino acids are prone to degradation reactions such as decarboxylation and deamination. The higher the filming temperature, the more it promotes protein hydrolysis while also causing amino acid hydrolysis and transformation. To improve the quality of green tea, it is recommended to retain as many amino acids as possible during the processing; For black tea, the requirement for amino acid content is not very high. Therefore, during the drying process, it is possible to promote the mutual conversion of amino acids and other compounds, which can not only enhance the taste of the tea, but also facilitate the formation of tea aroma.

(4) Storage

Amino acids will undergo further degradation and transformation during tea storage due to changes in environment and time. For example, amino acids will continue to be oxidized by catechins and quinones, leading to Strecker degradation and decrease. The higher the temperature and humidity, the greater the degradation amount. The content of theanine often significantly decreases after 10 years of storage, indicating that low temperature and dry environment are conducive to preserving the content of most amino acids.

In summary, amino acids are not only flavor compounds, but also undergo various enzymatic or non enzymatic chemical reactions during different processing and storage processes of tea, and are important precursor substances for tea aroma and pigments. At present, there have been many in-depth studies on the synthesis, metabolism, and transport mechanisms of amino acids, especially theanine, in tea tree varieties, environmental responses, and post harvest processing and storage environments. At the same time, a series of advances have been made in the synthesis and metabolism mechanism of post harvest gamma aminobutyric acid, but the research on the metabolic regulation mechanism of other amino acids in tea by different processing techniques before and after harvest is still relatively vague and needs further in-depth study in the future.