Formation of Volatile Heterocyclic Compounds and Open-Chain Amides of Theanine in Model Systems with Glucose, Tea Leaves, and Tea Extract under Tea-Roasting Conditions

Abstract

Theanine is a non-proteinogenic amino acid found in the tea plant Camellia sinensis. At an elevated temperature (>90 °C), it released two major volatile compounds 1-ethyl-1,5-dihydro-2H-pyrrol-2-one and N-ethylsuccinimide. Other products were identified, including 10 pyrroles and 12 amides/imides. The formation of the two major compounds was proposed to be initiated by the deamination of theanine and through the intermediate α-keto acid. In the presence of glucose, the two major products and many other volatiles from theanine thermal degradation were accelerated and further Maillard reactions occurred. A total of 56 compounds were identified in the model system of theanine and glucose, including 12 amides/imides, 16 pyrazines, 16 pyrroles and other Nheterocycles, and 12 furans and other O-heterocycles. Although most of the reaction products were detected in tea leaves and in their aqueous extract with or without the addition of theanine under the same experiment conditions, imides and amides were considerably suppressed, left only minute amounts, or were even no longer detectable. Pyrazines and pyrroles were also shown at reduced concentrations as a result of the interaction with tea components but to a lesser extent. A total of 16 and 12 pyrazines were identified in the theanine/glucose reaction system and tea leaves/aqueous extract after roasting, respectively. The results indicated that pyrazines and other main volatiles in roasted tea leaves were formed from the Maillard reactions of the aqueous fraction of tea leaves. Theanine participated in the formation of pyrazines in tea leaves under roasting conditions.

Formation Mechanism of Di-N-ethyl-2-pyrrolidinone-Substituted Epigallocatechin Gallate during High-Temperature Roasting of Tea

“…Under high-temperature roasting conditions, theanine will not only react with EGCG to form EPSFs or di-EPSFs but also react with other substances. First, theanine degrades to form volatile pyrroles during tea roasting at high temperatures. − , Second, theanine will be involved in the Maillard reaction with glucose to form a variety of aroma substances, such as 2,5-dimethylpyrazine and 2,3-diethyl-5-methylpyrazine. , Third, theanine reacts with glucose to form nonvolatile compounds, known as Maillard intermediates, which may also contribute to the aroma or taste of tea. − …”

“…At present, there are many studies on the ARP formed in the process of high-temperature heating of amino acids and sugars, including their flavor characteristics and inhibition of bitterness or astringency taste. , Because of its strong nucleophilic properties, , catechin (EGCG) also reacts with ARP to form adducts during high-temperature roasting. − ARPs have been reported in tea, − which indicates that adducts of catechin (EGCG) and ARP may also be formed in tea. Therefore, the raw materials for producing LYT are mainly three leaves in one bud to six leaves in one bud, and the content of EGCG in fresh leaves of LYT is about 8 times that of theanine. , However, theanine is also involved in a variety of chemical reactions during tea roasting at high temperatures, ,, so the content of di-EPSFs in LYT with different degrees of roasting is low.…”

“…Therefore, the raw materials for producing LYT are mainly three leaves in one bud to six leaves in one bud, and the content of EGCG in fresh leaves of LYT is about 8 times that of theanine. 14,41 However, theanine is also involved in a variety of chemical reactions during tea roasting at high temperatures, 8,10,16 so the content of di-EPSFs in LYT with different degrees of roasting is low.…”

“…Catechin is the main polyphenol in tea, which is associated with bitterness and astringency. − Although tea is one of the three major nonalcoholic beverages in the world, the undesirable bitterness and astringency taste attributes of tea are still restricting its application in bakery and functional foods . To reduce the bitterness and astringency of tea, high-temperature roasting is usually applied during processing, which also contributes to improving the aroma of tea. − The chemical structure of catechin is not stable, especially upon heating. During roasting under high temperatures, the isomerization reaction on (2R,3R)-flavan-3-ols, which led to the transformation into (2S,3R)-flavan-3-ols, would occur. ,− In addition, the degradation reaction involving the removal of gallic acid would also occur. ,− …”

“…In recent years, the adducts ( N -ethyl-2-pyrrolidone-substituted flavan-3-ols (EPSFs)) of tea catechins with Strecker degradation products of theanine have become a popular research compound in tea. At present, these substances are found in six types of tea, and their health effects (anti-inflammatory, anticancer, antioxidation, prevention of senile dementia and hypoglycemia), ,,,,, as well as changes in the content of tea during processing and storage times (white tea, green tea, dark tea, and black tea), were widely studied. ,− Catechin has a strong bitter and astringent taste, while theanine possesses a umami taste. − In addition, theanine is the amino acid with the highest level in tea. ,,,− , However, the taste properties of EPSFs were less studied. From a review of the formation mechanism of EPSFs, it was found that the formation of these compounds was related to temperature and microbial activities, especially during the drying stage of tea processing. ,,− ,,, Under higher temperatures, the contents of EPSFs significantly increased.…”