The ability to determine the quality of tea is not only an art form, but also a clear scientific procedure that combines several approaches such as sensory analysis, chemical methods, technological standards and instrumental analytics. Below we look at 15 key criteria used by tea industry professionals, researchers and producers to certify and classify tea quality.
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Appearance of the dry leaf
Criteria: shape, curl, integrity, uniformity, gloss, colour.
This is the first indicator of the origin and level of processing.
That is, the shape, curl, integrity, uniformity, gloss, and colour of quality tea should correspond to the grade being evaluated. In the Chinese tea market, appearance has up to 30% of the weight in the overall quality score («Research on the Correlative Factors of Sensory Quality of High-quality Green Tea», авторы: WANG Tong-he, HU Min (College of tea and food technology, Anhui agricultural university, Hefei, China). ZHANG Jiu-qian, LI Hong (Tea office in Xinyang, Henan province, Xinyang, China), Journal of Tea Science ” 2008, https://www.tea-science.com/EN/10.13305/j.cnki.jts.2008.01.005).
High-quality tea should be uniform in size, colour, leaf shape, aroma and flavour within a single batch. Uniformity is determined both in the dry leaf and in the brewed form, which is especially important for standardisation of premium varieties. Deviations between samples may indicate blending, poor processing or impurities («Tea Quality: An Overview of the Analytical Methods and Sensory Analyses Used in the Most Recent Studies», авторы: Juan Moreira, Jyoti Aryal (School of Nutrition and Food Sciences, Louisiana State University Agricultural Center, USA), Luca Guidry, Yan Chen (School of Plant, Environmental, and Soil Sciences, Louisiana State University, USA), Sujinda Sriwattana (Product Development Technology Division, Faculty of Agro-Industry, Chiang Mai University, Thailand), Foods 2024, https://pubmed.ncbi.nlm.nih.gov/39593996/#full-view-affiliation-3).
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Brew colour
The colour of brewed tea depends on the level of fermentation, brewing temperature, polyphenol and chlorophyll content of a particular type or variety.
Brewed green tea should be light green or yellowish, black tea should be red-amber, oolongs – golden (turquoise) and copper (dark), white tea should be slightly yellowish, shen pours should be greenish and shu-ting should be brown.
According to the correlation analysis, the colour of the infusion is closely related to the aroma and taste of tea («Research on the Correlative Factors of Sensory Quality of High-quality Green Tea», авторы: WANG Tong-he, HU Min (College of tea and food technology, Anhui agricultural university, Hefei, China). ZHANG Jiu-qian, LI Hong (Tea office in Xinyang, Henan province, Xinyang, China), Journal of Tea Science ” 2008, https://www.tea-science.com/EN/10.13305/j.cnki.jts.2008.01.005).
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Aroma
Perhaps the key factor in the quality and, accordingly, the market price of tea (which can differ by up to 10 times between aromatic – seasonal and ordinary – ‘flat’ varieties) is its aroma («Studies of leaf surface fungi in relation to flavour of tea», автор: U. K. Wickremasinghe (Dibrugarh University, India), Abinash Hazarika (Kaziranga University
Jorhāt, India), 1988, The Australian National University, https://www.researchgate.net/publication/365903891_Seasonal_occurrence_of_leaf_surface_microflora_in_tea_C_sinensis_L_Kuntze_cultivars_TV1TV23_and_P126_of_Meleng_tea_EstateJorhat_and_the_effect_of_fungicide_Propiconazol_on_those_leaf_surface_microflor).
An exhaustive review of the aroma and unpleasant taste characteristics associated with 184 flavours was conducted using instrumental methods: GC-MS, HS-SPME, GC-Olfactometry, allows the identification of hundreds of aromatic compounds. It is described in the study “Flavor of tea (Camellia sinensis): A review on odorants and analytical techniques”, авторы: Xiaoting Zhai, Liang Zhang (State Key Laboratory of Tea Plant Biology and Utilization, International Joint Laboratory on Tea Chemistry and Health Effects of Ministry of Education, Anhui Agricultural University, Hefei, China.), Michael Granvogl (Institute of Food Chemistry, Faculty of Natural Science, University of Hohenheim, Stuttgart, Germany), Chi-Tang Ho (Department of Food Science, Rutgers University, New Jersey, USA), COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY, July 2022, https://pubmed.ncbi.nlm.nih.gov/35810334/#full-view-affiliation-2.
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Taste and aftertaste
The tea taste is formed by a combination of bitterness (catechins), astringency (theaflavins, gallic acid), umami (L-theanine, amino acids). For professionals, it is usually investigated by sensory descriptive analysis (QDA), bitterness index and the results of electronic tongues. Amateurs and gourmets usually use their own mouths, comparing different flavours and aftertastes, similar to a sommelier’s wine tasting.
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Chemical composition balance
It is determined by the key tea metabolites:
– Catechins
– Caffeine
– L-theanine
– Polysaccharides
– Vitamins (C, E)
The various key chemical components are clearly linked to the sensory characteristics of the six most consumed teas in the world (black and dark and green, oolong, yellow and white tea. During the processing of tea leaves, volatiles are altered by enzymatic and thermal processes and account for most of the final aroma, while non-volatile components, which are more common in fresh leaves, contribute to a relatively lower aroma. In general, the content of total volatile, aliphatic, aromatic and terpenoids increases with the degree of fermentation during tea processing, while jasmine lactone and indole have been found in high amounts in oolongs.
In terms of taste, the sweet and umami flavours are usually well received by consumers, while the bitter and astringent flavours are often undesirable, but they make up the whole sensory experience of Camellia tea. In tea brews, the main astringents are flavonol-O-glycosides, tannins and halogenated catechins; caffeine and non-halogenated catechins are responsible for bitterness. The sweetish taste is a unique perception of green tea, which is explained by the hydrolysis of halogenated catechins. L-theanine, succinic acid, gallic acid and theogaline contribute to the umami flavour.
The quality of tea as a function of the specific variety and genetic diversity, regions, processing methods or production practices (e.g. the impact of fertilisation, shading and harvesting methods) is crucial for the quality of the final tea product («Tea Quality: An Overview of the Analytical Methods and Sensory Analyses Used in the Most Recent Studies», авторы: Juan Moreira, Jyoti Aryal (School of Nutrition and Food Sciences, Louisiana State University Agricultural Center, USA), Luca Guidry, Yan Chen (School of Plant, Environmental, and Soil Sciences, Louisiana State University, USA), Sujinda Sriwattana (Product Development Technology Division, Faculty of Agro-Industry, Chiang Mai University, Thailand), Foods 2024, https://pubmed.ncbi.nlm.nih.gov/39593996/#full-view-affiliation-3).
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Microbiological parameters and roast level
The influence of Rhodotorula glutinis yeast on tea aroma during the flavouring season is also significant, as this yeast enriches the leaf with carotenoids, which form aromatic compounds when oxidised «Studies of leaf surface fungi in relation to flavour of tea», автор: U. K. Wickremasinghe (Dibrugarh University, India), Abinash Hazarika (Kaziranga University
Jorhāt, India), 1988, The Australian National University, https://www.researchgate.net/publication/365903891_Seasonal_occurrence_of_leaf_surface_microflora_in_tea_C_sinensis_L_Kuntze_cultivars_TV1TV23_and_P126_of_Meleng_tea_EstateJorhat_and_the_effect_of_fungicide_Propiconazol_on_those_leaf_surface_microflor).
Roasting (or ‘baking’) is an important and critical step in the processing of semi-fermented teas (e.g. oolongs), which forms the appropriate flavour profile, reduces moisture and activates/deactivates certain enzymes. The choice of temperature, time and method of roasting directly determines the chemical composition, taste, aroma and functional activity of tea.
Light and medium roasting is dominated by floral and fruity compounds such as linalool, α-farnesene, and benzaldehyde.
High roasting leads to the formation of pyrroles and pyrazines, compounds characteristic of a ‘roasted’ or “caramelised” aroma («Characterization of volatile composition, aroma-active compounds and phenolic profile of Qingxin oolong tea with different roasting degrees», авторы: Xiaoye Lan, Zhibin Liu, Daoliang Wang (Institute of Food Science & Technology, Fuzhou University, Fujian, China), Food Bioscience, December 2022, https://www.sciencedirect.com/science/article/abs/pii/S221242922200445X).
In yellow tea, the method of roasting (charcoal, electric, drum) creates different dominant notes: ‘rice crust’, ‘corn’ or “burnt” («Differences in the aroma quality of large-leaf yellow tea subjected to different roasting methods», автори: Caiyan Sheng, Mingxia Lu (State Key Laboratory of Tea Plant Biology and Utilization, Anhui Agricultural University, Hefei, China), Qiuyan Liu, Huan Zhou (School of Tea and Food Science and Technology, Anhui Agricultural University, Hefei, China), LWT, July 2024, https://www.sciencedirect.com/science/article/pii/S0023643824007540).
The depth and method of roasting critically determine the quality of the future tea, namely its aromatic profile, bioactivity, and taste. Optimal roasting is a balance between creating characteristic flavours and preserving nutrients. Too much roasting, although it adds roasted notes, is often accompanied by a loss of functionality.
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Heat stability of aroma
This parameter evaluates the stability of aromas during repeated brewing. For example, in green tea, 1-2 brews retain an intense aroma, 3-4 brews reduce astringency and aroma to a mild profile («Sensory and Instrumental Flavor Changes in Green Tea Brewed», автори: Jeehyun Lee (Department of Food Science and Nutrition, Pusan National University, Busan, Korea), Delores Chambers, Edgar Chambers IV (Sensory Analysis Center, Kansas State University, USA), Foods, 2013, https://pmc.ncbi.nlm.nih.gov/articles/PMC5302280/).
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Brewing water conditions
Water quality has a significant impact on flavour perception. For example, water with a low TDS (total dissolved solids) improves the quality of green tea, prevents oxidation and darkening («Effects of natural spring water on the sensory attributes and physicochemical properties of tea infusions», авторы: Sihan Deng (College of Tea Science, Yunnan Agriculture University, Kunming, China), Qing-Qing Cao, Yan Zhu (Tea Research Institute Chinese Academy of Agricultural Sciences, Hangzhou, China), Food Chemistry September 2023, https://pubmed.ncbi.nlm.nih.gov/37037130/).
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Real-time aroma analysis
The optimal time for the release of aromatic substances during brewing is 2-3 minutes, especially for green tea («Profiling Real-Time Aroma from Green Tea Infusion», Litao Sun (Tea Research Institute, Qingdao Agricultural University, Qingdao, China), Xue Dong (Institute of Agricultural Resources and Environment, Jiangsu Academy of Agricultural Sciences, Nanjing, China), Yonglin Ren (College of Science, Health, Perth, Australia), Foods, Feb 25, 2022, https://www.mdpi.com/2304-8158/11/5/684).
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Instrumental methods of the future: AI, sensors, spectroscopy
Integration of AI analytics, GC-MS, NIR spectroscopy, electronic noses and tongues provides automated quality assessment that is on par with expert tasting (“Artificial Intelligence Sensing: Effective Flavor Blueprinting of Tea Infusions for a Quality Control Perspective”, авторы: Andrea Caratti, Angelica Fina, Fulvia Trapani (Dipartimento di Scienza a Tecnologia del Farmaco, Università degli Studi di Torino, Turin, Italy), Molecules 2024, https://www.mdpi.com/1420-3049/29/3/565).
Tea quality assessment is a complex process that combines sensory characteristics, chemical composition, processing technology and innovative analytical tools. For professional certification and quality control, it is recommended to combine traditional sensory analysis with high-precision laboratory methods to obtain a complete and objective picture of the quality of the tea product.
