Interannual Variation in Key Quality Constituents in Shiqian Taicha Manufactured as Green and Black Tea (2021–2023)

Shiqian Taicha (Camellia sinensis) is a local tea cultivar originating from Shiqian County and Guizhou (China) that is suitable for both green and black tea. The year-on-year manufacturing conditions, which affect chemical quality, were elucidated through the analysis of 78 green tea and 38 black tea commercial batches manufactured in 2021–2023. The batches were manufactured by the same process, but these naturally varied in raw-leaf status and factory parameters. The moisture content, water-soluble extract, free amino acids, tea polyphenols, caffeine, gallic acid, total ash, total catechins and individual catechins were predicted using a calibrated near-infrared (NIR) spectroscopy model and membership function evaluation, which integrated multiple indices to produce an overall quality score for each year and tea type. The amino acids of green tea peaked in the year 2022, (with 4.55%) whereas the polyphenols (which refers to carbon-based molecules) was in the year 2021, (with 24.22%), and the total catechins was in the year 2021, (with 16.71%); due to these observations, the ratio of phenol-to-amino was high in the year 2021, with (10.09); while the year 2022 had a lower ratio with (3.41). Although there were fewer differences from region to region with black tea, 2022 was better in terms of moisture control, amino acids retention and composite score with a value of 0.585. The assessment of the membership function indicated that 2022 was the most ideal tea production year for green tea (0.506) as well as black tea (0.477), with 2021 tea (0.486) and 2023 tea (0.488) following next based on type. The data presents quantitatively stable fixation and moisture/fermentation management targets to improve Shiqian Taicha value and consistency.