Compositional Analysis and Regulation of Foam Formed during Black Tea Juice Fermentation

In order to clarify the biochemical and volatile constituents of foam formed during the aerobic fermentation of black tea juice, foam was collected at different stages of aeration and analyzed for non-volatile compounds such as tea polyphenols, amino acids, tea saponins, catchins and theaflavins by spectrophotometry and high performance liquid chromatography (HPLC), and for volatile compounds by head space solid-phase microextraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC-MS). The results showed that the concentrations of tea saponins, tea polyphenols, caffeine, catechins and theaflavins in the foam formed during tea leaf homogenization and aeration were higher than those in the residual tea juice. The total amount of volatile compounds; the contents of decanoic aldehyde, citral, linalool, geraniol and β-ionone, responsible for the sweet and flowery aroma; and the contents of hexanolaldehyde, 1-octyl-3-alcohol, phenylacetaldehyde, methy salicylate and (E)-2-hexenal, responsible for the green aroma, were also higher than those in the residual tea juice, indicating that the foam took away many quality components. In order to retain the quality components, an antifoaming agent was added in black tea juice to prevent foam formation. It was found that 0.2% of the defoamer effectively controlled foam formation while having no significant effect on tea polyphenols or amino acids, and no adverse effect on the fermentation process, and promoted the release of volatile compounds in black tea juice.