Development of an aroma-rich functional green tea concentrate by supercritical CO₂ extraction and subsequential hot water extraction

Abstract

Tea, as the second most consumed beverage worldwide, provides refreshing aroma, unique taste and potential health-promoting effects. Research shows that green tea has the function of antibacterial, anti-mutagenic, anti-inflammatory, anti-diabetic and hypocholesterolemic properties, as well as body weight control, increase bone mineral density, and solar ultraviolet protection. However, tea is a heat-sensitive substance. Leaching at a high temperature would cause aroma loss and oxidation of the contained components. This research was carried out to develop a volatile-rich green tea concentrate by applying supercritical carbon dioxide extraction (SC-CO2), which has excellent quantitative recovery of volatiles, essential oils and fats in food processing. Orthogonal array design with four factors (extraction temperature, extraction time, pressure, and water content) and three levels were applied to optimize extraction conditions. From the range analysis, the highest yield could be achieved by using 3.5 hours extraction time and 10 mL water content at 60°C extraction temperature and pressure of 35 MPa. The only factor that was seen to be statistically significant (p < 0.05) was extraction time. On the other hand, the optimum extraction conditions for the green tea volatiles were at 40°C and 15 MPa, using 10 mL water and extraction time of 3.5 hours. The composition of the volatiles was compared with those extracted by steam distillation and hot water tea infusion using the solid-phase micro extraction gas chromatography-mass spectrometry. The volatile yields from the three methods were 2.91 ± 0.26 μg/g dw for hot water tea infusion, 23.75 ± 1.77 μg/g dw for SC-CO2 and 14.84 ± 1.09 μg/g dw for steam distillation, respectively. Among the results from the three extraction methods, tea infusion extracted the most compounds (44), followed by steam distillation (33) and SC-CO2 (32). β-Ionone was the most abundant volatile exhibiting 'woody and violet' aroma in SC-CO2 extracts, while linalool with strong floral and rose notes was the richest in the extracts by steam distillation. On the other hand, dimethyl sulfide with cabbage, sulfur, gasoline odour occupied the largest portion in the extracts obtained by tea infusion. Tea polyphenols are moderately polar substances, soluble in water (especially in hot water) and ethanol. The results showed that water and aqueous ethanol had excellent recovery of catechins according to the result of total phenolic content, total flavonoids content, antioxidant actives and high performance liquid chromatography detection. Additionally, after SC-CO2 processing, a large quantity of catechins could still be extracted by water extraction from the treated tea samples. In conclusion, the present study has shown that combining SC-CO2 and conventional solvent extraction to develop an aroma-rich green tea concentrate was feasible, which made up the shortage of volatiles in tea processing, as well as providing abundant antioxidants.