Optimization of ultrasound‐assisted extraction of cold‐brewed black tea using response surface methodology

Abstract

Black tea is considered as good source of bioactives like phenolics and antioxidants. In this study, cold brewing of black tea is coupled in combination with ultrasound‐assisted extraction (UAE) to maximize extraction efficiency of bioactives. Response surface methodology was designed to optimize the conditions for extraction using UAE for cold‐brewed black tea. A multiparameter face‐centered central composite design was used to predict the conditions (amplitude: 10–70%, solvent volume: 25–100 ml, and sonication time: 10–60 min). The results obtained indicated a second‐order polynomial model. The optimized conditions for maximum extraction of bioactive for cold brewing of black tea were found to be 69.9% amplitude, 25 ml solvent volume, and 30 min of sonication time. The experimental values from the optimal process were as follows: 72.2 mg gallic acid equivalent (GAE)/g, 6.77 mg GAE/g, 37.67%, 62.97% for total phenolic content (TPC), total tannin content, 2,2‐diphenyl‐1‐picrylhydrazyl, and 2,2′‐azino‐bis(3‐ethylbenzothiazoline‐6‐sulfonic acid). The validation results agreed with the predicted models and showed an increase of 4 folds for TPC and 1.5 folds increase in antioxidant activity of cold‐brewed black tea. The displayed outcomes can remain as an extension for designing techniques for the accelerated extraction process for cold‐brewed black tea. Practical Application The application of cold brewing is limited due to the low extraction efficiency and longer extraction time. Ultrasound‐assisted extraction (UAE) improves the extraction of valuable nutrients with higher efficiency and shorter extraction time, helping the industry to produce larger amounts of cold‐brewed tea with lesser time. For other researchers, UAE‐treated cold‐brewed tea is a technique, which yields higher antioxidants and can be used as a method to study other cold‐brewed teas in the market.

Processing Technologies for the Extraction of Value-Added Bioactive Compounds from Tea

“…UAE has been in use for a long time to extract bioactive compounds from various tea leaves, as referred to in Table 1 . Most of the research work was on green tea leaves [ 121 , 134 – 143 ] and Black tea [ 31 , 136 – 138 , 143 ]. In addition, research has been carried out to extract bioactive from tea infusions [ 39 ], matte leaves [ 144 , 145 ], tea solids [ 146 – 148 ], tea seeds [ 131 ], yellow tea [ 122 ], white tea [ 136 , 137 , 149 ], and Oolong tea [ 136 , 137 ].…”

“…Solvents used for the extraction process were dependent on the targeted bioactive compounds for the process. Water remained the most used solvent for extraction [ 31 , 121 , 122 , 138 , 142 , 143 , 146 – 148 ] followed by methanol [ 138 , 143 , 144 , 148 , 149 ], ethanol [ 122 , 138 , 142 , 147 , 149 ], hexane [ 131 , 144 ], and acetone [ 147 ]. Sometimes solvents like water/acetone and methanol/ethanol were used in combination for better yields.…”

“…UAE experiments were designed to understand the effect of various processing parameters like temperature, sonication time, power, and solid solvent ratio on the extraction efficiency of tea [ 39 , 122 , 131 , 134 , 140 , 141 , 144 , 146 , 147 ]. UAE was explored as an extraction method to extract compounds like catechins [ 134 , 135 , 138 , 139 , 142 , 143 ], caffeine [ 136 , 142 , 143 ], tannic acid [ 148 ], oil [ 131 ], flavonoids [ 141 ], volatile compounds [ 137 ], theophylline and theobromine [ 136 ], theaflavins [ 138 ], antioxidant polyphenols [ 31 , 140 ], total phenolic content [ 149 ], and antioxidant capacity [ 138 ] of tea extracts were analyzed with UAE. Among these, the catechins were found to be more stable in deep eutectic solvent (DES) extracts than in any other solvents (e.g., water, methanol, and ethanol) used for the extraction process [ 139 ].…”

“…Among these, the catechins were found to be more stable in deep eutectic solvent (DES) extracts than in any other solvents (e.g., water, methanol, and ethanol) used for the extraction process [ 139 ]. In addition, UAE has been studied for increasing the total phenolic content and antioxidant capacity in cold-brewed black tea [ 31 ].…”

“…Application of UAE at high pressure, such as 90 to 338 kPa, increased the extraction yield to 200% and reduced extraction time and efficiency of extraction to 74% [ 145 ]. UAE significantly reduced the cold brewing time from 16 h to 30 min providing an energy-efficient method of extraction from black tea leaf matrix [ 31 ]. The application of UAE enhanced the extraction yield of EGCG to 15%, and the highest oxidative stability was observed in the nanoemulsion sample prepared with green tea with peanut oil [ 135 ].…”