Instant tea is a Tea product made from finished tea, by-products of tea processing, or fresh tea leaves through processes such as extraction, filtration, concentration, and drying. It retains the flavor and efficacy of tea while being convenient to carry and brew. China has become the world's largest producer of instant tea, and since the 13th Five-Year Plan period, continuous innovations in instant tea processing technology have further propelled the development of the instant tea industry.

Extraction is a critical step in the processing of instant tea, significantly impacting its color, aroma, taste, and solubility. To enhance the value of tea dust and optimize the extraction process for instant Green Tea, this study uses tea dust generated during the processing of An Kang green tea as raw material and employs ultrasonic extraction technology to prepare instant green tea. The effects of extraction time, extraction temperature, and tea-to-water ratio on the quality and extraction rate of instant green tea are investigated. The optimal extraction process for instant green tea is determined based on sensory evaluation (Table 1).

Results of Single-Factor Experiments

1. Effect of Ultrasonic Extraction Time on Sensory Score and Yield As shown in Figure 1, when the ultrasonic extraction time is between 10-20 minutes, both the sensory score and yield show an increasing trend; after 20 minutes, the sensory score decreases, but the yield continues to rise. Considering these factors, the extraction times selected for the orthogonal experimental design are 15, 20, and 25 minutes.

2. Effect of Ultrasonic Extraction Temperature on Sensory Score and Yield As shown in Figure 2, with the increase in ultrasonic extraction temperature, the sensory score and yield of instant tea both exhibit a trend of first increasing and then decreasing. The highest sensory score is achieved at an extraction temperature of 40°C. Within the range of 30-60°C, the yield continues to rise, but it decreases after exceeding 60°C. Considering all factors, the ultrasonic extraction temperatures of 30°C, 40°C, and 50°C are chosen for the orthogonal experiment.

3. Effect of Ultrasonic Extraction Tea-to-Water Ratio on Sensory Score and Yield As shown in Figure 3, when the ultrasonic extraction tea-to-water ratio ranges from 1:5 to 1:20, the sensory score of instant tea shows an increasing trend with significant changes, while the yield first increases and then decreases. The sensory evaluation score reaches its peak at a ratio of 1:20, and beyond this point, the score decreases, while the yield increases again. Considering all factors, the ultrasonic extraction tea-to-water ratios of 1:15, 1:20, and 1:25 are chosen for the orthogonal experiment.

Orthogonal Experiment Results

As shown in Table 2, Group 2 has the highest sensory evaluation score for the instant green tea, which is 91.09 points. Therefore, the optimal combination of parameters for achieving the highest sensory quality of instant green tea using ultrasonic-assisted extraction is A2B1C2 (extraction time of 20 minutes, extraction temperature of 30°C, and tea-to-water ratio of 1:20). Through comparison of the range values R, it can be concluded that C > A > B, indicating that the order of impact on the quality of instant green tea is tea-to-water ratio > extraction time > extraction temperature. Based on the comparison of K values, the best combination of parameters is A2B1C3 (ultrasonic-assisted extraction time of 20 minutes, extraction temperature of 30°C, and tea-to-water ratio of 1:25).

Verification Experiment

The optimal combination of parameters obtained from the orthogonal analysis (time of 20 minutes, extraction temperature of 30°C, and tea-to-water ratio of 1:25) was used for ultrasonic-assisted extraction, yielding a rate of 24.67%. The sensory evaluation composite score of the instant green tea produced under the optimal combination of parameters was 91.56 points (Table 3), with tea polyphenol content of 13.78%, soluble Sugar content of 9.04%, Caffeine content of 3.32%, and free amino acid content of 2.30% (Table 4). Compared to the raw material, there was a significant increase in the soluble sugar content in the instant green tea; the contents of tea polyphenols and caffeine decreased significantly, reducing the bitterness and astringency, thus improving the taste and mouthfeel; the change in free amino acid content was not significant.

This study demonstrates that the instant green tea prepared through optimization of the ultrasonic-assisted extraction process (time of 20 minutes, extraction temperature of 30°C, and tea-to-water ratio of 1:25) has a tender green appearance with luster, a yellow-green and bright liquor, and a fresh, full-bodied, and sweet aftertaste.

Tea dust and fragments generated during traditional green tea processing, if not effectively utilized, can lead to resource waste. This study provides a theoretical basis for the processing of instant green tea and offers a reference for utilizing tea dust resources generated during traditional green tea processing.