Coffee leaves contain various bioactive compounds that are beneficial for human health. However, there are very limited researches related to the extraction of the bioactive phytochemicals from coffee leaves. In the present study, the extraction conditions for bioactive components from coffee leaves were optimized using Taguchi design and response surface methodology (RSM). Taguchi design was used to screen significant factors that affected the yield of phytochemicals including trigonelline, caffeine, chlorogenic acids, mangiferin, and rutin, total phenolic content (TPC) and antioxidant activity. Sequentially, a Box‐Behnken design (BBD) was used to optimize the extraction conditions. Three factors including Liquid‐to‐solid (L:S) ratio, ethanol concentration, and extraction temperature that significantly affected most of the phytochemical yields and antioxidant activity were selected from the six variables using Taguchi design. The optimal extraction conditions obtained from RSM were 30.3:1 L:S ratio, 54.5% ethanol, and 80 °C when simultaneously considered four responses, including TPC, the yields of mangiferin and 5‐CQA and DPPH scavenging capacity. Under the optimal conditions, the experimental results for the above four responses were 62.1 mg gallic acid/g, 4.1 mg/g, 11.4 mg/g, and 356.9 µmol Trolox/g, respectively, which were close to the predicted values. About 97% of phytochemicals can be extracted in the first two times of extraction. In conclusion, the combination of Taguchi design and response surface methodology can be successfully used to screen and optimize the significant factors that affected the bioactive components extracted from coffee leaves. Practical Application Coffee leaves, the byproducts of coffee plants, are considered no‐ or low‐value although it has a long history for using them as tea‐like beverage and ethnomedicine by locals in the coffee plant growing countries. Bioactive components extracted from coffee leaves can be used as ingredients in functional beverages, functional food, and natural health products. These applications will add values to coffee leaves as well as increase the incomes of coffee farmers and workers.

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In this study, the accumulation of γ-aminobutyric acid (GABA) in coffee leaves under ultrasonication and the impacts on total phenolic content (TPC), the activities of glutamate decarboxylase (GAD) and GABA transaminase (GABA-T), and angiotensinconverting enzyme (ACE) were investigated. The pH, incubation time, and ultrasonic power were the three most important factors that affected the accumulation of GABA. Coffee leaves treated with 20/40 kHz ultrasound have the highest GABA and the greatest GAD and ACE inhibitory activity. After optimization with a Box-Behnken design, GABA content was increased by 1.6 times under the optimal conditions of pH 5.9, incubation time 6.6 hr, and ultrasonic power 150 W. Our study indicated that ultrasonication activated GAD while inhibited GABA-T, thereby leading to an increase in GABA and ACE inhibitory activity. Ultrasonication also enhanced TPC; however, long time incubation caused the oxidation of phenolic compounds, and thus, affected the ACE inhibitory activity.Novelty impact statement: This is the first research that studied the enrichment of Gamma-aminobutyric acid (GABA) in coffee leaves using ultrasound stress. GABAenriched coffee leaves can be used to prepare tea-like beverage or natural health products to prevent hypertension, anxiety, and sleeplessness.

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Summary It is well known that the choice of extraction solvents greatly affects both phytochemical composition and related bioactivity of plant extract. Herein, we show that coffee leaf extracts derived from 80% methanol have no effect on basal nitric oxide production in Raw 264.7 cells, whereas, a different result was observed with a previous hot water extract. Phytochemical mixtures formulated to contain equivalent amounts of CQAs, mangiferin and rutin know to present in coffee leaves showed a relatively low (3.5–18.5%, TPC; 17.6–31.6%, ABTS; 7.9–13.6%, DPPH; and 8.5–29.7%, ORAC) contribution to antioxidant activities when compared to the coffee leaf extracts. We conclude that the solvent polarity is particularly important to the unique phytochemical mixture recovered, which in turn governs the bioactivity. This study also demonstrated that other phytochemicals in coffee leaves that were not measured in this study are important in assessing total antioxidant activity of this potential plant beverage.