Ultrasound accelerated γ-aminobutyric acid accumulation in coffee leaves through influencing the microstructure, enzyme activity, and metabolites.

Gamma-aminobutyric acid (GABA) is a non-protein amino acid that possesses various physiological functions. Our previous study has shown that ultrasound increased GABA accumulation in coffee leaves. In this study, we aimed to uncover the GABA enrichment mechanism by investigating the surface microstructure, cellular permeability, enzyme activities, and metabolomics of coffee leaves under ultrasound treatment. The results showed that ultrasound increased the electrical conductivity and the activities of glutamate decarboxylase, γ-aminoaldehyde dehydrogenase, and diamine oxidase by 12.0%, 265.9%, 124.1%, 46.8%, respectively. Environmental scanning electron microscope analysis demonstrated an increased opening of stomata and the rougher surface in the leaves after ultrasound treatment. UPLC-qTOF-MS/MS-based untargeted metabolomics analysis identified 82 differential metabolites involved in various metabolism pathways. Our results indicated that ultrasound changed the surface microstructure of coffee leaves, thereby accelerating the migration of glutamate into the cells; activated related enzymes; regulated C/N metabolism pathways, which led to an increase of GABA.

The effects of ultrasonication on the phytochemicals, antioxidant, and polyphenol oxidase and peroxidase activities in coffee leaves.

In the present study, we investigated the impacts of ultrasonic conditions on the phytochemical profiles, antioxidant activity, and polyphenol oxidase (PPO) and peroxidase (POD) activities in coffee leaves. Ultrasonic frequency, power, and time, pH, and incubation time affected PPO and POD differently, thus resulting in different ABTS scavenging capacity and phenolic content in coffee leaves. Triple-frequency (20/35/50 kHz) ultrasound significantly (P < 0.05) inhibited trigonelline, caffeine, mangiferin, rutin, chlorogenic acids, antioxidant activity, and PPO activity, while the single frequency of 35 kHz increased the phenolics compounds, which was associated with the lowest POD activity. Increasing the incubation time after ultrasonication gradually decreased phenolic compounds and antioxidant activities, however, POD activity followed a temporal pattern of first increase and then decrease. Our results showed that PPO and POD were temporally inactivated after ultrasonication, which leading to the continuous decrease of phenolics in coffee leaves.

Optimization of ultrasonic-assisted extraction conditions for bioactive components from coffee leaves using the Taguchi design and response surface methodology.

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.