No abstract

Drying is a key step that affects the chemical composition and quality of tea. In the present study, we reported the impacts of drying temperature and time on drying and color change kinetics, phytochemical composition, antioxidant activity, and surface microstructure of coffee leaves during hot-air drying. The results showed that drying temperature was positively (p < 0.05) correlated with the drying rate (DR), color index a* and total color change ΔE, and total soluble sugar (TSS), while negatively correlated with color indexes b* and L*, soluble protein content, and the DPPH scavenging capacity. Drying time has similar impacts on the color indexes and soluble protein as drying temperature. The content of total free amino acid and TSS increased by 62.5% and 47.4%, respectively, when coffee leaves were dried at 160 • C for 24 min, under which the total phenolic content and DPPH and ABTS scavenging capacities reached the maximum of 108.04 mg GAE/g, 515.07 µmol Trolox/g, and 606.70 µmol Trolox/g, respectively. Drying significantly decreased the contents of chlorogenic acids and mangiferin and antioxidant activity, while high-temperature short-time drying helped retain phenolic compounds in coffee leaves. The DR fitted Page kinetic model. The color changes fitted the first-order kinetic models and the activation energies ranged from 16.00 to 31.06 kJ⋅mol −1 . Prolonged drying time caused serious wrinkles on the surface of coffee leaves and the stomata closed.

The bioactive components and bioactivities of citrus peel can be enhanced with microbial fermentation. Accordingly, this study investigated the ability of Aspergillus niger CGMCC3.6189 to accumulate flavonoids in Citrus reticulata peel powder (CRPP) by solid-state fermentation (SSF). Under the optimal SSF conditions including 80% moisture, 30 °C, pH 4.0, 4 × 107 spores/g d.w. CRPP, and 192 h, the total phenolic content (TPC), total flavonoid content (TFC), and 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) scavenging activities of fermented CRPP significantly increased by 70.0, 26.8, 64.9, and 71.6%, respectively. HPLC analysis showed that after fermentation, the contents of hesperidin, nobiletin, and tangeretin were significantly increased from 19.36, 6.31, and 2.91 mg/g to 28.23, 7.78, and 3.49 mg/g, respectively, while the contents of ferulic acid and narirutin were decreased under the optimal fermentation conditions. Fermentation time is one of the most important factors that affect the accumulation of flavonoids and antioxidant activity; however, extended fermentation time increased the darkness of CRPP color. Therefore, our study provides a feasible and effective SSF method to increase the bioactive components and the antioxidant activity of CRPP that may be used in food, nutraceutical, and medicinal industries.

Roasting is a critical process that affects the quality attributes of coffee beans; however, how roasting conditions affect the physical, chemical, biological, and organoleptic changes of coffee pulp needs more research. In the present study, we investigated the effects of roasting temperatures and times on chemical compositions and quality attributes of coffee pulp. The results showed that the contents of total soluble sugar (TSS) and free amino acid (FAA) followed a temporal pattern of first increasing and then decreasing under the roasting temperatures between 100 and 160 • C. Total phenolic content (TPC) and antioxidant activity of coffee pulp significantly (p < 0.05) increased after roasting, reaching the maximum values of 83.09 mg gallic acid equivalent (GAE) /g and 360.45 µM 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox) /g, respectively, when coffee pulp was roasted at 160 • C for 18 min. Drying rates of coffee pulp fitted the Logarithmic kinetic model, while color (L*, a*, and b*) changes and 5-caffeoylquinic acid degradation followed the first-order kinetic model. Electronic nose analysis showed that the main aroma compounds of the coffee pulp are sulfur-containing organics that were reduced with the extended roasting time. Scanning electronic microscopy analysis presented the loosened, shrunk, and cracked microstructure on the surface of the roasted coffee pulp, which might contribute to the increased TSS, FAA, TPC, and antioxidant activity of coffee pulp roasted under specific conditions. In conclusion, our research provides valuable information for preparing high-quality coffee pulp tea.