Unraveling the temporal changes of Maillard reaction products and aroma profile in coffee leaves during hot-air drying

Drying is a critical process that affects chemical compositions, aroma, and quality of tea. Our previous study demonstrated the impact of hot-air drying on the physiochemical properties of coffee leaves. Herein, we investigated changes in carbonyl compounds, volatiles, and their precursors in hot-air dried coffee leaves. The drying process increased the content of 5-hydroxymethylfurfural, glyoxal, and methylglyoxal, while decreasing the levels of free amino acids and sugars. Electronic-nose analysis highlighted an increase in alcohols, aldehydes, and ketones in hot-air dried leaves compared to their freeze-dried counterpart. Using head-space solid-phase microextraction-gas chromatography–mass spectrometry, combined with relative odor activity values, we identified that hexanal, 2-hexenal, α-ionone, and limonene significantly contributed to the grassy, floral, fruity, woody, and citrus flavors of the freeze-dried leaves. In contrast, 1-octen-3-ol and ( E,E )-2,4-heptadienal play a key role for the mushroom and nutty aromas of hot - air dried leaves. Additionally, nonanal, ( E,Z )− 2,6-nonadienal, and phenylacetaldehyde contributed significantly to the fatty, grassy, rose, and honey aromas of both coffee leaves. Furthermore, interactive network analysis revealed the close correlation between aromas and the levels of alcohols, aldehydes, and ketones. This study provides valuable guidance for production of coffee leaf tea with a prominent aroma while minimizing harmful carbonyl compounds.