Abstract: The aim of this paper was to discuss the aging characteristics of raw dark tea and the safety of dark tea during storage. The moisture absorption characteristics of Hunan raw dark tea at 20, 30 or 40 ℃ over the water activity range of 0.010 3–0.981 8 were evaluated using the static gravimetric method and response surface models were developed. Moisture absorption isotherm curves were plotted and fitted to a nonlinear model. Furthermore, the mildew degree was examined during storage under simulated storehouse conditions: 25 ℃ and 85%, 87% or 90% relative humidity. The results showed that the moisture absorption isotherms of raw dark tea exhibited the characteristics of a type II isotherm and the equilibrium time (14, 10 and 6 days) was shortened with increasing temperature (20, 30 and 40 ℃). Equilibrium moisture content was reduced with increasing temperature. The response surface models at all three temperatures showed regular patterns. Among 8 classical mathematical models, GAB and Oswin models were the best ones to describe the isothermal moisture absorption process. During 10 days of storage under simulated storehouse conditions, the dark tea did not mildew at 85% relative humidity (RH), mildewed partially at 87% RH, and mildewed seriously at 90% RH.

Key words: raw dark tea, moisture absorption isotherms, mildew degree, model fitting