Objective To investigate the distribution characteristics of browning onset time in the upper leaves of flue-cured tobacco, and determine the relationships between key influencing factors and browning onset time.

Method A two-factor randomized combination design was employed, with different maturity levels (unripe, mature, overripe) and dry-bulb temperatures at the color-fixing stage (40, 42, 44 ℃). Image changes of tobacco leaves were continuously monitored during the curing process, with the appearance of brown spots on the leaf surface as the criterion for browning onset. The Kaplan-Meier method was employed for univariate analysis of browning onset time, and a model for browning onset time was established based on Cox regression analysis.

Result Maturity level had no significant effect on the risk of browning onset . Compared with unripe leaves, the hazard ratios for mature and overripe leaves were 0.85 and 1.10, respectively, and neither was statistically significant. The dry-bulb temperature at the color-fixing stage was a key risk factor affecting browning onset in the upper leaves of ‘Yunyan 87’. At 44 ℃, the proportion of browning occurrence was the highest, and the survival curve declined fastest. Compared with 40 ℃, 44 ℃ increased the risk of browning onset by approximately 1.52 times, while the difference between 42 ℃ and 40 ℃ was not significant. The model diagnostics showed that the P values for both individual variables and the global proportional hazards assumption test were greater than 0.05, indicating that the proportional hazards assumption was not violated and the parameter estimates of the Cox model were reliable.

Conclusion This study provides a quantitative relationship between the risk function of browning onset time and the influencing factors, providing technical support and a theoretical reference for optimizing the curing parameters for upper leaves.