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A mathematical model was presented to characterise heat transfer in high-humidity hot air impingement blanching (HHAIB) of cuboid carrots. The model accounts for condensation heat transfer and predicts the sample’s core temperature evolution with time. The simulation was performed at different relative humidity of 20%, 40%, 60%, 80% with constant temperature of 383 K. Results showed that the heat transfer process of HHAIB is divided into two stages based on vapor–liquid phase transition, namely a condensing heat transfer segment and a non-condensing heat transfer segment. In the initial stage of HHAIB, a comprehensive and intense condensation occurred on the sample surface leading to high heat transfer coefficient and greatly enhancing heat transfer, but at the same time creating a huge temperature gradient in the sample. The relative humidity of 40%–60% at 383 K could not only enhance the heat transfer rate but also improve the homogeneity of temperature distribution in the sample. Simulated core temperatures were compared with experimental measurements, showing good agreement with a coefficient of determination R2 of 0.991. The findings of current work provide theoretical basis to better design and control of the process conditions of HHAIB as it elucidates the heat transfer characteristic taking account condensation phenomenon and illustrates the temperature distribution and evolution profiles.

The current work presents energy efficient hot air drying by controlling the relative humidity (RH) including step-down and continuous dehumidification of RH as to specify material based on Weibull and Bi-Di models. The drying characteristics of yam slices of different thickness (6, 12, 18 mm) and longan were investigated using two RH control strategies. Results showed that continuous decrease of RH was energy efficient for longan, while the step-down strategy was more energy efficient for 12 or 18 mm thick yam slices drying. In terms of drying kinetics, the step-down control strategy of RH is more energy efficient for those materials whose drying rate as well as sample’s temperature curves display three distinct stages and sample thickness is relative large. In terms of β and Bi values in Weibull and Bi-Di models respectively, it is observed that the step-down control strategy of RH drying method is more efficient when the β value exceeds 1 and the Bi value was more than 0.1. The current investigation provided an assessment method for the convective hot air drying method by controlling RH to improve drying efficient and save energy consumption.