Evaluation of Thermal Resistance of Refractance Window (RW) Drying System and Effect of Drying Temperature on Strawberry Powder Characteristics

This study investigates the performance of an in-house fabricated Refractance Window (RW) drying system for processing strawberry puree, with a focus on heat transfer dynamics, material properties, and quality characteristics of the dried product. Spectral transmissivity of the Mylar film was measured using FTIR spectroscopy, revealing significant peaks within the wavelength ranges of 2.7-3.69, 5.29-11.16, and 12.44-15.09 µm, corresponding well with the absorption spectrum of water-rich strawberry puree. Thermal resistance components were evaluated, including resistance between water and Mylar film (R1), resistance due to Mylar film (R2), the strawberry puree (R3), and the convective resistance between puree and air (R4).  R1 exhibited the highest resistance at 0.653 W⁻¹ m² K, identifying it as the primary bottleneck for heat transfer. Conversely, the strawberry puree posed minimal resistance (R3 = 0.00162 W⁻¹ m² K), facilitating efficient energy absorption. The overall heat transfer coefficient of the system was calculated to be 1.187 W m⁻² K⁻¹, indicating effective heat utilization. Results showed that powders dried at higher temperatures (85°C) exhibited greater solubility (89.04%) and hygroscopicity (17%) due to enhanced porosity from rapid water evaporation. However, this was accompanied by significant color degradation, likely due to Maillard browning and anthocyanin degradation, as evidenced by a high total color difference (∆E > 4.0). Samples dried at 65°C retained better color but exhibited lower solubility and higher moisture content. The optimal drying temperature was found to be 75°C, balancing functionality and appearance with a solubility of ~85.8%, water activity <0.46, and a perceptible color difference below critical thresholds (∆E < 3.2). This temperature effectively maintained product integrity while ensuring efficient drying.