Enhanced productivity, quality and heating uniformity by stacking oil palm fruits under microwave irradiation

Studies on microwave sterilisation of oil palm fruits have predominantly been qualitative in nature, with little focus on profiling the temperature and heat transfer involved during the process in real-time. The present study investigates the influence of different oil palm fruit arrangements on the heat transfer and distribution under microwave irradiation and its effect on product quality. The heat transfer characteristics of single-layered and stacked layers of fruits were investigated through experimentation (response surface methodology) and simulation, followed by qualitative analysis of the products. Microwave power and heating time were recognised as significant factors (p-value < 0.05) influencing the temperature and moisture content of the oil palm fruits. The established computational model predicted significantly denser electric energy deposition in stacked fruits than a single layer of fruits. Oil palm fruits fit for storage purposes (moisture content <= 15 wt%) and palm oil meeting special quality palm oil standards (free fatty acid content <2.5%, moisture and volatile matter content <0.2%) were obtained. Stacking reduced sterilisation time by ~21% and produced ~11% softer fruits with ~27% less variation in hardness among the fruits than a single layer. The stacked arrangement proved advantageous in attaining a relatively uniform heat distribution, improved heating pattern, boosted the sterilisation process with a faster drying rate and eased the extraction stage with softer fruits. Thus, enhancement in both productivity and products was accomplished through stacking.