Selection of chemical and thermal pretreatment combination for plum drying at low and moderate drying air temperatures

Prunes produced by drying plums are healthy and tasty human diet rich in various nutritions. Drying plums is a slow and energy-intensive process because of its waxy skin having low permeability to moisture. Therefore, Stanley plums in the sample bags were dipped in one of four different pretreatment solutions (4% ethyl oleate, 1% KOH, 1% NaOH, or water) at two different dipping temperatures (23 or 60 °C) for 1 min to accelerate the skin moisture diffusivity by breaking down the waxy cuticular surface of plum. After the completion of pretreatment process, the plum samples were dried at low or moderate temperatures (<55 °C) of drying air.The plums treated by 1% KOH or 1% NaOH at 60 °C dipping temperature were the fastest ones reaching to the final weight loss percentage (75%). They dried in 54 h in the laboratory tray dryer providing 44.5 °C drying air while the untreated samples had the weight loss percentage of 28.81% at the same drying time. Plums dipped only in 60 °C hot water lost their weight by 55.54% under the same conditions. Solar drying and open sun drying of plums treated by 1% KOH or 1% NaOH at 60 °C dipping temperature completed approximately in 177 h and 250 h. The other plum samples could not reach to the desired weight loss percentage even in 441 h after initiation of solar drying and open sun drying. The pretreatment combinations caused slight variations in the prune colors.