Effect of freezing pretreatment on the drying characteristics and volume change of carrots immersed in a fluidized bed of inert particles under reduced pressure

• The carrots were dried in the fluidized bed of inert particles under reduced pressure.
• The freezing carrots were immersed in the fluidized bed without thawing.
• The drying rate was higher for freezing pretreated carrots than for untreated carrots.
• The freezing pretreatment reduced the shrinkage that occurs with drying.
• The carrot remained frozen in the drying process for freezing pretreatment.

A drying method with a low drying temperature and a high drying rate that induces small volume change in biological samples was developed for drying heat-sensitive foods and agricultural products. Freezing pretreatment was combined with fluidized-bed drying under reduced pressure. Cylindrical carrot samples were frozen and then immersed in a fluidized bed of inert particles under reduced pressure without thawing. A higher drying rate was achieved for carrots subjected to freezing pretreatment than for the untreated counterparts. The drying rate was higher at 12 kPa than at atmospheric pressure. The volume change induced during the drying process was smaller for carrots subjected to freezing pretreatment than for the untreated congeners. A high drying rate at a low temperature with minimal sample shrinkage was achieved via the combined freezing pretreatment and fluidized-bed drying under reduced pressure. This method can be applied for drying heat-sensitive materials such as food and agricultural products.