Effects of freezing pretreatment and hygroscopicity of fluidizing particles on drying characteristics of carrots in fluidized bed under reduced pressure

Abstract

A method for low-temperature drying with high drying rate was developed for heat-sensitive foods and agricultural products. A freezing pretreatment was combined with fluidized bed drying under reduced pressure. Cylindrical carrot samples were frozen and then placed without thawing in a fluidized bed of hygroscopic porous silica gel particles that adsorbed water from the sample during the drying process. The effects of the freezing pretreatment and hygroscopicity of fluidizing particles on the drying characteristics of carrots were examined. A higher drying rate was achieved when carrots samples were subjected to freezing pretreatment than without it. At 12 kPa, the volume change was smaller in carrots subjected to freezing pretreatment than in untreated samples. A larger amount of water was absorbed during rehydration by carrots dried at 12 kPa than at 101 kPa within 120 min. The properties of dried carrots were affected not only by freezing pretreatment but also by the pressure applied during the drying process.

Keywords:

• Fluidized bed drying
• vacuum-drying
• freezing pretreatment
• carrot
• shrinkage
• rehydration