Sequential Infrared Radiation and Freeze-Drying Method for Producing Crispy Strawberries

Sequential infrared and freeze-drying (SIRFD) as a new processing method was studied for producing high-quality crispy fruit pieces at reduced cost. This research investigated the drying characteristics of strawberry slices and the quality of the finished products under SIRFD. The 4 mm thick strawberry slices were pre-dehydrated to 30%, 40%, and 50% levels of weight reduction with infrared (IR) heating at each of the three different intensities (3000, 4000, and 5000 W m-2). The pre-dehydrated samples were then further freeze-dried to achieve a final moisture content of about 5%. For comparison, the slices were also pre-dehydrated with hot-air drying (62.8C) followed by freeze-drying (SHAFD) and dried with regular freeze-drying without pre-dehydration. The drying kinetics of strawberry slices under IR, hot-air, and freeze-drying were determined and modeled. The color, shrinkage, rehydration ratio, and crispness of finished products were measured. The IR radiation heating had a much higher drying rate than hot-air during the pre-dehydration. The product produced with SIRFD had more desirable color, higher crispness, and more shrinkage, but a lower rehydration ratio than regular freeze-drying, which, however, did not produce a high-crispness product. The microstructure characteristics of the dried products explained the differences in quality produced with the different methods. IR pre-dehydration to a 40% weight reduction level reduced required freeze time by 42%, indicating a great energy saving potential for SIRFD, since the energy efficiency of freeze-drying is very low. It has been concluded that SIRFD could be a desirable method for producing high-crispness strawberry pieces.