Comparison of freezing and convective dehydrofreezing of vegetables for reducing cell damage

Abstract

Freezing is a standard method to preserve perishable agricultural products such as fruits and vegetables, which increases off-season availability. Nevertheless, freezing of plant tissue with high water content causes cellular damage by the formation of ice crystals. This damage leads to drip loss and decreased firmness, which then reduces the quality of the thawed product. To maximize cell survival for industrial freezing processes, a promising freezing method, namely convective dehydrofreezing, was benchmarked against conventional freezing methods for different fruit and vegetables. We analyzed the final quality of thawed carrot, bell pepper, and cucumber cuts by quantifying drip loss and tissue firmness. The tissue microstructure was investigated by X-ray computed tomography after slow and fast freezing. We found that convective dehydrofreezing of bell pepper leads on average to a 52% firmer product in comparison with conventional freezing at −20 °C. For dehydrofrozen carrot, the firmness was similarly increased by 35%. Together with the significantly reduced drip loss for all tested species, these results are indicative of lower cell damage in dehydrofrozen samples. We found that dehydrofreezing of bell pepper, using different pre-drying times with resulting moisture content between 818% and 1303% dry basis, did not lead to a significant difference in drip loss or product firmness. Additionally, it was shown that freezing at an ultra-low temperature of −196 °C reduced product quality as the cucumber firmness decreased by 34% compared to conventional freezing. Freezing at low temperatures by convective freezing at −80 °C improved quality for bell pepper by producing 67% firmer products than conventional freezing. • Convective dehydrofreezing (CDF) is promising for freezing vegetables. • In contrast to conventional freezing, after CDF drip loss was reduced in carrot, bell pepper, and cucumber; tissue firmness was improved in dehydrofrozen bell pepper and carrot. • Cryogenic freezing provoked microstructural changes, including freeze-cracking and tissue breakdown. • For products with high moisture content, slow freezing or fast freezing at - 80 °C is preferred to cryogenic freezing to improve firmness retention.