Elucidating the Mechanisms Underlying Decreased Firmness of Broccoli Harvested at Low Temperatures

Abstract

Loss of vegetable texture after freezing represents a crucial challenge that demands attention and is influenced by various factors, such as cultivar, maturity, and harvest time. Therefore, understanding the factors significantly impacting texture after freezing is imperative. Our previous study revealed a correlation between the temperature at harvest and the firmness of frozen broccoli. However, the specific mechanisms contributing to the decreased firmness of frozen broccoli remain unclear. Therefore, this study aimed to elucidate the mechanism underlying the decreased firmness of frozen broccoli at low temperatures through field experiments. Two broccoli cultivars were selected for the study and harvested under three distinct conditions: spring, winter, and late winter. Among the frozen-thawed samples, those harvested in spring exhibited the highest breaking stress, followed by those from winter and late winter harvests. Analysis of pectin composition revealed that samples from winter and late winter harvests displayed weaker cell wall adhesion compared to spring samples, which could promote tissue breakdown via ice crystal formation. Moreover, the low water-holding capacity of late winter samples could inhibit the restoration of frozen-thawed samples to their original state, consequently leading to significant texture deterioration after freezing. The examination of tissue structure after freeze-thawing supports these considerations. The findings of this study can help stabilize the firmness of frozen broccoli through adjustments in harvest timing.