Effects of Size, Cultivar, and Harvest Season on the Tissue Softening in Frozen Broccoli

Textural deterioration in frozen vegetables is a result of softening after freeze-thawing and is a critical issue to address. In the present study, field experiments were conducted on broccoli using six cultivars of three sizes (i.e., different maturities) in two seasons (spring and winter) to evaluate the relationship between the level of tissue softening caused by the freezing process and composition of pectin, which is a main component of the cell wall. The mechanical properties after freeze-thawing depended more on the growing season than on the cultivar or maturity, i.e., the maximum stress after freeze-thawing of spring broccoli was higher than that of winter broccoli. Between the winter broccoli, the samples harvested at lower temperatures showed more pronounced softening after thawing. For all broccolis, the temperature during harvest was strongly correlated with the maximum compressive stress of frozen broccoli (R = 0.867). Broccoli harvested in winter had a higher ratio of galacturonic acid in water-soluble pectin than that harvested in spring, indicating weakened cell wall adhesion. However, the absence of a substantial correlation between the pectin content and mechanical properties of broccoli, suggests that other factors such as cold-responsive materials are also involved in the tissue softening of frozen broccoli.