Heat-shock Treatments Increase the Chilling Tolerance of Harvested Asparagus Spears

Abstract

Holding harvested asparagus spears at non-freezing temperatures below 2.5 °C induces chilling injury (CI), a physiological disorder that reduces quality and shelf life. CI can be quantified by subjective visual parameters, or by objective measurements of the increased rate of ion leakage from excised tissue into an isotonic bathing solution. The rate of ion leakage from apical (2–3 cm), middle (9–10 cm), and basal (15–16 cm) segments excised from 18-cm asparagus spears increased after 7 days of chilling at 2.5 °C. The increase continued and was similar for middle and basal segments after 14 days of chilling, but more pronounced from apical segments. Various heat-shock treatments (i.e., combinations of temperature and duration) decreased the chilling-induced increase in ion leakage from these 1-cm stem segments. Although the chilling tolerance of all spear segments was increased by specific heat-shock treatments, the optimal temperature and duration of exposure varied among the segments; some treatments that were effective in segments from one location were either ineffective or damaging to segments from another location. As the apical half of the whole spear is the predominant culinary portion and contains the most chilling sensitive tissue, heat-shock treatments that would increase the chilling tolerance of the upper half of whole spears were selected for further study. These heat-shock treatments were applied to freshly harvested whole 18-cm asparagus spears that were chilled at 2.5 °C for 14 days. Two treatment combinations (i.e., 45 °C for 4.0 ± 0.6 minutes or 50 °C for 2.9 ± 0.8 minutes) were identified that maintained the highest level of quality and significantly reduced the rate of chilling-induced ion leakage.