‘d’Anjou’ pear metabolism during ultra-low O2, low CO2 controlled atmosphere storage reflects disorder outcome

With increasingly restrictive regulation of postharvest crop protectants and expected year-round availability, pear (Pyrus communis L.) production is ever more reliant on precise controlled atmosphere (CA) storage technologies to provide both ripening and disorder control for long term storage. Ultra-low oxygen (ULO; <1 kPa O₂) CA storage meets many of these criteria affording control of ripening and superficial scald. However, internal browning often is an unintended outcome caused by atmospheric conditions and other events that occur, similarly to superficial scald, prior to symptom development. While this is often associated with elevated CO₂, it also can occur at lower partial pressures (<0.5 kPa CO₂) reducing its widespread use for storing pears. We expected levels of specific metabolites in ‘d’Anjou’, a cultivar susceptible to both disorders, would reflect specific disorder risk under ULO and low CO₂. The metabolic profile of pear peel stored at 0.5 °C in air, CA (1.5 kPa O₂; 0.5 kPa CO₂), or ULO (determined in the first week of storage by monitoring chlorophyll fluorescence; 0.5 kPa O₂; 0.5 kPa CO₂) was compared at multiple time points during 0−16 weeks storage. Final fruit quality and relative ripeness were assessed at 16 weeks plus 7 d at 20 °C. Quality and ripeness differences and related metabolism followed that established in existing literature regarding peel color, titratable acidity, ethylene evolution, respiration, and sucrose metabolism. Superficial scald developed on air stored fruit and core browning incidence most in pears stored in 0.4 kPa O₂. α-Farnesene oxidation increased most in air followed by 1.5 kPa O₂ storage indicating risk of this disorder on fruit from those storage conditions. As in other studies, γ-aminobutyric acid (GABA) was elevated most where internal browning had occurred, although levels were also elevated in the peel. Levels of glutamic acid, alanine, and, unique to this study, proline were also most elevated in peel from pears stored in 0.5 kPa O₂ indicating their association with conditions that lead to core browning. Monitoring α-farnesene oxidation coupled with GABA, glutamic acid, alanine, and proline levels in peel may be a means to indicate whether ULO CA atmospheres with reduced CO₂ levels are accomplishing the critical task of controlling pear superficial scald without causing internal browning before injury occurs.