Improvement of Apple Quality and Storability by a Combination of Heat Treatment and Controlled Atmosphere Storage

The effects of two nonchemical methods [controlled atmosphere (CA) storage and postharvest heating, alone or combined] on the quality (firmness, taste, color, and skin wax) and storability (losses resulting from bruising and fungal decay) of apples were investigated in a 3-year study. Fruits of two cultivars (cv. Aroma and cv. Ingrid Marie) were mechanically wounded on two opposing sides, inoculated with conidial suspensions of one of three pathogens [Pezicula malicorticis (bull's eye rot), Penicillium expansum (blue mould), and Colletotrichum gloeosporioides (bitter rot)], exposed to 40 °C for four different exposure periods (24, 48, 72, and 96 h), and stored either in air (21.0 kPa O2 + 0.03 kPa CO2) or in CA storage (2.0 kPa O2 + 2.0 kPa CO2) for 4 months. Effect of postharvest heating on bruise susceptibility of air- or CA-stored apples was also investigated. Cultivar Aroma apples generally showed higher bruise susceptibility than cv. Ingrid Marie. The sun-exposed side of apples was less sensitive to bruising than the shaded side and red phenotypes of these two cultivars also showed increased resistance to bruising as compared with standard phenotypes. Heat treatment and CA storage, either alone or in combination, decreased bruise occurrence in both cultivars. Pz. malicorticis was the more aggressive storage pathogen for both apple cultivars followed by P. expansum and C. gloeosporioides. The highest decay severity occurred in inoculated and nonheat-treated apples stored in air. Heat treatment, especially in combination with CA storage, showed an eradicative effect on the pathogens without any negative effects on apple quality. Heat treatment maintained flesh firmness during storage, reduced ethylene production, and caused clearly visible changes in epicuticular wax structure, resulting in a higher resistance to bruising or to natural and artificial infections with the pathogens. The effective exposure period could be reduced to 24 h, because a combination of heat treatment (at 40 °C for 24 h) and CA storage showed the best protective effect against bruising and fungal decay. This combined treatment decreased bull's eye rot by 86% and 60% and bitter rot by 73% and 65% in cv. Aroma and cv. Ingrid Marie, respectively, in comparison with untreated apples.