Performance evaluation of three controlled atmosphere storage equipment / techniques on Granny Smith apples

This study evaluated different long-term CA and DCA storage techniques and the effect of ethylene scrubbing on fruit quality and scald development

Fruit harvested in Elgin in 2019 and 2020 were stored under (T1) DCA – chlorophyll fluorescence without and with (T2) ethylene scrubbing; (T3) DCA – respiration quotient method and (T4) extreme low oxygen with ethylene scrubbing. Fruit were evaluated after 42 weeks (2019) and 43w (2020) at 0 °C, again after 6 weeks in RA (-0.5 °C), and after 10 days of shelf-life at 20 °C. Each system had three replicate bins, each bin containing 9 lugs. Unmanageably high ethylene levels were noted in T4 in 2019 and so lower O₂ levels set in the 2020 season to suppress ethylene production as the technology relies on low ethylene during storage.

No treatment exhibited superficial scald after 43w of storage in 2020. After 6w RA, a single replicate of T4 developed superficial scald. Similarly, to season 1, this replicate ran at the highest O₂ concentration. Superficial scald incidence did not differ significantly between treatments at any of the evaluations in season 2. None of the maturity indices differed in season 2 after 43w, likely due to low oxygen levels utilized in all treatments. Differences observed in season 1 for TSS:TA and firmness followed the pattern of higher degradation under higher oxygen levels. In season 2, differences were only seen in the RA and shelf-life examinations. Hue angle for Treatment 4 was significantly yellower than all other treatments but differences developed later than in season 1. Differences in chroma during shelf-life observed in the first season were not seen in season 2. Differences in firmness, TA, TSS:TA followed the pattern of highest degradation accompanying higher O₂ levels utilized in storage in both seasons. Although significant differences were seen in season 2 for the MI sample for superficial scald and internal browning incidence, these differences are thought to be due to sample size utilized in MI sampling. The same observation was made for internal browning incidence in season 1.

Low O₂ (<1 kPa) effectively suppressed superficial scald and quality degradation. T4 had an advanced maturity compared to the other treatments, irrespective of season. Differences took longer to emerge in season 2 due to the lower oxygen level utilized compared to season

1. Scald incidence in season 2 did differ significantly between T4 and other treatments (all very low). The oxygen levels utilized in season 1 resulted in unacceptably high levels of superficial scald. The higher ripening potential of fruit stored under T4 was a function of higher oxygen levels (double that of the DCA-CF treatments in season 2 and triple that of the DCA-CF treatments in season 1), which resulted in more ethylene production compared to other treatments. Cumulative oxygen showed potential in determining quality degradation risks and superficial scald potential.

Storing ‘Granny Smith’ scald free relies on cumulative oxygen. Therefore oxygen levels below 1% should be in place as soon as possible after harvest and correct temperatures were reached. Equipment that can deliver this as soon as possible and maintain these levels provided the best results (0.2- 0.4% O_2, depending on safety limits). This will benefit not only fruit quality but also scald incidence during long-term storage of ‘Granny Smith’.