Postharvest apple softening : effects of at-harvest and post-harvest factors : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science at Massey University, Palmerston North, New Zealand
'Cox's Orange Pippin' (COP) and 'Royal Gala' (RG) are rapid softening apple cultivars. This makes it difficult for growers to meet minimum firmness standards in the marketplace. Research was undertaken to characterise softening curves of COP and RG in relation to different at- and post-harvest factors, and to compare these cultivars with the slower softening cultivars 'Granny Smith' (GS) and 'Pacific RoseTM' (PR). Regular measurement of firmness during low-temperature storage showed that the postharvest softening curve for all cultivars was triphasic with an initial slow softening phase (I), followed by a phase of more rapid softening (II), and then a final slow softening phase (III). Phase I largely determined the fruit market life for firmness, as fruit with a short phase I had less market life than fruit with a longer first phase. Phase I of RG and COP was lengthened by harvesting fruit at an earlier rather than later maturity, by rapidly cooling fruit after harvest to 0.5-3°C, and by placing fruit in controlled atmospheres (CA). Rate of phase II softening was not affected by harvest maturity, but decreased as storage temperature was reduced from 22 to 0°C, and was reduced in CA relative to air. A modified Arrenhius equation described softening rates of COP and RG at different temperatures, where softening rate increased from 0°C to a maximum at 22°C, and then decreased through 35°C. In contrast, this equation could not describe softening rates of PR and GS at different temperatures, as both cultivars softened slowly at similar rates from 0-12°C, and phase II did not occur at 20-35°C. Prior cold or ethylene treatment induced phase II softening at 20°C for GS, but not PR. Internal ethylene concentration (IEC) may have a role in regulating onset of phase II softening in RG and COP at 0-35°C, while for GS and PR fruit sensitivity to ethylene may have a more important regulatory role than IEC. A prototype model was developed for estimating loss of RG and COP firmness through the postharvest handling chain. This model has potential to improve commercial management of the "soft fruit" problem in the marketplace.