Gas exchange, ripening behaviour and postharvest quality of coated pears : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Postharvest Physiology and Technology at Massey University, New Zealand

Abstract

Pear cultivars 'Bartlett', 'Beurre Bosc', 'Doyenne du Comice', and 'Packham's Triumph' were treated with different levels of deposits of a carnauba based wax on the skin and assessed for gas exchange, ripening behaviour and postharvest quality. The response to coating treatments was strongly dependent on cultivar, ripening stage and environmental temperature. 'Bartlett', 'Comice' and 'Packham's', with non-lignified skin, had substantial reductions in skin permeance (P'j) with small increases in coating deposit. Magnitudes of reduction in P'j to different gases were observed in the order: P'O2 > P'CO2>> P'H2O. The skin of 'Bosc', with lignified cells, had high P'H2O and low P'CO2' and increasing the amount of coating deposited on the skin resulted in small reductions of P'H2O and a gradual reduction of P'O2 and P'CO2. 'Bartlett' and 'Bosc' had a high risk of developing internal disorders caused by excessive internal accumulation of CO2 at low temperatures when treated with substantial coating deposits, as a result of high respiration rate ('Bartlett') or low P'CO2 of coated skin ('Bosc'). These cultivars were also less tolerant to hypoxia (expressed in terms of internal lower O2 limit, LOLi) created by high coating concentrations, and their level of tolerance reduced with increasing ripeness. 'Comice' and 'Packham's' were highly tolerant of hypoxia [the fruit did not ferment despite of an internal O2 partial pressure (piO2)@ 0 kPa]. Respiration rates, softening and colour change followed a Michaelis-Menten model when plotted against piO2, while internal CO2 partial pressure (piCO2) had virtually no explanatory power for these variables during shelf life. Variable cover of skin pores in cultivars having high P'j might result in variable P'O2 and, consequently, variable piO2. This could increase the naturally high ripening variability of pears treated with a given coating concentration. Softening had a lower Michaelis-Menten constant for pio2 than skin colour. Therefore, coated pears with intermediary pio2 might have variable postharvest quality mainly in terms of colour change, and the fruit may still soften while being unable to change in colour. For 'Comice', higher levels of coating deposit resulted in more substantial modification of internal atmosphere during cold storage, slightly increasing ripening delay. These treatments reduced wastage by diminishing the incidence of senescent breakdown and senescent scald after long term storage and by reducing skin friction discolouration during shelf life. Increasing the amount of coating deposit improved skin gloss and reduced senescent breakdown of 'Bartlett', 'Comice' and 'Packham's' during shelf life. The results show that optimisation of surface coatings should take into account differences between cultivars, ripening stage when the fruit is coated and storage temperature to avoid the risk of fermentation and physiological disorders. Even though there are some quality problems due to uneven ripening, wax coatings represent a technology with high potential for the pear industry, improving the finish of the skin, reducing water loss, delaying ripening and reducing the incidence of senescence related disorders.