Induction of ethylene production and ripening of winter pears at room temperatures requires prior exposure of fruit to cold storage. This chilling period is specific to each cultivar, and if inadequate or too long, abnormal ripening patterns and poor quality of the fruit will occur. The amount of chilling required for ripening may also be influenced by fruit maturity at harvest; while Chen and Mellenthin (1981) found distinct chilling time requirements and fruit quality of ripened 'd'Anjou' pears harvested over a three week period, no effect of harvest date on ripening of 'Flor de Invierno' pears harvested over a seven week period was observed by Recasens et al. (1989).

'Buerre Bosc' and 'Doyenne du Comice' are two winter pear cultivars which in New Zealand may each be commercially harvested over a period as long as four weeks. Although selective or spot picking may result in less advanced and reduced variability of maturity, as shown for apple fruit, pears harvested over this period are presumably at different stages of maturation. Little is known about effects of harvest date on chilling requirements for ripening of either 'Buerre Bosc' or 'Doyenne du Comice' fruit. The objectives of the present study were to investigate the relationships between harvest date and ripening capacity as indicated by changes in ethylene production, respiration rates, physical changes in fruit quality, and sensory analyses of the two cultivars.

Fruit from two winter pear cultivars, 'Buerre Bosc' and 'Doyenne du Comice', were harvested during maturation at weekly intervals during two seasons. Ripening behaviour during one week at 20°C was assessed at harvest and after 2, 4, 6, 8, 12, 16 and 20 weeks of storage at -0.5°C. During ripening the two cultivars showed distinctly different patterns of ethylene production after chilling. Ethylene production of 'Buerre Bosc' fruit followed typical climacteric patterns and generally exhibited progressively higher rates with longer storage. That of 'Doyenne du Comice' fruit increased with longer storage when initially transferred to higher temperature but tended to decline during the ripening period. Ripening of fruit of both cultivars, as judged by physical measurements, was affected by harvest date and storage period.

Early-harvested fruit required longer periods of storage before reaching full ripeness than late-harvested fruit (6 weeks cf. 2 weeks). Fruit still retained the ability to ripen fully after 20 weeks of storage. The extractable juice contents and titratable acidity concentrations of ripened fruit decreased with increasing storage, but these changes were less pronounced and occurred later than those for flesh firmness and background colour. Sensory analyses indicated that harvest date only affected eating quality of 'Doyenne du Comice' fruit. Fruit stored for 4 to 8 weeks generally had the highest flavour intensity, juiciness and overall quality. These results indicate that 'Buerre Bosc' fruit should be stored for a minimum of 4 weeks depending on harvest date, and best quality is obtained between 4 and 12 weeks after harvest. In contrast, optimum quality of 'Doyenne du Comice' fruit occurred between 8 and 20 weeks after harvest.

References

Recasens, D.I., Roig, J. & Graell, J., 1989. The effect of harvest date on 'Flor de Invierno' pears in cold storage. Acta Horticulturae, 256:213-221.

Prepared for HortNET - June 1998