Ripening of European pears: The chilling dilemma

The majority of European pears (Pyrus communis L.) show at some extent resistance to ripening after harvest. Low temperatures and/or ethylene treatments have to be applied to counteract this behavior. This review provides the main protocols that have been used experimentally to ripen European pears grown in the U.S. and summarizes important aspects to understand the ripening physiology of this fruit. Many factors interacting with pear ripening are discussed including, cold storage, controlled atmosphere storage, ethylene treatment, cultivar differences, preharvest temperature, growing region, harvest maturity, storage time and ripening temperature, cooling and warming of fruits, treatments to inhibit ripening, and plant growth regulators. © 2008 Elsevier B.V. All rights reserved. Most European pears (Pyrus communis L.), unlike other climacteric fruit, possess varying degrees of resistance to ripening at harvest even when harvested at the appropriate maturity, and require a period of chilling and/or ethylene exposure to ripen properly. This resistance to ripening poses a number of practical challenges for the pear industry in preparing their fruit for market, and therein rests the dilemma. The biochemical basis for this resistance and the specific requirements for cold and/or ethylene exposure are not well described or understood. Pears ripened on the tree generally do not develop the characteristic buttery and juicy texture required for marketing and consumption (Murayama et al., 1998). It has been demonstrated that ripe ‘Bartlett’ pears out-sell unripe pears by a ratio of three to one (California Pear Advisory Board, 2004). In addition, studies conducted across the U.S. showed that ‘d’Anjou’ pears treated with exogenous ethylene prior to marketing out-sold pears not treated with ethylene by 16% (Pear Bureau Northwest, 2002). This increase in marketability may be due to the perception of aromatic volatiles by consumers in ripened fruit on display at the market as well as the improved taste of the ripened pears (Rapparini and Predieri, 2003). There is an increasing interest in marketing European pears ∗ Corresponding author. Tel.: +1 530 752 7512; fax: +1 530 752 8502. E-mail address: ejmitcham@ucdavis.edu (E.J. Mitcham). in a partially ripened stage. Detailed ripening protocols have been developed for some cultivars of pears, but not for others, and the scientific basis of some of these protocols is unclear. This review summarizes the available research results related to European pear fruit ripening, from expression of ethylene biosynthetic genes to commercial methods for controlling the rate and uniformity of ripening. Physiological studies that increase our understanding of the underlying mechanisms of pear ripening, and the influence of preharvest and postharvest factors are presented. A listing of current ripening protocols involving cold storage and/or ethylene treatments is provided for several pear cultivars. 1. Effect of cold storage Proper temperature during storage is the most important factor for maintenance of high quality pear fruit during postharvest management. In European pears, temperatures ranging from −1 to 10 ◦C (low temperature conditioning) also play a crucial role in the stimulation of ethylene biosynthesis during subsequent ripening at room temperature (Sfakiotakis and Dilley, 1974; Chen et al., 1983; Knee et al., 1983; Blankenship and Richardson, 1985; Knee, 1987; Gerasopoulos and Richardson, 1997a,b; Lara and Vendrell, 1998; Agar et al., 2000a; Miró et al., 2001). However, for long-term storage of most pear cultivars, the quality and storage life are diminished by even a slightly elevated storage temperature above the opti0925-5214/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.postharvbio.2008.03.003 Author's personal copy 188 M. Villalobos-Acuña, E.J. Mitcham / Postharvest Biology and Technology 49 (2008) 187–200