SO UTH AFRICAN AVO CADO G RO WERS’ ASSO CIATIO N YEARBO O K 33, 2010 56 ABSTRACT A three year study considered the effects of production location, maturity, storage temperature, modified at- mosphere (MA) storage and a cold chain break on avocado ripening physiology. The aim of this study was to better understand post-harvest avocado physiology to enable improved post-harvest management to maintain fruit quality longer. Focus was placed on sugars, ripening enzymes, respiration, ethylene production and water relations. The seven carbon sugars, mannoheptulose and perseitol, were confirmed as the predominant sugars and possible energy source of avocado fruit. The six carbon sugars – glucose, fructose and sucrose – were not present in sufficient quantities nor do their trends suggest that they are used as an energy source. The seven carbon sugars are present in low concentrations in the mesocarp and need to be preserved during cold storage to enable proper ripening. The activity of three enzymes (cellulase, polygalacturonase and pectin methylesterase) was measured during ripening. The activity of these ripening enzymes is essential for softening and palatability. Polygalacturonase (PG) was not affected by different storage treatments although increased because of storage but cellulase activity was highly dependent. Fruit that were not stored in MA storage had a higher cellulase activ- ity post-storage and fruit stored at 1°C had a slightly higher activity. The activity of pectin methylesterase (PME) was initially higher in fruit that were not stored in MA bags, but PME showed only slight variations post-storage. The 1°C and MA storage both reduced the respiration rate, ethylene production rate, mass loss, water loss and heptose consumption. The MA storage also reduced protein synthesis and external chilling injury. These two treat- ments together were found to be an effective means of maintaining fruit quality. These treatments negated the symptoms of long-term cold storage: the reduced storage temperature reduced the respiration rate and the MA storage reduced desiccation, but had a minimal effect of the ripening rate. A cold chain break, at any time during cold storage, increased mannoheptulose usage and water loss, and caused a spike in respiration and ethylene production. A cold chain break also increased shrivelling, greatly reducing fruit quality. Water content of fruit was measured using near infrared spectroscopy (NIRS). The model was validated in 2009 after two years of model development. It was found that the model was more accurate for fruit from KwaZulu-Natal, probably due to less Post-harvest avocado physiology RJ Blakey, JP Bower and I Bertling Horticultural Science, School of Agricultural Sciences and Agribusiness University of KwaZulu-Natal, Pietermaritzburg Private Bag X01, Scottsville 3209, South Africa E-mail: robblakey@gmail.com; bowerj@ukzn.ac.za time loss between harvesting and measurement than fruit from other areas. INTRODUCTION The avocado (Persea americana Mill.) is a fruit crop with physiology which does not always appear to fol- low that of other fruit. The popularity of the avocado fruit has grown during the last fifty years and the fruit is produced extensively in a broad range of climatic zones, including semi-desert, temperate, subtropical and tropical. Avocados are also an important export crop in most of the countries of production. The cold storage of avocados has brought about many chal- lenges for horticultural scientists. Unfortunately for the consumer, there are still many unsolved problems. These problems can only be solved once the avocado physiology is properly understood. Progress in under- standing avocado physiology has been slow because of its highly variable nature and high concentrations of oil and phenols, requiring large datasets and compli- cated assays. Research resources are also divided into pathology, agronomy, cultivar development and post- harvest technology, further slowing advances in post- harvest management. The South African avocado industry is export driven. However, export also results in high costs of shipping and handling. Profits can be increased substantially with optimised management of the fruit. Management decisions need to have a physiological basis, because fruit quality is imperative for maintaining – and grow- ing – market share. Two low-cost post-harvest tech- niques were implemented in this study: 1°C storage and modified atmosphere (MA) storage. The MA was achieved by enclosing fruit in micro-perforated poly- propylene bags (polybags). Avocados are climacteric fruit, generating large amounts of carbon dioxide and ethylene during rip- ening. The texture of fruit changes substantially dur- ing ripening. These changes are brought about by the