Evaluation of maturity parameters of 'Fuerte' and 'Hass' avocado fruit.
Olarewaju, Olaoluwa Omoniyi.
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Avocado fruit is one of the most important horticultural crops produced in South Africa. The fruit does not give obvious indication of maturity as it does not ripen as long as it remains attached to a tree. Harvesting avocado fruit at full physiological maturity, a stage at which it will continue normal development, plays a vital role in the postharvest physiological processes and the successful postharvest management of the fruit. Common maturity parameters used in various avocado fruit industries include mesocarp oil content, moisture content (MC) and dry matter (DM). However, the difficulty of measurement (oil content) and unreliability (MC and DM), can result in immature fruit reaching the consumer. To ensure that avocado fruit of good quality are delivered to the market and for growers to maximise profits, possible factors indicating optimal harvest maturity were investigated during the South African 2013 and 2014 avocado growing season. Additionally, the growth pattern of fruit, beyond what is currently regarded as physiological maturity, was examined for the possibility of the fruit exhibiting a double sigmoidal growth pattern, typical of nut crops. Fruit were harvested from two commercial orchards in the cool subtropical area of KwaZulu-Natal, South Africa. Fruit were harvested bi-weekly from February to March and then monthly from April to October, 2014. The MC decreased over the harvesting period, (p < 0.001), while oil content increased (p < 0.001). The study of the pattern of avocado fruit growth and development over the eight months observation period revealed that the fruit exhibits a single sigmoidal growth pattern. It could also be deduced from the experimental results that MC is a better indicator of maturity compared with oil content. In a quest to find an alternative maturity indicator that could provide a more reliable measure of avocado harvest maturity, total soluble solids (TSS) was evaluated for the possibility of providing an objective maturity index. Seven carbon (C7) sugars, D-mannoheptulose and perseitol, are dominant sugars in avocado fruit and have been suggested as likely indicators of avocado fruit maturity. D-mannoheptulose, a major component of mesocarp TSS, has been suspected to be responsible for the continued growth of the fruit. 'Fuerte‟ and „Hass‟ avocado fruit were harvested during the early, mid and late harvesting period in 2013 from Bounty Farm and during the 2014 season (February to August) from Bounty Farm and Everdon Estate. Sample ws taken along the equatorial region of each fruit and analysed for TSS, measured by squeezing juice out of the mesocarp using a garlic press and determining its °Brix using a digital refractometer. A high level of significant difference was observed between TSS and harvesting period for „Fuerte‟ during both seasons (p < 0.001) and a significant difference was found between the two production locations during the 2014 growing period (p < 0.001). There was no significant difference (p = 0.344) between production sites for „Hass‟ fruit harvested during the 2014 season. The results of the study reveal that TSS cannot be used as an indicator of avocado fruit maturity. In an attempt to non-destructively predict maturity parameters of avocado fruit, a total of 150 intact avocado fruit were scanned in reflectance mode of near-infrared spectroscopy (NIRS) during the 2013 and 2014 growing seasons. Reference maturity parameters, including MC, DM and oil content were measured using conventional destructive methods. Calibration models developed during 2013 season were used to predict the dataset acquired during 2014. NIRS prediction results showed that MC and DM were predicted with significant accuracy compared with oil content, prediction of which was not accurate. The prediction statistics for NIRS predicted MC and DM content demonstrated the potential of this system for non-destructive evaluation of avocado fruit maturity parameters (MC and DM). The high prediction accuracy recorded when models developed in 2013 were used to predict maturity of fruit harvested during the 2014 season demonstrated robustness of partial least square (PLS) models. Where speed and accuracy are required for assessing the maturity status of individual, intact avocado fruit, the method developed in this study is recommended.