Evaluating the Streif index against commercial subjective predictions to determine the harvest date of apples in South Africa

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Abstract
For the local deciduous fruit industry, which mainly exports fruit, determination of the optimum physiological harvest maturity for apples to ensure high fruit quality after long-term storage is still an important goal. This is performed by a subjective determination based on a weekly rate of change of all or some of the ripening parameters such as fruit firmness. In Europe, the Streif index (SI) has been implemented to reduce this subjectivity. The aim of this project was to evaluate this index under local conditions. Historical physiological data for Golden Delicious and Starking apples (Malus domestica Borkh.) from various orchards in the Ceres area were analysed. Significant correlations were obtained between the SI and subjective predictions of the release date for harvesting, after adapting the local starch breakdown values. However, during seasons when the starch conversion rates were very low during the early season, the SI alone may not be sufficient to indicate the release date for harvesting. A multilinear regression equation was also fitted to the maturity data. Likewise the accuracy of this model was influenced by season and variation between individual sites. Both methods can be applied commercially to support the otherwise subjective determination of the harvest dates.

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