Relation between temperature and the beginning of peel color change in grapefruit (Citrus paradisi Macf.)

• Degreening process begins when minimum temperatures (average of the preceding 14 days) fall below 15 °C.
• Lightness of grapefruit peel begins to increase for minimum temperatures (average of the preceding 14 days) below 17 °C.
• Degreening process accelerates when the mean minimum temperatures (average of the preceding 14 days) fall below 14 °C.
• High thermal integral is required while color change needs temperatures below 15 °C.

The grapefruit varieties Marsh (white-fleshed) and Redblush (pink-fleshed) are the most widely cultivated varieties in the world. In this work we attempt to determine the temperature below which degreening begins in these two varieties of grapefruit, first studying the influence of radiation on the colorimetric coordinates and then the influence of minimum temperatures on the change from green to yellow, based on the data gathered during the 2005/6, 2006/7, 2010/11 and 2011/12 growing seasons. The results show that net radiation alone does not explain when degreening begins, but that the process depends on the mean of the minimum temperatures for the 14 days preceding the sampling being below 14 °C.Finally, we discuss the relationships between coordinate a, and each of the following variables: maturity index (MI), °Brix, and percentage of acidity, concluding that the indices show that grapefruit are mature at the beginning of autumn when the peel is still green. Therefore, these indices are not related with natural degreening, which occurs in autumn.This knowledge will help predict the color that fruit will reach according to the temperatures of a given geographical area and whether or not degreening chambers will have to be used, with the additional costs that this implies.