The loss of greenness (degreening) in citrus fruit during ripening is associated with autumnal falls in
temperature, and there is a close correlation between the color of the fruit measured in the HunterLab
space (colorimetric coordinate a) and the mean daily minimum temperatures recorded for the 7 and 21
days prior to the measurement being made. The term colorimetric coordinate a expresses the variation
from green to yellow during lemon fruit ripening.
Inthisworkweattempttodeterminethetemperaturebelowwhichdegreeningbeginsinthreevarieties
of lemon fruit on the tree: Eureka, Lisbon and Fino. Only the results for Eureka are described since all
three varieties showed the same behavior. The colorimetric coordinate a was measured weekly in the
peel of previously marked and identified lemon fruit over a period of 7 years, and the temperature was
recorded in a weather station situated on the same farm.
Theresults showthatdegreeningbeginswhentheminimumtemperatures fallbelow10.5◦
Cfor2days
(notnecessarily consecutive).Inour experimentthemeandailyminimumtemperature recorded for the
previous 7 and 21 days were 12.14◦
C and 13.64◦
C, respectively.
This implies that a knowledge of autumntemperatures should enable growers to know whenitis best
to harvestthe fruit – when degreening has just begun (with a shortened artificial degreening period) or
when more time has elapsed, although this may mean thatthe commercial opportunity associated with
early fruit may be lost.