The biochemical basis of colour as an aesthetic quality in Citrus sinensis.
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Date
2001
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Abstract
The development of fruit colour in the sweet orange (Citrus sinensis) is a cultivar
characteristic affected by climate and environment. Although external colour is not always
an indication of internal quality or maturity, it is probably the most important factor
determining consumer acceptance. In the present investigation, efforts were made to
determine the biochemical basis of colour as an aesthetic quality in C. sinensis 'Navel'
and 'Valencia'. Furthermore, the changes in pigment content and composition during the
period of colour development were recorded. Finally, in an attempt to manipulate citrus
colour, the effect of dehydrating agents (alcohols), plant hormones, micro-nutrients and
low temperature on flavedo carotenoid content was determined.
Saponification of the two major colour-imparting components resolved by thin layer
chromatography, followed by reversed-phase high performance liquid chromatography
revealed that the principal colour-imparting pigments in C. sinensis flavedo are the yellow-coloured
xanthophyll 9-Z-violaxanthin and the red C(30) apocarotenoid β-citraurin. Both
pigments occur in the flavedo in esterified form. Identification of the chromophores was
based on co-chromatography and online spectral analysis. The colour quality of mature
fruit was dependant on the content and relative amounts of 9-Z-violaxanthin and Pcitraurin.
Quantitative results revealed that increased colour intensity of citrus flavedo
was associated with a decline in the 9-Z-violaxanthin : β-citraurin ratio from greater than
50 to below 10, and an increase in 9-Z-violaxanthin and β-citraurin content. Measurement
of the mass and ratio of these pigments can be used to accurately colour-grade orange
fruit for local and export markets. These parameters will also aid in the evaluation of
colour manipulatory techniques.
Visual colour break in C. sinensis appears to be associated with a minimum in total
pigment as well as total carotenoid content. The period prior to colour break is
characterised by a reduction in chlorophylls, carotenes and free xanthophylls usually
associated with photosynthetic activity. Following colour break, a massive increase in
xanthophyll acyl esters (particularly 9-Z-violaxanthin) is observed.
Efforts were made to manipulate carotenoid content of citrus flavedo in vivo, with a view
to manipulate fruit colour commercially. It was shown that the micro-nutrients tungsten
and molybdenum, and the plant hormones abscisic acid and jasmonic acid increased
carotenoid content of flavedo discs; whereas the plant hormone gibberellic acid
decreased carotenoid levels. The dehydrating agents ethanol and butanol increased
carotenoid content in whole fruit flavedo and flavedo discs. Optimum concentrations were
shown to be 20-30% (v/v) for ethanol and 5-10% (v/v) for butanol.
Description
Thesis (M.Sc.Agric.)- University of Natal, Pietermaritzburg, 2001.
Keywords
Citrus fruits--Colour., Oranges--Colour., Plant pigments., Theses--Horticultural science.