Accumulation of beta-carotene and trans-cis isomerization of ripening mango mesocarp were investigated as to concomitant ultrastructural changes. Proceeding postharvest ripening was shown by relevant starch degradation, tissue softening, and a rising sugar/acid ratio, resulting in a linear decrease (R (2) = 0.89) of a ripening index (RPI(KS)) with increasing ripening time. A modest accumulation of all-trans-beta-carotene and its cis isomers resulted in a slight pigmentation of the mango chromoplasts, because ambient temperatures of 18.2-19.5 degrees C provided suboptimal ripening conditions, affecting color development and beta-carotene biosynthesis. The ultrastructures of chromoplasts from mango mesocarp and carrot roots were comparatively studied by means of light and transmission electron microscopy. Irrespective of the ripening stage, mango chromoplasts showed numerous plastoglobuli varying in size and electron density. They comprised the main part of carotenoids, thus supporting the partial solubilization of the pigments in lipid droplets. However, because different pigment-carrying tubular membrane structures were also observed, mango chromoplasts were assigned to the globular and reticulotubular types, whereas the crystalline type was confirmed for carrot chromoplasts. The large portions of naturally occurring cis-beta-carotene in mango fruits contrasted with the predominance of the all-trans isomer characteristic of carrots, indicating that the nature of the structure where carotenoids are deposited and the physical state of the pigments are crucial for the stability of the all-trans configuration.

Accumulation of beta-carotene during postharvest ripening of nine Thai mango cultivars was assessed after verifying extraction and high-performance liquid chromatographic quantification of the beta-carotene stereoisomers for this sample matrix. No relevant trans-cis isomerization was induced by the analytical procedure. The vitamin A potential of mangoes was evaluated at different ripening stages unequivocally defined by a ripening index (RPIWB). Being rather stable throughout postharvest ripening, the cultivar-specific proportion of cis-beta-carotene isomers ranged from 14 to 40% of the total beta-carotene content. Subjected to the same postharvest ripening conditions, only the cultivars Kaew, Maha Chanok, Chok Anan, and Nam Dokmai #4 developed a bright yellow-orange mesocarp coloration at their fully ripe stage (RPIWB = 1.5-1.8, sugar-acid ratio approximately 50), resulting in total beta-carotene contents of 6544-11249 microg/100 g mesocarp dry weight (DW) and vitamin A values of 892-1573 retinol equivalents (RE)/100 g DW. Contrarily, poor-colored cultivars Mon Duen Gao, Rad, Kiew Sawoei, Okrong Kiew, and Okrong Thong reached total beta-carotene contents of 1019-2195 microg/100 g DW and vitamin A values of 136-298 RE/100 g DW at comparable sugar-acid ratios. Exponential development of mesocarp color (hue angle, H degrees ) and all-trans-beta-carotene levels, respectively, with RPIWB was described for each cultivar, allowing good prediction of mesocarp color and vitamin A value at consumption ripeness.

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