Freezing induces reversible colour-change in both zeaxanthin-biofortified and yellow sweet-corn

Zeaxanthin-biofortified sweet-corn is a deeper golden-orange colour than regular yellow sweet-corn due to a greater ratio of orange carotenoids (zeaxanthin, β-cryptoxanthin and β-carotene) to yellow carotenoids (lutein, zeinoxanthin). Freshly-harvested zeaxanthin-biofortified and yellow corn-cobs were subjected to freezing 
(-20°C) and assessed for colour-change (hue angle). Freezing kernels at -20°C was observed to significantly reduce hue angle by approximately 4-6°, with decline slightly greater in zeaxanthin-biofortified kernels than in yellow kernels. Frozen kernel hue angle could be predicted by linear correlation with fresh kernel hue angle (r2=0.90). Carotenoid-analysis of zeaxanthin-biofortified and yellow corn confirmed no significant change in carotenoid profile due to freezing, apart from slight changes to antheraxanthin, β-cryptoxanthin and β-carotene. This indicates that colour changes may be due to physical, rather than chemical changes. In order to investigate if cell membrane disruption was the cause of colour change, fresh kernels of zeaxanthin-biofortified and yellow sweet-corn were blended into a slurry, and hue angle measured after freezing to -20°C. Decline in hue angle was similar to that of intact kernels, indicating membrane disruption was unlikely to be responsible. Fresh cobs were subsequently subjected to freezing and subsequent thawing. Thawing to room-temperature resulted in a reversal of hue angle to the original values of fresh kernels. The results indicate that colour change due to freezing may be due to the formation of ice-crystals, possibly interfering with light absorption, as the effect disappeared once the ice-crystals had melted. Rapid freezing with liquid nitrogen had less effect on hue angle than freezing at -20°C, indicating that ice-crystal size may also play a role. The observation of why orange cobs are affected more than yellow cobs remains to be elucidated.