Zeaxanthin biofortification of sweet-corn and factors affecting zeaxanthin accumulation and colour change.

Zeaxanthin, along with its isomer lutein, are the major carotenoids contributing to the characteristic colour of yellow sweet-corn. From a human health perspective, these two carotenoids are also specifically accumulated in the human macula, and are thought to protect the photoreceptor cells of the eye from blue light oxidative damage and to improve visual acuity. As humans cannot synthesise these compounds, they must be accumulated from dietary components containing zeaxanthin and lutein. In comparison to most dietary sources, yellow sweet-corn (Zea mays var. rugosa) is a particularly good source of zeaxanthin, although the concentration of zeaxanthin is still fairly low in comparison to what is considered a supplementary dose to improve macular pigment concentration (2mg/person/day). In our present project, we have increased zeaxanthin concentration in sweet-corn kernels from 0.2 to 0.3mg/100g FW to greater than 2.0mg/100g FW at sweet-corn eating-stage, substantially reducing the amount of corn required to provide the same dosage of zeaxanthin. This was achieved by altering the carotenoid synthesis pathway to more than double total carotenoid synthesis and to redirect carotenoid synthesis towards the ß-arm of the pathway where zeaxanthin is synthesised. This resulted in a proportional increase of zeaxanthin from 22% to 70% of the total carotenoid present. As kernels increase in physiological maturity, carotenoid concentration also significantly increases, mainly due to increased synthesis but also due to a decline in moisture content of the kernels. When fully mature, dried kernels can reach zeaxanthin and ß-carotene concentrations of 8.7 mg/100g and 2.6 mg/100g, respectively. Although kernels continue to increase in zeaxanthin when harvested past their normal harvest maturity stage, the texture of these 'over-mature' kernels is tough, making them less appealing for fresh consumption. Increase in zeaxanthin concentration and other orange carotenoids such as ß-carotene also results in a decline in kernel hue angle of fresh sweet-corn from approximately 90° (yellow) to as low as 75° (orange-yellow). This enables high-zeaxanthin sweet-corn to be visually-distinguishable from standard yellow sweet-corn, which is predominantly pigmented by lutein.

Increase in ß-ionone, a carotenoid-derived volatile in zeaxanthin-biofortified sweet corn.

Carotenoids are responsible for the yellow color of sweet corn (Zea mays var. saccharata), but are also potentially the source of flavor compounds from the cleavage of carotenoid molecules. The carotenoid-derived volatile, ß-ionone, was identified in both standard yellow sweet corn ('Hybrix5') and a zeaxanthin-enhanced experimental variety ('HZ') designed for sufferers of macular degeneration. As ß-ionone is highly perceivable at extremely low concentration by humans, it was important to confirm if alterations in carotenoid profile may also affect flavor volatiles. The concentration of ß-ionone was most strongly correlated (R(2) > 0.94) with the ß-arm carotenoids, ß-carotene, ß-cryptoxanthin, and zeaxanthin, and to a lesser degree (R(2) = 0.90) with the α-arm carotenoid, zeinoxanthin. No correlation existed with either lutein (R(2) = 0.06) or antheraxanthin (R(2) = 0.10). Delaying harvest of cobs resulted in a significant increase of both carotenoid and ß-ionone concentrations, producing a 6-fold increase of ß-ionone in 'HZ' and a 2-fold increase in 'Hybrix5', reaching a maximum of 62 µg/kg FW and 24 µg/kg FW, respectively.