Factors affecting carotenoids in zeaxanthin-biofortified sweet-corn (Zea mays L.)

Abstract

Zeaxanthin-biofortified sweet-corn hybrids are novel super sweet-corn that have been developed in Australia with the objective to provide sufficient zeaxanthin (2 mg per day), equivalent to that recommended for synthetic supplements) with the consumption of one cob daily, to reduce the progression of macular degeneration. Macular degeneration is the leading cause of blindness in the developed world and is responsible for 50 % of all cases of blindness in Australia. A decline in the macula carotenoid pigments, lutein and zeaxanthin, has been associated with macular degeneration. Humans cannot synthesize carotenoids, and therefore need to obtain these from dietary sources (or synthetic supplements). Generally, lutein is easier to obtain in a normal diet as it can be found in most green vegetables. By contrast, zeaxanthin is relatively scarce and only present in some yellow and orange-pigmented food sources. The zeaxanthin-biofortified sweet-corn was developed by conventional breeding to increase total carotenoid synthesis and accumulation, but also to increase synthesis of b-branch carotenoids (which includes zeaxanthin) at the expense of a-branch carotenoids (which includes lutein). Because of the relative novelty of zeaxanthin-biofortified sweet-corn, there is currently no information related to factors that may affect carotenoid concentration in these hybrids, or any information in regard to the effect of these factors on general product quality. This thesis represents the first approach to generate knowledge about the accumulation of carotenoids in four zeaxanthin-biofortified hybrids and how they can be influenced by factors such as kernel maturity, summer and autumn growth environments, kernel position on the cob, kernel structure, and storage temperature. The work was divided into separate studies, in which the assessment of quality parameters such as external kernel colour, moisture, total soluble solids, total protein, and total starch content were included, to meet the objectives of each study. Significant differences (p l 0.05) between the carotenoid accumulation and profile of the four zeaxanthin-biofortified hybrids were shown in this study. The carotenoid profile of all hybrids was significantly affected by the harvest season. Under summer growing conditions, zeaxanthin and total carotenoid concentration (TCC) were up to 2-fold higher compared to growth in autumn. Kernel maturity was shown to have a significant influence on TCC, with most carotenoids peaking in concentration at later stages of kernel maturity, when measured on a fresh weight (FW) basis. Carotenoids measured on a dry weight (DW) basis appeared to peak earlier and then decline at later stages of kernel maturity. These differences were due to the concurrent loss of moisture content as kernels matured, thus concentrating any carotenoids based on FW, despite their apparent reduction on DW. Kernel position along the cob also affected TTC, though to a lesser degree than either seasonal growing conditions or kernel maturity. A trend was observed in which, kernels located at the tip-end of the cob had higher TCC, compared to kernels located at the middle and the base. Despite the above trend, the higher concentration only was significant for zeaxanthin (and TCC), such that kernels located at the tip-end of the cob had 20 % more zeaxanthin. Influence of kernel structure on carotenoids was further evaluated to determine the contribution of the germ and endosperm fractions to the whole kernel carotenoid content. Significant differences between the carotenoid profile and TCC in the endosperm compared to the germ were found. The germ showed a carotenoid profile with a higher proportion of b-branch carotenoids compared to the endosperm, but despite this difference, the carotenoid contribution of the germ to the whole kernel was relatively small, due to a substantially lower weight in comparison to the endosperm. The proportion of vitreous to starchy endosperm was observed to be larger in kernel sections of the zeaxanthin-biofortified hybrids compared to a commercial yellow sweet-corn. This higher proportion of vitreous endosperm was linked to a greater concentration of carotenoids and protein, and was potentially a means of allowing greater carotenoid accumulation in zeaxanthin-biofortified kernels. Postharvest storage temperature at -20 dC was further shown to have a significant impact on carotenoid retention. Zeaxanthin and b-branch carotenoid concentration decreased in cobs that were immediately stored at -20 dC after harvest. However, when cobs were preconditioned at 4 dC before being transferred to -20 dC, carotenoids were shown to be stable at this lower temperature over an assessment period of three months. Overall, this study has shown that zeaxanthin and TCC were affected by all the factors evaluated. Taking all these factors into account, it is possible to supply sufficient zeaxanthin (2 mg), equivalent to a synthetic supplement, in a cob of sweet-corn. It should be noted that this will be more difficult to achieve under autumn growing conditions, and the option of using cobs harvested in autumn with kernels of more advanced maturity to offset this reduction in zeaxanthin concentration, should be measured against an associated potential loss in eating quality.