Harvesting time and roasting effects on colour properties, xanthophylls, phytates, tannins and vitamin C contents of orange maize hybrid

Maize is a globally important staple crop, particularly in sub-Saharan Africa and Latin America. Biofortified orange maize varieties, enriched with micronutrients, offer a potential solution to micronutrient deficiencies in at-risk populations. However, processing methods can alter the nutritional composition and sensory attributes of the final product. Previous research has demonstrated the impact of processing on provitamin A carotenoids in orange maize, but less is known about its effects on non-provitamin A carotenoids (lutein, zeaxanthin), other bioactive compounds (tannins, phytic acid, vitamin C), and color properties – a crucial factor for consumer acceptance. This study aimed to evaluate the combined effects of roasting (with and without husks) and harvesting time on the color properties and bioactive component levels of biofortified orange maize hybrids, addressing a critical knowledge gap in the optimal processing and utilization of biofortified maize for maximum nutritional benefit.

The literature highlights the importance of maize as a global staple and the potential of biofortification to combat micronutrient deficiencies. Studies have shown the efficacy of biofortified maize in improving micronutrient status, but the impact of processing methods on the nutritional value is not fully understood. While the effects of processing on provitamin A carotenoids have been explored, research on non-provitamin A carotenoids like lutein and zeaxanthin, along with other bioactive components such as tannins, phytic acid, and vitamin C, remains limited. The role of these components in health is discussed, including the importance of lutein and zeaxanthin for eye health, the potential anticancer properties of phytic acid, and the antioxidant effects of vitamin C and tannins. Existing studies on the impact of heat treatment on phytic acid content in maize show variable results, emphasizing the need for further investigation.

Eight orange maize hybrids (including a control) were planted at two locations in Nigeria under rainfed conditions. Three harvest maturity stages (20, 27, and 34 days after pollination (DAP)) were selected based on literature on maximum dry weight accumulation in kernels. For each hybrid and replication, samples were divided into three sets: unprocessed, roasted with husks, and roasted without husks. Roasting was conducted using hot charcoal according to local practices. Roasting times varied depending on harvesting time and husk presence. Samples were freeze-dried, milled, and stored at −80°C until analysis. Color properties (L*, a*, b*) were determined using a colorimeter. Carotenoid content (lutein, zeaxanthin, β-cryptoxanthin) was quantified using RP-HPLC. Vitamin C, phytic acid, and tannins were analyzed using AOAC, a modified ferric chloride method, and Folin-Dennis methods, respectively. Statistical analysis involved ANOVA and LSD tests using SAS software.

Harvesting time and roasting significantly (P ≤ 0.001) affected color properties. In unprocessed maize, orange color intensity (b*) increased with delayed harvesting. Roasting without husks impacted lightness at later maturity stages. Roasted maize with husks had higher L*, b*, and ΔE* values compared to those roasted without husks. In unprocessed maize, lutein, zeaxanthin, β-cryptoxanthin, tannin, and vitamin C levels increased with maturity, while phytate showed a slight decrease at 34 DAP. Roasted maize without husks showed increased xanthophylls and phytic acid with maturity, while tannin and vitamin C levels initially decreased before increasing at 34 DAP. Roasted maize with husks exhibited more variable patterns, with some bioactive compounds decreasing at 27 DAP before increasing at 34 DAP. The study found a highly significant (P<0.001) effect of maturity and roasting method on color and bioactive properties, with a significant three-way interaction between location, maturity, and method.

The findings highlight the importance of both harvesting time and processing methods in influencing the nutritional and sensory qualities of biofortified orange maize. The observed increases in bioactive components with maturity in unprocessed maize align with previous research. The impact of roasting, particularly the husk's role in preserving color and bioactive compounds, requires further investigation. The decrease in certain bioactive compounds at earlier roasting times suggests potential for optimization of roasting parameters to maximize nutrient retention. The significant interactions observed underscore the need for considering both location and environmental factors when developing processing guidelines.

This study demonstrated that biofortified orange maize contains considerable quantities of non-provitamin A carotenoids and other bioactive compounds. Harvesting time significantly influenced the levels of these components in unprocessed maize, while roasting affected their retention differently depending on the presence of the husk. Optimizing harvesting time and roasting methods is crucial for maximizing the nutritional value and consumer acceptance of biofortified orange maize. Future research should focus on optimizing processing techniques to enhance nutrient retention and sensory attributes, further exploring the role of environmental factors, and conducting human efficacy trials to evaluate the impact of the different processing conditions on the nutritional value for consumers.

The study was conducted in two locations in Nigeria, which might limit the generalizability of the findings to other geographical regions with varying environmental conditions. The sample size might also be considered in the context of the wide range of maize types and the variability inherent in agricultural experiments. Further research could explore a wider range of processing methods and genotypes.