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

Maize is a major staple in sub-Saharan Africa and Latin America, and biofortified orange maize can help address micronutrient deficiencies. Green (fresh) maize is often consumed boiled or roasted, and processing can influence nutrient retention and consumer-acceptable traits such as colour. While prior work quantified effects of processing on provitamin A carotenoids in orange maize, less is known about impacts on non-provitamin A xanthophylls (lutein, zeaxanthin), other bioactive components (phytate, tannins, vitamin C), and colour metrics across harvest maturity stages. Given the role of lutein and zeaxanthin in ocular health and known processing-related losses, and variable behavior of phytate and phenolics under heat, this study aimed to determine how roasting (with and without husk) and harvesting time (20, 27, 34 days after pollination) affect colour properties (L*, a*, b*) and bioactive components in biofortified orange maize hybrids with high provitamin A content.

The macular pigment of the human retina comprises lutein and zeaxanthin, implicated in filtering blue light and antioxidation, with lutein linked to reduced cataract risk. Zeaxanthin, abundant in orange maize but scarce in typical diets, is key in the macula. Processing commonly reduces carotenoids. Phytic acid (IP6) occurs widely (1.5–6.4% in cereals) and may confer anticancer and other health benefits; heat treatments can lower phytate content and levels vary with maturity. Maize can be relatively rich in vitamin C compared to some fruits. Phenolics, including tannins, also vary by cultivar and environment, and are sensitive to processing and maturation. Prior studies reported increasing β-cryptoxanthin and lutein with kernel maturation, inconsistent patterns for zeaxanthin, and heat-related changes in colour during roasting in other seeds. There is limited information on how roasting with or without husk and harvest maturity affect xanthophylls, phytates, tannins, vitamin C, and colour in roasted fresh orange maize, motivating the present study.

Study materials: Eight orange maize hybrids (including Obatampa II as control) from IITA’s high provitamin A pipeline were grown at two Nigerian locations (Ibadan: 7°22′N, 3°58′E, 150 m; Ikenne: 10°40′N, 8°77′E, 730 m) across two rainy seasons under rainfed conditions in RCBD with three replications; no fertilizers or herbicides were applied. Plants were pre-labeled for harvest at 20, 27, and 34 days after pollination (DAP), based on known windows of maximal kernel dry weight accumulation. Harvest occurred at 08:00 h, with samples transported to the lab. Processing: For each hybrid and harvest stage, two batches of 15 cobs (3 reps) were roasted over hot charcoal on wire gauze either with husk (undehusked) or without husk (dehusked), following local practice. Average roasting times (min) varied with maturity: dehusked 20/27/34 DAP = 15/12/10; undehusked 20/27/34 DAP = 20/15/10. All cobs were processed within 12 h of harvest. Unprocessed and roasted cobs for chemical assays were shelled, freeze-dried (−54 °C, 0.45 mbar), milled (0.5 mm), and stored at −80 °C. Colour measurement: CIE L*a*b* measured using a Colour Tec PCMTM meter, standardized on Xerox white paper; multiple measurements on 3.0 g milled samples. Reported parameters included L*, a*, b*, and deltas (ΔL*, Δa*, Δb*), total colour difference (ΔE*), and hueness (ΔH*). Carotenoids (lutein, zeaxanthin, β-cryptoxanthin): RP-HPLC following Howe & Tanumihardjo with modifications. Extraction from 0.6 g dried maize using ethanol with 0.1% BHT; reconstitution in methanol/dichloromethane (50:50), 100 µL injection. Waters HPLC system; solvent A methanol:water (92:8) with 10 mM ammonium acetate; solvent B MTBE; gradient 1 mL/min (29 min 83→59% A; 6 min 59→30% A; 1 min hold; 4 min 30→83% A; 4 min hold). Detection at 450 nm; identification/quantification by external standards (CaroteNature). Vitamin C: AOAC method using 2,6-dichloroindophenol titration after extraction with metaphosphoric/acetic acid. Phytic acid: TCA extraction, FeCl3 precipitation, heating, washing, NaOH treatment, filtration, dissolution in HNO3, KSCN addition, absorbance at 480 nm; iron quantified versus standard to compute phytate phosphorus assuming 4:6 Fe:P. Tannins (total extractable polyphenolics): Methanolic extraction, Folin–Dennis reagent with sodium carbonate, absorbance at 760 nm. Statistics: Data are means ± SD of at least duplicate independent extractions across two locations and two seasons; ANOVA with SAS 9.2, LSD for mean separation; assessed main effects (location, maturity, method) and interactions, including location × maturity × method.

Colour: In unprocessed maize, L* decreased while a* and b* increased with maturity; b* (yellowness) rose from 30.7 (20 DAP) to 36.0 (27 DAP) to 38.1 (34 DAP). Δb* values were positive and increased with maturity (24.5, 29.8, 31.9), indicating intensifying orange hue; ΔE* and ΔH* also increased with maturity (no significant ΔE* difference between 27 and 34 DAP). Roasting effects: Compared to roasting without husk, roasting with husk generally yielded higher L*, b*, and ΔE* across maturity stages, suggesting a protective effect of husk on colour retention. Overall, roasted samples had lower L*, b*, and ΔE* than unprocessed. Significant effects (P ≤ 0.001) of maturity and roasting method on all colour properties were observed, with significant location × maturity × method interactions. Bioactive components: Unprocessed maize showed increases with maturity in lutein, zeaxanthin, β-cryptoxanthin, tannin, and vitamin C; phytate increased at 27 DAP then decreased at 34 DAP. Representative unprocessed means (20/27/34 DAP): lutein 6.85/9.02/10.4 µg/g; zeaxanthin 9.24/10.6/12.5 µg/g; β-cryptoxanthin 1.99/2.83/4.26 µg/g; phytate 2.08/2.43/1.53%; tannin 1.79/2.17/2.72%; vitamin C 32.6/35.1/44.3 mg/100 g. Roasted without husk: xanthophylls and phytate increased with maturity; tannin and vitamin C dipped at 27 DAP. Means (20/27/34 DAP): lutein 4.80/7.11/7.62 µg/g; zeaxanthin 6.94/7.91/8.58 µg/g; β-cryptoxanthin 1.87/2.70/2.77 µg/g; phytate 1.73/2.37/2.57%; tannin 2.85/1.56/2.76%; vitamin C 49.0/38.4/49.9 mg/100 g. Roasted with husk: lutein decreased at 27 DAP then increased by 34 DAP; β-cryptoxanthin and phytate increased across maturity; zeaxanthin decreased slightly with maturity; tannin varied slightly; vitamin C dipped at 27 DAP then increased. Means (20/27/34 DAP): lutein 8.85/8.61/11.5 µg/g; zeaxanthin 13.01/12.6/12.3 µg/g; β-cryptoxanthin 3.11/3.29/4.35 µg/g; phytate 0.970/1.83/2.54%; tannin 1.51/1.61/1.52%; vitamin C 42.3/26.5/45.1 mg/100 g. Across treatments, non-provitamin A carotenoid concentrations were generally higher in maize roasted with husk than without husk.

Findings demonstrate that both harvest maturity and roasting method substantially influence the colour and bioactive profile of biofortified orange maize. Increasing Δb*, ΔE*, and ΔH* with maturity reflects greater pigment accumulation, enhancing orange hue desirable for consumer acceptability. Roasting reduces lightness and yellowness relative to unprocessed kernels, but retaining the husk mitigates these losses, likely by shielding kernels from direct heat and oxidative degradation. Xanthophylls (lutein, zeaxanthin, β-cryptoxanthin) generally increased with maturity, consistent with carotenoid biosynthesis during kernel development. Under roasting, with-husk treatment better preserved non-provitamin A carotenoids compared to without husk, aligning with the protective role against heat and oxygen. Variations in zeaxanthin versus β-cryptoxanthin across maturity may reflect pathway interlinks where β-cryptoxanthin is a precursor to zeaxanthin. Phytate showed processing-related decreases at earlier maturity relative to unprocessed, though increased with maturity during roasting; tannins and vitamin C were sensitive to roasting, particularly showing dips at 27 DAP, suggesting combined effects of moisture, exposure time, and temperature. Significant location × maturity × method interactions indicate environmental modulation of these effects. Overall, selecting later harvest maturity and roasting with husk can improve colour and retain higher carotenoid levels in roasted fresh maize.

Non-provitamin A carotenoids (lutein, zeaxanthin) and other bioactives (tannins, phytic acid, ascorbic acid) are present in considerable amounts in biofortified orange maize. In unprocessed fresh maize, lutein, zeaxanthin, tannin, and vitamin C increased with maturity, while phytate decreased at later maturity. Roasting affected retention of these compounds and colour properties; roasting with husk generally preserved higher levels of non-provitamin A carotenoids compared to roasting without husk, though phytate, tannin, and vitamin C tended to be lower with husk. Overall, processing (dry heating) and harvest time significantly influence nutrient retention and colour, informing best practices for preparing biofortified orange maize. Future work could explore additional processing methods, optimize roasting conditions, and assess sensory acceptability alongside nutrient bioavailability.