Influence of thermal treatment on Anthocyanin, total phenolic content and antioxidant capacity of Pigmented Maize (Zea mays L.)

Pigmented maize (Zea mays L.) is a healthy crop due to its perfect proximates and phytochemicals. Thermal treatment was widely used to enhance phytochemical constituents in different kinds of crops. This research evaluated the impact of temperature (100, 115, 130 °C) and duration (10, 15, 20 min) in roasting to anthocyanin, total phenolic content and antioxidant capacity of pigmented maize. Results showed that thermal treatment at 115 °C in 10 min significantly improved anthocyanin in pigmented maize; however, this content would be lower at higher temperatures or prolonged exposing time. Meanwhile, total phenolic content and antioxidant capacity in the pigmented maize were recorded at the highest level when being roasted at 100 oC for 10 min. This research proved that phytochemical constituents and antioxidant capacity inside the pigmented maize would be seriously damaged at high temperatures and extended duration in roasting. By this, producers should pay more attention to thermal conditions in roasting.


Introduction
Pigmented maize (Zea mays L.) is an important crop in Vietnam. It contains high carbohydrates, proteins, lipids, anthocyanins, minerals and phenolics (1). Abundant anthocyanins and phenolics are located in the aleurone and pericarp monolayer of the cereal contributing to the pigment of the maize species (2). Pigmented maize seed has a high content of anthocyanin in the aleurone layer and lowered in the starchy endosperm (3). Anthocyanins and phenolics play functional properties against chronic diseases due to their antioxidant and anti-inflammatory activity (4,5). They offer protection against mutagenesis (6). Maize is commonly utilized for animal feed, cornmeal, grits, starch, flour, tortillas and snacks (5). Pigmented maize is roasted to turn into bread in the bakery industry.
Roasting involves applying dry heat to change the physicochemical, nutritional and phytochemical properties of raw material (7). It is one kind of thermal treatment widely applied in grain processing to improve nutritional bioavailability, phytochemical efficiency, organoleptic property and deduct the toxic components (8). The roasting of maize grains improved aroma, antioxidant capacity, food quality of semi and final products (9). Phytic acid in oat flour was greatly minimized by roasting to support calcium bioavailability (10). The mineral bioavailability in millet and biofortified bean flour was also significantly improved by roasting treatment (11,12). Roasting induced modification in the proximate composition and biological properties of the coffee bean, supporting the release of derivative antioxidants (13). Roasted rice wine had a better flavour compared to unroasted rice wine (8). Rice powder had decreased levels of free amino acids by roasting at high temperatures and extended duration (14). Roasting had a positive impact on bioactive constituents in soybean (15), wheat (16), barley (17), pistachio nuts (18), cocoa beans (19), coffee beans (20) and wattle seeds (21). The objective of this study was to verify the influence of temperature (100, 115, 130 o C) and time (10, 15, 20 min) in convective roasting to anthocyanin, total phenolic content and antioxidant capacity of pigmented maize.

Material
The pigmented maize was collected from Mỹ Xuyên district, Soc Trang province, Vietnam. It was harvested at maturity and dehydrated in an infrared drying oven to 15% moisture content. Chemical reagents were all analytical grade supplied from Merck (Germany) and Sigma Aldrich (USA).

Researching method
500 gm of each pigment maize seed sample was roasted at temperature (100, 115, 130 o C) and duration (10, 15, 20 min) in the oven (Memmert, model Universal oven UF30). The chamber load was exposed to defined temperatures at atmospheric pressure in the interior of a drying oven. Thermal energy was transferred to the chamber load by convection and radiation. The roasted seed was then cooled to an ambient condition, ready for analysis. Anthocyanin content (mg/100 gm) was measured following Abdel-Aal and Hucl (22). Total phenolic content (TPC, mg GAE/100 gm) was examined by Folin-Ciocalteu reagent assay (23). Free radical scavenging activity (DPPH, mg Trolox/100 gm) was evaluated by the method described by Bakar (24).

Statistical analysis
The experiments were run in 5 replications with different groups of samples. The data were presented as mean±standard deviation. Statistical analysis was performed by the Statgraphics Centurion version XVI.

Results and Discussion
Anthocyanin exerted a major activity against oxidative stress (6). The impact of roasting temperature and duration on anthocyanin content in the roasted pigmented maize was shown in Table 1. It was realized that the highest anthocyanin content (31.15±0.02 mg/100 gm) in the pigmented maize was noticed by roasting at 115 o C for 10 min. This content would be lower at higher temperatures or prolonged exposing time. Anthocyanin was highly sensitive to high temperatures (25). Anthocyanin was seriously damaged at higher roasting temperature (> 115 o C) and longer drying times (> 10 min) due to the decomposition of anthocyanin molecules through the hydrolysis of glycosidic links (26). The temperature could seriously affect anthocyanin's stability and its pigment intensity (27). Anthocyanin in black rice grain decreased dramatically with an increase in temperature from 100-140 °C (28). Anthocyanin in glutinous rice powder was seriously decomposed by spray-drying at 160-180 °C (29). One study reported the decomposition of anthocyanin in potatoes treated at a temperature >100 o C (30). In a similar research, anthocyanin decomposed after the rupture of glycosidic moiety and the establishment of other chalcones (31). The most suitable roasting temperature and duration for achieving the highest yield of anthocyanin was 100 °C up to 20 min for pigmented rice and 200 °C for 20 min for nonpigmented rice (32).
Major phenolics from maize were ferulic acid and anthocyanin (5). The efficiency of roasting temperature and time on total phenolic content in the roasted pigmented maize was presented in Table 2. It is noticed that the highest total phenolic content (169.52±0.03 mg GAE/100 gm) in the pigmented maize was noticed by roasting at 100 o C for 10 min. Longer exposure time and higher temperature resulted in low phenolic content. It is suggested that the roasting process significantly decreased total phenolic content. Roasting induced an accumulation of total phenolic content due to thermal modification in chemical constituents via cell wall disruption in quinoa seed (33). Degradation in insoluble phenolics and an accumulation of soluble ones were noticed in peanut seeds under roasting at 170 °C (31). The total phenolics in black soybean were greatly degraded by roasting at 210 °C for 30 min (34). Phenolics would be accelerated by roasting from 30-90 min at 150 °C, however, it was significantly decomposed by roasting at an extended period (35). Roasting was considered one of the most innovative processing techniques to improve total phenolics in broomcorn millet (36). Phenolic content in pistachio was accelerated under roasting at 110 °C for 16 min (37). Total phenolic content in fenugreek seed was greatly improved by roasting at 130 °C for 7 min (38). In another research, the total polyphenol content in roasted maize dramatically increased with higher roasting temperature and longer roasting time (39).
The durable DPPH radical with maximum absorption at 515 nm is commonly applied to estimate the free radical scavenging activity of hydrogen-donating antioxidants in cereal (40). The efficiency of roasting temperature and time on DPPH antioxidant capacity in the roasted pigmented maize is presented in Table 3. It is noticed that the highest DPPH antioxidant capacity (89.15±0.02 mg Trolox/100 gm) in the pigmented maize was recorded by roasting at 100 o C for 10 min. Longer exposure time and higher temperature induce low antioxidant capacity. Antioxidants in pigmented maize might mostly be  covalently bonded with insoluble polymers (41). Mild heating induced cell wall disruption and release of antioxidants from insoluble particles of maize. The difference in antioxidant capacity could originate from the formation of derivative elements with potential antioxidant capacity and decomposition under excess temperature and duration (42,43). Heated samples showed chain-breaking and oxygenscavenging activities (44). Roasting was reported to enhance antioxidant potential by alteration of biochemical ingredients of cereal grains (33). Antioxidant activity in fenugreek seed was greatly improved by roasting at 130 °C for 7 min (38). Antioxidant capacity in the unpolished grain of nonpigmented rice was increased by roasting at 60 °C for 3 min (45). The most suitable roasting temperature and duration for achieving the highest total phenol content and antioxidant capacity was 100 °C in 20 min for pigmented rice and 200 °C for 20 min for nonpigmented rice (31).

Conclusion
Pigmented maize included a significant amount of nutrients, minerals, vitamins and specific flavours. The anthocyanin, total phenolic and antioxidant capacity in pigmented maize were unstable and susceptible to degradation by high temperature. They would be maintained effectively by roasting at temperature 100 0 -115 o C within 10 min. Bioactive constituents in the pigmented maize would be significantly damaged by excess thermal treatment. Therefore, cereal processors should concentrate on thermal conditions to minimize the harmful impacts on phytochemical components. Roasting could be considered an important pretreatment step to prolong the food stability and enhance the efficiency of further processing steps.