Methanol soluble phenolics
The methanol soluble phenolics content in various maize varieties varied from 31.0 ± 2.0 to 106.0 ± 8.3 mg FAE/100 g sample (Fig. 2). Among these, SAU Bhutta 3 and deep red maize exhibited the highest methanol soluble phenolic contents, measuring 106.0 ± 8.3 and 105.0 ± 9.0 mg FAE/100 g sample, respectively.
The SAU white maize variety exhibited the lowest quantity (31.0 ± 2.0 mg FAE/100 g sample) of methanol soluble phenolics. Conversely, the BARI hybrid maize 9, SAU red maize, and multicolored maize variants demonstrated comparable levels of methanol soluble phenolic content. The relatively lower concentration of methanol soluble phenolics observed in this study aligns with previous findings [20]. However, the reported levels of methanol soluble phenolics were notably higher (ranging from 33 to 680 mg/100 g of grain flour) compared to our study [20]. Similarly, we observed a consistent pattern where pigmented maize varieties harbored higher concentrations of methanol soluble phenolics compared to non-pigmented counterparts. The methanol soluble phenolic content in raw corn was 35 to 50 mg/100 g sample, a range similar to the lower end of methanol soluble phenolics observed in present investigation [21].
Acidified methanol soluble phenolics
The levels of acidified methanol-soluble phenolics ranged from 47.00 ± 4.4 mg to 165.10 ± 8.0 mg FAE/100 g sample. SAU Bhutta 3 and deep red maize exhibited the highest acidified methanol soluble phenolic, measuring 165.1 ± 8.0 mg and 153.00 ± 10.0 mg FAE/100 g sample, respectively. Conversely, BARI hybrid maize 9, SAU red maize, and SAU white maize varieties showed similar levels of acidified methanol soluble phenolics (Fig. 2). Notably, SAU white maize kernels contained the lowest amount of acidified methanol soluble phenolics among the samples. Additionally, in BARI hybrid maize 9 and SAU red maize varieties, the content of acidified methanol soluble phenolics was less compared to methanol soluble phenolics, even though this content was superior in other maize varieties (Fig. 2). The maize samples displayed a higher content of acidified methanol soluble phenolics, ranging from 206 to 270 mg/100 g of dry weight [20,21]. They noted that the raw corn with high carotenoid levels exhibited the highest phenolic content, whereas the red corn showed the lowest phenolic content, contrasting with our findings. Conversely, the acidified methanol soluble phenolic content in yellow corn was notably less compared to that observed in our study. Specifically, yellow corn contained 13.43 ± 1.0 µmol/g sample acidified methanol soluble phenolics [1].
Total phenolics
The various types of maize showed a range of total phenolic content, spanning from 77.41 ± 6.0 to 271.00 ±16 mg/100 g sample (Fig. 2). SAU Bhutta 3 and deep red maize exhibited the highest levels, with 271.00 ± 16.00 mg FAE/100 g and 258.29 ± 5.4 mg FAE/100 g sample, respectively. No significant variations were observed among BARI hybrid maize 9, SAU red maize, and multicolored maize varieties. SAU white maize possessed the lowest total phenolic content among the samples analyzed. In a separate study, much greater total phenolic content was detected in some maize varieties than in the present study [22].
Flavonoid content in the different maize varieties
The total flavonoid levels varied significantly among the maize varieties examined in this study. The range of total flavonoid content across the maize varieties was from 9.0 ± 2.0 to 77.00 ± 10.0 mg CE/100 g sample (Table 2).
Table 2. Total flavonoids and anthocyanin content in different pigmented and non-pigmented maize varieties
Maize Sample
|
Total Flavonoids (mg/100 g)
|
Anthocyanin (mg of cyaniding-3-glucoside equv./100 g)
|
SAU white maize
|
8.82 ± 1.5d
|
5.7 ± 0.8d
|
BARI hybrid maize 9
|
12.4 ± 1.8cd
|
4.7 ± 1.1d
|
SAU Red maize
|
14.51 ± 1.9cd
|
4.11 ± 1.2d
|
SAU purple maize (SAU Bhutta 3)
|
77.0 ± 10.0a
|
69.0 ± 5.0a
|
Deep red maize
|
52.1 ± 7.5b
|
50.14 ± 6.4b
|
Multicolored maize
|
18.22 ± 2.7c
|
15.1 ± 3.14c
|
All values are presented as the mean ± standard deviation (SD) on a dry weight basis; different letters in each column indicate significant differences (p < 0.05)
SAU Bhutta 3 exhibited the highest flavonoid concentration at 77.00 ± 10.0 mg CE/100 g sample, trailed by deep red maize at 52.0 ± 8.0 mg CE/100 g sample. Multicolored maize displayed a flavonoid content of 18.22 ± 3.0 mg CE/100 g. SAU red maize, BARI hybrid maize 9, and SAU white maize showed similar flavonoid levels while white maize demonstrated the lowest flavonoid content. Consistent with another study, multicolored maize followed by white maize exhibited the lowest flavonoid concentrations, while light and dark blue maize kernels possessed the highest. Additionally, lemon yellow, yellow, and orange maize kernels were observed to contain higher flavonoid levels compared to red and dark red maize kernels [22]. The flavonoid levels could differ based on the darkness of the maize kernels.
Anthocyanin in maize
The total anthocyanin contents of the different maize varieties ranged from 4.11 ± 1.2 mg to 69.00 ± 5.3 mg C3G/100 g maize (Table 2). The SAU Bhutta 3 contained 69.00 ± 5.3 mg of C3G/100 g anthocyanin which was the highest among the maize types examined. Following that the deep red maize contained 50.00 ± 6.4 mg of C3G per 100 g sample. In contrast, the other maize varieties studied showed minimal anthocyanin content compared to the SAU Bhutta 3. Multicolored maize kernels contained approximately four times less anthocyanin than the SAU Bhutta 3. Notably, there were no significant differences observed among the anthocyanin levels in SAU white, yellow, and SAU red maize varieties. Additionally, anthocyanins were not detectable in white, yellow, lemon yellow, or orange maize kernels in one of the studies [22]. The highest anthocyanin content was reported in maize with dark colored pericarp such as dark red, dark blue and even in light blue and multicolored maize [22]. According to reports, the quantity of anthocyanins determines the color of the pericarp. Likewise, in our study, we noticed higher anthocyanin levels in dark-colored maize types and lower levels in the non-pigmented maize varieties. Consistent with earlier findings, we also noted low anthocyanin content in SAU white maize, contrasting with high levels found in pigmented maize varieties such as SAU purple (SAU Bhutta 3) and deep red maize.
Percent contributions of phenolics and correlation analysis
Table 3 displays the proportions of methanol soluble and acidified methanol soluble phenolics relative to total phenolics, the proportion of flavonoids relative to insoluble phenolics, and the proportion of anthocyanins relative to total flavonoids.
Table 3. The percentage contributions of the free and bound fractions of maize to total phenolics, total flavonoids, and total anthocyanins
Maize Sample
|
Phenolic content (%)
|
Flavonoid content (%)
|
Anthocyanin content (%)
|
soluble
|
insoluble
|
SAU white maize
|
39.47
|
60.53
|
18.83
|
64.63
|
BARI hybrid maize 9
|
55.82
|
44.18
|
21.39
|
37.90
|
SAU red maize
|
57.62
|
42.38
|
26.08
|
28.33
|
SAU purple maize
|
39.07
|
60.94
|
46.34
|
89.66
|
Deep red maize
|
40.72
|
59.28
|
34.03
|
96.24
|
Multicolored maize
|
44.67
|
55.32
|
19.61
|
82.88
|
The proportion of methanol soluble phenolics relative to the total phenolic content varied from 39.1% to 56.00% across maize varieties, while the proportion of acidified methanol soluble phenolics ranged from 42.4% to 61.0%. With the exception of BARI hybrid maize 9 and SAU red maize varieties, the contribution of acidified methanol soluble phenolics to total phenolics exceeded that of methanol soluble phenolics. Notably, the SAU Bhutta 3 exhibited the lowest percentage contribution of methanol soluble phenolics to total phenolics, despite having high methanol soluble phenolic content. Interestingly, the percentage contributions of acidified methanol soluble phenolics to total phenolic content in SAU white maize were akin to those in SAU Bhutta 3, yet the acidified methanol soluble phenolic content in SAU white maize was least among them.
Regarding flavonoids, their contribution to acidified methanol soluble phenolics ranged from 19.0% to 46.34%. SAU Bhutta 3 showed the highest proportion of flavonoids relative to acidified methanol soluble phenolics, while SAU white maize exhibited the lowest. This pattern was consistent with the total flavonoid content across the samples.
Furthermore, the proportion of anthocyanins to flavonoids ranged from 28.0% to 96.0%. The deep red maize ranked the first whereas the SAU red maize exhibited the lowest, despite SAU Bhutta 3 having the highest anthocyanin content and BARI hybrid maize 9 having the lowest. The proportion of free phenolics to the total phenolic content ranged from 18% to 23%, whereas bound phenolics constituted 77% to 82% [20]. In corn, bound phenolics accounted for 85% of the total phenolic content, with free phenolics comprising 15% [1]. Previous studies also indicated that bound flavonoids contributed 91% to insoluble phenolics in corn. Our current results regarding methanol soluble and acidified methanol soluble phenolics align with prior findings. However, the proportion of total phenolics to acidified methanol soluble phenolics in our study was lower than what has been previously reported.
The correlation matrix for all the components analyzed (Table 4) was highly positive.
Table 4. Correlation coefficients between functional constituents
|
TP
|
MSP
|
AMSP
|
TF
|
A
|
TP
|
1
|
|
|
|
|
MSP
|
0.933
|
1
|
|
|
|
AMSP
|
0.981
|
0.849
|
1
|
|
|
TF
|
0.936
|
0.805
|
0.956
|
1
|
|
A
|
0.940
|
0.780
|
0.975
|
0.992
|
1
|
Pearson’s correlation analysis was conducted using averaged values of each variable (p=0.05). TP-total phenolics; MSP-methanol soluble phenolics; AMSP-acidic methanol soluble phenolics; TF-total flavonoids; A-anthocyanin
The correlation between soluble phenolics and anthocyanin content was lower than the correlation between acidic methanol-soluble phenolics and anthocyanin content. Similarly, in the present study, there was a highly positive correlation (r=0.99) between the total phenolic and total flavonoid contents in the free, esterified and insoluble bound phenolic fractions obtained from herbs used in traditional Chinese medicine [23]. This indicates that the total flavonoids were the major contributors to the total phenolics. However, a moderately positive correlation (r=0.619, p<0.05) between the content of total phenolics and flavonoids in ear sections of sweet corn was observed [24]. A negative correlation between free and bound phenolics and between soluble conjugated and bound phenolics in maize was observed [25]. However, similar to the present study, a small significant positive correlation between free and conjugated phenolics indicated that an increase in bound phenolics resulted in a decrease in both free and conjugated phenolics [25]. A significant positive correlation between total flavonoid content and total phenol content (r=0.730, p=0.01) and total anthocyanin content (r=0.343, p=0.05) in maize was also established [26]. However, they reported a non-significant correlation (r=0.241) between total polyphenol content and total anthocyanin content in pigmented maize.