3.4. Quantification of Phenolic Compounds in Buckwheat Fractions
A total of 25 free phenolic compounds were quantified in whole grain flour (GSTQ) and its fractions (GS215, GS160, GS85, and GS45) (
Table 4). Flavonoids are the most abundant free phenolic compounds in buckwheat, which represented 73%, 66.2%, 65.6%, 75.8, and 75.8% of total phenolic content in whole grain flour and fractions (GSTQ, GS215, GS85, and GS45). The most concentrated flavonoid was epiafzelchin–epicatechin-
O-dimethylgallate, which corresponded around 14–16% of total free phenolic compounds in whole grain flour and its fractions. The highest content of epiafzelchin–epicatechin-
O-dimethylgallate was obtained in GS215 (225.36 mg kg
−1 d.w.), in which the value was 58.4%, 17.3%, 63.4%, 79.5% higher than in GSTQ, GS160, GS85, and GS45. Besides that, the most concentrated phenolic acid derivative was protocatechuic-4-
O-glucoside acid, which represented 11.1%, 15.7%, 15.4%, 9.4%, and 9.4% of the total free phenolic content, in which the highest value was obtained for GS215 followed by GS160, GSTQ, GS85, and GS45. Rutin was the second most abundant phenolic compound in whole grain flour and its sieved fractions with 45 µm and 85 µm, whereas this compound was the third most abundant in sieved fractions with 215 µm and 160 µm. Concentration of rutin in buckwheat flours decreased in the following order: GS215 > GS160 > GSTQ > GS85 > GS45 (195.47, 175.70, 87.33, 77.84, and 43.59 mg kg
−1 d.w.).
Total free phenolic concentration decreased in the following order: GS215 > GS160 > GSTQ > GS85 > GS45. Therefore, the greatest content of free phenolic compounds was obtained in GS215 (1153.52 mg kg−1 d.w.), in which the value was 14.7%, 66.9%, 61.9%, and 81.5% higher than that obtained in GS160, GS85, GSTQ, and GS45.
Comparing our results of phenolic content in whole buckwheat flour (GSTQ) with previous works, Verardo et al. (2011) [
23] obtained a total free phenolic content in whole buckwheat flour of 1008.91 mg kg
−1 d.w., which was 41.43% higher than that obtained in our work. But these differences could be due to the different cultivar. Verardo et al. (2011) [
23] reported that the highest free flavonoid was the rutin, whereas in our work, the most concentrated flavonoid was epiafzelchin–epicatechin-
O-dimethylgallate followed by rutin. Kalinová et al. (2019) [
13] reported the phenolic content in different parts of common buckwheat, in which the content of catechin, epicatechin, and rutin (20.87, 56.51 and 52.48 mg kg
−1 d.w.) in groat was in the same order of magnitude as that obtained for whole buckwheat flour in our work [
13]. Liu et al. (2019) [
27] reported the phenolic profiles and antioxidant capacities of common buckwheat and Tartary buckwheat, in which the content of rutin in common buckwheat was 62.19 mg kg
−1 d.w. and this value was similar to that obtained in whole buckwheat in the present study (87.33 mg kg
−1 d.w.) [
27]. Hence, our results are in accordance with the previous studies.
Second, a total of 26 bound phenolic compounds were quantified in whole buckwheat flours (GSTQ) and its fractions (GS215, GS160, GS85, and GS45) (
Table 5). Among them, flavonoids represented 63–68% of total bound phenolic content. The most concentrated flavonoid was catechin, in which the highest value was obtained in GS215 (320.22 mg kg
−1 d.w.), followed by GS160 (241.04 mg kg
−1 d.w.), GS85 (80.05 mg kg
−1 d.w.), GSTQ (77.79 mg kg
−1 d.w.), and GS45 (36.05 mg kg
−1 d.w.). The second most abundant flavonoid was epicatechin, in which the greatest value appeared in fraction with 215 µm (202.64 mg kg
−1), this value was 32.4, 75.3, 76.3, and 89.1% higher than that obtained in GS160, GS85, GSTQ, and GS45. Rutin was an abundant flavonoid in all fractions that represented 10–14% of total phenolic compounds, in which the highest value was obtained in GS215 (173.97 mg kg
−1 d.w.), follow by GS160 (127.24 mg kg
−1 d.w.), GS85 (59.09 mg kg
−1 d.w.), GSTQ (40.09 mg kg
−1 d.w.), and GS45 (27.09 mg kg
−1 d.w.). The most abundant phenolic acid derivative was syringic acid, in which the greatest concentration was obtained in GS215 (100.73 mg kg
−1 d.w.), this concentration was 21.5%, 68.9%, 69%, and 89.4% higher than that obtained in GS160, GSTQ, GS85, and GS45. There were no significant differences between the concentration of syringic acid obtained in GSTQ and GS85.
Total bound phenolic content was higher in GS215 (1565.22 mg kg−1 d.w.), in which the value was 25%, 69.6%, 74.9%, and 87.6% higher than that obtained in GS160, GSTQ, GS85, and GS45. Therefore, bound phenolic content decreases as the particle size falls.
By comparison of bounds phenolic compounds analyzed in whole buckwheat flours, Verardo et al. (2011) [
23] reported that the total bound phenolic compounds in buckwheat was 612.33 mg kg
−1 d.w. and this value was in the same order of magnitude as that obtained in our work. Catechin, epicatechin, and syringic acid were the most concentrated bound phenolic compounds; these results coincided with ours.
Figure 1 shows the sum of free and bound content of phenolic acid derivatives, flavonoids, and phenolic compounds in whole grain flour and its sieved fractions.
From total phenolic content in GSTQ and its fractions GS215, GS160, GS85, and GS45, the total phenolic acid derivatives corresponded to 27.6–33.5% of its total, in which the highest content was obtained in GS215 (1029.92 mg kg
−1 d.w.), in which the value was 18.6%, 71%, 71.1%, and 87.1% higher than in GS160 (838.03 mg kg
−1 d.w.), GS85 (298.61 mg kg
−1 d.w.), GSTQ (297.96 mg kg
−1 d.w.), and GS45 (133.07 mg kg
−1 d.w.) (
Figure 1a).
Whereas, flavonoids are the most abundant phenolic compounds in buckwheat, which represented 66.5–72.4% of total compounds in all fractions. The greatest flavonoid content in GS215 (2088.92 mg kg
−1 d.w.) was 20.5%, 66.9%, 67.2%, 83.3% higher than that obtained in GS215, GS160, GS85, GSTQ, and GS45 (2088.92, 1661.08, 690.86, 685.19, and 348.23 mg kg
−1 d.w.) (
Figure 1b). There were no significant differences between the total content of flavonoids and phenolic acids obtained in GSTQ and GS85.
Total phenolic content was obtained in GS215 (3118.84 mg kg
−1 d.w.), which was 19.8%, 68.5%, 68.3%, 84.5% higher than that obtained in GS160 (2499.11 mg kg
−1 d.w.), GS85 (989.46 mg kg
−1 d.w.), GSTQ (983.15 mg kg
−1 d.w.), and GS45 (481.31 mg kg
−1 d.w.). Hence, according to these results, as the particle size decreases from 215 µm there is a decrease in the phenolic content (
Figure 1), this trend was similar to that obtained in previous works. Bressiani et al. (2017) [
19] evaluated the total phenolic concentration in sieved whole grain wheat flours, which was higher in the fraction with the particle size of 194.9 µm (3.06 mg gallic acid/100 g flour), followed by 608.44 µm (2.23 mg gallic acid/100 g flour), 830 µm (2.11 mg gallic acid/100 g flour), and finally at 82.67 µm (1.69 mg gallic acid/100 g flour); therefore, as the particle size decreases from 194.9 µm, the phenolic content decreases. Bolea and Vizireanu (2017) [
17] evaluated the phenolic content in different black rice flours that were sieved at 630, 550, 315, 180, 125, and 90 µm, the fraction with 180 µm had the highest phenolic content (483 ± 5.32 mg gallic acid/g flour), closely followed by the fraction with 315 µm (432.13 ± 7.32 mg gallic acid/g flour); whereas fractions with 125 µm and 90 µm had almost the same content (402.26 ± 8.01 and 405.32 ± 6.32 mg gallic acid/g flour, respectively). Therefore, it has been reported in the previous works that the highest phenolic content was obtained in flours sieved with a particle size of 180–194.9 µm, whose particle size was similar to our enriched fraction (215 µm), concluding that as the particle size of the fractions decreases a decrease in the concentration of phenolic compounds is obtained. This could be due to the fact that the most enriched fraction contains bran in a high proportion which possess a higher phenolic content than endosperm, and bran could be lost with the sieving at lower particle size, obtaining thereby a fine fraction which is composed mainly of endosperm that contain lower phenolic content than bran.