Relationship between Color Change and Rutin Content in Roasted Tartary Buckwheat Bran

Abstract

Tartary buckwheat bran is an underutilized byproduct of buckwheat flour production that is extremely rich in rutin. In this study, the relationship between color change and rutin content in Tartary buckwheat bran sample amounts of 20 to 100 g during roasting between 10 to 30 min at 160 to 240 °C was investigated with the aim of obtaining rutin-rich Tartary buckwheat bran. Increasing the roasting time and temperature and decreasing the sample amounts caused reductions in the rutin content and the L* and b* values. However, even after roasting for 10 min at 160 to 230 °C and for 20 min at 160 °C in sample amounts of 100 g, Tartary buckwheat bran retained high levels of rutin. As a higher rutin content has been closely associated with higher values of L* and b*, analyzing color parameters using a chromaticity meter is promising for predicting the rutin content in roasted Tartary buckwheat bran.

Introduction

Rutin is a flavonol glucoside consisting of the aglycone quercetin and the disaccharide rutinose, which is composed of one molecule of rhamnose and one molecule of glucose. It is of considerable interest due to its potential health benefits. Some researchers have indicated that rutin has antioxidative (Jiang et al., 2007; Lee et al., 2016; Morishita et al., 2007; Zhou et al., 2015), anti-hypertensive (Matsubara et al., 1985), and α-glucosidase inhibitory activities (Li et al., 2009). Although rutin is widely distributed in plants, buckwheat has long been considered to be its most economical and promising source (Couch et al., 1946). Tartary buckwheat (Fagopyrum tataricum Gaertn.) seeds contain approximately 100 times the amount of rutin found in common buckwheat (Fabjan et al., 2003; Morishita et al., 2007). In general, Tartary buckwheat seeds are processed into flour to manufacture food products, such as noodles. However, Tartary buckwheat seeds contain high levels of rutinosidase, which rapidly hydrolyzes rutin to quercetin and rutinose after the addition of water to Tartary buckwheat flour (Suzuki et al., 2002; Yasuda and Nakagawa, 1994). To overcome this issue, we have developed a markedly low-rutinosidase variety of Tartary buckwheat, ‘Manten-Kirari’ (Suzuki et al., 2014a, b). Foods made with flour of the Tartary buckwheat variety ‘Manten-Kirari’ are reportedly rich in rutin (Ishiguro et al., 2016; Suzuki et al., 2015, 2020). Moreover, a clinical study has revealed that ingesting rutin-rich noodles containing ‘Manten-Kirari’ leads to decreases in body weight, body mass index, and levels of 2-thiobarbituric acid reactive substances (Nishimura et al., 2016). During the production of refined Tartary buckwheat flour, whole grains are milled with a roll mill, and the crushed grains are divided into flour, bran, and residue according to the particle sizes. Tartary buckwheat bran is often regarded as a byproduct of buckwheat flour production. Buckwheat bran is known to be a concentrated source of nutritional components, including rutin (Steadman et al., 2001). We recently reported that the rutin content of ‘Manten-Kirari’ bran is five to ten times higher than that of flour (Morishita et al., 2020). Thus, Tartary buckwheat bran appears to be a particularly promising source of rutin-rich raw materials for making functional foods. The activity of lipase, which breaks the links between fatty acids and glycerol esters, has been found in common buckwheat seeds (Suzuki et al., 2004) and flour (Suzuki et al., 2010). The production of free fatty acids, presumably caused by lipase in Tartary buckwheat bran, is considered to make the bran unsuitable for human consumption. Additionally, even a trace amount of rutinosidase activity in ‘Manten-Kirari’ bran is strong enough to hydrolyze rutin into quercetin, to some extent. Therefore, the complete inactivation of lipase and rutinosidase in Tartary buckwheat bran is desirable, and may be performed by using a roasting procedure for the stabilization of Tartary buckwheat bran. The stability of rutin is decreased in common buckwheat seeds when they are thermally processed (Zielinski et al., 2009). Thus, the thermal degradation of rutin may occur in Tartary buckwheat bran upon roasting.

The main objective of the present study was to quantify rutin and quercetin, a degradation product of rutin, in the bran of Tartary buckwheat variety ‘Manten-Kirari’ when roasted. Additionally, we evaluated the change in color parameters during roasting to investigate its relationship with rutin content.

Materials and Methods

Materials    Tartary buckwheat bran of the variety ‘Manten-Kirari’ was purchased from Kobayashi Shokuhin Co., Ltd., Okoppe, Hokkaido, Japan.

Bran roasting    ‘Manten-Kirari’ bran samples were roasted in stainless steel cans using a coffee bean roaster (Gene Café Model CBR-101, Genesis Co., Ltd., Seoul, South Korea). The experimental roasted brans were examined using different sample amounts, roasting times, and temperatures. For the test of a given sample amount, samples of 100 g were pretreated for 1 h at 105 °C, and were roasted for 10 to 30 min at 160 to 240 °C. To test the effect of the sample amount, samples of 20 to 100 g were pretreated for 1 h at 105 °C, and were roasted for 10 min at 180 to 220 °C. These treated bran samples were cooled to room temperature, then stored in an airtight container at -20 °C for further analysis.

Determination of bran characteristics    Rutin and quercetin contents of the bran samples were analyzed by HPLC as described previously (Suzuki et al., 2002). All measurements were made in duplicate. A color value (L*, a*, and b*) analysis of the bran samples was performed using a chromaticity meter (CR-13, KONICA MINOLTA, Tokyo, Japan). Color values were measured five times.

Regression analysis    A regression analysis was performed to model the relationship between the rutin content and color components using Excel statistics.

Results

The rutin and quercetin contents determined by HPLC in non-treated and roasted Tartary buckwheat bran samples are presented in Table 1. The rutin content of non-treated bran samples ranged from 5 861 to 6 322 mg/100 g DW, with a mean value of 5 991 mg/100 g DW. In contrast, the content of quercetin was very small in the non-treated samples, ranging from 0 to 56 mg/100 g DW with a mean value of 17 mg/100 g DW. Next, we examined the effects of roasting temperature (160 to 240 °C) and time (10 to 30 min) on the rutin and quercetin contents during the process of roasting of a fixed sample amount (100 g). When the roasting time was 10 min, varying the roasting temperature from 160 to 230 °C resulted in almost constant rutin (5 727 to 6 315 mg/100 g DW) and quercetin (0 to 117 mg/100 g DW) contents, which were in good agreement with those of the non-treated bran samples. A slight reduction in rutin content as well as a small enhancement of quercetin content was noted while roasting Tartary buckwheat bran at 240 °C for 10 min. For bran samples roasted for 20 min, a roasting temperature of 160 °C did not impact the rutin and quercetin contents. A clear decrease in the rutin content of the roasted bran samples occurred when the roasting temperature was raised between 160 to 240 °C. Namely, the rutin content of the bran sample roasted at 240 °C was around one-tenth of that roasted at 160 °C. Conversely, an increased roasting temperature of up to 230 °C resulted in an increase in the quercetin content. The quercetin content of the bran sample roasted at 240 °C was slightly lower than that of bran roasted at 230 °C. As the roasting time increased to 30 min, a modest reduction in the rutin content and very little change in the quercetin content occurred during the roasting process, even at 160 °C. At higher temperatures, the rutin content dramatically decreased. For example, bran samples roasted at 180, 220, and 240 °C exhibited rutin contents of 3 974, 265, and 27 mg/100 g DW, respectively. The quercetin content increased with increasing roasting temperature up to 220 °C. In contrast, when the roasting temperature increased from 220 to 240 °C, the quercetin content was clearly reduced. Moreover, we clarified the influence of a decrease in sample amounts from 100 to 20 g on rutin and quercetin contents during the roasting process at 180 to 220 °C for 10 min. When the bran samples were roasted at 180 °C, almost constant values of rutin (5 787 to 6 210 mg/100 g DW) and quercetin (39 to 125 mg/100 g DW) contents were noted with decreasing sample amounts up to 40 g. These values of rutin and quercetin contents agreed with those of the non-treated bran samples. Sample amounts up to 20 g led to a slight reduction in the rutin content and a modest increase in the quercetin content. From the data of the samples roasted at 200 °C, no differences in the rutin and quercetin contents were observed between sample amounts of 100 and 80 g. The reduction in sample amounts from 80 to 20 g caused a decreased rutin content and increased quercetin content. As the roasting temperature rose to 220 °C, the sample amount greatly impacted the rutin and quercetin contents. A markedly decreased rutin content (769 mg/100 g DW) and a remarkably enhanced quercetin content (761 mg/100 g DW) were found in samples of 20 g, whereas the rutin and quercetin contents were as high as 5 750 mg/100 g DW and as low as 98 mg/100 g DW, respectively, in samples of 100 g.

Table 1. The rutin and quercetin contents in non-treated and roasted Tartary buckwheat bran samples

	Sample amount (g)	Temperature (°C)	Time (min)	Rutin content (mg/100g DW)	Quercetin content (mg/100g DW)
Non-treated sample	-	-	-	5991 ±2222)	17±262)
Roasted sample (Effects of temperature 100 and time)1)	100	160	10	5728(96)	0(0)
	100	160	20	5560(93)	62(366)
	100	160	30	5122(86)	78(461)
	100	180	10	5983(100)3)	39(229)3)
	100	180	20	4847(81)	210(1238)
	100	180	30	3974(66)	363(2137)
	100	200	10	5727(96)3)	76(447)3)
	100	200	20	3962(66)	354(2082)
	100	200	30	2132(36)	721(4243)
	100	220	10	5750(96)3)	98(576)3)
	100	220	20	2032(34)	642(3775)
	100	220	30	265(4)	799(4699)
	100	230	10	6315(105)	117(690)
	100	230	20	1379(23)	739(4346)
	100	230	25	238(4)	838(4931)
	100	240	10	5068(85)	250(1473)
	100	240	20	557(9)	609(3581)
	100	240	30	27(0)	445(2620)
Roasted sample (Effects of reducing sample amounts)1)	20	180	10	5190(87)	210(1294)
	40	180	10	5787(97)	125(737)
	60	180	10	6210(104)	94(555)
	80	180	10	6109(102)	62(367)
	20	200	10	3175(53)	505(2969)
	40	200	10	4720(79)	287(1690)
	60	200	10	5219(87)	220(1294)
	80	200	10	5856(98)	95(561)
	20	220	10	769(13)	761(4475)
	40	220	10	2773(46)	232(1363)
	60	220	10	3714(62)	221(1300)
	80	220	10	5071(85)	101(592)

1)  Values in parentheses indicate percentage to non-treated sample.

2)  Values are mean ± standard deviation of four samples.

3)  Values are mean of two samples.

The color components (L*, a*, and b*) of non-treated and roasted Tartary buckwheat bran samples were determined using a color meter; the results are given in Table 2. L* measures lightness in the range of values from 0 (dark) to 100 (light). The value of a* indicates red (positive a*) and green (negative a*), whereas b* indicates yellow (positive b*) and blue (negative b*). Among non-treated bran samples, the L*, a*, and b* values varied from 58.3 to 59.2, from 3.6 to 3.8, and from 18.4 to 21.3, respectively. The average values of L*, a*, and b* were 58.9, 3.7, and 19.6, respectively. Next, we evaluated the effects of the increase in roasting temperature from 160 to 240 °C and in roasting time from 10 to 30 min on the color components during the roasting process with a certain sample amount (100 g). When the roasting time was 10 min, the bran sample roasted at 160 °C had a slightly lower L* (55.7) and a modestly higher a* (5.6) than the non-treated bran samples. In contrast, no change in b* was observed during the roasting process at 160 °C. Small decreases in L* and b* and a slight increase in a* were found when the roasting temperature was increased from 160 to 230 °C. The increase in the roasting temperature from 230 to 240 °C, when slight rutin degradation occurred, led to decreased values of L* and b*, and no change in a*. As for the bran samples roasted for 20 and 30 min, a roasting temperature of 160 °C resulted in a modestly lower value of L*, a slightly higher value of a*, and a value of b* similar to that of non-treated bran samples. A decrease in L* and b* occurred when the roasting temperature was increased from 160 to 240 °C. In particular, the values of b* of bran samples roasted at 240 °C for 20 and 30 min were 3.5 and 0.3, respectively. Increases in a* were observed in bran samples roasted for 20 and 30 min when the roasting temperatures were increased to 200 and 180 °C, respectively. Above 220 and 200 °C, the values of a* decreased in samples roasted for 20 and 30 min, respectively. Additionally, we evaluated the effect of reducing the sample amounts from 100 to 20 g on the color components for the bran samples roasted at 180 to 220 °C for 10 min. When the bran samples were roasted at 180 °C, slight reductions in L* and b*, and a small increase in a* were noted with the decrease in sample amounts from 100 to 40 g. The reduction in sample amounts from 40 to 20 g, when the rutin content decreased slightly, caused decreases in L* and b*, and a modest increase in a*. For bran samples roasted at 200 and 220 °C, lowering the sample amounts from 100 to 20 g decreased L* and b*. However, when the roasting temperature was 200 °C, a* increased slightly with the decrease in sample amounts up to 60 g, and the lowering of sample amounts from 60 to 20 g resulted in an almost constant a* (9.5 to 9.7). For bran samples roasted at 220 °C, decreased sample amounts up to 60 g led to a slight change in a* (8.5 to 9.4). A small reduction in a* was noted in sample amounts of less than 60 g.

Table 2. Color components (L*, a*, and b*) of non-treated and roasted Tartary buckwheat samples

Sample No.	Sample amount (g)	Temperature (°C)	Time (min)	L*	a*	b*
Non-treated sample	-	-	-	58.9±0.42)	3.7 ± 0.12)	19.5 ± 1.32)
Roasted sample (Effects of temperature and time)1)	100	160	10	55.7(95)	5.6(152)	19.6(101)
	100	160	20	52.3(89)	7.6(204)	19(97)
	100	160	30	50.2(85)	8.8(238)	18.2(93)
	100	180	10	54.0(92)3)	6.6(178)3)	19(97)3)
	100	180	20	48.6(83)	9.1(247)	17.6(90)
	100	180	30	45.1(77)	9.8(264)	15.4(79)
	100	200	10	51.8(88)3)	7.5(203)3)	18.3(94)3)
	100	200	20	43.7(74)	9.6(259)	14.4(74)
	100	200	30	37.6(64)	8.9(242)	9.6(49)
	100	220	10	48.7(83)3)	8.5(230)3)	17(87)3)
	100	220	20	37.6(64)	8.4(228)	8.5(44)
	100	220	30	32.5(55)	5.5(148)	3.8(19)
	100	230	10	50.1(85)	8.5(230)	18.1(93)
	100	230	20	35.1(60)	7.1(191)	6.5(33)
	100	230	25	32.5(55)	4.4(119)	2.7(14)
	100	240	10	46.8(79)	8.3(224)	14.9(76)
	100	240	20	33.0(56)	4.9(131)	3.5(18)
	100	240	30	30.2(51)	2.3(62)	0.3(1)
Roasted sample (Effects of reducing sample amounts)1)	20	180	10	45.9(78)	9.8(264)	15.4(79)
	40	180	10	49.6(84)	8.9(240)	17.7(91)
	60	180	10	51.8(88)	7.9(214)	18.5(95)
	80	180	10	53.3(90)	7.2(195)	19.0(97)
	20	200	10	40.1(68)	9.4(255)	11.2(58)
	40	200	10	45.4(77)	9.7(263)	15.0(77)
	60	200	10	46.7(79)	9.5(258)	16.2(83)
	80	200	10	50.6(86)	8.5(229)	17.7(91)
	20	220	10	34.3(58)	7(189)	5.6(29)
	40	220	10	38.3(65)	8.3(225)	9(46)
	60	220	10	39.9(68)	9.1(245)	10.5(54)
	80	220	10	44.4(75)	9.4(254)	14.5(74)

1)  Values in parentheses indicate percentage to non-treated sample.

2)  Values are mean ± standard deviation of four samples.

3)  Values are mean of two samples.

Next, the relationships between the color characteristics (L*, a*, and b*) and the rutin content were determined in four raw and 33 roasted Tartary buckwheat bran samples (total of 37 bran samples). No relationship was observed between the value of a* and the rutin content, while L* and b* showed a significant relationship with the rutin content. Scatter diagrams of the color characteristics (L* and b*) and the rutin content in all 37 bran samples examined are presented in Figure 1. A linear increase in the rutin content of the bran samples was noted with L* values up to 50. The rutin content was almost constant (around 6 000 mg/100 g DW) with the increase of L* from 50 to 60. The rutin content increased linearly as the value of b* increased. The regression analysis clearly revealed that L* and b* could be used to monitor the rutin content in roasted Tartary buckwheat bran. Increases in the lightness and yellowness of the roasted bran samples indicated higher rutin content.

Fig. 1.

Scatter diagrams of color characteristics (L* and b*) and the rutin content in raw and roasted Tartary buckwheat bran samples

Discussion

Rutin, the principal phenolic compound of buckwheat, is well-known to possess a variety of health benefits. As compared to common buckwheat, Tartary buckwheat has much higher levels of rutin (Fabjan et al., 2003; Morishita et al., 2007). However, the transformation of rutin to quercetin occurs in Tartary buckwheat due to the high activity of rutinosidase. To solve this problem, we have newly developed a markedly low-rutinosidase variety of Tartary buckwheat, ‘Manten-Kirari’ (Suzuki et al., 2014a, b). Moreover, we found that the rutin content of Tartary buckwheat ‘Manten-Kirari’ bran, a byproduct of the buckwheat milling process, was five to ten times higher than that of flour (Morishita et al., 2020). The values of the rutin content of bran are much higher than those of Tartary buckwheat leaves (Kitabayashi et al., 1995) and sprouts (Kim et al., 2008; Zhou et al., 2015). Other research groups have also paid attention to the utilization of Tartary buckwheat bran as a source of rutin (Cho et al., 2014; Liu and Zhu, 2007). Roasting is one of the promising methods for the stabilization of Tartary buckwheat bran. However, the thermal degradation of rutin during the process of roasting Tartary buckwheat bran is an important negative aspect. Thus, we investigated the changes in rutin and quercetin, a degradation product of rutin, contents in Tartary buckwheat ‘Manten-Kirari’ bran sample amounts of 20 to 100 g during roasting for 10 to 30 min at 160 to 240 °C. We also determined the color characteristics (L*, a*, and b*) to evaluate the relationship between color and the rutin content in these bran samples.

On average, the rutin content of non-treated Tartary buckwheat bran samples used here was 5 991 mg/100 g DW. The data were similar to those of Cho et al. (2014) and Guo et al. (2012), who reported that raw Tartary buckwheat bran contained a high level of rutin (5 170 and 7 431 mg/100 g DW, respectively). Oh et al. (2019) reported a lower level of rutin (3 680 mg/100 g DW) in raw Tartary buckwheat bran. This was presumably due to the difference in the cultivar, cultivation conditions, and/or method of preparing the bran. We previously succeeded in obtaining Tartary buckwheat ‘Manten-Kirari’ bran with higher amounts of rutin (around 8 000 mg/100 g DW) by adjusting the grain moisture content before milling (Morishita et al., 2020). Our present study revealed that the roasting time and temperature as well as the amounts of Tartary buckwheat bran samples were important factors affecting the rutin and quercetin contents and color characteristics. Generally, an increased roasting time and temperature, and a decreased sample amount led to reductions in the rutin content, L*, and b*, and increases in the quercetin content. In contrast, roasting for 10 min at 160 to 230 °C and for 20 min at 160 °C in sample amounts of 100 g caused no or only slight changes in the rutin content, L*, and b*. Moreover, roasting for 10 min at 180 °C in sample amounts of 40 to 100 g and for 10 min at 200 °C in sample amounts of 80 to 100 g did not largely impact the rutin content, L*, or b*. Thus, the procedures mentioned above were suggested to be relatively better conditions for obtaining roasted Tartary buckwheat bran with a high rutin content. More importantly, the measuring of L* and b* was found to be a suitable method for predicting the rutin content. Bhinder et al. (2019) reported that the rutin content was not altered greatly in flours derived from Tartary buckwheat grains, even after infrared roasting at 130, 150, and 170 °C for 10 min. Another report also indicated that an increase in frying temperature from 150 to 190 °C did not largely affect the rutin content in an instant fried noodle system (Cho and Lee, 2015). Fujita and Yoshihashi (2019) studied the effect of heat-treating Tartary buckwheat grain with high rutinosidase activity using a circulating fluidized bed at 200 °C for 4 min. They observed a slight decrease in the rutin content and an increase in quercetin content by heat treatment. In contrast to these findings, Zielinski et al. (2009) indicated that the rutin content of roasted (160 °C for at least 30 min) common buckwheat groats was only around one-sixth of that of raw groats. It is apparent from the previous and present data that longer roasting times (> 30 min) and higher roasting temperatures (> 160 °C) produce Tartary buckwheat bran with decreased rutin content. In this study, the bran roasted at 240 °C for 30 min in sample amounts of 100 g exhibited an extremely decreased rutin content, but a relatively low quercetin content. According to the report of Buchner et al. (2006), who studied the degradation of rutin and quercetin in an aqueous solution at 100 °C, quercetin exhibited faster degradation than did rutin. Therefore, the thermal degradation of quercetin possibly occurred in the bran sample roasted at 240 °C for 30 min in sample amounts of 100 g.

The functionality of rutin in Tartary buckwheat ‘Manten-Kirari’ bran could be maintained by optimizing the roasting time, temperature, and bran sample amounts. Moreover, analyzing L* and b* using a chromaticity meter appears to be a suitable method for predicting the rutin content in roasted Tartary buckwheat bran. The roasted Tartary buckwheat ‘Manten-Kirari’ bran obtained here is considered to be a rutin-rich ingredient without rutinosidase activity. Hence, the present investigation facilitates the use of roasted Tartary buckwheat bran for the development of rutin-fortified foods and beverages, which have benefits for human health.

Conclusion

Tartary buckwheat bran is a potentially cheap source of dietary rutin, which can be used for the development and commercialization of value-added functional ingredients. The objective of this study was to evaluate the change in rutin content induced by roasting Tartary buckwheat bran for 10 to 30 min at 160 to 240 °C in sample amounts of 20 to 100 g to obtain Tartary buckwheat bran with a high rutin content. Color parameters and their relationships with the rutin content were also studied in four raw and 33 roasted Tartary buckwheat bran samples. Increasing the roasting time and temperature, and decreasing the sample amounts generally decreased the rutin content as well as the L*, and b*. However, no or only slight changes in the rutin content, L*, and b* were noted during the roasting process for 10 min at 160 to 230 °C and for 20 min at 160 °C in sample amounts of 100 g. A higher rutin content was closely associated with higher values of L* and b*, indicating increases in lightness and yellowness. The above results provide important information for the expanded utilization of Tartary buckwheat bran.

Acknowledgements    This work was supported by a grant from the Research Project on Development of Agricultural Products and Foods with Health-promoting benefits (NARO), Japan. We are grateful to Ms. T. Ando, Ms. K. Fujii, and Ms. M. Hayashida for their technical assistance. We thank Mr. J. Ashizawa (Kobayashi Shokuhin Co., Ltd.) for useful advice.

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