So-called “functional foods” are designed to maximize the availability of particular macro- or micronutrients while also incorporating acceptable physical qualities and sensory appeal. In the case of Tartary buckwheat, researchers have often focused on maximizing one or more flavonoid compounds, while minimizing the physical and sensory differences between the functional version and some familiar cereal-based recipe. Frequently, investigators have compared levels of those flavonoids in the buckwheat seed or groat with those in the final product. Sometimes concentrations were also measured at intermediate stages during preparation or storage. Some investigators also compared antioxidant activity in the raw seed and final product. Organoleptic qualities were often assessed by specialized laboratory procedures and/or by trained panels of evaluators. Because of its bitterness and unfamiliarity to many consumers, the taste of Tartary buckwheat is implicitly considered to be undesirable. Perhaps broad acceptance of “functional” Tartary buckwheat products will await the development of more non-bitter varieties.
Traditional users of common and Tartary buckwheat accepted or even relished the organoleptic qualities of these crops—that is, their appearance, odor, taste, or feel. Today, many researchers assess the bitterness, earthy flavor, and cooking properties of Tartary buckwheat negatively, or at best, neutrally. Consequently much of the recent interest in Tartary buckwheat focuses instead on the species’ nutritional properties. Researchers have mostly sought to incorporate its healthful components into recipes that had not previously contained any buckwheat ingredients. These so-called “functional foods” maximize the availability of particular macro- or micronutrients while minimizing or masking other characteristics. Investigators have compared levels of those nutrients in the raw ingredients with those in the food as consumed. In many cases, these concentrations were also measured at intermediate stages during preparation and storage. While the section on FLAVONOIDS AND ANTIOXIDANT ACTIVITY reports concentrations of rutin and other flavonoids found in whole seeds or fractions from seeds after minimal processing (i.e., grinding), studies have also examined those concentrations in food products or in the partially-prepared foods. Assays of antioxidant activity were included in many of those studies, because the level of activity is not always closely correlated with the concentration of any individual compound. These investigations frequently compared the physical and sensory properties of the prepared foods with those of “controls,” which were usually familiar all-cereal versions of the same dishes. If panels of evaluators are asked to score some attribute of the Tartary buckwheat and all-wheat versions of a dish, it is difficult not to consider the latter as the ideal. While it is challenging to create an all-Tartary buckwheat dish that looks, feels, and tastes as good as its all-wheat counterpart, the inclusion of some quantity of Tartary buckwheat might actually enhance some of those qualities. The extensive work at the Education Center Pyramid Maribor (Maribor, Slovenia) is evidence of that. The breeding of more non-bitter varieties may lead to more attention to and appreciation of Tartary buckwheat’s other physical and sensory attributes.
Ni et al. (1995) substituted Tartary buckwheat flour for some or all of the wheat flour in recipes for crisp biscuits, butter soda biscuits, and sesame crackers. In the first recipe, the flour was added after margarine and sugar had been pre-mixed, and prior to mixing, roller shaping, and baking (240-260ºC for five minutes). In the recipe for butter soda biscuits, half of the flour was mixed with water and yeast prior to fermentation for six hours at 28-30ºC. Then the oil, egg, milk solids, and the balance of the flour were added to the dough prior to a second fermentation (2-4 hours), sheeting, cutting, and baking (260-280ºC for five minutes). For the sesame crackers, roasted flour was mixed with a hot sugar syrup, cooled, sheeted, and shaped. After soaked sesame seeds were applied to the top, the crackers were baked for five minutes at 180ºC.
Compared to all-wheat crisp biscuits, Ni and co-workers found those containing 25 percent Tartary buckwheat flour to be “a little bitter but appetizing.” With greater substitutions of buckwheat flour, the authors reported increasingly course texture and bitter taste. The butter soda biscuits containing 20 percent Tartary buckwheat flour were judged to be “light green, tidy but a little shrinker [sic], crisp, luscious, and a little bitter.” Those containing 40 percent buckwheat flour—“yellow and green, shrinker, rigid, coarse, and bitterer”—would likely be unacceptable on all counts. The sesame crackers containing 25 percent Tartary buckwheat flour were judged to be “light green, tidy, luscious, sweet, crisp, and a little bitter.” With 50 percent buckwheat flour, the crackers were described as bitter. The authors did not attempt to measure how much antioxidant was present in the flour or in the finished products.
Concentrations of flavonoids and similar compounds are affected not only by growing conditions, but also by conditions during the storage and subsequent processing of the harvested material. For example, Dietrych-Szostak and Oleszek (1999) observed the changes in flavonoid levels in groats of common buckwheat due to dehulling by four different processes: heating at 22 percent moisture and 150ºC for 10, 70, or 130 minutes; or steaming for 70 minutes. All treatments were dried to 13 percent moisture prior to milling, and were compared to seeds that had been manually dehulled without heating. In the manually-dehulled controls, the authors detected rutin and isovitexin at 17.76mg/100g and 1.04mg/100g, respectively. Concentrations of both flavonoids were reduced by all four treatments—-in the steam treatment by three-quarters. The authors detected seven flavonoids in the buckwheat hulls. At 32.95mg/100g, rutin constituted 45 percent of total flavonoids. Treatments had less effect on hulls than groats, with a minimum rutin concentration in the former of 26.25mg/100g following the steam treatment. The authors noted that the tested treatments affected not only flavonoid content, but also color. They suggested that heating could convert proteins and sugars into compounds more readily digested or absorbed.
Liu and Zhu (2007) ground whole seeds of Tartary buckwheat (cv ‘Jingqiao 2’) in a Brabender mill, and separated the fines with 38GG and 7XX sieves. Particles that passed through the former (less than 500 μm) but were retained by the latter (greater than 200μm) were identified as crushed embryos, bran, aleurone, and a few hulls. These constituted about 16 percent of the total kernel weight; whereas hulls constituted about 26 percent and flour about 58 percent. This intermediate material was refluxed with ether (80ºC for eight hours) to remove oils and chlorophyll. Then the de-oiled material was extracted with a 61.57 percent ethanol solution, and subsequently washed with water to remove carbohydrates. This procedure was able to extract over 95 percent of the constituent flavonoids, themselves representing about 47 percent by weight of the flavonoids in the whole seed. Rutin was identified as the predominant flavonoid compound. The authors suggested that such an operation could be used to produce both flour and a high-flavonoid concentrate for inclusion in functional foods.
Phenolic content of Tartary buckwheat bran extract was reported in an article by Wang et al (2009). This bran was a byproduct of the production of flour from the variety ‘Kuqiao 1’ that had been grown in Sechuan Province, China. Bran was extracted three times with 95% ethanol at 55-65°C. The weight of dried “TBBE” was 9.85 percent of the weight of the extracted bran. Assayed by HPLC, the concentrations of rutin and quercetin in TBBE were 8.96 and 83.84 mg/g, respectively. Following hydrolysis by reflux (one hour at 80°C) with hydrochloric acid, the extract contained 19.47 percent quercetin. (Rutin was not determined following hydrolysis, nor was an explanation given for the increased concentration of quercetin. ) Measured by the Folin-Ciocalteu colorimetric assay, the total phenolic content of the TBBE was 158.44 μmol of gallic acid equivalent per gram.
Kočevar Glavač et al. (2017) reported concentrations of rutin and quercetin during production of roasted Tartary buckwheat groat tea in South Korea. Dry samples of raw seeds, steamed whole seeds, the steamed seeds after de-hulling, and the groats after roasting were ground, extracted with methanol, and analyzed by HPLC. On a dry weight basis, the raw grain contained 1.41 percent rutin and 0.004 percent quercetin. Steaming for 30 minutes reduced rutin almost by half, while increasing quercetin almost six-fold. Seeds dehulled after steaming contained 0.826 percent rutin and 0.007 percent quercetin. After roasting, different lots of groats contained from 0.327 to 6.28 percent rutin and from 0.039 to 0.082 percent quercetin. Large fragments of steamed hull contained 0.036 percent rutin and 0.040 percent quercetin. The very fine hull fragments were much richer in ruin (1.61 percent) while containing less quercetin (0.022 percent).
Intestinal α-glucosidase is a key enzyme in the digestion of carbohydrate. Its inhibition can delay that digestion, thereby diminishing the absorption of glucose from the gut into the bloodstream. By moderating plasma glucose levels, α-glucosidase inhibitors in the diet might delay or prevent the onset of type II diabetes. Qin et al. (2013) measured the capacity of Tartary buckwheat to inhibit α-glucosidase. Phenolic compounds were extracted with ethanol from material collected during several stages in the commercial production in China of Tartary buckwheat tea, and their total concentration was measured by the Folin–Ciocalteu method. Individual compounds were identified and quantified by HPLC.
The materials sampled by Qin et al. (2013) included 1) raw whole seeds; 2) whole seeds after soaking in 40ºC water (12-14 h); 3) whole seeds after steaming at 100ºC (40-60 min); 4) whole seeds after drying at 150-200ºC (5 min); 5) dehulled groats (without bran); and 6) Tartary buckwheat tea (without bran) after roasting at 120-150ºC (6-8 min). Extractable total phenolics, 13.0 mg/g DW in the raw seeds, increased to 16.2 mg/g DW in the soaked whole seeds and then declined during each successive process. Extractable total phenolics constituted 4.8 mg/g DW in the roasted groats (i.e., tea). Methanol-extractable total flavonoids showed a similar pattern. Rutin was the predominant flavonoid, except in the soaked whole seeds, in which quercetin predominated.
Using a spectrophotometric assay, those authors measured the activity of α-glucosidase from rat intestine in a 5 mM solution of p-nitrophenyl-α-D-glucopyranoside that also contained one of the extracts of total phenols described above. Percent inhibition—almost 30 percent with the raw seeds—increased to 37.9 percent with the steamed seeds, then declined to 15.6 percent with the tea. Therefore, in these samples, the inhibition of α-glucosidase was directly related to the concentration of total phenolics. Similarly, antioxidant activity, as measured by DPPH-scavenging, increased with the soaking of the whole seeds, and then decreased with each successive stage of tea production. Over the course of processing, there was a strong relationship between α-glucosidase inhibition, antioxidant activity, and the concentration of total phenols. However, weaker relationships were observed between α-glucosidase inhibition or antioxidant activity and the concentration of any individual flavonoid (i.e., rutin, isoquercitrin, quercetin, or kaempferol).
Ikeda et al. (2017) sought to clarify which compounds in buckwheat act as inhibitors of α-glucosidase. These researchers ground seeds of one variety of common buckwheat (‘Hashikami-wase’) and two of Tartary buckwheat (an un-named Chinese variety and ‘Manten-kirari’) in a Brabender milling machine with a 231 μm sieve. Flour was then extracted in an 80 percent methanol solution for one hour at room temperature. Following centrifugation, the supernatants were tested for effectiveness in inhibiting the enzymatic glycolysis of maltose. Exposure to increasing concentrations of the extract from the un-named Tartary buckwheat variety diminished the activity of α-glucosidase (an inhibitory effect that was not destroyed by heating the extract). In contrast, the activity of α-glucosidase was not substantially diminished by exposure to increasing concentrations of the extract from common buckwheat, nor by increasing concentrations of the extract from ‘Manten-kirari’ Tartary buckwheat. The latter variety contains virtually no endogenous rutin-degrading enzyme; therefore, compared to most Tartary buckwheat, its seeds contain higher levels of rutin but very low levels of quercetin.
These results suggested that quercetin is the effective agent inhibiting α-glucosidase. The authors tested various polyphenols for inhibitory effect on α-glucosidase, and reported a high inhibition constant for quercetin, a lower constant for tannic acid, but virtually no inhibition by rutin or catechin. These results are somewhat contradictory to those of Qin et al. (2013) (see above), who found that such inhibition correlated well with the concentration of total phenols, rather than with any individual compound.
Filipcˇev and co-workers (2011) investigated the inclusion of common buckwheat flour in recipes for ginger nut biscuits. Either buckwheat or rye flour was substituted for 30, 40, or 50 percent of the flour in an all-wheat (control) recipe. All were commercial flours: a type 500 soft wheat flour, a fine wholegrain rye flour (72 percent of particles <350μm), and a wholegrain buckwheat flour (moisture 12.31 percent; ash 2.2 percent dry weight). Flour blends, lecithin, NaHCO3 and spices were blended with a honey/sugar solution to a workable consistency (17-20 percent water in control and rye doughs, 15-15.5 percent water in buckwheat doughs). Dough samples were then covered, rested overnight at 15ºC, rolled to 1cm thickness, cut to 6cm rounds, and baked at 170ºC for 10-15 minutes.
The authors reported that the buckwheat-supplemented biscuits had a height and diameter similar to the control (all-wheat) biscuits but smaller than the rye-supplemented biscuits. The greater the percentage of buckwheat flour, the denser the biscuit. The hardness and fracturability of the buckwheat-supplemented biscuits were less than the controls, but greater than the rye-supplemented biscuits. Biscuits containing buckwheat flour received higher composite sensory scores than the control biscuits, but lower scores than those containing rye flour. These scores were based on shape, appearance of the upper and lower surfaces, fracture, structure, chewiness, and flavor (with factors varying from 0.3 for shape to 1.0 for flavor).
Hussain et al. (2017) prepared cookies in which 0, 10, 20, 30, 40, or 50 percent of flour was wheat flour, and the balance was common buckwheat flour (produced in Pakistan). A panel of six expert judges evaluated the cookies for color, texture, taste and overall acceptability according to a nine-point hedonic scale. Incremental increases in wheat flour consistently improved scores for all individual traits and for overall acceptability. The minimum score was 5.11 (color of all-buckwheat cookies); the maximum was 8.15 (color of 50-percent buckwheat cookies).
Maeda and co-workers (2004) studied qualities of cookies and pasta in which a fraction of the wheat flour in standard recipes was replaced by light- or dark-colored common buckwheat flour. The former was a commercially-milled product, and the latter was a mixture of the light flour and husk in an 81:19 ratio. Additions of dark buckwheat flour up to 50 percent, or light buckwheat flour up to 30 percent had little effect on the firmness of cookies; greater substitutions of the light flour decreased firmness. Those authors also concluded that the substitution of dark buckwheat flour for up to 30 percent of wheat flour made no significant difference in texture, taste, or appearance of noodles.
In their study of flavonoids during the processing of Tartary buckwheat, Vogrincic et al. (2010) mixed flour of ‘Wellkar’ Tartary buckwheat with wheat flour in ratios of 1:0, 0.5:0.5, 0.3:0.7, and 0:1. After 5 minutes of kneading, doughs were raised for 30 minutes, kneaded for an additional minute, and raised for an additional 29 minutes. Compared to all-wheat dough, loaf volume was significantly reduced only when composed entirely of Tartrary buckwheat flour. The color of the all-buckwheat bread was also darker.
Lin et al. (2009) substituted flour from common buckwheat (from either whole grains [“husked”] or groats[“unhusked”]) for 15 percent of the flour in high-gluten wheat bread. The bread containing either of the buckwheat flours was lower in carbohydrate, fat, and protein than the all-wheat bread, but higher in fiber. Bread containing “husked” buckwheat flour contained 1.75mg rutin and 0.03mg quercetin per 100g; bread containing “unhusked” buckwheat flour contained 0.9mg rutin and 0.04mg quercetin per 100g. Neither compound was detectable in the all-wheat bread. The iron-reducing power of ethanolic extracts was greatest for the bread containing “husked” buckwheat flour and lowest for the all-wheat bread. The same ranking was found for the ability to scavenge radicals of DPPH.
Lin and co-workers reported that all of the breads had acceptable specific volumes—greater than 6cm3 per g. The researchers questioned 48 Taiwanese consumers about the acceptability of these breads. For all three breads, the appearance, color, flavor, and mouth feel were rated as acceptable (between 5 and 6 on a 7-point hedonic scale). Scores for flavor and mouth feel were better for breads containing either of the buckwheat flours, compared to the all-wheat bread.
Brunori and co-workers (2010) attempted to introduce some healthful properties of buckwheat into traditional Italian staples like bread and biscuits. Traditional Tuscan biscuits were prepared in which a mix of common and Tartary buckwheat flours were substituted for 20 percent of the wheat flour; in the different treatments, Tartary buckwheat flour constituted 4, 8, or 12 percent of the total. As an additional prior treatment, the Tartary buckwheat flour was or was not heated to 80ºC for 30 minutes. (As explained in the section on FLAVONOIDS AND ANTIOXIDANT PROPERTIES, the treatment was intended to inactivate endogenous rutin-degrading enzymes. The concentration of rutin was 1091mg and 1149mg per 100g DW in the untreated and heat-treated flour, respectively. The corresponding concentrations of quercetin were 1.88mg and 2.73mg per 100g DW. ) The authors concluded that compared to all-wheat biscuits, those incorporating 20 percent buckwheat flour were not compromised in either appearance or taste. While the all-wheat biscuits contained no detectable rutin, those containing 12 percent tartary buckwheat flour contained over 70mg/100g—regardless of whether or not the flour had been heat-treated.
Those authors also prepared typical Tuscan bread in which 16 percent of the wheat flour had been replaced with common buckwheat whole flour and four percent of the wheat flour had been replaced with Tartary buckwheat whole flour (with or without prior heat treatment). After dough was mixed and proofed for 4-5 hours, loaves were baked at 190-200ºC for 45-50 minutes. The concentration of rutin in bread containing the untreated Tartary buckwheat flour was 4.44mg per 100g DW; the concentration in bread containing heat-treated flour was 6.45mg per 100g. The corresponding concentrations of quercetin were 17.4 and 15.8mg per 100g DW. No rutin or quercetin was detected in the wheat flour nor in the all-wheat control bread. Apparently the heat treatment did not protect rutin from degradation during bread-making. Most of the rutin had been hydrolyzed to quercetin while the dough leavened, and apparently much of the quercetin had been degraded as well. Compared to the all-wheat control, bread containing this ratio of flours exhibited somewhat reduced loaf volume and textural appeal.
Kočevar Glavač and co-authors reported concentrations of rutin and quercetin during production of all-Tartary buckwheat yeast bread from grain grown in West Germany. Dry samples of whole seeds, semolina (about 1/4 of grain mass, consisting largely of endosperm), flour (about 1/4 of grain mass, comprising endosperm, embryo, seed coat, and hull), dough from that flour (using method of Vogrincic et al., 2010), and the baked bread were analyzed as above. In the unmilled seed, the concentration of rutin was 1.63 percent and the concentration of quercetin was 0.005 percent. The semolina contained 1.15 percent rutin and 0.001 percent quercetin; the corresponding percentages in the flour were similar—1.23 percent and 0.003 percent. The dough only contained 0.024 percent rutin, while the percentage of quercetin had risen to 0.653. Baking little altered these levels; the finished bread contained 0.02 percent rutin and 0.629 percent quercetin.
Lukšič et al. (2016a) contrasted sourdough bread made from flour of common buckwheat (cultivar ‘Pyra’) with that made of Tartary buckwheat (unnamed cultivar from Luxembourg). Doughs of these flours were fermented for 10 hours with sourdough starter (a mixed culture of Lactobacillus heilongjiangensis and Pediococcus parvenus maintained on Tartary buckwheat flour). After the addition of salt, sugar, baker’s yeast, and more flour, the dough was risen for five hours and baked at 200ºC for one hour. The Tartary flour contained 1.47 percent rutin and 0.19 percent quercetin; neither flavonoid was detectable in the common buckwheat flour. Rutin concentration was reduced in the Tartary buckwheat dough (0.33 percent before fermentation), and continued to decline through bread-making until it was undetectable in the final product. The concentration of quercetin, in contrast, was higher in the dough (0.82 percent), remained fairly constant through the final rise, then declined to 0.51 percent after baking. The authors observed a different pattern in antioxidant activity (as measured against superoxide anion radicals generated from UV-stimulated luminol). The common buckwheat flour had two-thirds the activity of the Tartary buckwheat flour. From unfermented dough through successive stages, the antioxidant activity of common buckwheat increased, while that of Tartary buckwheat declined. The activity of the common buckwheat bread was half again that of the Tartary buckwheat bread. Apparently compounds other than rutin and quercetin contributed much to the antioxidant potential of common buckwheat. The authors reported that both the crust and crumb of Tartary buckwheat bread were more yellow and green than the common buckwheat bread. Loaves of the former were also denser (1.1g/cm3 versus 0.9g/cm3).
Lukšič and co-authors (2016b) reported flavonoid concentrations and antioxidant activity during bread-making with flour from hydrothermally-processed Tartary buckwheat. Processing entailed soaking seeds for 20 minutes in water at 95ºC, then air drying at 40ºC to 20 percent moisture. Samples were taken of the flour, the sourdough before and after a 10-hour incubation under refrigeration, the bread dough before and after a 5-hour rise at room temperature, and the baked loaf. Ground freeze-dried samples were extracted with 80 percent methanol for 20 minutes, two hours, and eight hours. Concentrations of rutin and quercetin were determined by HPLC.
Comparing rutin concentrations after the 8-hour methanol extractions, the authors reported that the hydrothermally-processed flour had slightly lower levels than unprocessed flour (7101 versus 8105mg/kg DM). At the beginning of the sourdough incubation, the rutin concentration had dropped to 2630 mg/kg; after baking, the bread contained 2092 mg/kg. The hydrothermally-processed flour also contained lower levels of quercetin than the unprocessed flour (531mg/kg versus 876mg/kg DM). At the beginning of the sourdough incubation, the level of quercetin had increased to 5346mg/kg; the baked bread contained 6165mg/kg. Contrasting these results with those of their earlier study, the authors noted that hydrothermal processing of seeds had limited the subsequent degradation of rutin into quercetin during bread-making. The methanolic extraction of rutin was slower in the processed flour than in untreated flour, suggesting that the processing promoted complexation of flavonoid molecules with proteins or amylose. The slower release of rutin apparently protected that compound from enzymatic degradation during bread-making.
As assayed against superoxide anion radicals generated by luminol photosensitized under UV light, the anti-oxidant activity of the methanolic extract of the processed flour (82 μg/mg Trolox equivalents) was not significantly different from activity at any stage during bread-making. The antioxidant activity of the extract was significantly lower (71 μg/mg Trolox equivalents) only after baking.
Xu et al. (2014) studied antioxidative substances in Chinese steamed breads in which 4, 8, or 12 percent of the wheat flour had been replaced by flour ground from either sprouted or unsprouted Tartary buckwheat seeds. Sprouting and bread-making procedures were described above (SOLUBLE CARBOHYDRATES). Compared with the all-wheat control, total flavonoids were increased with the inclusion of Tartary buckwheat flour in the bread, particularly the sprouted buckwheat flour. The all-wheat bread contained 7.9 mg rutin equivalents/100 g; the unsprouted tartary breads between 80.6 and 97.5 mg rutin equivalents/100 g; the sprouted Tartary breads between 121.7 and 149.2 mg rutin equivalents/100 g dry weight. A similar pattern was observed in the concentration of total phenolic compounds in these steamed breads. The all-wheat bread contained 31.2 mg gallic acid equivalents/100 g; the unsprouted Tartary buckwheat breads between 90.4 and 162.7 mg GAE/100 g; the sprouted Tartary buckwheat breads between 179.9 and 364.2 mg GAE/100 g dry weight. (Quercetin, protocatechuic acid, and rutin were the predominant phenolic compound in all the breads containing Tartary buckwheat. ) Ability of bread extracts to scavenge free radicals of DPPH or 2,2’-Azinobis-(3-ethylbenzthiazoline-6-sulphonate) (ABTS) followed the same pattern as the concentration of total phenolics.
In this study, Xu et al. (2014) reported that loaf volume and height diminished, but loaf diameter expanded with increasing concentration of buckwheat flour. Farinograph measurements included water absorption, development time, stability time, and degree of softening. Water absorption diminished only slightly with added increments of tartary buckwheat flour. Both development time and stability time were substantially reduced, particularly with increasing concentrations of the sprouted tartary buckwheat flour. In the all-wheat control, development time was 4.0 minutes, and stability time was 4.2 minutes. For dough containing 12 percent sprouted buckwheat flour, the respective times were 1.8 and 2.1 minutes. The authors also subjected these breads to texture analysis. Compared to the all-wheat bread, loaves containing sprouted Tartary buckwheat flour exhibited increased hardness, gumminess, and chewiness, but reduced adhesiveness, springiness, cohesiveness, and resilience. Loaves containing unsprouted flour were more similar to the control, but did exhibit consistently greater hardness, and consistently reduced adhesiveness, springiness, and cohesiveness. The texture of this bread was deemed satisfactory. Of the steamed breads containing buckwheat flour, the best tasting was that containing four percent unsprouted buckwheat flour, and the worst was that containing 12 percent sprouted buckwheat flour (described as “bitter” and “astringent”). Total phenolic content was negatively correlated with sensory evaluation (r=-0.92).
Ma et al. (2013) investigated antioxidant properties of noodles made from flour from three cultivars of common buckwheat (‘Yuqiao 4#’, ‘Wensha’, and ‘Dingbian red flower’) and two of Tartary buckwheat (‘Xinong 9940’ and ‘Xinong 9909’). Grains from buckwheat grown in Shanxi, China, were ground and passed through a 60-mesh sieve. Five parts of the resultant flour were blended in two parts of a 2 percent salt solution; dough was rested for 10 minutes prior to extrusion at 2mm diameter. Fresh noodles were cooked in boiling water until the core was translucent, then rapidly cooled in 10C water. The concentration of total phenolics in the fresh noodles ranged from 183 to 221 mg Gallic Acid equivalents/100g dry weight for the three common buckwheat cultivars, and from 970 to 1363 mg GAE/100g DW for the two Tartary buckwheats. (Total phenolic concentration in the Tartary buckwheat noodles was comparable to that in pepper, broccoli, and spinach. ) Total flavonoids ranged from 187 to 219 mg rutin equivalents/100g DW for the three common buckwheats, and from 503 to 601 mg rutin eq/100g DW for the two Tartary buckwheats. DPPH radical scavenging activity ranged from 0.9 to 1.1 mmol Trolox equivalents/100g DW for the common buckwheats and from 7.0 to 8.7 mmol Trolox eq/100g DW for the Tartary buckwheats. Chlorogenic acid and catechin were the predominant phenolic compounds in the common buckwheat noodles, p-hydroxybenzoic acid, protocatechuic acid, and quercetin in the latter.
Ma et al. (2013) examined the cooking properties, texture, and sensory acceptability of noodles made from 100 percent common buckwheat or Tartary buckwheat flour. The cooking loss of noodles of the three common buckwheat cultivars and two tartary buckwheat cultivars ranged from 3.7 to 6.2 percent. Water uptake ranged from 45 to 49 percent, except for noodles of the Tartary variety ‘Xinong 9909,’ for which water uptake was 65 percent. The latter exhibited the greatest adhesiveness—176 g, versus 101 g for ‘Xinong 9940,’ and between 27 g and 34 g for the common buckwheat cultivars. Both Tartary cultivars produced flours with less tensile strength than those from the common buckwheat cultivars. The authors had a panel of semi-trained evaluators score the noodles on a 1-to-9 scale for color, taste, odor, hardness, slipperiness, and chewiness. Noodles of all three common buckwheat cultivars scored at least 5 (neutral) and as high as 6.9 on all traits. In contrast, noodles of both Tartary buckwheat cultivars scored between four and five on all traits. The authors reported that the score for taste was negatively correlated with total phenolic content.
Han et al. (2013) investigated noodles in which 10, 20, or 30 percent (w/w) of wheat flour was replaced with one of four fractions of commercially-processed Chinese Tartary buckwheat flour. Flours was produced by passing raw dehulled buckwheat groats through break mills four times, with flour from each break passing through a 120-mesh sieve. The first (A) fraction represents “fancy” flour, whereas some mixture of fractions B, C, and D would constitute “dark” flour. Successive (A>D) buckwheat fractions varied from 80.6 to 34.7 percent total starch (dry basis), 9.3 to 21.1 percent protein, 5.7 to 33.4 percent crude fiber, 2.6 to 6.3 percent lipid, and 1.8 to 4.5 percent ash. (The wheat flour that constituted the balance of the noodles was 83.2 percent starch, 13.6 percent protein, 0.6 percent fiber, and 0.9 percent ash. ) Dough was prepared from 300g of flour blend, 102 ml distilled water, and 6g salt; the ingredients were mixed for 8 minutes and then rested for 30. Noodles were rolled at 1mm thickness, cut into 3mm-wide strips, and air-dried for 96 hr prior to storage and cooking.
Using a Mixolab (Chopin Technologies, Villeneuve-la-Garenne, France), Han and co-authors analyzed dough made from the all-wheat flour and 12 wheat/buckwheat mixes. Water absorption was defined as the amount of water (as a percentage of flour weight) that was required to create a dough that could generate a torque of 1.1 Newton-meters. The researchers reported that water absorption decreased with incremental substitutions of buckwheat flour. High water absorption is related to the preponderance of gluten (gliadins and glutenins) in wheat protein. Absorption was affected most by the inclusion of the first (A) fraction, and least by the inclusion of the fourth (D) fraction of buckwheat flour. The authors speculated that the high fiber content of the later buckwheat flour fractions increased solubility by providing hydroxyl groups attractive to water molecules. Dough development time was defined as the time required for the dough to generate maximum torque under a predefined temperature curve. Dough development was delayed by incremental substitutions of buckwheat flour. The delay was greatest with inclusion of buckwheat fraction D, and least with the inclusion of fraction A. Dough stability was defined as the duration during which torque stayed above 1.1 Nm. A decrease in dough stability followed the same pattern as development time; both parameters are related to the quantity of gluten contributed by wheat flour.
Han and co-authors also reported the Mixolab parameter C3, which is the torque generated during starch gelatinization at 90ºC. With addition of tartary buckwheat fraction A (the fraction with maximum starch content), the value of C3 increased. Tartary buckwheat starch has higher hot paste viscosity than does wheat starch. From fraction A through D, with reduced starch content there was a concomitant reduction in C3 value. Setback—the reduction of viscosity after cooling—was similarly diminished with each successive flour fraction from A through D. The authors speculated that the increasing concentration of lipids increased the formation of lipid-amylose complexes, and that these complexes interfered with the re-association and recrystallization of amylose molecules (i.e., retrogradation).
Compared with all-wheat noodles, those containing Tartary buckwheat were darker and more yellow in color, the effect increasing with increasing percentage of buckwheat and with successive fractions (D>C>B>A). Optimal cooking time of dry noodles was shorter for the wheat/buckwheat blends than the all-wheat controls, with noodles containing fraction D having shorter cooking times than those containing fraction A. Cooking loss, the percentage of noodle dry weight that is lost during cooking, was unaffected by the substitution of 10 percent of wheat flour with Tartary buckwheat flour. However, greater substitutions of buckwheat flour increased cooking loss; in noodles containing 30 percent buckwheat flour, losses ranged from 4.8 to 5.6 percent. Tensile strength of cooked noodles was diminished with substitutions of buckwheat flour for wheat flour. Later fractions of buckwheat flour showed greater effect. Firmness was similarly affected. Trained panelists scored the overall acceptability of the noodles on a 1-9 scale. Scores diminished with increasing substitutions of buckwheat flour, again with later fractions having greater effect. Only noodles containing 10 or 20 percent of fraction A or 10 percent of fraction B were statistically equivalent to the all-wheat controls (score >7.97).
Hatcher et al. (2011) compared the textural properties of soba noodles produced from either of two wheat flours (Canadian Western Red Spring or a lower-protein Canadian Prairie Spring Red) blended with any of four whole-groat buckwheat flours (common buckwheat variety ‘Koto,’ common buckwheat ‘Koma,’ “green testa-23” [F. emarginatum?], or “brown Tartary” buckwheat). All eight blends comprised 20 percent of one or the other wheat flour and 80 percent of one of the buckwheat flours. On a dry matter basis, the blends containing Tartary buckwheat flour were higher in starch (75.3-75.6 percent) than the others (67.8-69.5 percent), but lower in protein (11.7-12.1 percent versus 13.9-14.7 percent), ash (1.1-1.2 percent versus 1.9-2.0 percent), and dietary fiber (5.8 percent versus 6.6-7.8 percent).
Dough from all eight buckwheat-wheat blends required 40 percent water absorption (w/w) to reach a suitable consistency, in contrast to either all-wheat control, which required 34 percent water absorption. Dough was mechanically sheeted to 1.1mm thickness prior to being cut into noodles on an Ohtake B12 cutter. Fresh soba noodles were then cooked in boiling water for 80 seconds. Among the buckwheat-wheat blends, those containing Tartary buckwheat took up significantly less water during cooking. The authors attributed that difference both to the higher starch content and lower dietary fiber content of the Tartary buckwheat flour. They speculated that the lower water uptake by fiber allowed greater hydration of the protein in the Tartary buckwheat blend noodles. The all-wheat noodles had far greater water uptake and thickness than noodles containing buckwheat flour; the noodles containing green testa buckwheat flour were the thinnest.
Hatcher et al. (2011) tested textural parameters of these noodles, including the force required to cut through an individual noodle (“bite” or Maximum Cutting Strength in g/mm2). While much lower than the MCS of the all-wheat controls, the Tartary buckwheat blend noodles scored significantly higher than the other buckwheat blend noodles. The authors also measured to parameters related to stress relaxation profiles: K1, which is indicative of the initial decay of applied mechanical stress, and K2, which is indicative of the residual stress on the noodle. Higher values of these parameters, and lower values of the derived parameter SR% (at 20 seconds) are related to greater elasticity. While much lower than the K1 values of the all-wheat controls, the values of the various buckwheat blend noodles showed no clear pattern. However, the K2 values of the Tartary buckwheat noodles were the highest observed among the noodles containing buckwheat flour, whereas their SP% values were the lowest. K2 and SR% were both significantly correlated with flour pasting properties. Besides higher elasticity of the noodles containing Tartary buckwheat, that flour blend also displayed the highest final viscosity—another indication of noodle quality. The authors concluded: “Examination of the buckwheat blended noodles highlighted the unique composition of the tartary buckwheat; lower dietary fiber content, elevated starch content and lower protein content produced soba noodles of distinctive textural quality than the other three buckwheats evaluated.”
Yoo et al. (2012) investigated the effect of hydrothermal processing of whole Tartary buckwheat seeds on the concentration of flavonoids after flour milling. Seeds grown in Gangwondo, S. Korea were either steamed (10 minutes over boiling water) or autoclaved (10 minutes at 120ºC) prior to overnight drying at 25ºC. Seeds subjected to both treatments and native (untreated) seeds were subsequently ground in a laboratory blender; flour retained between sieves of 200 and 300 mesh was collected for subsequent analysis. A slurry containing 6 g of flour in 4 ml of water was agitated for 0, 10, 30, or 60 minutes. In freeze-dried residues, rutin and quercetin concentrations were measured by HPLC.
In flour from both hydrothermal treatments and the control, the rutin concentration was almost 4g/100g prior to agitation. That level remained constant over 60 minutes of agitation in flour from seeds subjected to either steaming or autoclaving. In contrast, the concentration of rutin in flour from native seed was reduced by three-quarters after 10 minutes of agitation, and by seven-eighths after an hour. The initial level of quercetin was negligible in flours from native seeds or those subjected to either of the hydrothermal treatments. After 10 or more minutes of agitation, flour from native seed contained about 2g of quercetin per 100g. There was no similar rise in the concentration of quercetin when flour from either hydrothermal treatment was subjected to agitation. According to the authors, grinding exposes rutin (concentrated in the embryo) to enzymes that are localized in the seed coat of intact seeds. Either hydrothermal process would be sufficient to break down enzymes that normally hydrolyze rutin during both milling and dough-mixing.
Yoo and co-authors produced batches of noodles comprising 35g all-purpose wheat flour, 15g Tartary buckwheat flour, 1g NaCl, and 20ml distilled water. Noodles made with steamed or autoclaved buckwheat flour contained at least 0.83g rutin per 100g, whereas noodles made with native buckwheat flour contained only 0.27g rutin per 100g. While noodles made from hydrothermally-treated buckwheat contained negligible concentrations of quercetin, those made from untreated buckwheat contained 0.43g per 100g.
Comparing the pasting properties of steam-treated or autoclaved flours to flour from untreated seeds, Yoo et al. (2012) found that the latter started thickening earlier, reached a higher peak viscosity, and retained a higher final viscosity. During dough mixing, the hydrothermally-treated flours had greater water absorption, longer development time, and reduced stability time than did the flour from untreated seeds.
Cho et al. (2014) measured the concentrations of rutin and quercetin in three milling fractions from Tartary buckwheat seeds that had been steam-treated as above. Bran (the fraction passing through a 16 mesh sieve but retained by a 150 mesh sieve) contained about 5 percent rutin but only 0.12 percent quercetin (dry weight basis). Neither hulls nor flour contained appreciable concentrations of either flavonoid.
Zhang et al. (2010) studied the effects of heating on the antioxidant properties of flour from whole Tartary buckwheat seeds grown in Shanxi, China. Batches of flour were roasted at 80º or 120ºC for 20 or 40 minutes. The ability to scavenge hydroxyl or superoxide radicals, or to inhibit lipid peroxidation were diminished by the longer treatment, but even more so by the higher temperature treatments. Compared to the raw flour, that roasted for 40 minutes at 120ºC was about 70 percent as effective scavenging radicals, and only half as effective inhibiting lipid peroxidation. Batches of flour were also autoclaved at 0.1 or 0.2 MPa for 20 or 40 minutes. Both higher pressure and longer treatment decreased the concentrations of rutin and total flavonoids. Similar results were observed after microwave heating at 700W for 10 minutes.
Wang et al. (2013) measured the ability of a bran extract from Tartary buckwheat and three of its constituent flavonoids to scavenge free radicals. Following soaking, steaming, and drying, hulls were removed from seeds of the variety ‘Heifeng 1’ (during the commercial production in China of buckwheat tea). The dehulled seeds comprised about 70 percent groats and 30 percent bran. This bran was passed through a 60-mesh screen and then extracted in 60 percent ethanol at room temperature. The resulting extract contained the flavonoids rutin, isoquercetin and quercetin at concentrations of 541mg/g, 9.3mg/g, and 66mg/g, respectively. Compared to the bran extract investigated by Wang et al. (2009) and discussed above, that reported in this study by Wang et al. (2013) contained slightly less quercetin, but about 60 times the concentration of rutin.
The authors measured the inhibition of free radicals of 1,1-Diphenyl-2- picrylhydrazyl (DDPH) in a methanol solution. The concentrations necessary to reduce free radicals by 50 percent were 5.12µg/ml, 7.63μg/ml, 8.36µg/ml, and 10.0μg/ml, for quercetin, isoquercetin, “bran extract,” and rutin, respectively. The same authors performed oxygen radical absorbance capacity (ORAC) assays, using 2,2′-azobis (2-amidinopropane) dihydrochloride (AAPH) as the source of peroxyl radicals. Quercetin was about twice as effective as the Tartary buckwheat extract, with isoquercetin and rutin exhibiting intermediate effectiveness.
The soluble carbohydrates found in buckwheat include free D-chiro-inositol (D-CI) and also its fagopyritol derivatives. The latter compounds comprise a D-CI molecule with one, two, or three galactose moieties attached (see SOLUBLE CARBOHYDRATES in NUTRITIONAL ASPECTS OF TARTARY BUCKWHEAT). The human stomach lacks the enzyme (⍺-galactosidase) that can hydrolyze fagopyritols and release free D-CI. Therefore, Yao et al. (2008) proposed to increase the free D-CI in an extract of Tartary buckwheat by chemical hydrolization of its constituent fagopyritols. The treated extract, enriched with free D-CI, could serve as a dietary supplement for the prevention or treatment of Type 2 diabetes mellitus (discussed on the page on ANIMAL STUDIES).
The authors first used a grain polishing machine to isolate bran. Containing seed coat and embryo tissue, the bran constituted 15 percent of the weight of whole Tartary buckwheat seeds. The isolated buckwheat bran was then extracted with a 60% ethanol solution at 50ºC for 30 minutes. Following concentration under reduced pressure, the crude extract solution was steamed in an autoclave at 1.6 MPa and 120ºC for one hour. Successive purification steps included passage through columns of activated carbon, a strongly acidic styrene cation ion-exchange resin, and strongly alkaline styrene anion ion-exchange resin. The resultant D-CI-enriched extract was then concentrated at 50ºC under reduced pressure. From a concentration of 0.032%, D-CI was enriched about seven-fold by the autoclave treatment. The sequential column chromatography and evaporation resulted in a further increase in the concentration of D-CI to 22 percent.
Advanced processing technologies such as freeze drying and electromagnetic wave sterilization have been used to manufacture a variety of powdered foodstuffs. Japan Food Products & Service Journal (25 Oct. 2004) reported that a medicinal Tartary buckwheat flour, “Yakuzen Dattan Sobako,” was being marketed for its rutin content.
As noted in the section FLAVONOIDS AND ANTIOXIDANT ACTIVITY, other plant parts contain flavonoids at concentrations much greater than those found in seeds. As described in that section, Merendino et al. (2014) measured the polyphenol content and the antioxidant capacity of dried sprouts of two cultivars of common buckwheat (‘Lileja’ and ‘Darja’) and one of Tartary buckwheat (‘Ljse’). The powdered dried sprouts were milled to a similar particle size and mixed (30%) with durum wheat semolina to make spaghetti. Measured as equivalents of gallic acid per gram DW, this spaghetti had about 12 times the polyphenol content of commercially-processed (all-durum) spaghetti. After cooking, the polyphenol content was significantly diminished in the buckwheat-enriched spaghetti, but was still significantly higher than in the cooked commercial product. When compared by three assays for antioxidant potential—DPPH activity, ORAC, and FRAP—raw or cooked samples of buckwheat-enriched spaghetti showed higher activity than the corresponding sample of all-durum spaghetti. The content of rutin was 0.8 mg/g DW in the uncooked buckwheat-enriched spaghetti and 0.6 mg/g in that spaghetti after cooking; the content of quercetin was 3.0 mg/g DW in the uncooked buckwheat-enriched spaghetti and 2.7 mg/g after cooking. Neither polyphenol was detected in either raw or cooked samples of the all-durum spaghetti.
Merendino and co-authors characterized the spaghetti that contained Tartary buckwheat sprout powder as having “. . . a slightly rough irregular surface with a light brown appearance.” Compared with the all-durum spaghetti, it was higher in ash (2.3% versus 0.9%) and in protein (15.2% versus 9.6%), but lower in carbohydrate (71.% versus 77.5%). The Total Organic Matter was 3.5% versus 0.9% for the all-durum spaghetti; representing a loss of dry matter to the cooking water, the TOM of the buckwheat-enriched spaghetti was considered unacceptably high.
Merendino and co-authors pointed out that in this experiment, the antioxidant capacity actually increased during the production and cooking of the buckwheat-enriched pasta. Different processes during food preparation can decrease or increase the concentration or the accessibility of the many constituent compounds with anti-oxidative potential.
Tartary buckwheat leaves and flowers have also been proposed as ingredients of functional foods. We hope in time to expand this discussion to such products. Moreover, flavonoids are not the only chemicals from buckwheats that have drawn interest because of their healthful effects. Inositol, vitamins, tripeptides, lipophilic low-molecular-weight antioxidants (LMWA), squalene, and tannins were also listed in a review of buckwheat-enriched foods (Giménez-Bastida et al., 2015).
ANTI-HYPERTENSIVE COMPOUNDS IN BUCKWHEAT
Common buckwheat flour contains a strong inhibitor of angiotensin I-converting enzyme (ACE), which converts angiotensin I to II. The latter is the pressor hormone in the renin-angiotensin (RA) system that controls blood pressure, and ACE is the rate-determining enzyme in the RA system. Therefore, inclusion of buckwheat in the diet was thought to help control high blood pressure (hypertension).
Aoyagi (2006) identified the active agent as 2”-hydroxynicotianamine (HNA), an analogue of nicotianamine (NA). While the latter is a known ACE-inhibitor widely distributed in plants, the former has been found primarily in the Polygonaceae. The two compounds have similar ACE inhibitory activity (IC50 0.08 – 0.085μM).
Higasa et al. (2011) investigated concentrations of NA and HNA in mature seeds, various milling fractions, sprouts, and various plant organs (at flowering) in a geographically-diverse sampling of varieties of both common and Tartary buckwheat. In the overall sample of 18 varieties, the concentration of HNA ranged from 16.1-27.7 mg/100g mature seeds (dry weight); the concentration of NA was much lower, ranging from 0.5-1.7 mg/100g dry weight. In the concentration of HNA in whole mature seeds, those authors did not find a significant difference between Japanese and imported varieties within either buckwheat species, nor between the two species. The concentration of NA in whole mature seeds was significantly lower in Japanese varieties of common buckwheat compared to imported varieties, but there was no such significant difference between common buckwheat and Tartary buckwheat.
With repeated grinding and sifting, Higasa and co-authors separated hulls, bran, and six flour fractions. Like the concentration of protein, the concentrations of HNA and NA were highest in the bran; these concentrations were lowest in the hulls, and at intermediate levels in the milling fractions derived largely from the interior of the groat. Fine flour from common buckwheat contained 54 percent of the seed’s total HNA; whereas, fine flour from Tartary buckwheat contained 45 percent of the seed’s total HNA. As sampled, the different organs of flowering plants contained much lower levels of HNA and NA than those in mature seeds. Flowers and roots contained higher levels than leaves, stems, or sprouts (measured 7 days after germination).
Higasa et al. (2011) also examined concentrations of HNA and NA in commercially-prepared soba noodles. In noodles prepared entirely from common buckwheat flour, the concentration of HNA was 14.69mg/100g DW; that of NA was 1.27mg/100g DW. After cooking 80g of noodles in 1000g of water, 20.1 percent of the HNA was lost, only 21.3 percent remained in the noodles, and 58.6 percent was recoverable in the cooking water. After cooking, 41.7 percent of the NA was lost, only 19 percent remained in the noodles, and 39.3 percent was recoverable in the cooking water.