Breeding of Buckwheat to Reduce Bitterness and Rutin Hydrolysis

plants Review Breeding of Buckwheat to Reduce Bitterness and Rutin Hydrolysis Tatsuro Suzuki 1,*, Toshikazu Morishita 2, Takahiro Noda 3, Koji Ishiguro 3, Shiori Otsuka 3 and Kenjiro Katsu 1 1 National Agriculture and Food Research Organization, Kyushu Okinawa Agricultural Research Center, Suya 2421, Koshi, Kumamoto 861-1192, Japan; [email protected] 2 National Agriculture and Food Research Organization, Institute of Crop Science, Radiation Breeding Division, 2425 Kamimurata, Hitachi-Omiya 319-2293, Japan; [email protected] 3 National Agriculture and Food Research Organization Hokkaido Agricultural Research Center, Memuro Upland Farming Research Station, Shinsei, Memuro, Kasai-Gun, Hokkaido 082-0081, Japan; [email protected] (T.N.); [email protected] (K.I.); [email protected] (S.O.) * Correspondence: [email protected] Abstract: Buckwheat (Fagopyrum esculentum) is recognized as an important traditional crop in some regions, and its taste is an important characteristic. Of the three cultivated buckwheat species, Tartary buckwheat (Fagopyrum tataricum) and perennial buckwheat (Fagopyrum cymosum) have strong bitterness in their seeds, which has prevented the wider use of the seeds of these varieties. In Tartary buckwheat, some studies have focused on the cause of strong bitterness generation. Tartary buckwheat seeds contain large amounts of the functional compounds rutin and rutinosidase, and rutin hydrolysis by rutinosidase has been found to be the trigger of rutin hydrolysis. Therefore, a variety with only a trace of rutinosidase and with reduced bitterness is required. The rutinosidase in Tartary buckwheat seeds consists of two major isozymes with very similar enzymatic characteristics, which can hydrolyze flour rutin within several minutes after the addition of water. Recently, the trace- Citation: Suzuki, T.; Morishita, T.; rutinosidase variety Manten-Kirari in Tartary buckwheat was developed. The trace-rutinosidase Noda, T.; Ishiguro, K.; Otsuka, S.; characteristics were dominated by a single recessive gene. In ‘Manten-Kirari’ dough and foods, Katsu, K. Breeding of Buckwheat to such as breads, confectionaries, and noodles, the rutin residual ratio was higher and bitterness was Reduce Bitterness and Rutin Hydrolysis. Plants 2021, 10, 791. reduced compared to that of the normal-rutinosidase variety. In this review, we summarize the https://doi.org/10.3390/ detailed research on the breeding of buckwheat related to reducing bitterness and rutin hydrolysis. plants10040791 Keywords: buckwheat; bitterness; rutin; rutinosidase Academic Editor: Ana Butrón Received: 24 March 2021 Accepted: 14 April 2021 1. Introduction Published: 16 April 2021 Buckwheat is an important cereal in several countries, such as Japan, China, Russia, and in Europe [1,2]. Buckwheat is recognized as an important traditional crop in some Publisher’s Note: MDPI stays neutral regions [1,2], and its taste is an important characteristic. Generally, sweetness, saltiness, with regard to jurisdictional claims in sourness, bitterness, and ‘umami’ are important elements for food taste. For example, published maps and institutional affil- the typical stimuli are sugars (sweetness), sodium chloride (saltiness), hydrochloric acid iations. (sourness), and either caffeine, quinine, or some kind of flavonoid (bitterness) [3–6]. Among them, bitterness is also an important trait for buckwheat. In the Fagopyrum genus, three major species are cultivated: common buckwheat (Fagopyrum esculentum), Tartary buckwheat (F. tataricum), and perennial buckwheat (F. cymosum). The degree of bitterness in seeds Copyright: © 2021 by the authors. varies by species. Common buckwheat, which is cultivated over the largest area in the Licensee MDPI, Basel, Switzerland. world [7], has only slight bitterness in seeds, flour, and leaves compared to the other species. This article is an open access article On the other hand, Tartary buckwheat has strong bitterness in seeds [8]. Tartary buckwheat distributed under the terms and has been called ‘bitter buckwheat’, and it has many advantages compared to other species, conditions of the Creative Commons such as self-pollination and high seed yield in the Hokkaido prefecture in Japan [9]. In Attribution (CC BY) license (https:// addition, Tartary buckwheat is considered a healthy food because it contains a greater creativecommons.org/licenses/by/ amount of rutin compared to common buckwheat. Tartary buckwheat grains have a higher 4.0/). Plants 2021, 10, 791. https://doi.org/10.3390/plants10040791 https://www.mdpi.com/journal/plants Plants 2021, 10, x FOR PEER REVIEW 2 of 8 Plants 2021, 10, 791 2 of 8 a greater amount of rutin compared to common buckwheat. Tartary buckwheat grains have a higher antioxidative activity compared to common buckwheat grains. The contribution of rutin to antioxidative activity was about 90% [10]. Rutin is a flavonol with a high antioxidativeantioxidative effect activity [11,12]. compared It is widely to common foun buckwheatd in plants, grains. mainly The in contribution leaves and flower of rutin buds, to antioxidative activity was about 90% [10]. Rutin is a flavonol with a high antioxidative although buckwheat is known as the only cereal that contains rutin in its seeds. However, effect [11,12]. It is widely found in plants, mainly in leaves and flower buds, although strongbuckwheat bitterness is known is an important as the only unfavorable cereal that contains characteristic rutin in itsthat seeds. is widely However, rejected strong both by peoplebitterness and isanimals an important [5,6]. Therefore, unfavorable the characteristic strong bitterness that is widely of Tatary rejected buckwheat both by people seeds has preventedand animals its wide [5,6]. Therefore,use. Rutin the hydrolysis strong bitterness has been of Tatary identified buckwheat as a mechanism seeds has prevented for the gen- erationits wide of strong use. Rutin bitterness hydrolysis in Tartary has been buckwheat identified and as a it mechanism is catalyzed for by the strong generation rutinosidase of activitystrong in bitterness seeds (Figure in Tartary 1) [13]. buckwheat Rutinosidase and it is is sufficient catalyzed to by hydrol strongyze rutinosidase the rutin activ-contained in itybuckwheat in seeds (Figureflour within1)[ 13]. a Rutinosidasefew minutes isafter sufficient the addition to hydrolyze of water the rutin[13–15]. contained Recently, a trace-rutinosidasein buckwheat flour variety within was a few developed minutes in after Tartary the addition buckwheat, of water and [13 showed–15]. Recently, reduced bit- a trace-rutinosidase variety was developed in Tartary buckwheat, and showed reduced terness [16–19]. In this review, to improve studies about breeding of buckwheat in terms bitterness [16–19]. In this review, to improve studies about breeding of buckwheat in terms of preventingof preventing bitterness bitterness generationgeneration and and rutin ruti hydrolysis,n hydrolysis, we summarized we summarized the present the present status sta- tusand and future future prospects prospects of buckwheatof buckwheat in terms in terms of the of mechanism the mechanism and breeding and breeding for reducing for reduc- ingbitterness bitterness and and rutin rutin hydrolysis. hydrolysis. OH OH OH OH HO O Rutinosidase HO O O OH + Glc - Rha HO O Glc - Rha HO O Rutin Quercetin Rutinose Figure 1. Rutin hydrolysis in Tartary buckwheat flour. Figure 1. Rutin hydrolysis in Tartary buckwheat flour. 2. Bitterness Generation and Rutin Hydrolysis in Buckwheat 2. Bitterness Generation and Rutin Hydrolysis in Buckwheat Bitterness is an important quality characteristic, even in buckwheat. In common buckwheatBitterness flour, is an sensory important analyses quality show that characteristic, bitterness increases even within buckwheat. the storage temper-In common buckwheatature of the flour, flour sensory [20], and analyses this feature show has that also bitterness been observed increases in instrument with the storage evaluation. temper- atureHowever, of the varietalflour [20], differences and this in feature bitterness has in al commonso been buckwheatobserved in have instrument not yet been evaluation. re- However,ported. Severalvarietal reports differences have shownin bitterness that Tartary in common buckwheat buckwheat has a strong have bitter not tasteyet been in reported.its seeds Several and processedreports have foods, shown such that as noodles. Tartary buckwheat Quercetin, the has hydrolysis a strong bitter product taste of in its rutin by rutinosidase, has been identified as the principle bittering compound in several seeds and processed foods, such as noodles. Quercetin, the hydrolysis product of rutin by fruits [21,22]. Kawakami et al., 1995 [23] reported that Tartary buckwheat dough contains rutinosidase,at least three has bitter been compounds: identified unidentified as the prin compoundsciple bittering F3 and F4compound and quercetin. in several Among fruits [21,22].them, Kawakami quercetin is theet al., bitter 1995 compound [23] reported generated that by Tartary rutinosidase buckwheat activity. dough Suzuki contains et al., at least2014 three [16] bitter hypothesized compounds: that rutin unidentified hydrolysis triggerscompounds bitterness F3 and generation. F4 and Theyquercetin. prepared Among them,steam-treated quercetin Tartary is the buckwheatbitter compound flour in generated which rutinosidase by rutinosidase was completely activity. inactivated, Suzuki et al., 2014and [16] they hypothesized also prepared that a highly rutin purified

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