Buckwheat, a pseudo-cereal crop belonging to the family Polygonaceae, genus Fagopyrum, is a popular health food in Asian and European countries (Kreft et al., 2003). Together with well-balanced chemical composition (Bonafaccia, Marocchini and Kreft, 2003; Zhu, 2016) including the optimal amino acid composition of seed storage protein (Prakash et al., 1987; Jiang et al., 2007) buckwheat manifests a high level of antioxidant activity due to the content of flavonoids (Holasova et al., 2002). In buckwheat grain it was identified rutin, quercetin, and flavone C-glycosides (Zielińska et al., 2012). It has several pharmacological functions such as anti-inflammatory, anti-diabetic (Lee et al., 2016; Kamalakkannan and Prince, 2006), blood capillary strengthening properties (Chua, 2013), and lipid-lowering activity (Tomotake et al., 2015). Also, rutin has cardioprotective effects (He et al., 1995; Wojcicki et al., 1995; Annapurna et al., 2009). It allows considering buckwheat as a functional food. Tartary buckwheat (Fagopyrum tataricum Gaertn.) contains approximately 100-fold higher amounts of rutin in its seeds compared to common buckwheat (Fabjan et al., 2003). Sometimes buckwheat flour is used for the improvement of wheat-based products. So, wheat bread with buckwheat flour shown the level of antioxidant activity depends on the percentage of buckwheat flour, and the rutin content in such bread ranged from 7.76 to 26.90 mg kg^-1 (Lin et al., 2009; Bojňanská et al., 2009; Brindzová et al., 2009). Such products are recommended especially for people who live in conditions of oxidative stress (Kuznetsova et al., 2018).

In Russia, at present, the traditions of buckwheat consumption mainly as a groats crop promote cultivating only common buckwheat which grain is more suitable for groats production. In China and several other Asian countries where buckwheat grain is used mainly for the production of flour the Tartary buckwheat is also cultivated. Grinding of non-hulled grain is one of most perspective approach for use of buckwheat since it is a simple method which allows producing various types of flour for making noodles, pasta, bread, confectionery, etc. (Steadman et al., 2001). Grinding grain with hulls reduces the relative content of nutrients in the flour obtained but increases the content of dietary fiber (Dziedzic et al., 2012). Buckwheat grains can be milled using any equipment designed for grinding cereals, for example, the millstones or roller mill (Mazza and Oomah, 2005). Millstones are more often used to produce whole grain flour for making pancakes at home. White flour can be obtained from such flour by removing the bran by sieving. The fineness of grinding with millstones can be different and is adjusted by changing the gap between them. Grinding with millstones is a one-step process, unlike grinding with a roller mill, where the process can be divided into several stages with the release of several flour fractions (Ohinata et al., 2001).

It seems promising to search, create, and evaluate new buckwheat samples which could be more suitable raw materials for the development of deeper processing products. Using hybridization F. tataricum (4x = 32) × F. giganteum together with selection in late generations (F₁₀ and later) recently was created a new buckwheat species F. hybridum (Fesenko and Fesenko, 2010). The artificial species manifests competitive yield ability and may be considered for registration as a cultivar (Fesenko et al., 2017).

An objective of this paper was the evaluation of F. hybridum grain properties, including amino acid composition, antioxidant activity, and characteristics of fragments of kernels and hulls after milling by both roller mill and millstones, compared to ones of F. tataricum and F. esculentum.

The work involved one sample of each species. F. tataricum – an accession k-17 from the Federal Research Center N.I. Vavilov All-Russian Institute of Plant Genetic Resourses (VIR) collection (St.Petersburg); F. hybridum is a new species of hybrid origin which has been created at the Federal Research Center of Grain Legumes and Groats Crops, Orel, Russia (Fesenko and Fesenko, 2010); F. esculentum was represented by cv. Devyatka (Federal Research Center of grain legumes and groats crops).

The grinding of the grain was carried out both on a millstone with gaps of 0.01 and 0.3 mm, and on roller mill with two frequencies of rotation, 6.5x10³ and 10x10³ turns per minute. Micrographs of the grains fragments were taken with an AxioCam MRc5 camera (Axio Imager microscope. Al, Carl Zeiss). The measurements were made using the AxioVision program.

Antioxidant activity (AOA) was measured using spectrophotometry in both alcohol and water extracts based on inhibition of the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical (Silva et al., 2005). Flavonoids were extracted with both 96% ethanol and water. 1g of milled grain was mixed with 25 mL of ethanol or water. Extraction was carried out for 24 hours with constant stirring. The extracts were filtered. 0.025 DPPH was dissolved in 100 mL of 96% ethanol. 10 mL of the solution was mixed with 90 mL of 96% ethanol. The optical density of the solution was measured after 30 minutes on a spectrophotometer in a cuvette with a thickness of 1 cm, at a wavelength of 515 nm. To determine the antioxidant activity of the studied extract, 0.1 mL of the filtrate was added to 3.9 mL of the solution of DPPH, mixed, and placed in a dark for 5 – 10 minutes. Then the optical density of the solution was measured using a spectrophotometer at a wavelength of 515 nm.

To determine the thermal stability of bioflavonoids, the extracts were gradually heated in a water bath (t = 20 – 100 ºC), and AOA was measured at 20, 40, 60, 80, and 100 ºC. The antioxidant activity (AOA) of the extracts is calculated by the formula AOA= A1−A2 A1 ×100% , where A1 is the optical density of the DPPH solution before adding the investigated extract; A2 – optical density of the DPPH solution after adding the investigated extract.

The amino acid composition was evaluated using amino acid analyzer BIOCHROM (Biochrom Ltd., Great Britain).

So, the new species F. hybridum is better in some biochemical characteristics in comparison to cultivated buckwheats, F. esculentum, and F. tataricum. The total antioxidant activity of ethanol extract from F. hybridum flour was higher even compared to F. tataricum. Water extract from F. hybridum flour manifested only 17% AOA of ethanol extract. Probably, minimal efficiency of water extraction may indicate protection from dissolution by water and, accordingly, from the destruction of the flavonoids by enzymes, but the alternative explanation about the higher activity of antioxidants degradation enzymes is not yet rejected. In terms of the content of all essential amino acids, the studied sample of F. hybridum exceeds the studied sample of F. tataricum by 5.2% (Cysteine) – 46.2% (Valine), and the studied sample of F. esculentum by 3.7% (Lisyne) – 39.2% (Arginine). The milling fragments of F. hybridum seeds hulls, as well as F. tataricum ones, have no pronounced acute angles, probably due to less compact structure of the hulls compared to F. esculentum. The non-hulled grain of both F. hybridum and F. tataricum is more suitable for production of whole-grain flour than the non-hulled grain of F. esculentum. Using the whole grain flour allows making the products with a high share of dietary fibers.