The Importance of Buckwheat as a Pseudocereal: Content and Stability of Its Main Bioactive Components

Abstract

The production of various bakery and non-bakery products based on buckwheat with components that positively affect health (fiber, antioxidants, and/or minerals), the optimization of recipes and technological process parameters, as well as giving character to final products in terms of their sensory acceptability and potential functional properties, gained significant interest last few years. Therefore, buckwheat products such as bread, biscuits, snacks, noodles, and cakes are commercialized and increasingly consumed. In addition, the use of non-bakery buckwheat products, such as tea, sprouts, honey, and other products, is becoming more common. In order to obtain potentially functional food with buckwheat of high nutritional quality, it is important to understand the effect of processing on bioactive components. The baking process, inevitable in the production of bakery products, is especially important. It is also important to understand the effect of storage on bioactive components. To this end, in the light of available literature, this chapter will provide an overview of bioactive components in buckwheat and discuss their stability in buckwheat and its products during processing and storage.

1. Introduction

Buckwheat is an annual herbaceous plant that botanically belongs to the order Polygonales, family Polygonaceae, genus Fagopyrum [1], but in terms of processing, similar use, chemical composition, and the seed structure itself, it is similar to cereals. It is therefore often classified as a pseudocereal [2, 3]. In the genus Fagopyrum, 15 species were discovered and described, among which nine have agricultural and nutritional value. However, only two species are most commonly grown: common (Fagopyrum esculentum Moench) and Tartary buckwheat (Fagopyrum Tataricum L.). Common buckwheat is the most common and cultivated species from temperate Europe to Japan, while Tartary buckwheat is grown in some mountainous areas [4].

Buckwheat grains are the main form of consumption of this pseudocereal. Hulled grains are mostly used for human consumption in the form of breakfast cereals or as flour for the production of various bakery products (bread, cakes, snacks) and noodles enriched with buckwheat flour (0.3–60%), buckwheat-improved non-bakery products (tea, honey, and tarhana) [5] and products made of buckwheat husks such as pillows, quilts, mattresses, collars, eye masks, and children’s toys [6]. In addition to flour and groats, buckwheat sprouts are increasingly used to improve bakery products [5, 7].

Since buckwheat is gluten-free, these products can be included in a gluten-free diet for patients with gluten intolerance [8, 9].

The addition of buckwheat into bakery products is of particular importance. This pseudocereal is gaining increasing attention as potentially functional food [4, 10]. Namely, buckwheat is recognized as a good source of nutritious proteins, lipids, dietary fiber and minerals and, in combination with other components that have a positive impact on health such as phenolic components and sterols, it is attracting increasing attention as a functional food. In vitro and in vivo studies have shown that the consumption of buckwheat and products enriched with this pseudocereal is associated with a wide range of biological and health activities: hypocholesterolemic, hypoglycaemic, anticancer, and anti-inflammatory. According to the recent phytochemical and pharmacological researches polysaccharides of buckwheat present also important bioactive components with numerous biological activities [11].

Buckwheat is the only pseudocereal that contains rutin, which has shown anti-inflammatory, anticancer, antiatherogenic, and antioxidant activity [4, 12]. Buckwheat protein extracts are associated with anticancer and cholesterol-lowering effects in animals [13, 14]. Apparently, the incorporation of buckwheat into bread results in significantly lower blood glucose and insulin responses compared to white wheat bread [15]. Buckwheat grain contains very rare D-chiro-inositol, which has been associated with reducing the symptoms of type 2 diabetes mellitus (non-insulin-dependent diabetes mellitus) [16].

2. Bioactive components in buckwheat

2.1 Proteins

Buckwheat proteins have a high biological value thanks to a well-balanced amino acid composition. The protein content of buckwheat is relatively lower than the protein content of legumes. However, the amino acid score of buckwheat protein is 100 and the content of essential amino acids corresponds to the recommended amino acid intake for children and adults [17].

They are rich in lysine, which is the first limiting amino acid of plant proteins, and arginine [18, 19]. However, the content of glutamine and proline is much lower compared to wheat [20], and threonine and methionine are the first and second limiting acids in buckwheat. Furthermore, Giménez-Bastida et al. [5] stated that buckwheat proteins are rich in albumin and globulin and that they are very poor in prolamin and gluten. Therefore, buckwheat flour is suitable for use in the diet of people with celiac disease due to its low non-toxic prolamin content [21]. The protein content in buckwheat flour is significantly higher compared to rice, wheat, corn, millet, and sorghum flour. While it is lower only compared to the protein content of oat flour [19]. Guo et al. [22] pointed out that the average protein content in buckwheat is 12.94%.

The digestibility of buckwheat proteins is about 80%, which is lower compared to proteins of animal origin such as hemoglobin and ovalbumin. However, it is higher than cereal proteins (e.g., sorghum 55–59%; corn 66–75%) and has a value approximate to rice bran (89%) and wheat germ (77–93%). Despite the balanced composition of essential amino acids, the bioavailability of buckwheat protein after digestion is not complete. Relatively low digestibility is attributed to the molecular structure of buckwheat protein and the presence of antinutritive factors in flour and protein isolates [17].

Buckwheat, along with other pseudocereals such as quinoa and amaranth, is recommended for use in creating new value-added bakery products because it can provide high levels of essential amino acids in the human diet [23].

The literature states that buckwheat proteins have many unique physiological functions, such as treating chronic diseases in humans, reducing serum cholesterol, suppressing gallstones and tumors, inhibiting angiotensin I-converting enzyme (ACE), and so on [17, 24, 25]. An ACE inhibitory tripeptide (Gly-Pro-Pro) was isolated and identified from common buckwheat [26]. In humans, buckwheat consumption has also been associated with a lower prevalence of hyperglycemia and improved glucose tolerance in people with diabetes [27]. Since many health benefits of buckwheat are inherently related to peptide radical binding activity from digested proteins, it is hypothesized that buckwheat protein hydrolysis may release peptide fragments capable of stabilizing reactive oxygen kinds and inhibiting lipid oxidation. By in vitro digestion of buckwheat protein six peptide fractions were obtained, whereas LC-MS/MS identified Trp-Pro-Leu, Val-Pro-Trp, and Val-Phe-Pro-Trp (IV), Pro-Trp (V), and tryptophan (VI) as the prominent peptides/amino acid in these fractions [28]. Six peptides DVWY (H-Asp-Val-Trp-Tyr-OH), FDART (H-Phe-Asp-Ala-Arg-Thr-OH), FQ (H-Phe-Gln-OH), VAE (H-Val-Ala-Glu-OH), VVG (H-Val-Val-Gly-OH), and WTFR (H-Trp-Thr-Phe-Arg-OH) identified from buckwheat sprouts fermented with Lactobacillus Plantarum revealed significant blood pressure-lowering effect, thereby the most potent were DVWY, FQ, and VVG [29]. From the seeds of common buckwheat were purified two peptides (Fa-AMP1 and Fa-AMP2) which have antimicrobial properties [30]. Studies have shown that buckwheat protein extracts show anticancer activity, activity of lowering cholesterol, as well as anti-constipation and anti-obesity activities in animals [13, 17, 24]. An unusual antitumor protein TBWSP31 isolated from water-soluble Tartary buckwheat extracts was also examined [31].

2.2 Phenolic components

Many health benefits of buckwheat are attributed to the high content of phenolic components and high antioxidant activity [18]. Whole grain buckwheat was found to contain 2–5 times more phenolic components than barley and oats, while the husk and bran of buckwheat have 2–7 times higher antioxidant activity compared to barley, triticale, and oats [32, 33]. The research by Begić et al. [34] showed that Tartary buckwheat contains about 20 times more total phenol content and that it shows antioxidant activity nine times higher than common buckwheat.

Among the polyphenolic components present in buckwheat, those from the group of flavonoids, and among them rutin, are the most important ones.

2.2.1 Flavonoids

The presence and amount of flavonoids in buckwheat grain make it specific compared to cereals, which contain small amounts of flavonoids. This group of polyphenolic components is the basic antioxidant of buckwheat [35, 36]. Buckwheat is considered to be one of the best dietary sources of rutin [36]. The content and composition of flavonoids are different in different types of buckwheat. In general, the flavonoid content in F. Tataricum (40 mg/g) is higher than in F. esculentum (10 mg/g), reaching concentrations of 100 mg/g in flowers, leaves, and stems [37]. The content of flavonoids in Tartary buckwheat can be up to 7% [38].

Flavonoids demonstrate a protective effect in lipid oxidation in vitro as “scavengers” of free radicals and metal chelators [39]. They generally occur as O-glycosides in which one or more hydroxyl groups are bound to sugars.

Six flavonoids were isolated and identified from whole buckwheat grains: rutin, quercetin, orientin, isoorientin, vitexin, and isovitexin. The presence of rutin and isovitexin was found in hulled grain while buckwheat husk contained all 6 flavonoids [40, 41]. Buckwheat is the only pseudocereal that contains rutin and is, therefore, a useful source of this flavonoid [25]. Except in buckwheat, rutin has not been detected in cereals and pseudocereals [41, 42]. Rutin (quercetin-3-O-β-rutinoside), a secondary metabolite present in buckwheat, is the best-known glycoside derived from flavonol quercetin. Buckwheat is considered the best source of dietary rutin. Buckwheat grains (groats and husk) and sprouts are important sources of rutin and their content depends on the type and conditions of growth [43, 44]. It is important to develop new well-adaptive varieties with a high content of rutin, and improved biological value of the proteins [45].

Tartary buckwheat groats contain more rutin—80.94 mg/g dry matter (DM) than common buckwheat groats—0.20 mg/g DM [46, 47] while Tartary buckwheat sprouts have 2,2 times more rutin than common buckwheat sprouts [48]. Li et al. [49] stated that Tartary buckwheat can contain up to 100 times more rutin than common buckwheat.

Rutin has attracted much attention mainly because of its many health benefits observed in vitro and in vivo: anti-inflammatory, antidiabetic, hypocholesterolemic, antiatherogenic, antiatherosclerotic, and anticancer ones [4, 12, 36, 50, 51, 52, 53] and its activity are related to antioxidant capacity [54]. Furthermore, rutin may be effective in preventing the toxic effects of methotrexate on the kidneys [55].

Rutin has relaxing effects on smooth muscles and is effective in preventing capillary apoplexy and retinal bleeding, lowers high blood pressure and shows antioxidant activity and lipid peroxidation activity. It also has lipid-lowering activity by reducing dietary cholesterol absorption, as well as reducing plasma and liver cholesterol [56, 57]. In addition, possibilities of rutin as a new strategy for the prevention of type 2 diabetes are noted [58]. Alkaline luminol chemiluminescence and electron spin resonance analysis revealed the formation of the rutin-ovalbumin complex which significantly increases the radical-binding activity in rutin. Rutin has also demonstrated antioxidant activity against hydroxyl radicals in a DNA protection test [59].

Quercetin (quercetin-3-ramnoside) is another glycoside present in buckwheat in concentrations ranging from 0.01 to 0.05% DM in Tartary and from 0.54 to 1.80% DM in common buckwheat [46, 60]. Isoquercetin (quercetin-3-glucoside) is present in buckwheat hypocotyl (1.4 μM/g DM) [61] and has been shown to exhibit antidiabetic and anticancer activity [36, 49, 62, 63]. Quercetin, an aglycone of rutin, is present in hulled grain (semolina) of buckwheat (0.001 mg/g DM) and husk (0.009–0.029 mg/g DM) in lower concentrations than rutin [18, 47]. Quercetin is the most studied flavonoid, primarily due to its pronounced antioxidant activity, as well as significant absorption in the digestive tract. It is predominantly in the form of glycoside as rutin (quercetin-3-O-beta-rutinoside). In addition to this, kaempferol-3-O-rutinoside and quercetin 3-O-rutinoside-3’-O-β glucopyranoside have been found in buckwheat seeds [49, 64].

Three flavonoids from Tartary buckwheat bran: quercetin, isoquercetin and rutin were evaluated as R-glucosidase inhibitors (controlling blood glucose) using fluorescence spectroscopy and enzyme kinetics. The R-glucosidase activity was clearly influenced by extractives (mostly rutin) and their hydrolysis products (a mixture of quercetin, isoquercetin, and rutin) from buckwheat bran [65].

Recent research relating to the examination of the antiviral activity of rutin in the treatment of patients with COVID-19 symptoms have been topical [66, 67].

In addition to rutin, catechins, the antioxidant activity of which is higher than the antioxidant activity of rutin, were isolated from ethanol extracts of buckwheat groats. Four catechins were isolated and their structures were determined as: (−)-epicatechin, (+)-catechin-7-O-β-D-glucopyranoside, (−)-epicatechin 3-O-p-hydroxybenzoate, and (−)-epicatechin 3-O-(3,4-di-O-methyl)gallate [68]. The following components from the catechin group were identified in buckwheat: catechin, epicatechin, catechin glucoside (A or B isomers), catechin gallate, epicatechin gallate, epicatechin-O-3,4-dimethylgallate, epiaphzelchin-(4─8)-epicatechin-3,4-O- dimethylgallate, while catechin-3,4-O- dimethylgallate was identified in thermally treated buckwheat and epiaphzelchin- (4─6) -epicatechin was identified in green buckwheat [69].

These ingredients in plant tissue are influenced by numerous environmental factors such as ultraviolet (UV) radiation, harvest time and damage caused by pests, and genetic and aging-related factors. Studies have shown significant positive correlations between the mean altitude of the growth site and the amount of individual phenolic antioxidants [70, 71]. Buckwheat, as a source of rutin, can be successfully grown in Mediterranean conditions, too [72]. Flavon-3-glycosides present in buckwheat (vitexin, isovitexin, orientin, and homoorientin), anthocyanin and proanthocyanin content [61] and the presence of squalene, epicatechin, and vitamin E [73] make buckwheat a good antioxidant source in the human diet.

2.2.2 Phenolic acids

Phenolic acids in buckwheat also contribute to its antioxidant activity. In the grain of different varieties of Tartary buckwheat, p-hydroxybenzoic, ferulic and protocatechuic stand out, and other acids, including p-coumaric, gallic, caffeic, vanillic, and syringic acid, were also detected [74]. Several phenolic acids have been described during the flowering of different varieties of buckwheat: chlorogenic, p-anisic, salicylic, and methoxycinnamic [75].

2.3 Vitamins

Buckwheat is also an important source of vitamins, especially those of the B group. The total content of B vitamins, including B1 (thiamine, 2.2–3.3 μg/g DM), B2 (riboflavin, 10.6 μg/g DM), B3 (niacin, 18 μg/g), B5 (pantothenic acid, 11 μg/g) and B6 (pyridoxine, 1.5 μg/g) is higher in Tartary buckwheat compared to common buckwheat. The levels of vitamin C indicated for it in the literature go as high as 50 μg/g DM while its content, as well as the total amount of vitamins B1 and B6, increases by germination of buckwheat and, consequently, the content of vitamin C in buckwheat sprouts reaches 250 μg/g DM [4, 54, 76, 77]. Vitamin B1 is found in thiamine-binding proteins of buckwheat grains, which, according to Li and Zhang [37] increases the availability of vitamin B1 and improves its stability during storage. The content of vitamin E (tocopherols) in buckwheat is higher compared to wheat, barley, oats, and rye [18]. The most common tocopherol in buckwheat is γ-tocopherol. In addition to γ-tocopherol, α- and δ-tocopherol have also been identified in buckwheat [78]. The concentration of total tocopherols in buckwheat grains ranges from 14.3 to 21.7 mg/kg [79]. Tocopherols, along with the other components mentioned above, make buckwheat a good antioxidant source in the human diet. Tocotrienols were not detected in buckwheat [80, 81], while Piironen et al. [82] identified traces of tocotrienols in whole buckwheat grains. High levels of vitamin E intake are associated with a reduction in cardiovascular disease, a reduction in the risk of Alzheimer’s disease, and an improvement in the immune system [73].

2.4 Isoprenoids

Squalene is an isoprenoid component that has six isoprene units and antioxidant activity and is widely produced in buckwheat plants. Squalene protects cells from radicals, strengthens the immune system, and reduces the risk of various types of cancer. There are some differences between buckwheat species, especially in the content of squalene and rutin [73].

2.5 Phagopyritols

Buckwheat grains contain a very rare D-chiro-inositol, which is mostly found in the form of phagopyritols [18]. Phagopyritols are mono-, di- and trigalactosyl derivatives of D- chiro-inositol and are called phagopyritol B1, B2, and B3. Phagopyritols A1, A2, and A3 have been identified as isomers of phagopyritols B1, B2, and B3 [83, 84, 85]. Phagopyritols are concentrated in aleurone and embryonic grain cells, with phagopyritol B1 (0.392 mg/g DM of integral common buckwheat semolina) being the most abundant. D-chiro-inositol, free form, is present in lower concentrations (0.21–0.42 mg/g DM) [18].

So far, several studies have described the role of D-chiro-inositol and phagopyritol as molecules that exhibit insulin-like activity [18, 86, 87]. In particular, D-chiro-inositol has attracted much interest due to its ability to lower glucose in animal models [88], and an important number of studies have shown that chemically synthesized D-chiro-inositol lowers elevated plasma glucose levels [89, 90]. Although no studies have been conducted to date to explore the effects of D-chiro-inositol and phagopyritol in humans, these components may have positive effects in the treatment of diabetes. In animal models, D-chiro-inositol has been associated with a reduction in the symptoms of type 2 diabetes mellitus [16]. The potential of D-chiro-inositol in the treatment of polycystic ovary syndrome has been extensively investigated [57, 91, 92, 93].

2.6 Phytosterols

Buckwheat is also significant for its phytosterol content. Phytosterols present in buckwheat, although in low concentrations, also show a positive effect in lowering blood cholesterol levels. In addition, phytosterol intake significantly reduces in vivo cholesterol absorption. Buckwheat phytosterols are present throughout the grain, but their content varies by grain parts [37]. The most abundant phytosterol in buckwheat flour is β-sitosterol (0.86 mg/g DM) and makes up about 70% of total sterols, followed by campesterol (0.11 mg/g DM) and stigmasterol (0.02 mg/g DM) [94].

According to the research of Dziedzic et al. [95]. the sterols content in Tartary buckwheat whole grains was 15,398 mg/kg of lipids and the most prevalent was the β-sitosterol (10,944 mg/kg of lipids).

2.7 Iminosugars

D-fagomin is a minor component from the group of iminosugars detected in the dehulled grain of common buckwheat which shows a glucose-lowering effect [96, 97].

Similar to D-fagomin, other imino sugars such as 1-deoxynojirimycin (DNJ) are intestinal glucosidase inhibitors, associated with a reduced risk of developing insulin resistance, gaining weight, and suffering from excess potentially pathogenic bacteria [97, 98]. Anthraquinone emodin is present in buckwheat concentrations between 1.72 and 2.71 mg/kg DM [99]. Due to the wide range of biological activities that emodin exhibits, it can be considered an important bioactive factor in buckwheat [100].

2.8 Other components

Along with vitamins, other components were detected, such as glutathione (1.10 mmol/g DM in buckwheat groats), phytic acid (35–38 mg/g DM in bran), carotenoids (2.10 mg/g DM in grain), and melatonin (470 pg/g DM in groats). These components may contribute to the antioxidant activity of buckwheat [4, 47].

Both types of buckwheat, common and Tartary, have a high tannin content (1.76 and 1.54%). Tannins isolated from buckwheat showed a relatively high level of activity against Listeria monocytogenes [101].

It has also been found that γ-aminobutyric acid (GABA) and 2″-hydroxynicothianamine (2HN) serve as functional components in buckwheat. Grains and sprouts contain GABA, while 2HN was identified in flour. Literature states that these components lower blood pressure in humans and inhibit the activity of angiotensin I-converting enzyme (ACE) [102, 103, 104]. Suzuki et al. [105] quantified GABA and 2HN concentrations in common and Tartary buckwheat leaves 14, 28, and 42 days after sowing (DAS). The concentration of GABA reached a peak at 42 DAS, while the concentration of 2HN decreased with the age of the plant.

3. Stability of bioactive components in buckwheat and its products during processing

It is well known that processing can cause chemical changes in food products. Therefore, it is important to consider the effects on bioactive components in buckwheat. Today, there are several technological processes related to buckwheat, which will be presented below.

3.1 Milling

Milling is one of the technological processes that is inevitable during the processing of buckwheat into flour. During the processing of buckwheat grains into white flour, the husk and outer layers are separated, which lowers the ratio of fibers, minerals, and polyphenolic components.

Hung and Morita [106] explored the possibility of improving the functionality of buckwheat flour by successively milling buckwheat and they found that in 16 different fractions of flour the content of ferulic acid and rutin increases with an increased ratio of outer grain layers. The same authors found that the antiradical activity on DPPH extracts of free and bound polyphenolic components of buckwheat, the fraction of the successive milling of buckwheat is highest for fractions containing external grain parts. Additionally, better antiradical activity on DPPH was registered for extracts of free polyphenolic components compared to extracts of bound polyphenolic components in buckwheat grain.

Inglett et al. [107] examined the antioxidant activity of ethanolic extracts of four types of commercial buckwheat flour and found the highest antiradical activity on DPPH in buckwheat flour containing a high ratio of husk and aleurone layer, while the lowest antiradical activity was registered in white flour consisting exclusively of the endosperm. The highest content of total polyphenolic components and total flavonoids was registered in whole buckwheat flour. Gallardo et al. [108] established that the content of rutin in buckwheat flour is 0.7 mg/100 g and 11.2 mg/100 g in buckwheat husk.

A recent study found that the buckwheat protein contents decreased from the exterior to the interior parts of the groats [109]. Significantly higher content of amino acids, fatty acids, polyphenols, and flavonoids was found in the bran of Tartary buckwheat, compared to the flour [110].

It should be noted that the milling conditions should be adapted to the type of buckwheat. The granulation composition of common and Tartary buckwheat flour differed under the same milling conditions and affected the physical characteristics of the obtained flour fractions. Tartary buckwheat flour contained larger fractions compared to common buckwheat flour under the same milling conditions [111]. By adjusting the grinding and knowing the content of different components in the fractions of Tartary buckwheat, it is possible to obtain products of different nutritional value [45, 111].

3.2 Heat treatment

The number of studies researching the effects of heat treatment on buckwheat foods has increased significantly. Today, many new thermal techniques are used in the food industry to improve the quality of functional buckwheat food. The extrusion process has become important in the production of pasta, ready-to-eat cereals, snacks, animal feeds, and textured plant proteins. Microwave heating has gained popularity in food processing due to the ability of this technique to achieve high heating rates, significantly reduce cooking time, provide more uniform heating and safe handling. This technique could change the taste and nutritional properties of food to a lesser extent, as opposed to conventional heating during the cooking process [112]. However, data on the effects of heat treatments on the antioxidant capacity of buckwheat and its products are still limited. In general, most studies are aimed at determining the effect of heat treatment on the content of total phenols and flavonoids due to their role in the management of human health and diseases.

It was established that the heat treatment of buckwheat causes changes in its chemical composition and, above all, that it affects the functional properties of the selected bioactive components. The results published so far on the effects of the heat treatment on buckwheat grain and processed flour are contradictory.

One of the first studies was conducted by Dietrych-Szostak and Oleszek [40] who examined the effect of heat processing on flavonoid content in hulled grains and buckwheat husks by removing the husk using heat. Removing the husk from buckwheat grain by applying heat treatment resulted in a product that was both visually and chemically different. The peeling process removed primarily the multitude of tannins and crude fibers that are naturally present in the husk. As for the concentration of total flavonoids, dehulling process with different temperature treatments caused a drastic reduction of the total flavonoid concentration in the grain (by 75% of the control) and smaller but significant (15–20%) reduction in the hulls.

Kreft et al. [12] compared the content of rutin in buckwheat products with its content in the raw materials used to obtain these products. Noodles prepared with 70% of integral buckwheat flour contained much less rutin (78 mg/kg DM) compared to the integral buckwheat flour (218 mg/kg DM) out of which they were produced. As a possible explanation for this reduction in rutin content in the product, the authors cited the presence and activity of an enzyme that degrades rutin, flavonol 3-glucosidase, during dough mixing. The presence of this enzyme in buckwheat was confirmed by Suzuki et al. [113]. In raw (uncooked) hulled buckwheat grain (raw buckwheat semolina) the rutin content was 230 mg/kg DM, while in pre-cooked hulled buckwheat grain its content was 88 mg/kg DM. The aforementioned authors explained the established reduction of rutin content in hulled buckwheat grain due to hydrothermal treatment by possible degradation of rutin molecules or its combination with some other molecules, in such a way that it becomes insoluble in the applied solvent. A similar reduction in rutin content was observed during bread production in different combinations of wheat and Tartary buckwheat flour (100:0; 70:30; 50:50; and 0:100) where the effect of making bread and baking on the content of rutin, quercetin, and polyphenols and the antioxidant activity of said loaves was examined. After baking, rutin (0.47 mg/g) was present in bread which is made of 100% Tartary buckwheat flour, together with quercetin (4.83 mg/g). The dough that this bread was made of contained a lower concentration of rutin and greater concentration of quercetin compared to flour used to prepare it; wherein 0.0175 mmol of rutin degraded with the addition of water and yeast to Tartary buckwheat flour, and 0.0149 mmol of quercetin was obtained at the same time. This indicates that 85% of the rutin was converted to quercetin by adding water and yeast to the flour [114].

Degradation of rutin can be the result of activities of the rutin-degrading enzyme found in buckwheat. This enzyme is stable and active at pH 5–7 and below 40°C. Based on the comparison of the level of concentration, it appears that quercetin is more stable compared to rutin in the process of proofing and baking bread. There were no significant differences in the content of rutin and quercetin between the bread crumb and crust. Additionally, the results showed a reduction in the total polyphenol content in all samples of bread as a result of the heat treatment in the baking process [112].

The obtained results were consistent with the results of the authors Alvarez-Jubete et al. [21] which showed a significant decrease in the concentration of total polyphenols, particularly phenolic acids in bread made of common buckwheat flour (0.65 mg GAE/g) compared to the concentrations of these components in buckwheat grain (3.23 mg GAE/g). During the process of mixing and proofing the bread, there was a modest increase in the concentration of total polyphenols in bread samples made with 100% wheat flour and 100% Tartary buckwheat flour, and a slight reduction in the other samples, containing a combination of both kinds of flour (70:30 and 50:50).

Reductions in polyphenol content and antioxidant activity were also reported during baking of bread samples prepared in different combinations (90:10; 80:20; and 70:30) of rice and buckwheat flour (wholemeal and white) relative to their content in flours. It was noticed that the baking process resulted in a higher percentage of reduction in total polyphenols in bread samples made white buckwheat flour, while only minor or insignificant changes were observed in lower percentages (10 and 20%) in samples with wholemeal buckwheat flour, and only the sample with 30% of wholemeal buckwheat flour had a decrease of about 17%. In addition, the decrease in antioxidant activity was more pronounced in bread samples prepared with white buckwheat flour. During baking, there was a loss in rutin content in bread samples relative to its assumed (calculated) content, and this loss increased with increasing the proportion of buckwheat flour, both types (wholemeal and white), in the range of 4.57–40.4%. The opposite trend was observed in the quercetin content, which increased from 1.5 to 7 times, probably due to the hydrolysis of rutin into quercetin [115].

Similar results were shown in bread samples produced with the addition of buckwheat in the amount of 15 g/100 g and 30 g/100 g. A decrease in the content of total phenols, total flavonoids, and antioxidant activity in bread samples relative to their content in flour was found. The content of total flavonoids in bread samples was 2 to 4 times lower compared to its content in flour [116].

The thermal treatment of Tartary buckwheat bran and flour significantly reduced the content of fatty acids, polysaccharides, and polyphenols. As for the content of amino acids and total flavonoids, their content in bran after heat treatment decreased, while increased in the flour of Tartary buckwheat [110].

In addition to bread and cakes, a decrease in the content of bioactive components was observed in the production and cooking of other products with the addition of buckwheat such as spaghetti, pasta, noodles, etc. A decrease in the content of free (about 74.5%) and bound (about 80%) phenolic components “farm to table”, i.e., from flour to cooked spaghetti with buckwheat was found. Regarding the content of total phenolic components, the spaghetti production process (mixing, extrusion, and drying) caused a loss of 45.9%, which the authors explain by the increase in temperature during the extrusion process and the high temperature (about 95°C) reached during drying. Further degradation of phenolic components was found after cooking spaghetti. The boiling process caused the degradation of 52.9% of the total phenolic components. This degradation was significantly different (p < 0.05) compared to post-production degradation. This effect can be attributed to the solubility of phenolic components in boiled water. Of the total phenolic components that were present in the spaghetti after the drying process, 11.6% were dissolved in water after cooking [117].

Biney and Beta [118] also reported that the production and cooking process led to a reduction in phenol content and antioxidant activity in spaghetti enriched with buckwheat flour and bran. The production process did not cause statistically (p < 0.05) significant changes in the content of total phenols between flour mixtures and uncooked products. However, cooking significantly reduced total phenols in all spaghetti samples. Although the addition of buckwheat flour resulted in a significantly higher content of these components in all spaghetti samples, the average percentage of decrease in total cooking phenols due to cooking was higher in samples containing buckwheat flour or bran, compared to control samples prepared from semolina. The production and cooking process also led to a significant reduction in the content of rutin and total flavonoids in spaghetti samples. The higher the proportion of buckwheat in the spaghetti formulation, the greater the losses in rutin content.

The results of similar research (pasta enriched with buckwheat flour in the amount of 20%) showed a reduction of about 44% of total phenolic components after cooking compared to their content in pasta after drying; 8.37% of total phenols from dried pasta was present in the water in which it was boiled, and 35.63% was degraded. The cooking process reduced the rutin content by about 8.50%. During cooking, rutin was converted from its bound form to quercetin, which is shown in the increase of quercetin content by about 20%. The results also showed that catechin showed a minimum tolerance to the cooking process, with a loss of about 57% [119].

Furthermore, the autoclaving of buckwheat grains caused a decrease in free and an increase in bound phenolic forms in flour. Similarly, this was found in noodles produced by adding this flour to the formulation with wheat flour, compared to the content of these components in noodles produced in the same way with flour obtained from untreated buckwheat grains. Although autoclaving caused an initial reduction in rutin in treated grain flours, it prevented further degradation and conversion of rutin to quercetin in uncooked and cooked samples obtained from these flours, causing a possible improvement in the sensory acceptability of noodles. The loss of phenolic components in noodle samples with added buckwheat flour during cooking (48.1–61.1%) was at the same level as in the control sample with wheat flour only, indicating that buckwheat-containing pasta can maintain the quality during cooking [120].

Cho and Lee [121] examined the thermal stability of rutin in wheat instant fried noodles fortified with rutin-enriched material (REM) from buckwheat milling fractions. The noodles were fried at different temperatures (150, 170, and 190°C) during different periods of time (1, 2, and 3 minutes). Also, noodles were placed in boiling water at different periods (0, 3, and 6 minutes) to examine the effect of cooking on rutin content. The results showed that different temperatures and frying times did not negatively affect the rutin content, while a marked loss of rutin was observed after cooking the noodles.

However, the results of another study showed a reduction in bioactive components in buckwheat products during various heat treatment processes and reported an increase in the total antioxidant activity in buckwheat sprouts and shoots after autoclaving treatment. Furthermore, an increase of 20% and a reduction of 7% of total phenols were observed in buckwheat sprouts and shoot respectively [122].

Contradictory results were also found by Zieliński et al. [123] during extrusion of buckwheat, which showed a decrease in antioxidant capacity accompanied by a reduction in rutin and isovitexin, but at the same time an increase in free phenolic acids and those freed from ester bonds. The authors stated that the reported increase in phenolic acids could be due to the increased release of these bioactive components from the matrix, making them available for extraction. The same authors report that, although the extrusion caused a marked reduction in antioxidant content in hulled buckwheat grain, the amount of bioactive component in hulled buckwheat grain after thermal treatment was still significant, resulting in a decrease in antioxidant activity of only 10%.

Hes et al. [124] also reported contradictory results when testing the impact of cooking in water on the antioxidant properties and dietary fiber of hulled buckwheat grain. It was shown that cooking in water for 30 minutes in a ratio of 2:1 (water: grain) has no negative effects on the nutritional characteristics of the hulled buckwheat grain. Extracts of cooked hulled grain showed a significantly higher content of polyphenols and total dietary fiber compared to raw grain. The detected higher content of polyphenols in cooked hulled grain is explained by the authors as a result of their partial release from the bound form of the protein as a result of cooking. Additionally, phenols can also be associated with other components such as carbohydrates. In terms of individual phenolic components, a significantly higher content of catechin particularly stands out, and, in contrast to that, a considerably lower content of p-coumaric acid in the extracts of cooked buckwheat grain compared to the extracts of the raw buckwheat grain. Cooking did not cause any changes in rutin content.

It has been recognized that the possible beneficial effects of phytochemicals present in buckwheat may be related to the inherent antioxidant capacity of these components. Therefore, during the last decade, the relationship between antioxidant capacity and these components after heat treatment has been exposed. The antioxidant capacity of buckwheat products is linked to flavonoid concentrations after hydrothermal treatment [125]. Kreft et al. [12] described significant correlations between rutin content and antioxidant activity of buckwheat grain and buckwheat food products. Chlopicka et al. [116] found positive and significant correlations between total phenols and antioxidant activity in buckwheat bread samples, as well as between total phenols and antioxidant activity of buckwheat bread samples, and, finally, between antioxidant activities themselves. Zhang et al. [126] reported that the baking, heating under steam pressure, and microwave heating of integral buckwheat flour had a statistically significant (P < 0.05) effect on the decrease in total flavonoids and antioxidant activity of flour, while the decrease in total phenols in buckwheat flour was less pronounced for all three applied treatments. As a possible explanation, the authors cited the creation of Maillard reaction products, which react with Folin-Ciocalteu reagent, resulting in masking the actual decrease in polyphenol content.

Similar conclusions regarding the formation of Maillard reaction products were reached by the authors Constantini et al. [127] during the production of bread with the addition of Tartary buckwheat flour, where a loss in the total antioxidant capacity and content of total polyphenols and flavonoids was observed, relative to their values in flour mixtures. The aforementioned authors pointed out that it is possible that the real reduction is greater than what was found in this study. As an explanation, they stated that heat treatment of cereals and pseudocereals, such as during baking, can also result in the synthesis of substances with antioxidant properties, including certain products of the Maillard reaction that occur in the crust of bread. These syntheses can mask the actual decrease in the content of total phenols and flavonoids (which are able to react with Folin-Ciocalteu reagent), as well as any loss in total antioxidant capacity.

Aside from phenols, other components, such as proteins, appear to be involved in the formation of the antioxidant activity of buckwheat products. The frying hulled buckwheat grain, in addition to reducing antioxidant activity, also resulted in a decrease in protein content and quality, while heat treatment did not show an effect on whole grain proteins [125]. In addition, during thermal treatment, Maillard components are generated due to a chemical reaction between the free amino groups of lysine and the carbonyl groups of reducing sugars [128]. It was observed the formation of Maillard products was caused by heat treatment of both whole and hulled buckwheat grains. Although Maillard components may be harmful to health, they may contribute to an increase in antioxidant activity, masking the actual decrease in total phenolic components, as highlighted in the above studies [125, 126, 127]. In addition, it has been suggested that antioxidant capacity may increase as a result of dissociation (separation) of phenolic forms and release of phenols bound to cell walls due to heat treatment followed by polymerization/oxidation of phenolic constituents or by-product generation [122].

The influence of baking on the content of tocopherols in buckwheat bread was investigated. Vitamin E loss was found to be about 30%. Smaller losses were observed in bread samples of 100% buckwheat flour compared to samples in which the share of buckwheat flour was 50% [81]. A significant reduction in vitamin E content (about 63%) in buckwheat was also found during the extrusion process [123].

The importance of common and Tartary buckwheat is generally recognized. However, one should also keep in mind some disadvantages of their application in the bakery in terms of sensory impression. This primarily refers to the particle size and the proportion of bran that can negatively affect the rheological properties of the dough and result in an inappropriate texture of bakery products. In addition, the finished products with Tatary buckwheat may appear a slightly bitter taste [6].

Based on all of the above, it is indicative that the contradictory results obtained so far greatly emphasize the importance of determining the exact composition and ratio of bioactive components. In addition, more studies are needed to identify the effect of heat treatment on the functional components, including proteins and phenolic components, of buckwheat products, in order to ultimately obtain buckwheat of consumption quality. Therefore, processing conditions, such as time and temperature, need to be optimized to preserve the functionality of bioactive components.

3.3 High pressure

High pressure has been shown to be a viable alternative to heat treatment, with no adverse effects such as forming an off flavor, loss of vitamins and phytochemical properties, and discoloration [129]. The effect of high hydrostatic pressure treatment (200 MPa at 2, 4, and 9 minutes) on total antioxidant capacity (TAC), reducing capacity (RC), and rutin content of raw and roasted buckwheat groats were examined. After high-pressure treatment, the content of TAC and rutin differed in the case of raw and fried semolina. The TAC of raw and fried semolina subjected to high-pressure treatment was 16–20% and 12.5–17% lower, respectively, compared to the TAC of untreated semolina. Hydrophilic antioxidants were the main components contributing to the TAC of raw and fried semolina subjected to high-pressure treatment. RC decreased in the case of raw buckwheat (raw semolina), while the rutin content dropped in a shorter time compared to fried semolina. In contrast, overpressure in fried semolina increased the RC formed by hydrophilic antioxidants by 18% when measured by cyclic voltammetry on average and decreased the concentration of rutin after treatment [130].

The results of Zhou et al. [131] suggested that treatment under high pressure at 45 °C improves the nutritional properties of buckwheat compared to untreated and treated under high pressure at room temperature.

3.4 Ionization and radiation

Radiation is a method of treating food to make it safer to eat and to extend its shelf life. Traditionally, this process is used to control surface microorganisms on vegetables and fruits without affecting nutritional quality. Hayashi et al. [132] reduced the microbial load to a lower level by exposing buckwheat grains to soft-electrons without affecting their quality. Chun and Song [133] conducted a study in which aqueous chlorine dioxide, fumaric acid, modified packaging atmosphere enriched with CO₂, and ultraviolet radiation (UV) were combined in the treatment of buckwheat sprouts to improve microbiological quality. A decrease in total aerobic bacteria, yeasts and moulds, and enterobacteria to low levels was observed without affecting sensory quality. However, after the treatment, there was an increase in the concentration of rutin. A comparative study by Orsák et al. [134] studied the effects of UV, microwave, and γ-radiation on three buckwheat samples. Different effects were observed depending on the radiation system and the applied dose on the content of polyphenols and rutin. In addition, it has been described that the content of rutin and flavone C-glycosides is improved in sprouts after exposure to LED (light-emitting diodes) [135].

Therefore, radiation could be offered as a way to increase the half-life of food, maintain sensory quality, improve microbiological quality and increase nutritional value due to bioactive components in buckwheat products. Although public knowledge about radiation remains limited, interest in buying “safe—radiation-enhanced food” is increasing, especially after obtaining information about the potential benefits and risks.

4. Stability of bioactive components in buckwheat and its products during storage

A detailed review of the literature showed that the data on the stability of the most important bioactive components in all types of buckwheat products (bakery and others) during storage is quite limited. Unlike stability during processing, which can still be stated to have been the subject of a significant number of studies in recent years, and that the number of studies is constantly increasing, stability during storage is still almost unexplored. One of the studies that could be related to a certain extent to the mentioned topic is the one conducted by the authors Cho and Lee [121].

Namely, these authors established experimental extraction procedures for preparing rutin-enriched material (REM) from buckwheat milling fractions. Then the REM was used for the fortification of wheat instant fried noodles with rutin. After frying, the noodle samples were stored for 14 days at 60°C and its peroxide value was measured every other day in order to examine the effect of REM on the oxidative stability of the noodles. The monitoring showed that the peroxide number of noodle samples tended to increase with increasing storage time. However, it was noted that the rate of increase in peroxide value was markedly lower in noodle samples with incorporated REM compared to the control sample which did not contain REM. This indicates that the oxidizing improvement of instant fried noodles in storage was reduced using REM. These results can be expected from the high level of rutin in REM, since rutin has strong antioxidant activity [32].

Tabaković et al. [136] found that the most suitable way to store buckwheat seeds for a long period is paper material in order to retain their physiological and morphological properties to the greatest extent.

5. Conclusion

Buckwheat, a pseudocereal belonging to the Polygonaceae family, is used as an important raw material for the development of functional foods due to its functionality and content of components such as proteins, flavonoids, phytosterols, phagopyritols, phenolic acids, vitamins, and others. Scientific research confirms many biological and health properties of buckwheat: hypocholesterolemic, hypoglycaemic, anticancer, and anti-inflammatory activity. The increase in the amount of data focused on the positive nutritional and functional characteristics of buckwheat, which affect the prevention and treatment of chronic diseases, reduction of plasma and liver cholesterol, etc., results in an increase in the number of newly created buckwheat-based food products and their increased use in the population’s diet. In the category of bakery products, the following ones stand out: buckwheat-enhanced bread, wheat bread enhanced with fermented buckwheat sourdough, buckwheat biscuits and snacks, and many others. However, in order for these products to be of high nutritional quality, it is important to understand the effect of processing on bioactive components. And it is also important to know the effect of storage on bioactive components.

When it comes to processing, there are several processes associated with buckwheat, the first and inevitable one being the milling process. Milling is the processing (conversion) of buckwheat grains into flour, the primary raw material for the production of bakery products. During the processing of buckwheat grains into white flour, the husk and outer layers are separated, which removes the ratio of fibers, minerals, and polyphenolic components. Then there is the process of heat treatment, and baking, which belongs to the category of heat treatment, is also inevitable during the production of bakery products. In general, when it comes to the effects of heat treatment on bioactive components in buckwheat, a considerable amount of research has been conducted so far. However, the results of the conducted studies are contradictory. Although in most cases heat treatment resulted in a decrease in bioactive components, on the other hand, there are studies in which heat treatment did not have a significant effect on bioactive components. Therefore, it is necessary to conduct more studies with an emphasis on optimizing the conditions of heat treatment in order to ultimately obtain buckwheat and buckwheat-based products of appropriate quality for use. Recently, high-pressure treatments and the application of radiation are becoming increasingly important. However, although knowledge about radiation remains limited, there is an increased interest in buying “safe—radiation-enhanced food”, especially after obtaining information about the potential benefits and risks.

Although a considerable number of studies have been conducted on the effects of processing, and primarily on the effects of heat treatment, there are still significant gaps in this area. They are primarily related to the fact that most of the conducted studies are aimed at determining the effects of processing on the content of phenolic components, especially flavonoids, due to their recognized role in health benefits. Therefore more research that focuses on the effects of processing on all other bioactive components in buckwheat is needed in the future.

When it comes to the effects of storage, this topic is still almost unexplored. A review of the literature revealed that there is a small number of studies dealing with this topic, and therefore further research is needed to identify the effect of storage on bioactive components of buckwheat products, and ultimately to preserve their quality for as long as possible.