**1. Introduction**

Buckwheat is a pseudocereal of renewed interest that gained popularity as it often provides a distinguished nutritional and health-promoting value [1,2]. The genus *Fagopyrum* includes three species (*F. esculentum*, *F. tataricum,* and *F. cymosum*), but *F. esculentum* Moench is widely cultivated for human consumption [3,4]. Buckwheat seeds flour is considered to be a remarkable resource for functional food development and was used as an ingredient in bread, pasta, cake, and pancake [5–8] due to its nutritional quality and bioactive compounds. The major nutritional components in buckwheat seeds are represented by the carbohydrates that vary between 63.1 and 82.1%, of which from 54.5% to 54.7% was found to be starch [9] with a relatively high level of amylose (18.3–47% of total starch) [10]. Moreover, buckwheat has high levels of resistant starch (27–33.5%) which provides health benefits [11]. Buckwheat starch is concentrated in the endosperm, similarly to common cereals, while protein and lipids are located in the embryo that extends through the starchy endosperm [12]. Buckwheat grains represent an excellent source of dietary fiber (17.8%) with a low ratio of soluble–insoluble dietary fiber (0.5–0.28) [13]. The protein content of buckwheat ranges from 5.7% to 14.2%, depending on the variety and environmental conditions [11]. Buckwheat protein is characterized by a high biological value due to its excellent amino acids balance [14] and it is close to the optimum composition suggested by FAO/WHO [15], making buckwheat an essential contributor to human protein intake. This gluten-free high-quality protein is rich, especially in lysine, methionine, and tryptophan,

**Citation:** Cot,ovanu, I.; Mironeasa, S. Buckwheat Seeds: Impact of Milling Fractions and Addition Level on Wheat Bread Dough Rheology. *Appl. Sci.* **2021**, *11*, 1731. https://doi.org/ 10.3390/app11041731

Academic Editor: Mike Boland

Received: 31 January 2021 Accepted: 12 February 2021 Published: 15 February 2021

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which are limiting amino acids in most cereals [14,16]. Lipid content from buckwheat (0.75–7.4%) is higher than that of wheat, being characterized by a high degree of unsaturation, a fact preferable from a nutritional point of view [2,14]. The main mineral compounds in buckwheat are potassium, magnesium, and phosphorous followed by calcium and other important minerals [2,17,18], showing variability among genotypes. Buckwheat grains contain a higher content of vitamins, especially the group of B vitamins, and buckwheat flour was highlighted by the presence of vitamins B2, B3 and B6 [1,19]. In addition to being a source of nutrients components, buckwheat contains non-nutrient components that present health benefits recognized as bioactive compounds. One group of bioactive components include the phenolic compounds that are of great interest, considering the design and the development of functional food products. Due to the high levels of phenolic compounds, buckwheat can be considered an excellent source of phenols [20,21]. The supplementation of common cereals like wheat with buckwheat possessing better nutritive value and nutraceutical characteristics would be a feasible strategy to improve the features of final products [22] and dietary diversification. In the literature, most of the study focuses on the levels of buckwheat flours added in wheat flour because the addition in high amounts led to various technological challenges related to the fiber content and due to the gluten dilution effect. As a gluten-free protein, buckwheat has negative effects on dough due to the absence of gluten proteins that form the structure, resulting in poor dough strength. To minimize such an undesirable effect, an appropriate level of buckwheat needs to be used to supplement wheat flour.

Nowadays, it is a challenge to produce flour at different particle sizes, because milling fractions varying in physical and chemical properties, and thus offering the possibility to produce flours with specific features for diverse use products [23]. The flour separated on different granularities with high concentrations of valuable components of interest such as proteins, soluble dietary fiber and polyphenols could be used to obtain bakery products with desired features [24]. The functionality of buckwheat milling fractions is essential. Functional properties describe the behavior of components during the technological process, and how they affect the final products in terms of rheological and sensorial properties [25]. Very few studies regarding the functionality of buckwheat milling fractions have been published [22,26], without studying their effects on wheat bread dough. Flour particle sizes influence dough behavior with an impact on the quality attributes of the finite products. In some previous investigations [27–29] the impact of the chemical composition and functionality of different particle size and their effect on rheological properties was highlighted, but there was no evident tendency.

Knowing the rheological behavior of composite flour is necessary for designing and developing new baked products because, in this study, the dilution of wheat gluten when buckwheat flour is added in the wheat flour dough, can impair proper dough rheological characteristics. In order to obtain information on the rheological characteristics, the Mixolab technique can be taken into account since, in only one test, it can predict the quality of the finite product, simulating dough behavior in the bread-making process. The Mixolab technique has been used to assess the impact of different milling fractions from whole buckwheat grains on the rheological properties [30], as well as the effect of amaranth and quinoa milling fractions on wheat flour dough rheology [31,32].

Response surface methodology (RSM) is recognized as a useful statistical tool that provides more information for a few numbers of experiments that require assessing the factors that influence the process and their interaction that implies mathematical models and needs small time resources [33]. In order to optimize the various composite flour formulation, multiple-response optimization, in conjunction with the desirability function, has been successfully applied [34,35].

An optimal amount of buckwheat flour that could be added in wheat flour has tried to be established by various researches, but, to the best of our knowledge, the impact of particle size of buckwheat flour on wheat flour dough rheology, assessed by applying the Mixolab technique, is yet unknown. Moreover, to our knowledge, the effect of the interaction between particle size and the level of buckwheat flour on wheat dough rheology to find the optimal values that give the best technological parameters has not yet been studied. The results will provide millers and manufacturers with scientific evidence of an optimal buckwheat flour particle size range and of an optimal supplementation level of refined wheat flour to improve the quality of baked products.

The aim of this study was to investigate the impact of buckwheat milling fractions on the chemical composition and functional properties, and to evaluate the effect of wheat flour substitution level by buckwheat flour at 0, 5, 10, 15, and 20% at three different milling fractions (large, medium, and small) on thermo-mechanical characteristics and to find the optimal particle size and level added, which inform on the bread-making potential of buckwheat—wheat composite flour.

#### **2. Materials and Methods**

#### *2.1. Raw Materials*

Commercial refined wheat flour of 650 type that has been used to formulate buckwheatwheat composite flour was acquired from a local company S.C. Mopan S.A. (Suceava, Romania). Buckwheat seeds (*Fagopyrum esculentum* Moench) were purchased from a local market (S.C. SANOVITA S.R.L) and they were grinded with a laboratory machinery (Grain Mill, KitchenAid, Model 5KGM, Italy) to obtain buckwheat flour and sieved through a Retsch Vibratory Sieve Shaker AS 200 basic (Haan, Germany) to obtain three milling fractions: large (L > 300 μm), medium (M > 180 μm, < 300 μm) and small fractions (S < 180 μm).
