Influence of Buckwheat Seed Fractions on Dough and Baking Performance of Wheat Bread

Abstract

The study was conducted to determine the influence of buckwheat fractions (BF) on the physicochemical characteristics of wheat flour (WF), dough rheology, and bread quality parameters. Buckwheat seeds’ fractionation influenced the microstructure and molecular conformation depending on the particle size (PS). The protein content of the WF–BF improved when the medium PS was added and decreased for large and small PS. Lipids and ash increased with the increase in BF amount in all samples in comparison with the control. Dough tenacity increased with BF addition, being higher than in WF dough only when large PS were added, while samples with medium and small PS presented a lower tenacity in comparison with the control. Dough extensibility decreased significantly in all samples when BF increased, as follows: M ˃ S ˃ L. Dough viscoelastic moduli increased proportionally when adding large PS, while the addition of medium PS (5–15%) and small PS decreased it. Bread firmness, springiness, and gumminess rose proportionally with the addition level. Bread volume decreased when BF increased, and medium PS had a good influence on this parameter. Bread porosity and elasticity presented higher values than for the control bread, but these decreased when the BF amount increased. Flour and bread crust and crumb color parameters were also influenced by different fractions of BF addition.

1. Introduction

The development of a nutrition culture increases the consumption of healthy foods, which leads to continuous research to discover new innovative ingredients that can confer functional properties. It is well known that bread is one of the most consumed foods, but the refined wheat flour from which it is usually made is very poor in nutrients. That is why many studies focus on the nutritional improvement of this product by incorporating various raw materials, with a positive impact on consumer health. One of these is a pseudocereal, buckwheat, that has recently attracted attention as a novel material in functional food formulations because of its outstanding health properties [1,2]. Buckwheat (Fagopyrum esculentum Moench), a dicotyledonous crop of the Polygonaceae family, is widely distributed in China, Russia, South Korea, Japan, Europe, and other regions [3,4] due to its superior chemical composition compared to that of wheat varieties [5,6,7]. Buckwheat is one of the potential food ingredients for the functional food industry. A growing trend for consumer demand highlights the need for the development of composite flour-based bakery products such as bread. Buckwheat flour is recognized as a good source of nutritionally valuable protein, lipid, nonfibrous carbohydrates, dietary fiber, vitamins, and minerals, as compared with other cereals such as rice and wheat [8]. The protein of buckwheat flour consists of 2S albumin and 8S and 13S globulin but is very poor in prolamin, glutelin, and has very little or no gluten [9]. Buckwheat is a source of high-quality protein because it contains many essential amino acids such as lysine, histidine, valine, and leucine [10,11,12]. The kinetics of protein cross-linking depends on their structure and the availability of lysine and glutamine residues, so to achieve a good result it is necessary to provide a substrate rich in these amino acids. Buckwheat flour fulfills this requirement [13,14,15]. 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 [6]. Buckwheat has been reported to possess higher amounts of potassium, magnesium, and phosphorous followed by calcium and other important minerals [4,16,17], and a group of B vitamins (B2, B3, and B6) [3,18]. Therefore, buckwheat may have a beneficial effect on various metabolic disorders such as diabetes, coeliac disease, and so on that are closely associated with the gluten-free diet. It should be highlighted that buckwheat is one of the best pseudocereal sources of phenolic compounds [19], which present antioxidant activity higher than other cereals, mainly due to rutin, quercetin, ferulic acid, gallic acid, and p-coumaric acid content [20,21]. Incorporating buckwheat flour into bread has the advantage of maintaining antioxidant capacity after thermal treatments [22]. Buckwheat flour 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 [23]. Moreover, there is an important role when incorporating the mixing of these flours for the type of sieving and the size of the particles. The particle size of the flour greatly influences the chemical composition and morphology of the flour obtained, as well as the rheological properties of the dough and the physical, textural, and bread-keeping properties, offering the possibility to produce flours with specific features for diverse use products [24]. For the right preparation and storage of bakery products, it is important to know the rheological, thermal, and other functional properties of dough and bread.

Dough rheology provides information about its processability and could be affected by the incorporation of buckwheat flour in wheat flour. Viscoelasticity is an essential physical property in products such as bread since this must have the capacity to form an ideal structure that retains gas inside the bubbles formed during the kneading and baking processes. In order to provide information on the importance of pseudocereal particle size and its addition level in refined wheat flour, other studies have previously been conducted for amaranth and quinoa. [25,26]. A complete evaluation of incorporating buckwheat flour fractions at different addition levels is necessary because there is still a lack of research on the effect of this composite flour on dough rheology and baking performance, about the microstructure, molecular conformation, and physicochemical characteristics of the raw materials.

2. Materials and Methods

2.1. Materials

Wheat flour (WF) with an extraction rate of 65% (Type 650) acquired from S.C. MOPAN S.A. (Suceava, Romania), buckwheat seeds (S.C. SANOVITA S.R.L., Valcea, Romania) and fresh Saccharomyces cerevisiae yeast (S.C. ROMPAK, Pascani, Romania) were purchased from the local market. WF was analyzed according to the International Association for Cereal Chemistry (ICC) methods (110/1, 105/2, 136, 104/1, 107/1) [27] for the following analytical characteristics: moisture (14.08%), protein (12.45%), fat (1.41%), ash (0.69%), and Falling number index (312 s), while Romanian method (SR 90:2007) [28] was used to determine wet gluten (30.00%) and gluten deformation index (6.00 mm). Buckwheat grains’ analytical characteristics included 13.28% moisture, 13.26% protein, 2.00% ash, and 3.40% fat [23].

2.2. Milling of Buckwheat Kernels

Cleaned buckwheat kernels were ground with a laboratory mill (Grain Mill, KitchenAid, Whirlpool Corporation, Benton Harbor, MI, USA), then sifted by a sieve shaking machine Retsch Vibratory AS 200 basic (Haan, Germany). Three different buckwheat flour (BF) particle sizes: large (L > 300 µm), medium (M > 180 µm, <300 µm), and small fractions (S < 180 µm) were obtained and used in study. The proximate composition of these three BF fractions was determined and reported in a previous work [23].

2.3. Sample’s Formulations

Buckwheat flours at each particle size (L, M, and S) were added at 5, 10, 15, and 20% in refined wheat flour and mixed for 30 min in a Yucebas Y21 mixer (Izmir, Turkey) in order to obtain the following coded samples: BL_5, BL_10, BL_15, BL_20, BM_5, BM_10, BM_15, BM_20, BS_5, BS_10, BS_15, and BS_20. The flour with no buckwheat flour added was considered the control sample.

2.4. Physicochemical Characterization of the Formulated Flours

The routine composition determination of WF–BF was carried out according to International Association for Cereal Chemistry (ICC) methods [27]: moisture content (110/1), protein content (105/2), fat content (136), ash content (104/1), and carbohydrate content, which were calculated by difference, as % of the total weight. The color parameters of the samples were recorded by a colorimeter CR-400 (Konica Minolta, Osaka, Japan) using the color scale characteristics: L*—lightness/darkness (0 = black/100 = white), a*—intensity of green (−a* = more green) or red (+a* = more red), and b*—the intensity of blue (−b* = more blue) or yellow (+b* = more yellow). Three replicate measurements were performed.

2.5. Dough and Bread Manufacturing

WF or formulated composite flours (0.3 kg), salt (1.8%), and yeast (3%) were used in the bread manufacturing process. The flours were previously tested on the Mixolab device for the water absorption capacities (BL_5 = 57.60%, BL_10 = 57.80%, BL_15 = 57.70%, BL_20 = 5 8.90%, BM_5 = 58.40%, BM_10 = 58.30%, BM_15 = 59.20%, BM_20 = 57.80%, BS_5 = 58.70%, BS_10 = 58.30%, BS_15 = 57.90%, and BS_20 = 57.30%). Bread samples were prepared following the biphasic method by mixing all amount of water and yeast, and half amount of composite flour for the sourdough development at 30 ± 2 °C and 85% relative humidity (RH) for 2 h in a leavening chamber (PL2008, Piron, Cadoneghe, Padova, Italy). The leavened dough, the other half part of WF–BF flour, and salt were kneaded together for 10 min with a Kitchen Aid mixer (Whirlpool Corporation, Benton Harbor, MI, USA) and leavened at 30 ± 2 °C and 85% relative humidity (RH) for another 60 min in the same leavening chamber [29]. When fermentation was finished, the dough was divided into 400 g per piece, molded by hand, and leavened in aluminum trays for another 60 min (30 ± 2 °C and 85% RH). The leavened dough was baked at 220 ± 5 °C for 25 min in an oven (Caboto PF8004D, Cadoneghe, Padova, Italy).

2.6. Flours’ Microscopy

Scanning electron microscopy (SEM) was determined according to the method described previously by Cotovanu et al. (2021) [30], the samples being fixed on double-sided adhesive carbon bands. Briefly, the SEM images of samples were analyzed under a scanning electron microscope (VEGA II LSH device, Tescan, Brno, Czech Republic) at an acceleration tension of 30 kV, and the images were collected at 2000×, 1000×, 500×, and 100× magnifications.

2.7. Flours’ ATR FTIR Spectra Collection

The Fourier transform infrared FTIR spectra of the WF and BF fractions were collected in triplicate using a Thermo Scientific Nicolet iS20 (Waltham, MA, USA) device in attenuated total reflectance ATR mode, with the range of the wavenumber from 4000 to 650 cm⁻¹ at a resolution of 4 cm⁻¹ by 32 scans. The molecular characteristics of the samples were identified according to previous data from the literature [31,32,33] by using OMNIC software (9.9.549 version, Thermo Fisher Scientific, Waltham, MA, USA).

2.8. Empirical Dough Rheology

The dough rheological parameters for extension were determined with the Alveograph device (Chopin Technologies, Villeneuve-la-Garenne, France) according to AACC International approved method 54-30.02 at constant hydration to a 14% moisture basis and 2.50% salt [34]. The main alveographic parameters were: dough resistance to deformation (P), dough extensibility (L), deformation energy (W), and configuration ratio of the Alveograph curve (P/L).

2.9. Dynamic Dough Rheology

Dynamic oscillatory measurements as a non-destructive method were performed using HAAKE, MARS 40 (Thermo Scientific, Karlsruhe, Germany) with parallel plate–plates geometry. Dough samples were tested for the linear viscoelastic region (LVR), from 0.00 to 100 Pa, at a constant oscillation frequency of 1 Hz. The dough prepared at optimum water absorption capacity (previously tested on the Mixolab device), but without the addition of the salt and fresh yeast, was left to rest for 5 min before testing [35,36]. The excess dough was trimmed just before the measurement, and a layer of Vaseline was applied to the exposed edge to avoid evaporation of moisture during the resting period. To determine dough storage (G’) and loss modulus (G”), a frequency sweep test, from 0.01 to 20 Hz at 10 Pa stress, in the LVR was applied. The maximum gelatinization temperature (T_max) was determined by heating the dough from 20 to 100 °C (4.0 ± 0.1 °C/min) performed at a constant strain of 0.10% and a frequency of 1 Hz.

2.10. Bread Quality Parameters Analysis

After cooling, the obtained bread samples were analyzed for their physical, textural, and color characteristics. The bread physical characteristics (volume, porosity, and elasticity) were determined according to the Romanian SR 90: 2007 standard method [28]. Bread volume was determined following the rapeseed displacement method, bread crumb porosity was calculated based on a sample cylinder volume (60 mm height and 45.50 mm diameter), and crumb elasticity was determined using a crumb cylinder that was pressed for 1 min until half of its height, and left to return to its shape one minute more [37]. The bread color parameters (L*, a*, b*) were analyzed for crumb and crust by using the Konica Minolta CR-400 colorimeter (Tokyo, Japan). The textural properties of bread samples were evaluated based on the TPA mode by a TVT-6700 texture analyzer (Perten Instruments, Hägersten, Sweden). The test speed of the probe was 1.0 mm/s. Bread sample slices of 50 mm in height were submitted to two cycles of compression, and the compression strain was set at 20% while the auto-trigger force was 5.0 g, with an interval of 15 s between compressions. The data were recorded and processed by TexCalc 5 software (5.1.0.x. version, Perten Instruments, Hägersten, Sweden). Firmness, springiness, gumminess, and cohesiveness were registered.

2.11. Statistical Analysis

The statistical calculations were performed using the SPSS 25.0 software (IBM, Chicago, IL, USA). Mean differences were determined by a two-way ANOVA followed by Tukey’s tests at a p ≤ 0.05 significance level. Principal component analysis (PCA) was applied to observe the correlations between the WF–BF flour routine composition, dough rheological measurements, and bread characteristics, and to reach a preliminary definition of parameters that can characterize and discriminate the flour, dough, and bread samples.

3. Results

3.1. Microstructure of Flours

SEM is used to observe the buckwheat fractions flour microstructure and to deeply explore the intrinsic mechanism on how buckwheat flours can influence the baking performance of wheat bread. The microstructure of BF milling fractions is presented in Figure 1. Buckwheat starch appeared to be grouped in clusters, especially at a medium particle size of BF (Figure 1(c1–c4)), and became rough and irregularly shaped when PS decreased. There were differences in shapes, sizes, and granule surfaces among different flour particle sizes, according to the results presented by Alvarez-Jubete et al. (2010) [6]. It can be observed in Figure 1 that the small milling fractions had irregular surfaces of buckwheat starch granules, and the number of irregular surfaces increased with the decrease in particle size (Figure 1(d1–d4)). When PS decreased, the size and volume of starch agglomerations gradually increased.

3.2. Fourier Transforms Infrared Spectrometry Analysis of Flours

Infrared spectroscopy is used to detect structural changes in organic compounds, particularly in terms of the presence or absence of functional groups [38]. To detect the structure of buckwheat milling fractions, FTIR spectra were recorded, and the spectra of the wheat flour and buckwheat flour fractions are shown in Figure 2. For more comprehensive and intensive study about baking performance variation, the functional groups were investigated by FTIR [39]. The functional groups found on BF spectra indicated the presence of chemical components, such as resistant starch, that interfere in dough formation, which may contribute to low viscoelastic properties of dough and increased hardening [40].

There were differences in absorbances only between large fractions and the other two buckwheat fractions, while there were no differences between S and M fractions, and wheat flour. The intensities of the peaks increased as the PS decreased from L < S < M.

3.3. Physicochemical Properties of Composite Flours

Table 1. Physicochemical properties of composite flours as influenced by buckwheat flour fractions’ addition.

Sample	Moisture (%)	Protein (%)	Lipids (%)	Ash (%)	Carbohydrates (%)	Color Parameters
						L*	a*	b*
Control	14.08 ± 0.08 e	12.45 ± 0.15 a	1.41 ± 0.01 a	0.69 ± 0.04 a	73.36 ± 0.01 e	91.46 ± 0.15 d	−5.13 ± 0.03 a	15.09 ± 0.07 c
BL_5	13.94 ± 0.00 dy	12.48 ± 0.00 bx	1.43 ± 0.00 bx	0.67 ± 0.00 bx	71.48 ± 0.00 dy	90.84 ± 0.06 cy	−5.22 ± 0.00 bx	14.64 ± 0.05 bx
BL_10	13.88 ± 0.00 cy	12.36 ± 0.00 bcx	1.45 ± 0.00 cx	0.69 ± 0.00 cx	71.61 ± 0.00 cy	90.11 ± 0.16 bcy	−5.10 ± 0.07 bcx	14.12 ± 0.02 abx
BL_15	13.82 ± 0.00 by	12.23 ± 0.00 cx	1.48 ± 0.00 dx	0.70 ± 0.00 dx	71.75 ± 0.00 by	89.69 ± 0.35 aby	−5.08 ± 0.06 cdx	14.18 ± 0.06 ax
BL_20	13.77 ± 0.01 ay	12.17 ± 0.06 cx	1.50 ± 0.00 ex	0.72 ± 0.00 ex	71.88 ± 0.00 ay	88.91 ± 0.33 ay	−5.19 ± 0.03 dx	13.47 ± 0.20 ax
BM_5	13.90 ± 0.00 dx	13.30 ± 0.00 by	1.61 ± 0.00 bz	0.83 ± 0.00 bz	70.36 ± 0.00 dx	89.74 ± 0.12 cx	−4.90 ± 0.05 by	14.61 ± 0.55 bx
BM_10	13.80 ± 0.00 cx	14.00 ± 0.00 bcy	1.82 ± 0.00 cz	1.01 ± 0.00 cz	69.36 ± 0.00 cx	89.53 ± 0.03 bcx	−4.94 ± 0.01 bcy	14.45 ± 0.10 abx
BM_15	13.70 ± 0.01 bx	14.70 ± 0.00 cy	2.03 ± 0.00 dz	1.18 ± 0.00 dz	68.38 ± 0.01 bx	88.66 ± 0.81 abx	−4.72 ± 0.09 cdy	14.19 ± 0.17 ax
BM_20	13.60 ± 0.01 ax	14.95 ± 0.44 cy	2.24 ± 0.00 ez	1.36 ± 0.00 ez	67.38 ± 0.01 ax	88.74 ± 0.07 ax	−4.80 ± 0.02 dy	14.18 ± 0.10 ax
BS_5	13.93 ± 0.00 dy	12.47 ± 0.00 bx	1.43 ± 0.09by	0.68 ± 0.00 by	71.48 ± 0.00 dy	89.85 ± 0.33c y	−4.96 ± 0.01 by	14.21 ± 0.15 bx
BS_10	13.87 ± 0.00 cy	12.35 ± 0.10 bcx	1.46 ± 0.00 cy	0.71 ± 0.00 cy	71.61 ± 0.01 cy	90.03 ± 0.05 bcy	−4.93 ± 0.02 bcy	14.19 ± 0.09 abx
BS_15	13.80 ± 0.00 by	12.22 ± 0.00 cx	1.49 ± 0.00 dy	0.73 ± 0.00 dy	71.74 ± 0.01 by	89.82 ± 0.60 aby	−4.94 ± 0.02 cdy	13.76 ± 0.00 ax
BS_20	13.73 ± 0.00 ay	12.10 ± 0.01 cx	1.52 ± 0.00 ey	0.77 ± 0.00 ey	71.87 ± 0.02 ay	89.18 ± 0.23 ay	−4.71 ± 0.10 dy	14.20 ± 0.55 ax
Two-way ANOVA p value
Factor I	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001
Factor II	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001	p = 0.018
Factor I × Factor II	p = 0.003	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001	p = 0.007

Factor I: level of BF addition; Factor II: BF particle size; mean value ± SD; ^a–e—indicates significant (p ˂ 0.05) difference between BF addition level; ^x–z—indicates significant (p ˂ 0.05) difference between BF particle size. L*—lightness; a*—greenness; b*—yellowness.

presents how particle size and addition level of BF influenced the final chemical composition of composite flours. It can be observed that there was a decrease in the flour’s moisture with the increase in BF amount, while, from a particle size point of view, it can be seen that the lowest moisture was found in the composite flour with medium PS. Protein content was significantly influenced by particle size and boosted when BF addition increased only in the flour-based composite with medium PS. In the formulated composite flours with large and small BF particle sizes, the protein content decreased when BF supplementation increased, and no statistically significant (p > 0.05) differences were found between these particle sizes. The highest protein content was observed at BM_20 (14.95 ± 0.44%), while the lowest value was given by BS_20 (12.10 ± 0.01%). The lipid content of the WF–BF flours ranged between 1.43–2.24% and was significantly (p < 0.001) affected by the BF addition level and PS compared to wheat flour (1.41%). This component increased gradually with the rise in BF amount, while particle size influenced lipid content as follows: M > S > L. The ash content, which ranged between 0.67–1.36%, followed the same trend as lipids in all of the samples except for BL_5 and BS_5, being richer in ash than WF (0.69%). The carbohydrate content for the composite flour ranged between 67.38–71.87% and presented a significant (p < 0.001) difference in comparison with wheat flour (73.36%). A decrease in carbohydrate content was observed when medium PS of BF addition was raised in wheat flour. For the composite flour with large and small PS, a rise in this component was observed when the BF addition level increased but presented a lower value than the control sample. Lightness, yellowness, and redness were the color parameters that were analyzed, and the results indicated that there was a significant (p < 0.001) influence had, in terms of more darkness, yellow, and whitish and less red, by the BF fractions and addition levels. The lightness L* values significantly (p < 0.01) decreased in all composite flours with the BF addition. The medium fractions of BF had a brightness effect on the composite flour, but all formulations with BF influenced the lightness of the wheat flour. This variation could be explained by the high ash content from these fractions [41], or due to the diverse chemical composition of central endosperm, aleurone layer, embryo, and cell wall tissues in grains and seeds [42].

3.4. Dough Rheological Properties

3.4.1. Alveograph Rheological Parameters

Evaluating the characteristics of the dough using the Alveograph device provides valuable information regarding the technological suitability and the baking properties of the formulated flours.

The addition level and PS of BF presented a significant (p < 0.001) effect on the dough’s alveographic properties (

Table 2. Alveograph rheological parameters as influenced by buckwheat flour fractions.

Sample	P (mm H2O)	L (mm)	G (mL)	W (10−4 J)	P/L (adim.)
Control	86.50 ± 0.50 d	94.00 ± 3.00 e	21.55 ± 0.35 d	253.00 ± 4.00 e	0.92 ± 0.03 a
BL_5	83.50 ± 1.50 az	48.00 ± 1.00 dx	15.45 ± 0.15 cx	155.50 ± 0.50 dx	1.74 ± 0.07 bz
BL_10	86.00 ± 1.00 bz	41.50 ± 0.50 cx	14.35 ± 0.05 bcx	131.50 ± 1.50 cx	2.07 ± 0.05 cz
BL_15	88.50 ± 0.50 cz	30.50 ± 0.50 bx	12.30 ± 0.10 bx	115.00 ± 0.00 bx	2.90 ± 0.03 dz
BL_20	93.50 ± 0.50 dz	26.00 ± 0.00 ax	11.40 ± 0.00 ax	95.00 ± 1.00 ax	3.59 ± 0.02 ez
BM_5	68.50 ± 0.50 ax	66.00 ± 4.00 dz	17.21 ± 1.54 cz	138.50 ± 1.50 dx	1.04 ± 0.05 bx
BM_10	70.50 ± 1.50 bx	52.50 ± 1.50 cz	15.46 ± 1.20 bcz	128.00 ± 6.00 cx	1.34 ± 0.01 cx
BM_15	78.00 ± 1.00 cx	50.00 ± 0.00 bz	16.38 ± 1.45 bz	123.50 ± 1.50 bx	1.56 ± 0.02 dx
BM_20	81.00 ± 1.00 dx	40.00 ± 0.00 az	14.78 ± 1.18 az	107.00 ± 5.00 ax	2.02 ± 0.02 ex
BS_5	82.50 ± 0.50 ay	52.00 ± 1.00 dy	16.05 ± 0.15 cy	168.50 ± 0.50 dy	1.58 ± 0.04 by
BS_10	83.50 ± 0.50 by	48.50 ± 0.50 cy	15.50 ± 0.10 bcy	145.50 ± 2.50 cy	1.72 ± 0.03 cy
BS_15	84.00 ± 1.00 cy	41.50 ± 1.50 by	14.35 ± 0.25 by	131.00 ± 2.00 by	2.02 ± 0.10 dy
BS_20	88.00 ± 1.00 dy	33.50 ± 0.50 ay	12.90 ± 0.10 ay	111.50 ± 0.50 ay	2.62 ± 0.01 ey
Two-way ANOVA p value
Factor I:	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001
Factor II	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001
Factor I × Factor II	p < 0.0001	p < 0.0001	p = 0.036	p < 0.0001	p < 0.0001

Factor I: level of BF addition; Factor II: BF particle size; mean value ± SD; ^a–e—indicates significant (p ˂ 0.05) difference between BF addition level; ^x–z—indicates significant (p ˂ 0.05) difference between BF particle size. P—resistance to deformation; L—dough extensibility; G—index of swelling; W—deformation energy; P/L—configuration ratio of the Alveograph curve.

). Statistically significant differences between the p-values of composite flours, regarding PS and the addition level, are noticed. The highest value for resistance to deformation (93.50 ± 0.50 mm H₂O) was observed for the BL_20 sample, while the lowest value was found in BM_5 (68.50 ± 0.50 mm H₂O). Resistance to deformation increased gradually in all samples when BF addition level increased, but in samples that incorporated medium and small PS (except BS_20), a lower value than the control sample was observed. Regarding dough extensibility (L), the ANOVA results showed significant effects of the addition level and PS (Table 2). The increment in BF decreased dough extensibility gradually. Statistically significant (p < 0.001) differences were observed between the dough extensibility among the flour samples. The highest value for L (94.00 mm) was found for WF, which is much higher than the values of the composite flours (26.00–66.00 mm). The index of swelling (G) values of the composite flours ranged between 11.40 and 17.21% and was significantly (p < 0.001) lower than for the control sample (21.55%). An increase in BF in the wheat flour is linked to a significant (p < 0.001) decrease in the deformation energy (W), from 253.00 × 10⁻⁴ J (control sample) to 95.00–168.50 × 10⁻⁴ J (composite flours). Regarding particle size, no significant (p > 0.05) differences were observed between the large and medium fractions, while these fractions were significantly (p < 0.01) different from small fractions.

3.4.2. Dynamic Rheological Parameters

Investigating further using two-way ANOVA, the results showed that the storage (G’) modulus was significantly (p < 0.001) higher in composite dough based on buckwheat large fractions than in control dough, while in doughs based on medium and small BF fractions, the storage modulus (G’) was lower in comparison with the control dough, except BM_20. G’ was higher than G’’ for all samples through all of the frequency range, showing only a slight increase with increasing frequency. Loss tangent (tan δ) presented an irregular trend regarding dough samples, and a significant (p < 0.001) difference was only found between the sample based on medium PS and the other two fractions, large and small. Maximum gelatinization temperature (T_max) during heating was influenced significantly (p < 0.001) by the PS and addition level of the BF (

Table 3. Elastic and viscous moduli, loss tangent, and gelatinization temperature as influenced by buckwheat flour fractions.

Sample	G′ (Pa)	G″ (Pa)	tan δ (adim.)	Tmax (°C)
Control	26,370.00 ± 257.00 ab	9488.00 ± 95.00 ab	0.3598 ± 0.00 a	82.74 ± 0.49 d
BL_5	23,600.00 ± 360.00 abz	8765.50 ± 196.50 abcz	0.3713 ± 0.00 ax	78.73 ± 0.63 ax
BL_10	34,780.00 ± 3450.00 bz	12,431.00 ± 1189.15 bcz	0.3590 ± 0.01 ax	79.75 ± 0.12 bcx
BL_15	28,560.00 ± 1570.00 az	10,730.00 ± 470.05 az	0.3760 ± 0.00 ax	79.03 ± 0.12 bx
BL_20	30,105.00 ± 2705.00 bz	10,645.00 ± 515.75 cz	0.3569 ± 0.02 ax	81.03 ± 0.91 cx
BM_5	26,055.00 ± 595.00 abyz	10,285.00 ± 255.50 abcyz	0.3947 ± 0.00 ay	80.91 ± 0.33 ay
BM_10	25,680.00 ± 310.00 byz	9509.50 ± 210.50 bcyz	0.3702 ± 0.00 ay	80.69 ± 0.31 bcy
BM_15	25,630.00 ± 1200.00 ayz	9590.50 ± 619.50 ayz	0.3739 ± 0.00 ay	82.63 ± 0.17 by
BM_20	30,255.00 ± 2445.00 byz	11,475.00 ± 825.04 ayz	0.3794 ± 0.00 ay	80.99 ± 0.10 cy
BS_5	26,240.00 ± 1260.00 abx	9752.00 ± 358.00 abcx	0.3455 ± 0.00 ax	79.42 ± 0.72 ay
BS_10	23,295.00 ± 525.00 bx	8339.00 ± 209.00 bcx	0.3579 ± 0.00 ax	81.74 ± 0.26 bcy
BS_15	19,680.00 ± 400.00 ax	7105.50 ± 307.50 ax	0.3608 ± 0.00 ax	79.67 ± 0.41 by
BS_20	24,360.00 ± 2676.00 bx	8911.00 ± 300.00 cx	0.3657 ± 0.00 ax	81.76 ± 0.60 cy
Two-way ANOVA p value
Factor I	p = 0.001	p < 0.0001	p = 0.222	p < 0.0001
Factor II	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001
Factor I × Factor II	p < 0.0001	p < 0.0001	p = 0.004	p < 0.0001

Factor I: level of BF addition; Factor II: BF particle size; mean value ± SD; ^a–e—indicates significant (p ˂ 0.05) difference between BF addition level; ^x–z—indicates significant (p ˂ 0.05) difference between BF particle size. G’—elastic modulus; G’’—viscous modulus; tan δ—loss tangent; T_max—maximum gelatinization temperature.

). It can be observed that T_max varied between 78.73–82.63 °C and was significantly (p < 0.001) different from the T_max of the control dough (82.74 °C). Regarding particle size, significant differences (p < 0.001) were found only between large fractions and the other two fractions, medium and small. Moreover, it can be observed that T_max increased when PS decreased.

3.5. Physical Properties of Bread

Baking results (

Table 4. Physical characteristics of bread as influenced by buckwheat flour fractions.

Sample	Loaf Volume (cm3)	Specific Volume (cm3/g)	Porosity (%)	Elasticity (%)
Control	378.70 ± 1.12 e	2.27 ± 0.03 a	64.33 ± 0.11 a	91.72 ± 0.07 b
BL_5	387.75 ± 0.25 dz	2.70 ± 0.03 by	75.07 ± 0.29 cy	94.15 ± 0.05 cz
BL_10	371.29 ± 0.71 cz	2.92 ± 0.20 cy	71.09 ± 0.77 dy	94.83 ± 0.17 dz
BL_15	350.37 ± 1.54 bz	2.53 ± 0.11 by	70.98 ± 0.13 cy	93.34 ± 0.01 cz
BL_20	314.05 ± 1.35 az	2.42 ± 0.00 ay	70.58 ± 0.26 by	92.08 ± 0.21 az
BM_5	374.57 ± 0.43 dy	2.71 ± 0.00 by	74.42 ± 0.57 cy	91.85 ± 0.19 cy
BM_10	369.14 ± 0.36 cy	2.86 ± 0.04 cy	73.27 ± 0.42 dy	93.54 ± 0.21 dy
BM_15	356.01 ± 0.98 by	2.70 ± 0.03 by	73.10 ± 0.14 cy	92.69 ± 0.04 cy
BM_20	296.90 ± 3.30 ay	2.26 ± 0.00 ay	65.21 ± 0.11 by	91.26 ± 0.39 ay
BS_5	291.27 ± 3.73 dx	2.37 ± 0.04 bx	72.52 ± 0.30 cx	93.89 ± 0.35 cx
BS_10	279.96 ± 1.03 cx	2.52 ± 0.04 cx	71.67 ± 0.29 dx	91.46 ± 0.19 dx
BS_15	231.77 ± 0.77 bx	2.51 ± 0.00 bx	68.13 ± 0.28 cx	91.25 ± 0.40 cx
BS_20	229.20 ± 1.70 ax	2.19 ± 0.02 ax	66.55 ± 0.19 bx	86.61 ± 0.66 ax
Two-way ANOVA p value
Factor I	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001
Factor II	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001
Factor I × Factor II	p < 0.0001	p < 0.0001	p < 0.0001	p < 0.0001

Factor I: level of BF addition; Factor II: BF particle size; mean value ± SD; ^a–e—indicates significant (p ˂ 0.05) difference between BF addition level; ^x–z—indicates significant (p ˂ 0.05) difference between BF particle size.

) showed differences in terms of the loaf and specific volume, crumb porosity, and elasticity between the control and composite WF–BF bread. The specific volume was modified significantly (p < 0.001) by increasing the amount of buckwheat flour and by the particle size. It can be observed that for bread with large and medium PS, specific volume increased gradually when adding 5–10% BF and decreased when 15–20% BF was added, all of the samples presenting higher values than the control bread, except for the bread with the S particle size. As it can be seen from the presented results, a significantly higher specific volume was obtained for the BL_5 sample compared to the control sample. Crumb porosity values of the bread based on composite WF–BF varied between 65.21–75.07% and were significantly (p < 0.001) different than the control bread. When the BF addition level in wheat flour increased, crumb porosity decreased gradually, but all samples presented a better porosity than wheat bread. Regarding crumb elasticity, it can be observed that the values for the WF–BF composite bread ranged between 86.61–94.83%, and for control bread it was 91.72%. A two-way ANOVA test reveals that the particle size of BF influenced this bread’s physical parameters as follows: L > M > S, while addition level improved the bread’s elasticity almost for all bread crumbs, except for BM_20 and with the addition of small fractions of BF at the proportion of 10–20%.

The bread crust color parameters were significantly (p < 0.001) influenced by the addition level and PS of BF (

Table 5. Crust and crumb color parameters of bread samples as influenced by buckwheat flour fractions.

Sample	Crust Color			Crumb Color
	L*	a*	b*	L*	a*	b*
Control	67.36 ± 0.19 d	0.78 ± 0.22 a	31.60 ± 0.28 b	72.30 ± 0.27 d	−4.48 ± 0.03 a	19.02 ± 0.23 b
BL_5	64.79 ± 0.67 c y	4.20 ± 0.40 bx	30.92 ± 0.59 bx	62.42 ± 0.13 bx	−3.12 ± 0.12 by	17.48 ± 0.31 ax
BL_10	63.91 ± 0.51 by	4.75 ± 0.23 cx	31.98 ± 0.44 bcx	64.55 ± 0.13 cx	−3.25 ± 0.06 by	17.53 ± 0.05 abx
BL_15	62.71 ± 0.26 ay	4.85 ± 0.23 cx	33.00 ± 0.82 bcx	57.28 ± 0.22 ax	−1.94 ± 0.43 cy	16.46 ± 0.57 bx
BL_20	61.19 ± 0.57 by	4.97 ± 0.26 cx	33.36 ± 0.14 cx	57.33 ± 0.51 ax	−1.57 ± 0.37 dy	17.14 ± 0.67 ax
BM_5	62.86 ± 0.67 cx	5.48 ± 0.24 by	29.03 ± 1.19 bx	69.72 ± 0.81 by	−3.77 ± 0.09 by	17.67 ± 0.33 ay
BM_10	57.66 ± 0.38 bx	6.84 ± 0.39 cy	31.18 ± 1.38 bcx	68.89 ± 1.25 cy	−2.69 ± 0.25 by	19.02 ± 0.71 aby
BM_15	55.07 ± 1.23 ax	6.72 ± 0.61 cy	32.66 ± 0.81 bcx	56.79 ± 0.34 ay	−1.38 ± 0.24 cy	17.69 ± 0.41 by
BM_20	61.19 ± 0.57 bx	6.78 ± 0.42 cy	33.54 ± 0.25 cx	56.41 ± 0.66 ay	−0.86 ± 0.26 dy	18.21 ± 0.69 ay
BS_5	64.81 ± 0.11 cy	2.08 ± 1.04 bx	28.91 ± 0.82 bx	59.96 ± 1.08 by	−2.81 ± 0.14 bx	17.63 ± 0.40 az
BS_10	61.79 ± 0.68 by	5.52 ± 0.82 cx	33.43 ± 2.03 bcx	67.33 ± 0.61 cy	−3.64 ± 0.09 bx	18.79 ± 0.32 abz
BS_15	59.21 ± 0.06 ay	5.73 ± 1.53 cx	33.64 ± 1.14 bcx	63.36 ± 0.96 ay	−3.10 ± 0.16 cx	22.41 ± 0.76 bz
BS_20	63.42 ± 0.40 cy	5.90 ± 1.15 cx	34.37 ± 0.90 cx	64.91 ± 0.92 ay	−2.60 ± 0.23 dx	18.35 ± 0.71 az
Two-way ANOVA p value
F1	p < 0.001	p < 0.001	p < 0.001	p < 0.001	p < 0.001	p < 0.001
F2	p < 0.001	p < 0.001	p = 0.062	p < 0.001	p < 0.001	p < 0.001
F1 × F2	p < 0.001	p = 0.02	p = 0.092	p < 0.001	p < 0.001	p < 0.001

Factor I: level of BF addition; Factor II: BF particle size; mean value ± SD; ^a–e—indicates significant (p ˂ 0.05) difference between BF addition level; ^x–z—indicates significant (p ˂ 0.05) difference between BF particle size. L*—lightness; a*—greenness; b*—yellowness.

). The increase in buckwheat flour was conducive to a significant decrease in lightness (L*) for the bread crust, while significant differences were found only between medium PS and the other two PS. Redness (a*) value of the control crust bread was significantly lower than the composite bread, the crust a* values showing significant differences (p < 0.001) only between the bread containing 5% BF fractions and between samples based on medium PS. Yellowness (b* value) increased gradually when BF addition increased, but only breads with 10–20% addition were significantly different than the values of the control bread, and PS had no significant influence on this bread parameter. Lightness L* of crumb from the composite bread was found to decrease when substituting wheat flour with BF; all crumb samples containing buckwheat milling fractions showed significantly (p < 0.001) lower lightness than the control sample. For crumb bread that contained small fractions, the lightness values increased with BF addition increasing. Greenness values of crumb bread presented a statistically significant (p < 0.001) difference only for buckwheat small fractions added in wheat flour, while these values increased with adding of BF. Crumb yellowness was influenced by BF sieved fractions as follow: S > M > L, which means that when particle size decreased, yellowness became greater. These results indicate that bread crumb tends to become darker and less yellow with the increase in the buckwheat flour addition level.

3.6. Textural Parameters of Bread

Bread samples’ texture profiles (Figure 3) were significantly influenced by the BF sieved fractions and addition level. It can be observed that crumb firmness increased with the rising BF amount, and when bread was replaced with 15–20% BF, it presented higher firmness than the control bread. The bread samples in which BL_5, BM_5–15%, and BS_5 were incorporated showed a lower firmness than the control bread. Bread springiness decreased proportionally with BF addition level. The lowest springiness values were given by the small fractions, while medium and large PS presented higher values and did not present significant differences. Bread gumminess decreased gradually when BF increased. The bread with 5 and 10% BF, for all fractions, presented higher values for bread gumminess than the control bread, while the bread samples based on 15 and 20% BF addition level, presented lower values for this parameter in comparison with wheat flour bread. Bread cohesiveness had an irregular fluctuation, presenting a decrease when BF amount increased and PS decreased.

4. Discussion

4.1. Microstructure of Flours

To study the structure of protein and starch composites, the morphology of buckwheat fractions was analyzed. Buckwheat starch granules have a polygonal shape. The small PS had a smooth granule surface with some small depressions or pores, and these may be due to amylase activity in the kernels [43]. This fact can explain why the integrity of buckwheat starch granules is diminished when particle size decreases. On the large BF fractions, roughness in some depressions can be observed. A great deal of material adhered to the surface of starch was observed, and irregular bulks were found. Similar findings were previously published by some authors [6,43,44]. The modifications observed in BF structure depending on the particle size as a result of buckwheat seeds’ fractionation could explain the physicochemical, rheological, and technological properties of flour, dough, and bread, respectively.

4.2. Fourier Transform Infrared Spectrometry Analysis of Flours

Fourier transform infrared (FTIR) spectroscopy is a powerful analytical and diagnostic technique and has been widely applied to the characterization of different tissue’s biochemical makeup due to its sensitivity to the chemical architecture and information about the molecule [45]. The spectral peak characteristics due to different types of bonds stretching on the spectrums of buckwheat starches were interpreted in the light of the available literature [46,47]. The bands present at 721, 764, and 864 cm⁻¹ could give information about the substitutions in aromatic rings characterized by aromatic C–H out-of-plane bending [33].The peak at 1006–1020 cm⁻¹ is correlated with the amorphous state in starch [48]. In the spectrum of the buckwheat fractions, in the functional groups region, one new band could be observed at 900–1550 cm⁻¹. This band corresponded to the unsaturated bonds C=C connected to the oxygen atoms O-C=C or the nitrogen atoms N-C=C. Bands at 1200 to 1000 cm⁻¹ are related to axial deformation vibrations of C-O in alcohols and the axial deformation vibrations of the O-C-O system and are bands normally found in starches [49], and bands at approximately 1650 cm⁻¹ are associated with H-OH bonds, referring to bound water [50]. The sugar and C-O stretching vibrations (crystalline/amorphous structures of starch) were identified at about 934, 1026, 1086, and 1160 cm⁻¹. The simple C-H bending vibrations of methyl (CH₂, 1470 cm⁻¹) and methylene (CH₃, 1380 cm⁻¹) groups of lipids and proteins side chains occurred between 1500 and 1300 cm⁻¹. The bands in the range from 1500 to 1300 cm⁻¹ are due to bending deformation modes of CH/CH₂/CH₃ groups coming from the various amino acid side chains and lipids [33]. Additionally, amide I (proteins β-structure) at 1600–1700 cm⁻¹, amide II (band originating from peptide N-H bending modes, coupled with C–N stretching modes) at 1550 cm⁻¹, and lipids at 1750 cm⁻¹ and 2800–3000 cm⁻¹ were identified [31,51] (Figure 2). The band at about 3300 cm⁻¹ is given by the stretching vibration of -OH, possibly due to the presence of water, galacturonic acid, arabinose, galactose, xylose, glucose, alcohol, phenols, starch, and gums from buckwheat fractions [50,52]. The bands in the region of 2900–3100 cm⁻¹, corresponding to the -CH stretch, refers to hydrocarbons [53]. However, according to Lian et al. [54] these ranges are attributed not only to starch but also to protein. An intense peak in the range 2858–2928 cm⁻¹ is related to vibrations of -CH₂ stretching of the glucose units of polysaccharides [48,49]. This result was represented by strong absorption bands of the white matter in the lipid and phospholipid region in the range of 3000–2750 cm⁻¹. It may be possible to determine the components of WF and BF fractions by measuring the intensity of characteristic absorption peaks.

4.3. Physicochemical Properties of Composite Flours

Composite WF–BF presented significant (p < 0.001) variance between protein, lipids, ash, and carbohydrates, which could be explained by the buckwheat seed’s morphological structure, where protein and lipids are mainly concentrated in the germ (embryo parts) [6]. The variation in ash content could be explained by the cell wall material from the broken endosperm [23]. Similar results for medium and small sieved fractions were found by Sciarini et al., (2020) [51]. The phenomena of lower carbohydrate content from composite flours based on buckwheat milling fractions at different levels could be explained by buckwheat’s endosperm concentration in starch. This decrease in carbohydrate content in composite WF–BF based on medium particle size was expected due to the rise in protein and ash content in these flours. Similar trends of the chemical compositions for buckwheat milling fractions were previously reported by Coţovanu and Mironeasa (2021a) [23]. The darker composite flour was observed at more than 15% BF addition level.

4.4. Dough Rheological Properties

4.4.1. Alveograph Rheological Parameters

The use of buckwheat flour in dough formulation is a challenge due to the gluten absence, and the resulting dough probably cannot have a good viscoelastic property. Resistance to deformation (P) is an indicator of the dough’s ability to retain air bubbles [55]. Dough tenacity was lower in dough samples supplemented with medium and small fractions, and for 5–10% for large PS. This fact could be explained by the effect of sourdough and the sugar substrate that the yeasts had available in these BF fractions. The extensibility of dough (L), as a predictor of the processing characteristics of dough, also points out protease activity [56]. The influence of BF addition level and PS on dough extensibility could be based on the high contribution of this protein content and, therefore, a minor gluten formation, results which are in line with earlier reports [29]. Gliadin from wheat flour contributes to the viscous nature of wheat doughs and its extensibility [57]. The index of swelling (G) is correlated with a preferential pathway for water absorption. This weakening can be explained by the fact that buckwheat flour does not contain gluten or contains a very small amount of it, which is the main cause of changes in the rheological properties of wheat dough, whose protein components create a three-dimensional sponge-mesh structure in the dough determining the physical properties of the dough. Similar findings on the alveographic proprieties of different buckwheat flour addition levels in wheat flour were found by Dziki and Laskowski (2005) [58] and Marioti et al. (2008) [59].

4.4.2. Dynamic Rheological Parameters

To deem buckwheat flours suitable for breadmaking, composite WF–BF at a different BF particle size and addition level were formulated, and dough rheology was measured. Dynamic rheology with small deformation was applied because this avoids gel network disruption [60]. Storage modulus (G’) accounts for the energy stored and released per cycle of deformation per unit volume, and it highlights the molecular properties of its elastic nature, while, the loss modulus (G”) accounts for the energy dissipated as heat per cycle of deformation per unit volume, and it highlights the molecular properties of its viscous nature [61]. G’ was higher than G’’ for all samples (Figure S1 from the Supplementary Materials), which is typical gel-like behavior [62]. This behavior can be attributed to the presence of different chemical compositions (protein, lipid, carbohydrates) of particle sizes and attractive bonds between the molecular structures of starch granules [35]. In general, the incorporation of BF markedly raises both viscoelastic moduli, G’ and G”, leading to values higher than the control dough, except the WF substitution with the small PS of BF. The higher viscoelastic moduli can be associated with stronger starch–gluten interactions in composite flour. Similar findings were reported by Sivaramakrishanan et al. [63], which state that rice starch granules in the rice–wheat composite dough can act as filler that reinforces the gluten and produces strong bonds, giving a higher modulus. The loss tangent increased significantly (p < 0.05) with BF addition of medium PS, from 0.35 (0% BF) to 0.37 (20% BF of medium PS) implying a decrease in the solid-like behaviour of BF-added doughs that decreased with the medium PS level increase. This could be related to the differences in protein contents, the medium PS presenting a high content compared to large and small, as was reported in a previous study [23]. T_max increased when PS decreased, which may be explained by the higher contents of lipids and lower carbohydrate content from these fractions, which is associated with starch (Table 1), similar data being reported by Singh and Kaur (2004) [64]. During controlled heating, G’ and G” of all samples started to increase at around 60 °C and reached a peak (T_max) before falling (Figure S2 from the Supplementary Materials). The initial increment of G’ and G” could be attributed to leaching amylose chains and swollen starch granules leading to the rising of the dough’s viscoelastic properties [36,65,66]. Further heating leads to the melting of the crystalline region in the swollen starch granule, and the original structure is destroyed [64,67]. During gelatinization, amylose, amylopectin, and starch-binding proteins and lipids leached out from the granules, resulting in increased storage and loss modulus and less tan delta [68]. It seems possible that changes in gel strength were the consequence of changes in the macromolecular organization due to segregative interactions between components of the system. Other authors have demonstrated that interactions between proteins and polysaccharides lead to changes in the profile of the mechanical spectrum (G’ and G’’ values) [69]. It can be observed that the increase in BF addition level in formulated bread led to a decrease in the quality of the protein structure, which was manifested in a harder crust of the bread. Lower values of T_max indicate a smaller retrogradation tendency [70], making WF–BF a suitable composite flour for bakery products. Starch retrogradation is desirable for some starchy food products in terms of its textural and nutritional properties [70].

4.5. Physical Properties of Bread

The specific volume is the ratio of the volume to the weight, and this has been adopted as a reliable measure of bread quality [71]. The improvement in bread volume in wheat bread with buckwheat flour could be explained by the sourdough that was formed through the biphasic method that was used, and the possibility of yeast forming CO₂ that expands the dough cell. The reduction in volume in some composite breads could be linked to the decrease in protein contents and gluten dilution from these composite flours (Table 1), which leads to the reduced gas-retention capacity of the dough [72]. Similar findings regarding white bread supplemented with whole-grain buckwheat flour were found by Lin et al. (2009) [73] and Sedej et al. (2011) [74]. Regarding crumb porosity, it can be observed from Table 4 that particle size and addition level of BF significantly affect (p < 0.001) this physical bread parameter. All of the bread based on BF had greater porosity (65.21–75.07%), being plumper and less stiff than the control sample (64.33%). Regarding PS, there was no difference between large and medium PS on the crumb porosity but there was a significant difference between them and the small particle size of BF. These changes can be explained by the indirect method that we used in the recipe, which formed a sourdough that changed the texture of the crumb and the pore structure due to air bubbles. The hydrolysis of buckwheat storage proteins and the minor hydrolysis of wheat glutenins and gliadins occurred during proofing upon the addition of buckwheat sourdough, resulting from the activation of endogenous proteases at the acidic pH, which can induce the increase in stiffness and elasticity in gluten model systems [75]. Similar results were previously obtained by other authors, which indicate that buckwheat sourdough represents a helpful tool for improving some of the textural features and/or properties of wheat bread, according to the addition level [75,76].

Texture profile analysis (TPA) was performed to test the textural properties of the bread crumbs, which are influenced by the addition level of BF and PS composition. Firmness is defined as the peak force that occurs during the first compression. The protein from buckwheat flours is closely associated with the textural characteristics of its resultant products [77]. Buckwheat flour is not able to develop viscoelastic dough with good elasticity and plasticity because its proteins have a modest content in prolamins, and gluten-like proteins are absent [78]. The absence of a gluten network shows the movement of water by forming an extensible protein network [79,80] through the crust, which leads to the hardening of starch gel, hence, causing the increase in the firmness of bread crumbs [81]. This statement falls in line with Brites et al. (2010) [82] who confirmed that compact crumb texture and low specific volume is typical for bread with buckwheat flour. Consequently, the bread’s firmness is related to the quality of flour proteins with the ability to provide unique viscoelastic and network-forming properties. Springiness (elasticity) is measured by the distance of the detected height during the second compression divided by the original compression distance [83]. Bran particles from large PS of buckwheat flour contain high concentrations of fiber and swell extensively. Some studies have demonstrated that bran supplementation contribute to a weak structure and worsen the baking quality of wheat dough, which decreases the bread’s volume and elasticity of the crumb [84,85]. Gumminess is described as the energy needed to disintegrate a semi-solid food until it is ready for swallowing. Cohesiveness is how well the sample withstands a second deformation relative to its resistance under the first deformation [86]. Our results regarding BF particle size are in concordance with those obtained by Cai et al. (2014) [87], which reported that the springiness, cohesiveness, and resilience values of wheat dough with different particle sizes decreased with the increase in fractions. Our results highlighted that the firmness, springiness, gumminess, and cohesiveness of formulated bread changed with different BF addition levels, which can also influence machine workability.

4.6. Relations between the Characteristics

A multivariate correlation test provided information on the significant (p ≤ 0.05) relationship between wheat–buckwheat composite flour, dough, and bread characteristics. By applying Pearson correlation analysis for proximate composition, rheological, and technological parameters, a series of correlation coefficients (0.58 > r < 0.99) were found. Between flour moisture and dough extensibility L (r = 0.68), and dough strength W (r = 0.84) significant positive correlations were found, while between the moisture content and protein (r = −0.57) and fats (r = −0.76) from flours negative relations were found. Dough tenacity was found to be negatively correlated with tan δ (r = −0.59). High positive correlations were found between bread volume and bread elasticity (r = 0.70) and porosity (r = 0.66), but a negative relationship between bread volume and bread firmness (r = −0.78) was observed. A similar correlation for flour chemical constituents, dough and bread parameters was found by other authors [51,77].

The principal component analysis (PCA) was applied to highlight the effect of BF addition and PS on wheat–buckwheat composite flour, dough rheological characteristics, and bread parameters (Figure 4). The loadings of the studied variables on the first two principal components, PC1 (28.16%) and PC2 (33.42%) (Figure 4) described 61.58% of the total variance.

The PC1 was associated with composite flour moisture, dough, bread’s physical properties (elasticity, porosity, volume), and bread’s textural properties (gumminess, firmness) while PC2 was associated with the composite flour’s protein, lipids, ash, carbohydrates, dough resistance to deformation (P), dough extensibility (L), and dough curve configuration ratio (P/L). It can be observed that there is a high opposition between protein and carbohydrates, P and L alveograph parameters, bread elasticity, and bread firmness. A good relationship was observed between the control sample and bread with a 5% addition level of all BF fractions (L, M, and S), and BL_10. Samples with medium PS (10–20%) were associated with protein, in opposition to the samples with 10–20% small fractions, which were associated with bread gumminess and firmness.

5. Conclusions

The results of this investigation highlighted that buckwheat flour particle sizes and addition levels in wheat flour determined remarkable changes in WF-BF proximate composition, dough rheology, and bread quality parameters. Different structural and molecular characteristics have been found depending on the different particle sizes of the buckwheat flour fractions, properties that will affect the dough formation and the bread baking process. The chemical composition of composite WF–BF was improved regarding protein content, only when replaced with medium PS, and the fats and ash content raised with the increase for all BF fraction addition levels. Empirical and dynamic rheology properties were significantly influenced by the PS and BF addition level that will affect the bread quality parameters. Bread technological properties generally improved when BF was added. These results could be of real interest for manufacturers conscious to develop novel bread formulations with superior characteristics and improved nutritional profile. It should be noted that the best formulation of wheat flour based on BF bread making will depend on particle size. From our research, we can conclude that a level up to 15%, for a medium fraction, could give the acceptable dough rheological characteristics and bread quality parameters.