Physicochemical properties of grain
The physical properties of the grain of primitive rye and common rye cv. Dańkowskie Złote (open-pollinated cultivar) and cv. KWS Bono (hybrid cultivar) are presented in Table 1. The thousand-kernel weight (TKW) of primitive rye was low (21.6 g) compared with common rye cultivars, and its bulk density was determined at 756.3 kg m−3. The kernels weight is dependent on the growth conditions and cultivar (Järvan et al. 2018). Primitive rye grain did not differ significantly in width (2.48 mm) from the grain of open-pollinated and hybrid cultivars, but it was characterized by smaller average kernel length (6.69 mm). The average thickness of primitive rye kernels (2.00 mm) was similar to the thickness of hybrid rye kernels and significantly smaller than the thickness of open-pollinated rye kernels. The kernels of the evaluated rye cultivars were characterized by considerably smaller width and thickness than the grain analyzed by Jouki et al. (2012), where the above parameters were determined at 4.65 mm and 3.18 mm on average, respectively.
In the color analysis, the average value L* of primitive rye grain (51.3) was significantly higher than in hybrid rye (47.3) and lower than in open-pollinated rye (54.3). The grain of primitive rye was characterized by a higher contribution of redness (a* = 5.24) in comparison with the grain of common rye cv. Dańkowskie Złote (a* = 4.24) and KWS Bono (a* = 3.29), but it did not differ significantly from the reference rye in the value of the b* (19.04). According to Zykin et al. (2018), the color of cereal grain is determined mainly by the content of anthocyanin pigments.
The mechanical properties of cereal grain play a very important role in the milling process (Ponce-García et al. 2016). The force needed to deform grain has to be accurately determined for the purpose of designing grain harvesting and processing equipment. In primitive rye grain, rupture force was determined at 68.9 N, force at the end of compression—at 95.3 N, rupture energy—at 35.6 mJ, and total compression energy—at 81.5 mJ (Table 1). The rupture force of primitive rye grain was significantly lower compared with the open-pollinated cultivar. The remaining parameters (Fe, Erc and Ee) did not differ significantly across the analyzed cultivars. The average specific milling energy of primitive rye grain (95.3 kJ kg−1) was approximately 11% higher relative to the open-pollinated cultivar and approximately 29% lower relative to the hybrid cultivar. Milling energy is determined by the milling method and the properties of grain, mainly moisture content, kernel size and hardness and the degree of milling (Dziki et al. 2014; Warechowska et al. 2016). In a study by Rydzak et al. (2012), energy consumption during the milling process of a mixture of rye grain with similar moisture content, ground in the same type of a mill, was higher than that noted for primitive rye grain in our study. In the work of Hameed Hassoon and Dziki (2017), the milling energy of rye grain ground in a hammer mill ranged from 66.2 kJ kg−1 (grain with 10% moisture content) to 133.6 kJ kg−1 (grain with 18% moisture content).
Particle size distribution of flour
The curves presenting the particle size distribution of extracted and wholemeal flours are shown in Fig. 1. All flours had quatrimodal size distribution with a trace amount of fractions smaller than 10 μm. The average particle size of wholemeal flour (74.8 µm) and extracted flour (73.8 µm) from primitive rye grain was greater in comparison with hybrid rye flours and smaller in comparison with flours made from open-pollinated rye (Table 2). Wholemeal flour from primitive rye was characterized by a higher proportion of fine particles (d(0.1) = 18.4 µm) and a higher relative span of volume-based size distribution (SPAN) than flours from the reference rye cultivars. Extracted flour from primitive rye was characterized by a smaller proportion of fine particles (d(0.1) = 14.5 µm) and lower SPAN values than hybrid rye flour, whereas the reverse was noted in comparison with the open-pollinated cultivar. The granulometric composition of flour significantly influences rheological and end-product properties (Bucsella et al. 2016). A higher proportion of fine flour particles intensifies dough fermentation because enzymes have easier access to starch and pentosans.
Flour yield, flour quality characteristics and baking quality
The quality characteristics of extracted and wholemeal flours from primitive rye and common rye grain are presented in Table 3. The extraction rate of primitive rye flour (49.8%) was similar to that of hybrid rye flour (48.9%), and significantly lower than the extraction rate of flour from open-pollinated rye (53.5%). Rye flour has a lower extraction rate than wheat flour because the endosperm is difficult to separate from the seed coat, especially in grain with a high content of non-starch polysaccharides. The surface of sifting rye flour is 25–30% larger than required for wheat flour (Arendt and Zannini 2013).
The protein content of wholemeal flour (9.59%) and extracted flour (5.91%) from primitive rye grain was significantly higher in comparison with the corresponding flours from open-pollinated rye and was similar to hybrid rye flours. The protein content of primitive rye flours was typical of rye flour (Konopka et al. 2017b; Järvan et al. 2018). The total protein content of rye is generally lower in comparison with wheat (6.5–14.5% on average) (Arendt and Zannini 2013). The ash content of wholemeal (1.92%) and extracted (0.59%) flours from primitive rye was significantly higher in comparison with flours made from open-pollinated and hybrid cultivars (wholemeal flour—higher by 16% and 27%; extracted flour—higher by 10% and 22%, respectively). The starch content of wholemeal flour and extracted flour from primitive rye (54.9% and 64.9%, respectively) was considerably lower relative to the corresponding flours from the reference rye cultivars. Similar observations were made by Konopka et al. (2017b).
The color analysis revealed that unlike wholemeal flours, extracted flours differed significantly in lightness (Table 3). The value of L* was highest in extracted flour from primitive rye grain. Both wholemeal and extracted flours from primitive rye were also characterized by the highest contribution of redness, which could be attributed to their high ash content. The value of the correlation coefficient between ash content and a* was determined at R = 0.98. Flour whiteness is determined by the values of L* and a*, and it is an important parameter which influences consumer acceptance. The most desirable flours are characterized by low values of a* and high values of L* (Drakos et al. 2017). Flour lightness is correlated with its ash content (Protonotariou et al. 2014). High ash content increases contamination with seed coat residues and decreases the value of L*. Particle size can also influence the color of flour. Flours with a higher proportion of fine particles are characterized by higher L* values (Gómez et al. 2009). The falling number denotes the presence of α-amylases in flour, and it was determined at 129 s in wholemeal flour and 137 s in extracted flour from primitive rye (Table 3). The falling number of both extracted and wholemeal flours from primitive rye grain was lower in comparison with the corresponding flours from the remaining rye cultivars. The extraction process clearly influenced the falling number, and wholemeal flours were characterized by lower falling numbers. In sourdough bread, a low falling number is more desirable due to high amylolytic activity which rapidly initiates the fermentation process (Zieliński et al. 2008). In the current study, primitive rye flour was most suitable for the production of sourdough bread. In wholemeal and extracted flours from primitive rye, starch gelatinization began at a temperature of 51.5 °C and 48.8 °C and was completed at 63.8 °C and 64.1 °C, respectively. In wholemeal and extracted flours from primitive rye, the temperature marking the beginning and end of starch gelatinization was 2.2 °C and 11.6 °C lower and 2.2 °C and 14.7 °C lower, respectively, than in the corresponding flours from rye cv. KWS Bono. Starch gel viscosity in primitive rye flour (288 in wholemeal flour and 610 in extracted flour) was lower than in flours from open-pollinated and hybrid ryes. Wholemeal and extracted flours differed significantly in starch viscosity. Wholemeal flours were less viscous, and similar results were noted in an earlier study (Konopka et al. 2017b). The optimal parameters of rye flour for breadmaking are determined in the following range: falling number: 125–200 s, viscosity: 400–600 AU, final starch gelatinization temperature: 63–68 °C (Beck et al. 2011; Konopka et al. 2017b). In view of the above, the analyzed flour from primitive rye grain was characterized by high baking quality, but its maximum amylograph viscosity was lower in comparison with the remaining rye flours, which can decrease bread volume (Stępniewska et al. 2018). The water absorption capacity of wholemeal and extracted flours from primitive rye grain was higher relative to the corresponding flours from open-pollinated rye, but lower in comparison with hybrid rye flours. The hydration capacity of flour is affected by its protein and polysaccharide content, degree of milling and starch damage (Drakos et al. 2017). Finely ground flour has a higher water absorption capacity.
Content of free phenolic compounds and antioxidant potential
Rye flour is a rich source of phenolic compounds (Zieliński et al. 2008; Pejcz et al. 2015; Konopka et al. 2017b; Pihlava et al. 2018) which deliver health benefits due to their antioxidant potential (Pejcz et al. 2015). The content of phenolic compounds and the antioxidant activity of flour are altered during baking. The relevant changes are induced by numerous factors, including fiber content, recipe, fermentation, baking process and the formation of Maillard reaction products (Banu et al. 2010; Konopka et al. 2014; Pejcz et al. 2015). The content of free phenolics and the antioxidant potential of extracted and wholemeal flours from primitive rye and common rye are presented in Table 4. The content of free phenolics ranged from 8.31 to 12.6 mg/100 g DM in samples of extracted flours, and from 48.16 to 58.57 mg/100 g DM in samples of wholemeal flours. Primitive rye flours were most abundant in free phenolics whose content was approximately 4.7 times higher in wholemeal flour than in extracted flour. Similar results were reported by other authors (Konopka et al. 2014). However, the above findings do not correspond to the antioxidant activity of primitive rye flour. The DPPH radical scavenging activity of extracted flour from primitive rye (52.7 µM TE/100 g) was approximately two-fold higher than in flour from open-pollinated rye, and comparable with hybrid rye flour. The DPPH radical scavenging activity of wholemeal flour from primitive rye (200 µM TE/100 g) was similar to that noted in the corresponding flour from open-pollinated rye (186.9 µM TE/100 g) and lower than in hybrid rye flour (235.7 µM TE/100 g). The analyzed parameter was approximately 3.8-fold higher (primitive rye), 8-fold higher (open-pollinated rye) and 4.2-fold higher (hybrid rye) in wholemeal flours than in extracted flours. Zieliński et al. (2008) observed a reverse trend in rye flours in the DPPH test. A positive correlation was observed between the content of free phenolic compounds and the antioxidant potential of the analyzed flours (R = 0.95).