2.1. Physical and Chemical Properties of Oat Flours
As shown in
Table 1, β-glucan contents of the oat flours are averaged at 4.30%, with a varying range within 3.92%–4.79%. Among them, the flours of Bayou 9 (4.70%) and Baiyan 19 (4.79%) have higher β-glucan contents while Bayou 1 (3.92%), Baiyan 13 (4.05%) and Baiyan 15 (3.98%) have lower β-glucan contents. β-glucan content of Zhengmai 129 (0.28%) is significantly lower than those of oat flours. The average protein contents of oat flours is 14.89%, with a varying range within 12.47%–16.30%. Oat flours with higher protein contents are from Bayou 9 (16.30%), Baiyan 2 (16.05%) and Baiyan 15 (15.94%) while the lower protein content was from Baiyan 14 (13.99%). The wheat flour of Zhengmai 129 has the protein content of 12.47%, significantly lower than those of oat flours. The average starch content of oat flours is averaged at 57.74%, with a varying range within 54.61%–60.50%. Among the oat flours, Baiyan 14 (60.50%) and Bayou 9 (54.61%) have the highest and lowest starch contents, respectively. Zhengmai 129 has a starch content of 73.09%, significantly higher than those of oat flours.
Of the oat flours, the Calibrations of unit Chopin Dubois (UCDc) of damaged starch is averaged at 9.78, with a varying range within 4.35–12.80. Samples containing larger amounts of damaged starch (expressed by UCDc) are from Bayou 1 (12.80) and Bayou 12 (12.10) and that containing a lower amount is Baiyan 19 (4.35). Remarkably, UCDc of Zhengmai 129 (25.10) was significantly higher than those of oat flours (9.78). Particle size of oat flour (60–80 mesh sieve) is larger than that of wheat flour (100–120 mesh sieve), thus, damaged starch content of oat flour is lower than that of wheat flour. Damaged starch is defined as the starch of which surrounding cell membrane and granules are damaged by extruding, cutting, shearing, rubbing, ripping and other action forces in the process of milling [
19]. In general, damaged starch granules can exert greater influence on flour qualities by absorbing several times more water than undamaged ones [
20,
21]. Native starch can absorb about 39%–87% water (by weight), while damaged starch is about 200%–430% [
20]. Therefore, we measured the damaged starch for the purposes of better understanding the SRCs of oat flours with different components and analyzing the correlation between the components and their SRCs. Oat flour swelling analysis needs to take into account β-glucan, protein, starch, damaged starch contents as well as sample differences.
2.2. SRCs Against Different Solvents and Their Relations with Oat Flours Compositions
The WSRC, SSRC, LASRC, SCASRC values of oat flours were measured using conventional SRC measurements are summarized in
Table 2. The LASRC, SCASRC, SSRC, WSRC values of oat flours are 132.78%, 110.71%, 91.46% and 90.12%, respectively. Flours with higher WSRC values are from Baiyan 2 (107.45%) and Bayou 9 (102.05%), while flours with lower WSRC values are from Bayou 1 (78.35%), Baiyan 14 (76.25%) and Zhengmai 129 (68.40%). Flours with higher SSRC values are from Zhengmai 129 (109.65%) and Baiyan 19 (96.75%), while flours with lower SSRC values are from Baiyan 14 (89.80%), Bayou 12 (89.20%), Bayou 9 (89.15%), Bayou 1 (88.40%), and Baiyan 13 (88.10%). Flours with higher LASRC values are from Baiyan 19 (159.30%) and Bayou 1 (154.15%), while the flour with the lower LASRC values is from Bayou 12 (85.95%). Flours with higher SCASRC values are from Bayou 9 (120.70%), Baiyan 2 (119.65%) and Baiyan 19 (118.75%), while the flour with the lower SCASRC value is from Zhengmai 129 (87.10%).
Modified SRC measurements were also used to identify the effects of metal ions, SRCs against different solvents are summarized in
Table 2. The FeCl
3 SRC (FCSRC), CaCl
2 SRC (CCSRC), NaCl SRC (SCSRC), sodium cholate SRC (SCHSRC) values of oat flours are 157.09%, 99.65%, 88.21% and 64.13%, respectively. The flour with the higher SCSRC values are from Baiyan 9 (95.95%) and Baiyan 2 (95.60%), while the flour with the lowest SCSRC value is from Zhengmai 129 (70.70%). The flour with the highest CCSRC value is from Baiyan 2 (110.85%), and the flour with the lowest CCSRC value is from Zhengmai 129 (85.95%). The flour with the highest FCSRC value is from Bayou 9 (181.10%), and the flour with the lowest FCSRC value is from Zhengmai 129 (109.80%). The flour with the highest SCHSRC value is from Zhengmai 129 (77.45%), and the flour with the lowest SCHSRC value is from Bayou 1 (53.55%).
Modified SRC measurements were used to identify the effect of different solvent with the same pH, SRCs against different solvents are summarized in
Table 2. The SHSRC, SCABSRC and SDSSRC values of oat flours are 91.74%, 91.70% and 72.01%, respectively. The flour with the highest SHSRC value is from Baiyan 2 (105.80%), and the flour with the lowest SHSRC value is from Zhengmai 129 (68.05%). The flour with the highest SCABSRC value is from Baiyan 2 (105.55%), and the flour with the lowest SCABSRC value is from Zhengmai 129 (67.70%). The flour with the highest SDSSRC value is from Baiyan 2 (80.05%), and the flour with the lowest SDSSRC value is from Zhengmai 129 (48.60%). With the three solutions kept at pH 10, the SHSRCs and SCABSRCs are consistent with each other and higher than SDSRC.
As shown in
Table 2, the CV of the solvent retention capacities of the eight oat flours rank in the decreasing order of 5% lactic acid (19.18), water (12.71), sodium cholate (11.86), NaOH (pH 10) (11.47), Na
2CO
3 (pH 10) (11.38), CaCl
2 (8.25), 5% Na
2CO
3 (8.13), FeCl
3 (8.10), SDS (pH 10) (7.84), NaCl (6.77), 50% sucrose (3.89). The coefficient of variation (CV) is a statistic that measures the degree of variability in the data, and can determine whether there are significant differences among samples. Significant differences among samples can help expected the experimental errors.
With their solvent pH turned acid (lactic acid) or alkaline (Na
2CO
3), the SRCs of the oat flours slightly increased. Zhang [
22] interpreted that hydrogen ions or hydroxyl ions could affect hydrogen bonds between dietary fiber molecules, resulting in partial breakage and combination of hydrogen ions inside fiber molecules with water and then increased WSRCs. With the lower ionic valencies of metal salt solvents, the SRCs of oat and wheat flours turn lower and are much higher than that against sodium cholate.
Because of the higher pentosan contained in wheat flour, the sucrose retention capacity is higher than those of oat flour [
15], which follows the characterization of sample-contained pentosan in the standard SRC assay that SSRCs can indicate sample-contained pentosan. The SRCs of wheat flour against all the other solvents are lower than those of the oat flours, which can be attributed to the more dietary fibers dominated by β-glucan in oat flours [
23].
In order to identify the effects of components in oat flours on SRCs, we conduct the coefficient of correlation between components and solvent retention capacities and summarize in
Table 3. β-glucan contents are extremely significantly positively correlated with the SCSRC values (0.83**), CCSRC values (0.82**) and SCHSRC values (0.80**), and significantly positive to the FCSRC values (0.78*). Protein contents are extremely significantly positively correlated with SCABSRC values (0.85**), SHSRC values (0.83**), FCSRC values (0.82**) and WSRC values (0.77**) and significantly positive to that SDSSRC values (0.76*), SCASRC values (0.75*) and SCSRC values (0.72*). Starch contents are significantly negatively correlated with the FCSRC values (−0.69*). There is no significant correlation between damaged starch and SRCs.
Based on the report of Guo et al. [
17], β-glucan in oat flour appeared to be correlated with three metal salts and could increase the SRC with lower solvent ionic valency for the ability of absorbing metal ions. The correlativity between β-glucan and cholate salts derives from the absorption of cholate salts.
2.3. SRCs and SRC Correlations with β-Glucan, Protein and Starch Additions
In order to find out SRC patterns of oat flour versus β-glucan, protein and starch, protein (with a content of 81.96%), β-glucan (with a content of 88.20%) and starch (with a content of 90.33%) were extracted from the oat flours and then added into the flours of Baiyan 2, Baiyan 15 and Bayou 9 (
Table 4). The prepared Baiyan 15 flours are with the differentiated β-glucan contents of 3.98%, 4.95% and 5.91%, the differentiated protein contents of 15.94%, 17.89% and 19.95%, and the differentiated starch contents of 57.15%, 59.15% and 61.11%. The prepared Baiyan 2 flours are with the differentiated β-glucan contents of 4.33%, 5.29% and 6.30%, the differentiated protein contents of 16.05%, 18.01% and 20.02%, and the differentiated starch contents of 58.22%, 63.23% and 68.28%. The prepared Bayou 9 flours are with the differentiated β-glucan contents of 4.70%, 5.66% and 6.71%, the differentiated protein contents of 16.30%, 18.25% and 20.23%, and the differentiated starch contents of 54.61%, 59.60% and 64.65%.
With the addition of β-glucan, flours from the three oat varieties present the values of WSRC, SSRC, LASRC, SCASRC, CCSRC, SCHSRC that tended to increase slightly (
Table 4). Similar increase is found in values of SCASRC, CCSRC and SCHSRC according to
Table 2.
With more protein added, the flours tend to show varying but generally increasing SRCs (
Table 4). The values of WSRC, SSRC, LASRC, CCSRC, SCHSRC (
Table 2) are not stable, while the SCASRC values increase.
With more starch added, all the SRCs of the three oat flours tend to decrease (
Table 4). However, oat flours with higher starch contents in
Table 2 present high SRCs for the combined effects of β-glucan and protein [
18,
24]. The water holding capacity of starch is weaker than protein and β-glucan.
With both β-glucan and protein added at 2%, the former exerts significantly stronger influence on the SRCs than the latter (
Table 4), indicating that β-glucan has the stronger ability of holding and absorbing water [
18,
25].
Based on the Pearson’s correlation coefficients in
Table 5, the β-glucan content are significantly correlated with the CCSRC values (0.64*), while the protein contents are not correlated with the SRCs against all the six solvents and the starch contents are significantly negatively correlated with SCASRC values (−0.74*). It follows that with flour differences among the oat varieties excluded, β-glucan can be evaluated in terms of CCSRC values. Protein contents do not present proper SRC patterns. SRC deceases with the increase of starch content, eliminating the possibility of SRC against starch exerting influence on β-glucan.
In one word, the CCSRC values are only related with the β-glucan content and have no relation with the protein and starch contents. As a result, CaCl2 has the potential to be used as a β-glucan assay reagent for the solvent retention capacities method.