*3.5. Starch Properties of Breads*

The contents of resistant and digestible starches are shown in Table 5. The quinoa bread had the highest amount of resistant starch of 3.28% d.w., while the rice bread had the highest amount of digestible starch of 81.48% d.w.

De la Hera et al. [51] prepared and characterized the gluten-free bread based on rice flour and reported resistant starch of 0.89–1.96 g/100 g and rapidly digestible starch of 82.07–96.54 g/100 g. They mentioned that the lowest values were obtained when coarse flour was used for samples preparation and dough had low hydration.


**Table 5.** Rapidly and slowly digestible starch of quinoa, sorghum, millet and rice breads.

When analyzing the red and white sorghum flour, Khan et al. [14] reported resistant starch contents of 2.95 and 2.21% d.w., respectively, higher than in durum wheat semolina (0.42% d.w.). The higher content of resistant starch of the sorghum flours could be the result of the inhibitory effect exerted on the digestive enzyme by the polyphenols or could be due to the interactions occurring between starch and proteins impeding the efficient enzyme recognition of the specific substrate [52]. The cooked pasta prepared with 40% red sorghum flour and with 40% white sorghum flour had 1.44 and 1.16% d.w. resistant starch, respectively.

Yousif et al. [17] found that wheat flour supplementation with red and white sorghum flour resulted in the increase of the resistant starch content of the bread samples from 30.1 g/100 dry starch up to 38.9 and 36.4 g/100 dry starch, respectively. The same authors reported lower rapidly digestible starch contents in the wheat bread samples containing red and white sorghum flour (27.7 and 29.0 g/100 dry starch), than in wheat bread (38.4 g/100 dry starch). Additionally, Yousif et al. [17] noted no correlation between the damaged starch content and starch digestibility of the sorghum containing breads. The activity of the in vitro digestive enzymes might have been limited when incorporating the sorghum flour into the wheat breads, through impeding the efficient starch gelatinization or through inhibition exerted by sorghum polyphenolics [52], therefore reducing the rapidly digestible starch levels. In fact, Taylor and Emmambux [52] mentioned no particular chemical or structural characteristics of sorghum starch, which might result in slow digestibility. It appears that the reduced digestibility arises from the presence of sorghum polyphenols and from the interactions between sorghum starch and proteins which are highly cross-linked through di-sulphide bridges [53].

Xu et al. [3] demonstrated that the wheat flour substitution with quinoa flour results in bread samples containing higher contents of slowly digestible starch and resistant starch and reduced in vitro digestibility with lower estimated glycemic index. The increase of the percentage of quinoa flour from 0 to 15% in wheat flour increased the resistant starch content of bread from 21.83 to 31.94%, and decreased the rapidly digestible starch from 54.41 to 39.23% [3]. The authors explained that the high amounts of dietary fibers and polyphenols found in quinoa flour might have contributed to the decreased starch digestibility in bread with quinoa flour. The polyphenols might inhibit the activity of the amylases, by impeding the contact between enzymes and starch. Quinoa seeds appeared to be rich in insoluble simple or highly polymerized phenols which can be associated with the carbohydrates [50]. In addition, as suggested by Li et al. [54], the enzyme assisted starch hydrolysis might be significantly affected by the presence of high proportion of short chains of amylopectin.

#### **4. Conclusions**

The composition and physical properties, resistant starch, antioxidant activity and total phenolic content of quinoa, sorghum and millet revealed the huge potential of these flours to be used for developing functional gluten-free bread. All these flours have higher contents of proteins, fat and fiber compared to the rice flour, usually used in the gluten-free products as main ingredient. Compared to the rice bread, the quinoa bread had higher specific volume and lower crumb firmness, while sorghum and millet bread had lower specific volume and higher crumb firmness. All breads had the total phenol contents and antioxidant properties higher compared to the rice bread. Quinoa bread had the best physical properties, and the highest total phenol content. Additionally, quinoa bread had the highest resistant starch content among all investigated bread samples. These results indicate that quinoa demonstrated the highest potential to be used for obtaining high-quality gluten-free breads.

**Author Contributions:** Conceptualization, I.A. and I.B.; methodology, I.B.; validation, I.A.; formal analysis, I.B.; investigation, I.B. and I.A.; writing—original draft preparation, I.B.; writing—review and editing, I.A.; supervision, I.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors thank to Vitănescu Maricel for supplying the quinoa seeds.

**Conflicts of Interest:** The authors declare no conflict of interest.
