3.4.2. Specific Volume of Bread

The specific volume of gluten-free bread varies depending on the ingredients and the preparation process [75]. The addition of by-products influences the specific volume of bread to a different extent (*p* < 0.0001). The specific volume of gluten-free bread ranged from 1.48 to 2.50 cm3/g. All formulas had a much higher specific volume compared to the gluten-free control bread (1.48 cm3/g), but a lower specific volume compared to the wheat control bread (3.39 cm3/g)

The specific volume was the highest for DPPP bread at 5% (2.50 cm3/g), followed by DOP (2.46 cm3/g) at 7.5%, and the lowest specific volume was recorded for DPP bread (1.69 cm3/g) with an addition of 2.5%. These results are supported by O'shea et al. [76] with 4% addition of orange pomace. There were no significant differences between the specific volume of DOP bread at 2.5%, DAP bread at 2.5%, and DPP bread at 5% addition.

Overall, the specific volume increased with increasing levels of by-products, with a maximum found at 7.5% (*w*/*w*). These results are supported by Arslan et al. [77], who studied the effect of powdered guava pulp on gluten-free bread, Singh et al. [78], who assessed the effect of dietary fiber from black carrot pomace on gluten-free rice muffins, and TÜrKEr et al. [79], who investigated the effects of the green banana peel on gluten-free cakes. According to these authors, adding more than 7.5% by-products reduced the specific volume of gluten-free products. Parra et al. [75] reported that the right balance between the amount of apple pomace and water allowed for a gluten-free bread with an acceptable specific volume to be obtained.

#### 3.4.3. pH Value of Bread

The pH value of bread ranged from 5.21 to 5.87. Except for 5% DPPP bread and 7.5% DPPSP bread, all formulas had a significantly lower pH value than gluten-free control bread (*p* < 0.05). The pH values were similar for DPP bread at 7.5% and DPPSP bread at 2.5% and 5% additive levels. DTP and DPPSP increased the pH value of bread.

The pH of the gluten-free bread was reduced with the addition of DOP and DAP. Similar results were obtained by Majzoobi et al. [47] for carrot pomace powder, where the pH value of gluten-free cakes was reduced due to the presence of organic acid [80], amino acids [81,82], and other acidic components.

#### 3.4.4. Moisture Content of Bread

As shown in Table 6, the bread moisture content ranged from 27.21% to 31.13%. The control wheat bread had the lowest value of moisture. The addition of DOP, DAP, DPPP, and DPPSP increased the moisture content of gluten-free bread. A lower value (28.71%) was noted for DPPSP with 2.5% addition. The moisture content of the gluten-free bread was gradually increased with the gradual addition of DPP and DPPSP and decreased by the addition of DOP. The moisture content measurements showed no significant differences between gluten-free control bread, DPP bread with 2.5% and 5% addition, and DTP bread with 5% and 7.5% addition. Increased bread moisture contributes to an increase in the bread weight.

#### 3.4.5. Crust and Crumb Color of Bread

The results concerning the color of the crust and crumbs are presented in Table 7. A golden brown crust and creamy white bread crumbs are the most important factors indicating the quality of a bakery product to consumers [5]. The addition of by-products had a significant (*p* < 0.05) effect on the color characteristics of crust and crumbs.

The lightness component (*L\** for crumb and crust) decreased with the addition of by-products compared to the control wheat and gluten-free bread. For each by-product, the lightness of the crust and crumb color deceased as the level of the additive increased.


**Table 7.** Crust and crumb colors of wheat bread and gluten-free bread with and without added by-products.

DOP: dried orange pomace; DAP: dried apple pomace; DTP: dried tomato peel; DPP: dried pepper peel; DPPP: dried prickly pear peel; DPPSP: dried prickly pear seed peel; CTRL: control; GF: gluten-free. *P: p*-value probability. *F*: F-value Fisher. a–n letters indicate a statistical different of means in the same column (*p* < 0.05).

> Increased levels of DOP, DPP, DTP, and DPPP allowed increasing yellowness (*b\*)* and redness (*a\**) of the gluten-free bread crumb. The gluten-free color of the bread crumb supplemented with by-products was directly correlated with the ingredients used in the production of the dough.

> The color of bread depends on the formulation or baking condition. Maillard reactions and caramelization of the crust are responsible for the changes in the color parameters between the crust and the crumb and the development of brown color on the surface at high temperature, but the color given to the ingredients used in bread formulation may mask this color [5,55,83,84].

#### 3.4.6. Crumb Structure of Bread

The number of cells/mm2, average size, pore area fraction, perimeter, circularity, and solidity of crumb cellular structure obtained from image analysis are shown in Table 8.

The number of cells/mm<sup>2</sup> ranged from 78 to 308 cells/mm2. A higher number of cells/mm2 is demonstrated for DPPP and DPP at 2.5% followed by DPPSP at 7.5%, and a lower number was demonstrated for DOP at 5% and DAP at 2.5%. A higher average size was obtained for the wheat control bread (1.672 mm). The average crumb size increased significantly with the addition of DAP, DPP, and DTP and decreased with the addition of DPPSP. Crumbs of the wheat control bread and DOP and DPPP at 5% were characterized by a large perimeter.

Gluten-free bread with a large number of cells is characterized by an aerated structure against the crumbs with a small number of cells. According to Jafari et al. [85] the lower number of cells and the higher average size reflected the aerated structure. Solidity is a measure of the shape of disorder gas cells. The solidity value is lower for irregular shape of gas cells and higher for regular shape [86,87]. The solidity of samples ranged from 0.85 to 0.92. The shape of the gas cells DPP at 5% and DPPSP at 2.5% had a regular structure and uniform shape (solidity = 0.91 and 0.92, respectively) and greater roundness (circularity = 0.91 and 0.93, respectively) compared to control gluten-free bread. The crumb appearance of control and enriched bread is shown in Figure 3.


**Table 8.** Crumb structure of wheat bread and gluten-free bread with and without added by-products.

DOP: dried orange pomace; DAP: dried apple pomace; DTP: dried tomato peel; DPP: dried pepper peel; DPPP: dried prickly pear peel; DPPSP: dried prickly pear seed peel; CTRL: control; GF: gluten-free. *P: p*-value probability. *F*: F-value Fisher. a–i letters indicate a statistical different of means in the same column (*p* < 0.05).

### *3.5. Cluster Analysis*

A hierarchical cluster analysis and constellation plot were performed to identity analogies of the effects of by-products on the properties of the bread (Figure 4). The constellation plot consisted of four clusters. Group A included one cluster (C1) containing only the wheat control bread, and group B included three clusters: C2 (gluten-free control bread), C3, and C4 (gluten-free bread containing different by-products). The C3 cluster contained the highest counts of similar bread. The similarity between DTP, DPP, DPPP, and DPPSP at the additive levels of 5% and 7.5% was observed in the C3 cluster and the gluten-free bread with the addition of DOP, DAP, and DPPPS at the 2.5% additive level in the C4 cluster. The wheat control bread and gluten-free control bread differed highly from those with all samples containing by-products. The wheat control bread was characterized by higher values of Vsp, average size, perimeter, Hm, h, Tx, and RCO2, and the control gluten-free bread was characterized by a long time at the maximum height of dough T1, a high value of Vl CO2, and a low value of Vs, average size, Hm, and RCO2 (Figure 5).

**Figure 3.** Crumb appearance of wheat bread and gluten-free bread with and without added byproducts. DOP: dried orange pomace; DAP: dried apple pomace; DTP: dried tomato peel; DPP: dried pepper peel; DPPP: dried prickly pear peel; DPPSP: dried prickly pear seed peel; CTRL: control; GF: gluten-free; V*sp*: specific volume.

**Figure 4.** Constellation plot and hierarchical clustering of the investigated wheat and gluten-free bread samples based on the Ward method (level increase: orange, level decrease: green). DOP: dried orange pomace; DAP: dried apple pomace; DTP: dried tomato peel; DPP: dried pepper peel; DPPP: dried prickly pear peel; DPPSP: dried prickly pear seed peel; CTRL: control; GF: gluten-free.

**Figure 5.** Cluster standard deviations (Cluster 1: wheat control; Cluster 2: gluten-free control; Cluster 3: DOP (2.5%, 5%, 7.5%), DAP (2.5%, 5%, 7.5%), and DPPSP 2.5%; Cluster 4: DPP (2.5%, 5%, 7.5%), DTP (2.5%, 5%, 7.5%), DPPP (2.5%, 5%, 7.5%), and DPPSP (5%, 7.5%)).

#### *3.6. Multivariate Analysis of the Rheofermentometer and Rheological Parameters of Dough and Bread Quality*

Figure 6a presents the correlation circle obtained from the principal component analysis on the rheofermentometer and the rheological parameters of dough and bread qualities measured in wheat and gluten-free samples. The first and second principal components justified 49.3% of the variability. Variables with high eigenvectors on principal component 1 were Tx, Hm, average size, perimeter, h, Vsp, H m, and *L\** crumb (Figure 6a). On PC1, Tx, Hm, average size, perimeter, h, and Vsp were negatively correlated with H <sup>m</sup> and highly significant (*p* < 0.001) and positive correlations were noted between Hm and h (r = 0.94), followed by the Tx and Hm pairs (r = 0.89), Tx and h (r = 0.89) and Hm and Vsp (r = 0.81). Variables with high eigenvectors on principal component 2 were W, WL, *L\** crust, T1, pH, and H%. Negative correlations were observed between variables W and WL with *L\** crust, T1, and pH. Highly significant (*p* = 0.001) and negative correlations were found between pH and WL (r = −0.67), T1 and % (r = −0.62), and T1 and W (r = −0.67). A scatter plot for bread (wheat and gluten-free bread) is shown in Figure 6b. Principal component analysis confirmed the hierarchical results of the cluster analysis, in which the wheat control bread and the gluten-free control bread were distinguished from gluten-free bread containing by-products. The wheat control bread showed higher Tx, Hm, average size, and h; glutenfree control bread showed higher T1, pH, and Vl CO2; gluten-free bread prepared with DOP and DAP (all percentage) and DPPPS at 2.5% presented higher W, T 1, Vr, and CO2; and gluten-free bread containing DPP, DTP, DPPP, and DPPPS at 5 and 7.5% showed higher H m, K, circularity, solidity, *a\** crumb, and *a\** crust (Figure 5).

**Figure 6.** Multiple factor analysis correlating the percentage of by-products, dough rheological properties, and bread properties. (**a**) Map of parentage of by-products (in black), rheofermentometer parameters (in green) and rheological parameters of dough (in red), bread qualities (in blue), crumb structure of bread (in yellow), and crust and crumb colors (in purple). (**b**) Map of the distribution of the 20 types of bread.

#### **4. Conclusions**

Gluten-free bread based on the corn/chickpea formula was developed by adding different types of by-products (pomace of orange and apple; peels of tomato, pepper, and prickly pear; and prickly pear seeds). The rheology of the dough was measured and the properties of bread were evaluated. It was concluded that orange pomace, apple pomace, and peel of prickly pear seeds at 2.5% *w*/*w* induced the same effect on gluten-free bread. Pepper, tomato, prickly pear, and prickly pear seed peels with 5% and 7.5% addition had the same effect on gluten-free bread. Overall, all by-products increased Vsp, Hm, H m, VtCO2, RCO2, WL, and circularity, and decreased T1, Tx, n, area fraction, *L\** crust, and *L\** crumb as compared to control gluten-free bread. The addition of prickly pear peel at a 5% addition level can result in bread with good characteristics. These by-products could be used as economical and inexpensive improvers for gluten-free bread.

**Author Contributions:** This study was designed by F.D. and M.N.Z. The analysis was done by F.D. and the manuscript was written by F.D. and H.B. and corrected by R.R., A.B., and W.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by La Direction Générale de la Recherche Scientifique et du Développement Technologique (DGRSDT).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author R.R.

**Acknowledgments:** F. Djeghim acknowledges the Institut de la Nutrition, de l'Alimentation et des Technologies Agro-Alimentaires (INATAA), Algeria, the University of Life Sciences in Lublin, Poland, and the Centre de Recherche en Biotechnologie, Constantine, Algeria.

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