*3.2. Gas Production and Dough Development Parameters during Fermentation*

Table 4 shows the effect of by-product addition on the fermentation characteristics of doughs. Gluten-free dough with DOP, DAP, DPP, DTP, DPP, and DPPSP was characterized by a higher maximum dough height (Hm) compared to the gluten-free control dough, except for the dough with 2.5% DOP and 7.5% DPP, while this parameter was lower for the control wheat dough. Overall, the gluten-free dough with DAP, DTP, and DPPP had higher maximum dough heights (Hm) than the other samples.

The maximum height of gas release (H <sup>m</sup> ) of the gluten-free dough increased with the addition of selected dried peels at various levels, except for the 5% and 7.5% DOP addition. The time (T1) to achieve the maximum dough development ranged from 25 min to 84 min. Overall, all by-products at different levels reduced the time at maximum height (T1) compared to the wheat dough (at 63 min) and gluten-free dough (at 84 min). The increase in the H <sup>m</sup> value and the decrease in the T1 value could be related to the accelerated kinetics of CO2 formation after adding the dried peels.

The time of maximum gas formation (T´1) was influenced differently by each dried peel at different levels of addition. The same T´<sup>1</sup> value (45 min) was demonstrated for the gluten-free control dough and the DOP dough with 2.5% and 5% addition. The time of porosity (Tx) of all samples with by-products appeared earlier than the Tx of the control wheat dough (55.30 mn) and the gluten-free control dough (31.30 mn). The maximum dough height (Hm) of all samples was reached after the Tx. Verheyen, Jekle [61] indicated that the time when gas started to escape from the dough Tx and its retention capacity was related to the CO2 volume and the dough rheology. The addition of 7.5% DOP reduced the final dough height (h) to 0.9 mm.

Loss of dough volume W (weakening coefficient) after 90 min was high for all samples compared to the gluten-free and wheat controls, except for the 2.5% DPPSP addition for which the T1 increased (61:30 mn). The lower weakening coefficient (3.4%) and the higher final height (h) (5.6 mm) indicated that gluten-free dough with 2.5% DPPSP produced a dough with a stronger structure compared to all other samples.


**Table 4.** Dough development and gas production parameters 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; Hm: maximum height of dough; T1: time at maximum height of dough; H´m: maximum height of gaseous release; T´1: the time of maximum gas formation; h: height of dough at the end of the test; W: weakening coefficient = (Hm − h)/Hm; V**t**: total CO <sup>2</sup> production volume; Vl: volume of CO2 loss; Vr: volume of CO2 retained; R: retention coefficient of CO2; Hmadj: adjusted maximum height.

> The addition of selected dried peels at various levels to gluten-free doughs increased the total volume of CO2 production (Vt), decreased the volume of CO2 loss (Vl), and increased the volume of CO2 retained (Vr) as well as the CO2 retention coefficient (R), which means that the addition of by-products improves the production and the retention capability of CO2 compared to the non-enriched gluten-free dough.

> Significant gas production and retention have a positive effect on the final quality of the bread, as reported by Martínez and Díaz [34]. The CO2 retention capacity depends on the ingredients used and internal structure of the dough. The adjusted maximum height (Hadjm) of all samples was found to be lower than Hm after the yeast activity effect was isolated, which explains that the Hm was related to yeast activity and not only to dough rheology, where the selected by-products accelerated yeast activity.

> The by-products used contained fermentable sugars such as glucose and fructose [20,62] and minerals such as K+, Ca2+, and Zn2+ [63,64]. The yeast cells consume the fermenting sugars in the dough directly from the by-products or after pre-hydrolyzing the corn starch or chickpea flour, and produce carbon dioxide and ethanol, which are responsible for acidifying the dough during fermentation and the growth phase in the oven. On the other hand, the presence of the Ca2+ in fermentation media allows for maximum fermentation efficiency through increased alcohol production [65,66].
