*3.3. Rheological Properties of Dough*

The coefficient of consistency (K), flow behavior index (n), r-value (statistical correlation coefficient), and chi-square (χ2) are presented in Table 5. The power-law model showed low chi-square (χ2) and high values of the correlation coefficient (r) for all samples. The value of consistency coefficient ranged between 103 and 1049 Pa.sn. The highest values of the K consistency coefficient were observed for DPPP addition and then DPPSP. The lowest values were found in the samples of DPP dough. There was no statistically significant difference between the K consistency coefficient of the control bread and DOP, DAP, and DPP at the 2.5% and 5% additive levels and between the K of DTP at the 5%

additive level and DPPSP at the 2.5% and 5% additive levels. As the amount of DOP, DAP, DPP, or DTP increased in the dough, the K consistency coefficient decreased. This could be related to the pre-hydration of the by-products before adding them to the dough, where the water absorption and the consistency of the dough are reduced. The flow behavior index ranged from −0.436 to 0.33. In accordance with Ronda, Pérez-Quirce [35], the n values of gluten-free dough ranged between 0.25 and 0.35. Then, values of flow behavior index of whole dough were less than 1, indicating pseudo-plasticity (shear thinning) of the samples. In the case of pseudoplastic substances, the viscosity decreases with the increase in the shear rate because the connections between the device components break down as a result of shearing, which, under the influence of shear, decomposes the interactions between the components of the system [67]. Increasing levels of DOP, DPP, and DPPP additive reduced the value of the flow behavior n index of these samples.


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; K: consistency coefficient; n: flow behavior index; r: statistical correlation coefficient; χ2: chi-square; *P*: *p*-value probability; *F*: F-value Fisher; a–g letters indicate a statistical different of means in the same column (*p* < 0.05).

> According to Cheremisinoff [68] and Ronda, Pérez-Quirce [35], the coefficient of consistency k and the flow behavior index n depend on temperature, pressure, and formulation. While k is more temperature-sensitive than n on the non-Newtonian liquid food [69], the suspension becomes less viscous as temperature increases and more viscous when pressurized [70]. The flow behavior index varies depending on the formulation, in particular the hydration of the dough. Some samples showed a negative value of the flow behavior index n. There are very few studies that explain the negative values for the flow behavior index. These negative values can be attributed to the presence of slippage in the fluid, viscosity dispersion, or molecular degradation of the sample [71–73].
