*3.1. Wheat Dough Extension Properties (WDEP)*

The effect of yoghurt (Yg) and curd cheese (Cc) addition on wheat dough extension properties at 30 ◦C after 60 min of fermentation time was studied, and the results obtained are presented in Figure 1. It is clearly observed that the Yg and Cc have different impacts on dough extension properties, most probably due to the different nature and structure of the proteins added—acid precipitated caseins from Yg and soluble whey protein precipitated by thermal denaturation from Cc. The extension properties are positively affected by Yg addition and are adversely affected by Cc incorporations.

Considering Figure 1A, the dough resistance values were similar to control dough up to 50 g of Yg addition, whereas for 70gareduction of about 21% was observed compared to control.

For Cc additions, all the levels evaluated result in dough resistance values lower than the control, with a significant reduction of about 64% (p < 0.05) for the highest level of Cc added (83 g).

From Figure 1B,C, it can be seen that, at 20 g of Yg addition, a steep increase both in extensibility (B) and deformation energy (C) is observed, and remained constant for the rest of the levels evaluated, with higher values compared to the control. Relative to Cc additions, lower values of extensibility and deformation energy than the control dough were observed, except at 30 g of addition, which showed values similar to the control.

In terms of R/E ratio values, the differences observed for Yg and Cc additions are clearly reflected on Figure 1D. The resistance versus extensibility is an important ratio to evaluate the balance between dough resistance (elasticity) and extensibility (viscosity), and a good combination of both parameters is required to obtain desirable dough properties and bread quality [13].

**Figure 1.** Effect of the Yg and Cc incorporation, after 60 min of fermentation time at 30 ◦C, on the extension properties of the dough, in terms of: (**A**)—resistance to extension (N), (**B**)—extensibility (mm), (**C**)—deformation energy (N.mm<sup>−</sup>1) and (**D**)—ratio R/E (N.mm<sup>−</sup>1), with different levels of dairy product (DP) additions: 10, 20, 30, 50, and higher replacements (HR) tested—70 g for Yg and 83 g for Cc—compared with control dough (0 g/100 g). Different letters indicate statistically significant differences at p < 0.05 (in Tukey test), compared with the control dough parameters (black bars).

Overall, the addition of Yg has a positive impact on the structure of the dough, which was seen by the increase of the extensibility and deformation energy. One can suggest a synergic interaction between the Yg proteins and wheat proteins on the gluten network development. In addition, the presence of the exopolysaccharides produced by the lactic acid bacteria present in yoghurt, can also act on the system as lubricants, together with gliadins, giving more stability and extensibility to the wheat dough. Similar results were observed by other authors [3,10] who also reported an increase in deformation energy due to the strengthening effect of hydrolyzed caseins and sodium caseinates from the added dairy product.

However, opposite results were obtained for Cc addition that affected the dough properties in extension, suggesting that denatured whey protein has an antagonistic effect with wheat proteins, strongly affecting the gluten matrix. These results can be attributed to the gluten proteins dilution effect [11,12] and protein competition for available water, impacting negatively on dough development [19]. Similar results were previously reported by other researchers [10], demonstrating that dairy by-products such as whey protein, in general, reduce the extensibility and the deformation energy.

Therefore, it can be stated that the Yg addition had no adverse impact on the technology characteristics of the dough, which will have a positive impact on bread quality. The incorporation of the Cc implied a significant reduction on extensibility properties, which would promote a significant depression on bread volume.

#### Dough Viscoelastic Behaviour

The changes of dough viscoelastic properties after 60 min of fermentation time were monitored by a controlled stress rheometer. The variation of storage (G´) and dissipative (G") moduli was recorded

by frequency sweeping from 0.001 to 100 Hz at a low constant stress value under linear conditions, and the G´ values were plotted at 0.1 Hz and 1.0 Hz to show the rheology changes observed by addition of the Yg and Cc to the wheat dough (Figure 2A1,B1).

**Figure 2.** Variation of storage (G´) at 0.1 Hz and 1.0 Hz (**A1**–**B1**) and frequency sweep curves—change of the storage (G'—close symbols) and loss (dissipative (G")—open symbols) moduli with frequency (**A2**,**A3**; **B2**, **B3**), for samples with differents levels of DP: A yoghurt doughs and B for curd cheese doughs.

As can be seen at Figure 2A1,B1, which are focused on G´ at 0.1 and 1.0 Hz, the behaviour of the doughs with Yg and with Cc were similar. In both, the steepest increase on slope arises from concentrations over 20 g of addition. The almost linear increase of G´ with Yg and Cc addition is characteristic of the dough structuring, indicating a reinforcement of higher density of the molecular links.

The presence of vegetable (cereals) and animal (milk) proteins adds complexity to the protein matrix with protein–protein interactions, which leads to an increasingly structured material [3,20].

Figure 2A2/A3,B2/B3 represent the mechanical spectra obtained with different Yg (A) and Cc (B) additions, where it can be observed that the G´ and G" moduli increase with dairy product addition. In general, all the systems show a certain degree of dependence on the frequency range applied, and the addition of Yg and Cc promote the reinforcement of the dough structure, expressed in terms of viscoelastic functions (G´ and G") increase.

The apparent disagreement between technology performances—empirical large amplitude extensional rheology (Kieffer Dough and Gluten Extensibility Rig) and small amplitude oscillatory fundamental rheology—can be explained, beyond the different nature and structure of the proteins in play, if the presence of a significant amount of exopolyssacarides in Yg is considered. In fact, the major differences in nature from Yg and Cc are the content of exopolyssacarides produced by lactic acid bacteria and protein structural characteristics of the Yg material. As was recently published [21], the impact of exopolyssacarides on the rheology of dough shows positive interactions with the gluten matrix development.

## *3.2. Evaluation of Bread Properties*
