3.2.1. Bread Texture and Aging Kinetics

The texture of the breads produced with different contents of yoghurt and curd cheese was evaluated in terms of firmness by a puncture test. The determination of firmness during a storage time of 96 h at room temperature was also performed to evaluate the effect of dairy product additions on the kinetics of bread aging (Figure 3A,B) [12]. From Figure 3A,B, it can be observed that both Yg and Cc additions had different impacts on the aging kinetics of the breads.

**Figure 3.** Variation of firmness (N) and aging kinetics for breads prepared with different contents of Yg (**A**) and Cc (**B**) addition, replacing the flour in bread formulation, at initial and 96 h of storage time, and respective linear equations: staling rate as crumb firmness with time for the different concentrations of Yg and Cc.

Bread aging kinetics was described as a function of the dairy products incorporation, and a positive linear relation (R<sup>2</sup> > 0.98) was observed, with parameters presented in Table 2, and clearly reflect the impact of the different levels of Yg and Cc addition on bread aging kinetics (A, the slope) and firmness (B, the interception).

As can be seen from Figure 3A,B and the respective linearizations on Table 2, the Yg breads are clearly less firm than Cc breads: the initial firmness of the control bread (1.21 N) is higher than low incorporations of Yg up to 50 g (1.15 N), and for higher replacements (70 g), the firmness is slightly higher (1.39 N) than control.


**Table 2.** Parameters of linear relationship: bread aging kinetics (A) and firmness (B) obtained for control bread (CB—0 g/100 g) and for the different levels of yoghurt (Yg) and curd cheese (Cc) tested.

The Yg breads aging kinetics (A) ranging from 0.78 <sup>×</sup> <sup>10</sup>−<sup>2</sup> <sup>N</sup>/h for the 10 g up to 1.36 <sup>×</sup> <sup>10</sup>−<sup>2</sup> <sup>N</sup>/<sup>h</sup> for the 70 g of Yg addition and are similar to control bread (0.94 <sup>×</sup> <sup>10</sup>−<sup>2</sup> <sup>N</sup>/h). In the case of Cc breads, a completely different effect can be observed, i.e., all breads are firmer and the rate of staling is higher than the control bread. The initial firmness up to 30 g of addition is similar to the control, but at 50 g of Cc addition, the increased is about 130%, and at 83 g, it goes up to 330%.

The rate of staling is also increasing substantially, ranging from 1.70 <sup>×</sup> <sup>10</sup>−<sup>2</sup> <sup>N</sup>/h for the 10 g, up to 3.72 <sup>×</sup> <sup>10</sup>−<sup>2</sup> <sup>N</sup>/h for the 83 g curd cheese addition, representing an increase of 120%.

Similar results were obtained in earlier studies [11], indicating that the whey protein significantly increase the bread crumb firmness. Our findings are also in agreement with those obtained by other authors [3,10] with the addition of dairy products in bread dough formulations.

This difference in bread texture with addition of Yg and Cc should reflect the complexity of the interactions of the major macromolecules at play and are in agreement with the results observed in extension properties evaluation (Figure 1). Starting from the gluten matrix, when yoghurt is added, some reinforcement of the structure was seen by the increase of extensibility and deformation energy. Values of dough resistance were similar to the control, not interfering substantially on the R/E values. The opposite is observed for curd cheese breads, where R/E values increased steeply, revealing an imbalance of the elastic and viscous components of the wheat dough, clearly reflected in bread texture and aging kinetics.

#### 3.2.2. Quality Parameters of the Bread

The quality features of breads, i.e., moisture content, water activity (aW), specific bread volume (SBV), and the percentage of bake loss (BL), were determined, as well as the crumb cells numbers. The results of quality parameters obtained for yoghurt and curd cheese breads are summarized in Table 3. No significant differences were observed in initial moisture content (43.5–42.6%) and water activity (0.95–0.92) for the different yoghurt breads and the control.

According to the results obtained for the Cc breads, significant differences were observed for the initial moisture content, varying between 43.5% and 38.9%, with a reduction of 11.0% of moisture, compared to the control bread and the higher Cc concentration (83 g). This effect can be explained by the water competition between wheat proteins and precipitated whey proteins during the dough development. Similar results were obtained by other authors [10] by testing the incorporation of dairy by-products on new bread formulations. Water activity had no significant variations (p > 0.05).


**Table 3.** Quality parameters, moisture, aw, bake loss, specific bread volume (SBV), and cell numbers of the control bread (CB—0 g/100 g) and breads produced with yoghurt (Yg) and curd cheese (Cc) addition \*.

**\*** Different letters (a, b, c, d) within the same column, for each dairy product, indicate statistically significant differences at p < 0.05 (Tukey test), compared with the bread control parameters.

No significant differences (p > 0.05) in specific bread volume (SBV) values between control and Yg breads, up to 50 g, were observed (Figure 4A), as well as in the crumb cell numbers (Table 3). The highest replacement tested, 70 g Yg, presented slightly lower SBV (3.30 cm3/g), compared to the control bread (4.44 cm3/g), representing a decrease of 26% (p < 0.05). In terms of bake loss values, no significant differences, up to 50 g of Yg addition, were observed. However, for 70 g of Yg addition the bake loss was lower (11.6%), compared to control bread (16.0%), and this is in line with the slightly higher density observed, leading to a less weight loss during baking.

**Figure 4.** Breads obtained with different levels of yoghurt (**A**) and curd cheese (**B**) incorporation: 10 g, 20 g, 30 g, 50 g and 70 g Yg, and 83 g Cc/100 g wheat flour, compared to control bread (without dairy product, 0 g/100 g wheat flour).

These good bread volumes of the yoghurt bread are probably due to the synergic effect of the different proteins on the system. In addition, the contribution of exolpolyssacarides, by building a structured polysaccharide network that interacts with the gluten matrix [21], can give more stability and extensibility to the wheat dough (Figure 1B), contributing to the gas retention and leading to a good appearance and desirable bread volume, as observed in Figure 4A.

The addition of Cc to the dough reduces significantly (p < 0.05) the specific bread volume (4.44–1.28 cm3/g) during the baking process compared to control bread at the highest Cc concentration (83 g). As expected, the crumb gas cells for these breads decreased significantly (p < 0.05), varying from 2748.5 (control bread) to 276.0 (83 g of Cc), a reduction of 90% that is a consequence of the depression of 70% in bread volume, clearly observed in Figure 4B. These results, caused by the addition of precipitated whey protein, could be attributed to the antagonistic interaction between these proteins and the gluten complex, reducing the flexibility and the extensibility of the network (Figure 1B), which increases the density of the bread reducing the volume [1].

Our results are in agreement with those obtained by other authors [3] who reported the effect of the whey protein on loaf volume decrease, and hence on the reduction of the gas cell numbers, but sodium caseinate and hydrolyzed casein additions had no significant impact on these values.
