**4. Discussion**

The rheological testing of pasta dough can give information about the interactions of wheat and GPF components and could predict dough behavior during handling and modelling. One of the advantages of using a small amplitude oscillatory shear test is that no damages on the structure occurs and that it provides information about the structural changes to the system. All the tested samples presented G' > G", suggesting the solid-like nature of the material, which was expected. The increase of HMT temperature, time, and moisture determined the increase of G' and G" dynamic moduli probably as a result of the starch amylose and amylopectin chains reorganizations and/or interactions between phases during treatment [4]. Higher dough viscosity could be explained by the disulfide bonds formed by amino acids and starch structure reorganization [25]. During HMT, starch aggregates are formed which lead to changes of starch and gluten interactions [33]. Lazaridou et al. [34] also reported higher G' and G" values for wheat-barley treated flour, probably due to the plasticization process that HMT promoted causing starch amorphousglassy cell wall stiffening. Dynamic moduli of pasta dough increase revealed a structure reinforcement, similar findings being reported for vermicelli dough from sweet potato starch treated at moisture levels up to 30% [7]. On the other hand, G' and G" moduli rise could be attributed also to the addition of GPF, a fiber-rich ingredient. This trend may be caused by the competition for water between gluten and GPF and/or to the high fiber content that can behave as filler in dough networks, a fact supported also by the significant (*p* < 0.01) correlations of DF content. Similar results were reported by Mironeasa et al. [19]. The interactions between flour mix components may be improved or inhibited by GPF presence, compounds such as organic acids causing dough strengthening and consequently increases of G' and G" values [17]. Fanari et al. [35] revealed the dependence of dough rheological properties with the water content added in the system. Lindahl et al. [36] reported no significant differences of G' values (>10,000 Pa at 1 Hz) between durum and common wheat dough, underlying at the same time the importance of water. Peressini et al. [37] reported G' values between 43,000 and 145,700 Pa for durum wheat pasta dough enriched with different fibers.

Dough firmness increased as HMT temperature and GPF addition level increased. Lazaridou et al. [34] also reported higher firmness of dough made of treated wheat-barley flour, which indicated a stiffer dough related to the macromolecular rearrangements of flour molecules during HMT. The chemical composition of GPF could also be responsible

for dough firmness increase, with the strong correlation with DF content (*r* = 0.51, *p* < 0.01) supporting this hypothesis. Polyphenols presence may possibly contribute to the viscoelastic characteristics of pasta dough. Mironeasa et al. [17] also reported higher firmness values with GPF addition level increase at particle size <180 μm.

The synergistic effect of HMT and GPF addition was observed on dry pasta luminosity. HMT caused an increase of *L\** with temperature and moisture level increase, which may be related to the modification of starch crystalline architecture which led to physicochemical property changes [38]. On the other hand, GPF decreased *L\** values as the addition level was higher due to its pigments and to the promotion of nonenzymatic browning reactions that would determine darker products. Similar observations were made by Aksoylu et al. [39] for grape seeds enriched biscuits. TPC from GPF may also have contributed to *L\** decrease, with a very strong correlation (*r* = −0.81, *p* < 0.01) being observed.

Pasta DF content was significantly increased (*p* < 0.01) as the HMT moisture and time and GPF level increased. There results may be related to the aggregate formation and amylose/amylopectin chain structure changes under heat energy and water molecule migrations, with similar results being reported by Zheng et al. [40]. Furthermore, starch may form complexes with other composite flour components such as lipids, proteins and polyphenols, intensifying the effects of aggregations, static, and hydrogen linkages [41]. In addition, GPF is a rich source of soluble and insoluble DF, which contributed significantly to the pasta DF content increase.

The polyphenolics content was negatively affected by HMT, but this decrease was countered by the addition of GPF, which resulted in a TPC increase. HMT may possibly cause bioactive compound damage at high temperatures. GPF is known to be a source of polyphenols such as malvidin-3-O-glucoside, peonidin-3-O-glucoside, cyanidin-3-O-glucoside, and catechin [42–44]. Our results are in agreement with those of Sant'Anna et al. [16], who reported higher TPC for fettuccini pasta enriched with grape pomace, and of Gaita et al. [12], who studied wheat pasta supplemented with grape peels.

Pasta texture is a good predictor of the sensory profile and represents a key factor in consumer acceptance. The increase of HMT temperature and duration determined a decrease in pasta firmness, which may be related to the denaturation of gluten proteins. Liu et al. [45] reported lower tapioca starch gel hardness, probably due to the interactions between phases that occur during HMT. Galvez and Resurreccion [46] stated that pasta should be neither too hard nor too soft. The interaction between moisture and GPF level factor significantly (*p* < 0.01) positively influenced pasta firmness. Tolve et al. [14] reported an increase in pasta firmness when grape pomace was incorporated. On the other hand, pasta firmness decrease may be related to GPF ability to bind water, similar to gluten proteins. Polyphenols may also contribute to pasta texture behavior as a result of interactions with other components, a fact supported by the positive correlation (*r* = 0.42, *p* < 0.01) between firmness and TPC. Our data were in agreement with those reported by Li et al. [47] for noodles with treated wheat flour, which presented a firmness of 3397.76 g. The gumminess parameter was significantly decreased (*p* < 0.01) by HMT conditions and GPF level increase. Chandla et al. [48] also obtained lower gumminess values for pasta made of HMT amaranth starch noodles. Using a small particle size of GPF (<180 μm) might have favorable effects on pasta texture by lowering gumminess values, similar to findings being reported by Chen et al. [49] for Chinese noodles with wheat bran addition.

RS is a fraction resistant to hydrolysis in the digestive system under the action of enzymes, having a physiological behavior similar to DF [50]. HMT and GPF addition significantly (*p* < 0.01) increased RS content of wheat pasta. Starch resistance to enzyme action could have been determined by the crystalline areas' perfection and more dense amorphous areas caused by HMT [5]. Similar to our results, Wang et al. [5] observed that RS formation is directly proportional to the moisture content of the sample. The interactions between flour components, especially starch, lipids, and proteins, could possibly be responsible for the higher RS content, since proteins films could have been formed on starch surface and lead to difficult amylase attack [4]. In addition, the presence of polyphenols from GPF

could have a decisive role in starch digestion. It was stated that polyphenols may interact with starch to form complexes through non-covalent linkages, which are not accessible for hydrolysis [51]. Furthermore, polyphenols may inhibit the digestive enzymes by means of proteins-polyphenols interactions [52]. Thus, the synergistic effects of HMT and GPF led to an improved functional value of pasta by increasing RS content.

The optimization of HMT regime and GPF level allowed the establishment of a formulation with maximum nutritional and functional benefits and with minimum quality impairment. Thus, at 87.56 ◦C with a moisture of 26.01% for 3 h and an addition level of 4.81% GPF, pasta presented high TPC content (11.36 mg/100 g) and RS content (2.80%). Chen et al. [4] reported an RS content of HMT wheat starch at 25% moisture of 2.64%. Nakov et al. [53] obtained a TPC value of 9.41 mg/100 g of cakes with 4% grape pomace powder.

#### **5. Conclusions**

HMT is a useful tool for physical modification of wheat flour that enhanced its nutritional and functional value. The synergistic effect of HMT and GPF addition resulted in high DF, TPC, and RS contents of pasta due to the intake of GPF and the starch and proteins modifications during treatment. Doug firmness and viscoelastic moduli increased with HMT temperature, time, moisture, and GPF level increases. Pasta firmness and gumminess lowering was proportional to HMT regime and GPF addition increase.

The optimal wheat flour HMT regime was found to be a temperature of 87.56 ◦C, a moisture content of 26.01%, and a time of 3 h. GPF can be incorporated at a quantity of 4.81% in order to achieve the best pasta quality with the most health benefits. These results could be helpful for the development of novel functional pasta by applying an easy, controllable process and by using an inexpensive fiber-rich ingredient like GPF. The data presented in this study could be helpful for further pasta processing automatization.

**Author Contributions:** Conceptualization, M.I. and S.M.; methodology, M.I., S.M. and A.B.; software, M.I.; validation, M.I., S.M. and A.B.; formal analysis, M.I.; investigation, M.I. and A.B.; resources, M.I., S.M. and A.B.; data curation, M.I.; writing—original draft preparation, M.I.; writing—review and editing, M.I. and S.M.; visualization, M.I. and S.M.; supervision, S.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Romania National Council for Higher Education Funding, CNFIS, project number CNFIS-FDI-2021-0357.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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

#### **References**

