*3.3. Rheological Measurements*

#### 3.3.1. Steady Shear Analysis

The flow characteristic curves of the PoS and PoS-PPC complexes are shown in Figure 2, and all the fitting parameters are listed in Table 2. Since the correlation between shear stress and shear rate was nonlinear and all *n* values were less than 1, the PoS and PoS-PPC blends were non-Newtonian fluids, exhibiting a shear-thinning property (a pseudoplasticity behavior). As for all the samples, stress shear hysteresis loops appeared in the shearing process, indicating a thixotropic effect. A large hysteresis ring area signifies a strong thixotropic property [38]. The *K* value is related to the starch viscosity, and the larger the *K* value, the poorer the system mobility and shear stability [39]. The hysteresis ring area of starch and consistency coefficient, *K,* decreased with the addition of PPC, which suggested that the starch paste thixotropy was reduced and stability was improved by adding PPC, and the effect was more noteworthy for PPC with a lower DP. This finding was in agreement with the BD result of RVA. The same trends of *K* value and hysteresis ring area of starch added with polysaccharides have been reported before, and these results indicated that the relative stability and uniformity of the starch gel structure was enhanced [4,19].


**Table 2.** The steady shear rheological parameters of PoS and PoS-PPC pastes modelled by the Power Law equation.

PoS: potato starch; PPC: polymeric proanthocyanidin; *K*: the consistency coefficient; *n*: the flow behavior index.

The value of *n* is inversely correlated with the pseudoplasticity degree of the entire system [31], PPC replacement decreasing the *n* value, for PPC1 the most. This meant that the presence of PPC made the entangled starch molecular chains have a greater tendency to be straightened or dispersed under the action of the external force, which was conducive to the flow of starch paste, consequently lowering the viscosity more obviously. The result was unanimous to the change of the RVA test.

**Figure 2.** Steady flow curve of PoS and PoS-PPC pastes. PoS: potato starch; PPC: polymeric proanthocyanidin.

#### 3.3.2. Dynamic Rheological Analysis

The viscoelastic properties were determined by dynamic rheological analysis (Figure 3). The storage modulus (G- ) represents the starch elastic properties and reflects the ability of the material to restore its original state after deformation, while the loss modulus (G--) represents the viscous nature of starch colloid and characterizes the ability of the material to resist flowing [4]. G and G- of all samples showed mild frequency dependence; in the total frequency range, the value of G was markedly higher than the G- value, indicating that PoS and PoS-PPC blends displayed weak gel-like behavior [40]. Replacement of PoS with PPC increased the mechanical modulus, especially G- . This result indicated that PPC prominently affected the elastic properties instead of the viscous characteristics of the PoS-PPC mixtures. As a higher G means greater rigidity and strength of the PoS paste network structure, proancyanidins with a slightly lower DP could strengthen the gel network of starch more significantly [41]. Because amylose is the main polymer that forms the cross-linked network [42], we can speculate that the connection of PPC and starch internal amyloses was enhanced through hydrogen bonds, resulting in the increase of the elastic property of starch paste. The more externally active hydroxyl groups of PPC1 increase the entanglement points between molecular chains, facilitating the formation of a three-dimensional gel network structure [40].

In summary, starch-PPC pastes had better gelling properties than starch paste, and the lower the degree of PPC polymerization, the more obvious the effect. PPC addition could make PoS more suitable candidates as gelling agents for food industry manufacturing, including soft candies, ice cream, and meat foods production, thereby improving these products' nutrition, texture, and stability [43].

**Figure 3.** Variation tendency of storage modulus (**G***-* ) and loss modulus (**G***--*) with frequency for PoS and PoS-PPC mixtures. PoS: potato starch; PPC: polymeric proanthocyanidin.

#### *3.4. Thermodynamic Properties*

The thermodynamic parameters obtained from DSC are shown in Table 3. The PPC addition increased the pasting transition temperatures (To, Tp, and Tc) of PoS within 2 ◦C, indicating that PPC could delay starch gelatinization [28]. The results were supposedly attributed to the partial undissolved PPC attached to the starch granules, which enlarged steric hindrance and suppressed the swelling and fracture of starch. Similar effects have been observed by Xiao et al. [44] and Zheng et al. [30], who found that black tea extract and proanthocyanidins from Chinese berry leaves discernibly increased the gelatinization temperatures of rice starch, and they ascribed it to the stability of interactions between these polyphenols and starch.



Different letters in a column signify significant differences (*p* < 0.05). PoS: potato starch; PPC: polymeric proanthocyanidin. To: onset temperature; Tp: peak temperature; Tc: conclusion temperature; Δ*Hg*: gelatinization enthalpy; Δ*Hr*: retrogradation enthalpy; *R*: the degree of retrogradation (Δ*Hr*/Δ*Hg*) × 100.

> The gelatinization enthalpy value Δ*Hg* represents the energy required for the melting of the starch crystallization zone, especially for the double helix structure of amylopectin crystals [45]. Compared to the control (13.50 J/g), the Δ*Hg* for starch gelatinized with 5% PPC1, PPC2, and PPC3 decreased to 12.17, 8.70, and 8.07 J/g, respectively. PPC has a polyhydroxyl structure. The OH groups of PPC interacted with the side chain of amylopectin and bound to the amorphous area of starch particles to varying degrees. They facilitated the transition from the water-absorbing swelling in the amorphous zone to the crystalline zone for the starch granules, thereby altering the coupling force between the

crystalline and amorphous matrix [46]. The consequent hydration easily led to a decrease in gelatinization energy.

The Δ*Hr* and *R* value of starch mainly reflect the recrystallization level of amylopectin and the long-term retrogradation degree of starch [40]. With the addition of PPC, the Δ*Hr* and *R* values of starch declined significantly, indicating that the degree of order and crystallinity of starch decreased during the retrogradation process. On the one hand, the strong hydrophilicity of PPC reduced the content of free or available moisture in the starch-PPC system. On the other hand, PPC associated with starch molecules by intense hydrogen bonding and hydrophobicity interactions and, consequently, played a role in steric exclusion, which all limited the activity of the starch chains and retarded retrogradation [47]. Among the three types of PPC, PPC3 had the strongest suppression effect on starch retrogradation, then PPC2, and finally, PPC1. This might be ascribed to the increased binding affinity and specificity for larger proanthocyanidins with a multidentate character [48], which also showed that macromolecular proanthocyanidins possibly had a greater impact on amylopectin and could better inhibit amylopectin recrystallization.

#### *3.5. X-ray Diffraction Patterns*

The results of XRD are shown in Figure 4. Native potato starch (NPoS) displayed a typical B-type crystal structure, with representative peaks appearing at 2θ = 5◦, 15◦, 17◦, 21◦, and 24◦. During gelatinization, the crystalline structure of starch granules was destroyed by hydrothermal treatment and converted into an amorphous form [33]. However, during retrogradation, the starch chains tended to recombine into an ordered crystal structure and show a B-type XRD spectra (2θ ≈ 17◦), which is principally due to rearrangement of the amylopectin part [6,49]. The supplementation of PPC led to the disappearance of starch Btype diffraction peaks, indicating that PPC could effectively restrain the recrystallization of amylopectin. Studies have shown that starch will form V-type crystals when combined with fatty acids, iodine, and other substances, and the V-type inclusion shows representative diffraction peaks at 20.0◦ and sometimes at 7.0◦ and 13.0◦ [50]. The complex had an obvious diffraction peak at 2θ = 13.1◦, which was primarily due to the formation of amylose–lipid complexes, further hindering amylose rearrangement [28].

**Figure 4.** XRD diagrams of PoS and PoS-PPC blends, and the relative crystallinity is shown next to the sample name. NPoS, native potato starch; PoS, potato starch; and PPC, polymeric proanthocyanidin. PoS: potato starch; PPC: polymeric proanthocyanidin; 2θ: Diffraction angle.

After adding PPC1, PPC2, and PPC3, the relative crystallinity of potato starch decreased from 17.94% to 16.86%, 15.54%, and 14.44%, respectively. It is speculated that when embedded in the starch-moisture matrix, PPC could have interfered with the rearrangement of starch chains, especially for amylopectins, and a hydrogen-bond interaction might have been the main cause of their mutual effects [51]. The result also showed that larger proanthocyanidin molecules exerted a more significant influence on starch crystallinity than smaller ones, which indicated that proanthocyanidins with high DP values were more likely to destroy the long-range ordered crystalline structure and, thus, retard starch long-term retrogradation. Due to their large steric hindrance, macromolecular polyphenols are difficult to insert into the cavity of the amylose double helix to form inclusion compounds through hydrophobic interactions [52,53]. In other words, long-chain PPCs affect the overall distribution and arrangement instead of short-range double helical construction of the starch chains [30]. This result was consistent with the DSC.

PoS is often served as a substrate for producing starchy packaging films due to its edibility and biodegradability. However, on account of the shortcomings of PoS in processing, starch film production faces the problems of poor mechanical properties and easy crystallization [54,55]. Through our research, we found that PPC as a functional compound can not only serve as a mechanical reinforcement to form an interpenetrating network with starch and enhance the mechanical characteristics of starch films, but it can also suppress the recrystallization and retrogradation of starch, thus maintaining the preserved starch film as flexible and stretchable.
