**3. Results and Discussion**

### *3.1. Qualitative Analysis of PPC*

On the basis of the difference in the molecular weight, molecular structure, and molecular polarity of proanthocyanidins with different DPs, we reported on the use of such a gradient with acetone/methanol/water solutions (respectively, 2:5:3, 4:3:3, and 7:0:3 *v/v/v*) to preparatively separate the proanthocyanidins [9,31]. With the increase of acetone volume fraction, the average degrees of polymerization of the fractionated proanthocyanidins increased successively, which were 6.39 ± 0.13, 8.21 ± 0.76, and 9.92 ± 0.21, respectively.

#### *3.2. Pasting Properties*

Table 1 and Figure 1 exhibit the pasting properties of PoS in the presence of PPC with three DPs. As shown in Figure 1, with the temperature increasing, starch granules began to swell under the action of water, the crystalline region was destroyed, and amylose subsequently diffused and leached out [27]. Visually, all of the viscosity curves increased markedly but showed an obvious difference between PoS and PoS-PPC mixtures, with 3491, 2397, 2736, and 2591 mPa·s for PoS, PoS-PPC1, PoS-PPC2, and PoS-PPC3, respectively. The addition of PPC1, PPC2, and PPC3 led to significant decreases in peak viscosity (PV)

and increases of pasting temperature (PT) of PoS from 68.63 to 68.66 and 69.00 and 69.53 ◦C, respectively, which indicates that PPC delayed the rapid swelling of starch granules. Similar results were reported in previous works [18], which found that the released polyphenols crosslinked starch with water through polyphenolic hydroxyl groups. On the one hand, PPC diminished water usability for swelling of starch particles via competition for water and then decreased the amount of leaching amyloses and overall viscosity. On the other hand, the connection between PPC and starch chains, especially amylose, weakened the direct interaction between starch granules and moisture [32]. The newly formed starch-PPC complex might have wrapped onto the surface of starch particles and further inhibited the hydration of amorphous regions and pasting process of starch. Moreover, the negative charges of phosphate monoesters in PoS repelled each other, which promoted the rapid swelling of PoS granules. However, interactions between phosphate monoesters of starch and hydroxy groups of proanthocyanidin might happen during hydration, thus retarding the blends' gelatinization [33].



Different letters in a column signify significant differences (*p* < 0.05). PoS: potato starch; PPC: polymeric proanthocyanidin; PV: peak viscosity; TV: trough viscosity; FV: final viscosity; PT: pasting temperature; BD: breakdown; SB: setback.

**Figure 1.** RVA gelatinization curves of native PoS and PoS-PPC pastes. PoS: potato starch; PPC: polymeric proanthocyanidin.

Among the three types of PPCs, PPC1 had the strongest effect on the pasting viscosity. This could be explained by the hygroscopicity of proanthocyanidins varying with their DP. PPCs with a slightly lower DP exhibited stronger moisture-absorption capacity because of a lot of external hydroxyl groups, thus reducing the exposure of starch molecules to water [19]. Presumably, PPCs of a smaller size could hamper the entanglement and formation of the double helix crystalline region of amylose, resulting in a decrease in

starch viscosity. As the DP value increases, proanthocyanidin molecules become larger and their spatial structures get more complicated [34]. Vernhet et al. [35] described that highly condensed proanthocyanidins showed a denser shape with ramifications. To maintain the system stability of PPC molecules, intramolecular hydrophobic interactions and hydrogen bonds between the groups tend to be formed, which lead to a less extended structure and lower solubility.

In the case of BD, it reflects the stability and integrality of the starch, and a high BD value suggests that granules were easily damaged [33,36,37]. PPC1, PPC2, and PPC3 addition caused decreases of the BD value from 1906 to 902, 1202, and 1061 mPa·s, respectively. The results showed that PPC could improve the thermal and shearing stability of potato starch. The effect was most pronounced for short-chain PPCs with the smallest molecules. This might be attributed to the increased conformational constraints for higher-DP proanthocyanidins, which impeded the interaction between proanthocyanidins and starch molecules to form a network structure within a relatively short time. The SB value reflects the degree of amylose reaggregation and recrystallization during short-term retrogradation [24]. The three types of PPCs could significantly reduce SB values to varying degrees, for PPC1 the strongest and PPC3 the least, from 397 to 248 and 330 mPa·s, respectively. This phenomenon might be because the connection of PPC and amyloses was stronger than the rearrangement between amylose molecules, resulting in a decrease of SB value [24]. In addition, PPC molecules perhaps generated a water layer around starch granules to limit the rearrangement of amylose during the gelatinization process; low-DP proanthocyanidin had a higher water retention capacity and reduced the fluidity of starch chain, thereby restraining short-term retrogradation of starch.
