*2.3. Determination of the Degree of Polymerization of Proanthocyanidin*

The average degrees of polymerization of PPC1, PPC2, and PPC3 were determined by the vanillin-hydrochloric acid method [20,21]. Briefly, the standard curves of catechin's mass concentration and molar concentration were drawn in sequence (the standard curve of catechin mass concentration was y = 2020.6x + 0.6963, R<sup>2</sup> = 0.9954; the standard curve of catechin molar concentration was y = 0.1756x − 0.0013, R2 = 0.9999). A total of 10 mg of the proanthocyanidin sample thoroughly dissolved in 20 mL methanol solution, which was then diluted 20 times with methanol and acetic acid and, respectively, named as liquid A and liquid B. One milliliter of liquid A and liquid B was applied to measure the content (*m*) and the amount of substance (*n*) of proanthocyanidin according to the methods of standard curves of catechin mass concentration and catechin molar concentration, respectively [22]. In this research, the value of DP was computed by the formula below (1):

$$DP = \frac{m}{M \times n} \tag{1}$$

where *m* refers to the proanthocyanidin content, *n* is the amount of substance of proanthocyanidin, and *M* is the relative molecular mass of catechin.

#### *2.4. Rapid Viscosity Analysis (RVA)*

The pasting characteristics of PoS and PoS-PPC blends were determined using RVA (Perten RVA 4500, Stockholm, Sweden), according to preceding papers [18]. Based on our previous research, we chose 20:1 as the mass ratio of starch to PPC [18]. Briefly, 2 g of starch was mixed with proanthocyanidins (PPC1, PPC2, and PPC3) at doses of 0% and 5% (*w/w*), and added with 20 mL of distilled water. The slurries of PoS and PoS-PPC samples were characterized using the RVA, in accordance with the description of Li et al. [23]. The preliminary stirring speed was 960 r/min; then, it was kept at 160 r/min after 10s. The sample was maintained at 50 ◦C for 90s, heated and maintained at 95 ◦C for 150s, and decreased and kept at 50 ◦C for 90s. The pasting curves and parameters of starch pastes were generated by TCW (Thermocline for Windows). Some of the gelatinized samples were transferred for rheological analysis, and others were reserved at 4 ◦C for a week to prepare the retrograded samples. In addition, the same mass ratio of starch to PPC was used to investigate DSC, XRD, and in vitro digestibility of starch-PPC samples.
