*2.6. Determination of Physicochemical Properties of APPH-WPI*

#### 2.6.1. Solubility

A protein sample was suspended in distilled water to obtain a final solution at 0.02 mg/mL and stirred for 1 h at room temperature, followed by centrifugation (6500× *g*, 4 ◦C) for 20 min. The protein content of supernate was measured by the micro-Kjeldahl method, each replicate was determined three times, and the solubility was calculated as the percentage of the soluble protein (g) compared to total protein sample (g) [5].

#### 2.6.2. Foaming ability (FA) and Foaming Stability (FS)

FA and FS of protein samples were measured according to Qian and Sun [8] with slight modification. A protein sample dispersion (1 mg/mL) was prepared by being suspended in PB (0.05 mol/L, pH 7.0), and after stirring for 1 h at room temperature, a 40 mL portion of the dispersion was homogenized at 10,000 rpm for 2 min. The volume of stirring solution at the first moment (0 min) and 60 min were recorded and calculated according to the follow equations to obtain FA and FS. This experiment was repeated with three different batches of samples.

$$\text{FA(\%)}=(\text{V}\_1-\text{V})/\text{V}\tag{1}$$

$$\text{FS}(\%) = (\text{V}\_2 - \text{V}) / (\text{V}\_1 - \text{V}) \tag{2}$$

where V1 is the volume of sample solution homogenized at the first moment, V is the initial volume of sample solution (40), and V2 is the volume of sample solution kept for 30 min after being homogenized.

#### 2.6.3. Water/Oil Holding Capacity (WHC/OHC)

An amount of 200 mg protein powder was mixed well with 10 mL water or soybean oil for 1 min in a weighted centrifuge tube using a vortex; afterward, the mixture was centrifuged at 3000× *g* for 20 min and the supernate was discarded, while the pellet was weighed. This trial was repeated three times and the WHC/OHC was expressed as the amount of absorbed water/oil (g) per gram protein sample [25].

#### *2.7. Mass Spectrometry Analysis*

Mass spectrometry analysis was carried out by nano liquid chromatography-Q Exactive (LC-QE) mass spectrum (Thermo Fisher Scientific, Q Exactive, Waltham, MA, USA). The control and oxidized WPI (10 mmol/L AAPH) samples were prepared by reduction and alkylation methods, followed by trypsin-hydrolyzed (1:50, *w/w*) at 37 ◦C for 20 h. The hydrolysates were lyophilized and re-dissolved in formic acid solution (FA, 0.1%, *w/w*) after desalting, and stored at −20 ◦C for further liquid chromatography (LC) separation. The LC conditions were as follows: Solution A, aqueous solution of 0.1% FA, solution B, 84% acetonitrile containing 0.1% FA. The hydrolysate was loaded into the trap column by automatic sampler after being balanced, then the full scan was performed and 20 fragment profiles were collected to obtain the mass charge ratios of peptides and peptide fragments. The mass spectrometry data were searched against the UniProtKB database by proteome discoverer software (Thermo Fisher Scientific, Version 1.4, Waltham, MA, USA).
