*2.8. Measurement of Hydration Properties*

Hydration properties including swelling capacity (SC) and water-retention capacity (WRC) were determined.

Swelling capacity was determined by referencing the method of Mateos-Aparicio, et al. [19]. Accurately weighed pea fiber (0.3 ± 0.001 g) was added into a 25 mL graduated cylinder containing 15 mL of distilled water. The sample was stirred gently, then left undisturbed at room temperature for 8 h to completely hydrate. The volume (mL) of the settled sample was recorded, and SC was expressed as the volume of the settled sample (mL) per gram of dry fiber.

Water retention capacity was measured by referencing the method of Morales-Medina, Dong, Schalow and Drusch [11], with some modifications. Accurately weighed pea fiber (0.3 ± 0.001 g) and 15 mL of distilled water were added in a 50 mL centrifuge tube, and the sample was stirred and allowed to hydrate at room temperature for 8 h. Subsequently, the fibre suspension was centrifuged at 1790× *g* for 15 min, and the supernatant of each tube was carefully decanted. The excess of liquid was drained by turning the tubes upside down on a filter paper for several minutes. The weight of the hydrated sample (m1) was recorded. Then the hydrated sample was freeze-dried, and the weight of the dried sample was labeled as m2. WRC was calculated according to Equation (2):

$$\text{WRC}(\text{g/g}) = \frac{\text{m}\_1 - \text{m}\_2}{\text{m}\_1} \tag{2}$$

#### *2.9. Measurement of Oil Holding Capacity*

Oil holding capacity (OHC) was measured following a modified procedure by referencing the report of Jiang, et al. [20] and Meng, et al. [21]. Pea fiber powder (0.2 ± 0.001 g) and oil (10 g ± 0.01 g) were put into a 50 mL centrifuge tube and fully mixed by vortex mixer for several minutes. Then the centrifuge tube was kept at room temperature for 4 h. Subsequently, the fibre suspension was centrifuged at 1790× *g* for 15 min, and the upper clear liquid was poured out gently. The weight of the pea fiber after absorbing oil was recorded. The OHC of pea fibers was calculated by Equation (3):

$$\text{OHC} \text{(g/g)} = \frac{\text{m}\_{\text{oiled}} - \text{m}}{\text{m}} \tag{3}$$

where m was the weight of the original ISMS-treated PeaF powder (0.2 g), and moiled was the weight of the ISMS-treated PeaF after absorbing oil (g).

#### *2.10. Statistical Analysis*

All experiments were carried out in triplicate using three samples, and then the mean and standard deviation were calculated by statistical analysis software (SPSS 25.0, SPSS Inc., Chicago, IL, United States). Significant differences between sample means (*p* < 0.05) were established according to Duncan's test using one-way analysis of variance (ANOVA).

#### **3. Results and Discussion**

#### *3.1. Particle Size Characteristics*

Initially, the influence of the ISMS treatment on the particle characteristics of pea fiber was examined. Particle size distributions for native PeaF and ISMS-treated PeaF aqueous suspensions are shown in Figure 1. Native PeaF aqueous suspensions exhibited a wide and asymmetric unimodal distribution (1.4~374.8 μm) with a shoulder around 8.1~37.7 μm. The pre-pulverizer treatment weakened the shape of the shoulder and slightly narrowed the particle size distribution. After ISM treatment, peaks of the distributions tended to be homogeneously distributed. Meanwhile, a progressive shift of the peaks to the left for ISMS-treated PeaF aqueous suspensions was observed as the treatment intensity increased, and the distribution gradually narrowed. Additionally, when ISM pressure was below 120 MPa, the peaks moved downward with the increasing of ISM pressure. Although the particle size distribution of ISM-120-T2 PeaF was located at the leftmost, it was not significantly deviated from that of ISM-120 PeaF. These phenomena indicated that the increasing treatment pressure resulted in a decrease in the overall particle size, and ISMS treatment could grind the pea fiber to a micron size to a limited extent. As displayed in Table 1, all mean diameters analyzed decreased with increasing ISMS treatment intensity, and similar results had been found in another study of treating soybean insoluble dietary fiber by high-energy wet media milling [22]. For example, along with the treatment intensity, D[4,3] values decreased from 92.6 μm of native PeaF to 38.3 μm of ISM-120-T2 PeaF. Furthermore, D[3,2] values of pea fiber significantly dropped from 34.5 μm to 19.3 μm, and those of ISM-60 PeaF, ISM-90 PeaF, and ISM-120 PeaF were 23.5, 20.4 and 19.4 μm, respectively. Bruno, et al. [5] reported that D[3,2] values of citrus fiber were approximately 26 μm and 20 μm after treatment by a M110P microfluidizer® at higher pressure for one pass (103.3 MPa and 172.2 MPa, respectively). The cumulative percentiles of D(50) and D(90) were also reduced to varying degrees. When pressures of ISM were 90 MPa and 120 MPa, D(90) the values of pea fiber were 93.2 μm and 82.5 μm, respectively. In the study of Morales-Medina, et al. [11], when D(90) values of pea hull fiber reached 100 μm and 80 μm, the conditions of treatment by the LM20 Microfluidizer® were predicted to be 109 MPa for two passes and 127 MPa for four passes, respectively. Meanwhile, D(50) values of pea hull fiber processed by the aforementioned conditions were slightly higher than these of ISM-90 PeaF and ISM-120 PeaF in this investigation. These phenomena indicated that ISM was a more powerful technique than conventional microfluidizers at disrupting fiber to a smaller particle size. Intriguingly, ISMS treatment did not cause a decrease in span, which was contrary to the observation of narrowing distributions from Figure 1. As depicted in Figure 1, the size range of native PeaF was actually larger than that of ISMS-treated PeaF, and native PeaF were mainly large size particles with concentrated distribution. Nevertheless, ISMS-treated PeaF possessed more homogeneous distribution with a high fraction of small sized particles and a slightly small size range. The concentrated distribution of large size particles and very low fraction of small size particles contributed to a lower span for native PeaF. As considered by Guo, et al. [12], the reduced particle size of pea fiber was possibly attributed to mechanical action initiated by ISMS, and higher processing strength provided a greater crushing effect. Meanwhile, the specific surface area of pea fiber was increased with the increasing of ISMS treatment intensity, which climbed from 173.6 m2/kg to 309.4 m2/kg when ISM pressure rose to 120 MPa (Table 1). The reduction in particle size and increase in particle specific surface area initiated by the crushing effect inevitably destroyed the structure of pea fiber. Therefore, the structure of ISMS-treated PeaF will be investigated in the next sections.


**Table 1.** Particle diameter size of ISMS-treated PeaF 1.

<sup>1</sup> Reported results correspond to mean ± standard deviation. Different letters within the same column indicate significant differences (*p* < 0.05).

**Figure 1.** Particle size distributions of ISMS-treated PeaF.
