*2.5. Browning Index and Grafting Degree Measurement of the Powders*

The browning intensity was measured using a spectrophotometric method with minor modifications [16]. A volume of 5 mL of a mixture consisting of equal volume of acetic acid (2% *v*/*v*) and formaldehyde (1% *v*/*v*) was added to 0.1 mL of 1 mg/mL of samples and centrifuged for 10 min at 4500× *g*. The resulting supernatant (5 mL) was mixed with 5 mL of ethanol and the mixture was centrifuged again. Absorbances of the supernatant at 420 and 600 nm were measured. The difference between the two absorbance values was used to evaluate the browning index.

The grafting degree was determined using the o-phthalaldehyde (OPA) method, according to Jia et al. [13]. A volume of 4 mL of OPA was added to 0.2 mL of the diluted samples (0.1 mg/mL) in test tubes. Upon homogenization, all tubes were placed in a water bath at 35 ◦C for 1 min. The absorbance was measured at 340 nm. A blank sample was also made using the same volume of ultrapure water. Grafting degree was determined using Equation (2):

$$\text{Grafting Degree} \left( \% \right) = 100 \times \frac{\text{A}\_0 - \text{A}\_\kappa}{\text{A}\_0} \tag{2}$$

where A0 and As are the absorbance of blank sample and absorbance of tested samples, respectively.
