The in vitro protein digestibility of instant noodle products was determined using a modified version described by Hur et al. (21 (link)). The phases of in vitro protein digestion model include mouth, stomach, and intestine, accompanied by the addition of artificial saliva (A7990, pH = 6.8, Solarbio Co., Ltd., Beijing, CN), and preprepared stimulated gastric, duodenal, and bile fluids, whose formulas are listed in Table 1 .
The degree of hydrolysis of the digested sample was analyzed using microplate OPA assay as described by Jadhav et al. (22 (link)). Appropriately 25 μL of diluted samples after centrifuging (10,000g, at 4°C for 20min) was mixed with 175 μL OPA reagent in the microtiter plate. The plate was incubated at 37°C for 2 min and the absorbance was measured at 340 nm. Standard was obtained using serine and degree of hydrolysis and was determined as follows:
where hsample is the concentration of free amino groups in the samples (mmol), htotal is the concentration of free amino groups per gram of completely hydrolyzed protein (8.83 mmol/g protein). X is the weight of the sample tested; P is the protein content of the sample tested; and α and β are expressed by the constants 1.00 and 0.40, respectively.
The degree of hydrolysis of the digested sample was analyzed using microplate OPA assay as described by Jadhav et al. (22 (link)). Appropriately 25 μL of diluted samples after centrifuging (10,000g, at 4°C for 20min) was mixed with 175 μL OPA reagent in the microtiter plate. The plate was incubated at 37°C for 2 min and the absorbance was measured at 340 nm. Standard was obtained using serine and degree of hydrolysis and was determined as follows:
where hsample is the concentration of free amino groups in the samples (mmol), htotal is the concentration of free amino groups per gram of completely hydrolyzed protein (8.83 mmol/g protein). X is the weight of the sample tested; P is the protein content of the sample tested; and α and β are expressed by the constants 1.00 and 0.40, respectively.