2.9. In Vitro Starch Digestibility

In vitro starch digestibility was analyzed following the method of Wang [23] with slight modification. Freeze-dried noodle samples (cooking under OCT) (containing 50 mg starch, dsb) and water (2 mL) were added into 50 mL polypropylene centrifuge tubes. (The starch content of noodles was determined using a K-TSTA 04/2009 kit from Megazyme (Bray, Ireland)). After equilibrating at 37.0 °C for 20 min, 7.5 mL acetate buffer (pH 6.0) and 0.5 mL of porcine pancreatic α-amylase/amyloglucosidase mixture were added to the tubes. The noodle was digested in a water bath at 37 °C with stirring at 300 rpm, and the digesta (0.1 mL) were collected at 0, 10, 20, 30, 60, 90, 120, 150, 180, 210, 240 and 300 min. The digestion was stopped by adding 0.5 mL of ethanol (99% v/v), followed by centrifuging at 4000× g for 5 min. The hydrolyzed glucose content was measured using GOPOD reagent.

The digestograms were fitted to first-order kinetics [24,25] as shown in Equation (2):

where C_t is the concentration of product at a given time (t), C∞ is the concentration of product at the end of the reaction, and k is the first-order rate constant. For ease of interpretation, C_t may be expressed as the amount of starch digested as a percentage of the total starch content of the sample. A Logarithm of Slope (LOS) plot was obtained by expressing the first derivative of the first-order equation in logarithmic form (Equation (3)).

where ln(dC/dt) represents the logarithm of the slope, and the equation describes a linear relationship between LOS and time of amylolysis, t. The resulting k and C∞ were used to construct model-fit starch digestion curves according to Equation.