2.6. In vitro digestion of starch (glycemic index)

The glycemic index was measured following the methods of Salgado‐Cruz et al. (²⁰¹⁷), and Goñi et al. (¹⁹⁹⁷) with some modifications. The jicama samples were weighed (10 mg), mixed with 2 ml of HCl‐KCl buffer (pH 1.5), and added with 0.04 ml of pepsin (1 g of pepsin, Merk 107185/10 ml of HCl‐KCl buffer). Samples were kept in an incubator shaker (Barnstead International‐SHKA4000‐7 IOWA, EUA) at 40ºC for 1 h.

After incubation, 5 ml of tris‐maleate buffer (pH 6.9) was poured to adjust pH (6.75–7.10) of the sample solution followed by the addition of 1 ml of tris‐maleate buffer containing 2.6 U of α‐amylase from porcine pancreas (A −3176, Sigma‐Aldrich Inc.); the sample was incubated at 37°C. Aliquots of 0.5 ml were taken every 30 min from 0 to 180 min. To inactivate the enzyme (α‐amylase), these aliquots were heated to 100°C and kept at this temperature for 5 min while shaking. Then, 0.6 ml of 0.4 M sodium acetate buffer (pH 4.75) and 12 µl of Aspergillus niger amyloglucosidase (A7420, Sigma‐Aldrich Inc.) were added; these samples were incubated at 60°C for 45 min.

Finally, glucose concentration was measured with the glucose oxidase‐peroxidase kit (GAGO20, Sigma‐Aldrich Inc.). The kinetics of in vitro starch digestion was followed by a nonlinear model. The results were multiplied by 0.9 (stoichiometric glucose/starch conversion factor) to convert glucose to starch, as Salgado‐Cruz et al. (²⁰¹⁷) reported. The glycemic index was calculated using the area under hydrolysis curve (AUC), which was obtained by fitting the experimental data to a two‐parameter exponential model. The (Equation 5) applied was:

where C is the percentage of hydrolyzed starch at time t. C_∝ is the percentage at equilibrium of hydrolyzed starch, normally after 180 min, k is the kinetic constant, and t is the time (min). The parameters C_∝ and k were estimated using the software SIGMAPLOT version 11 for MS Office and an exponential model was used to calculate AUC (Equation 6):

where, “t_f” is the final time (180 min), “t₀” is the initial time (0 min). The hydrolysis index (HI) was expressed as the ratio of the area under the hydrolysis curve (AUC) of the sample to the AUC of the white fresh bread. Then, the estimated glycemic index (eGI) was calculated by using the equation reported by Granfeldt et al. (¹⁹⁹²).