Confidence intervals for modeling anthocyanin retention in grape pomace during nonisothermal heating

ABSTRACT:  Degradation of nutraceuticals in low- and intermediate-moisture foods heated at high temperature (>100 °C) is difficult to model because of the nonisothermal condition. Isothermal experiments above 100 °C are difficult to design because they require high pressure and small sample size in sealed containers. Therefore, a nonisothermal method was developed to estimate the thermal degradation kinetic parameter of nutraceuticals and determine the confidence intervals for the parameters and the predicted Y (concentration). Grape pomace at 42% moisture content (wb) was heated in sealed 202 × 214 steel cans in a steam retort at 126.7 °C for > 30 min. Can center temperature was measured by thermocouple and predicted using Comsol software. Thermal conductivity (k) and specific heat ( C_p ) were estimated as quadratic functions of temperature using Comsol and nonlinear regression. The k and C_p functions were then used to predict temperature inside the grape pomace during retorting. Similar heating experiments were run at different time–temperature treatments from 8 to 25 min for kinetic parameter estimation. Anthocyanin concentration in the grape pomace was measured using HPLC. Degradation rate constant ( k _{110 °C}) and activation energy ( E_a ) were estimated using nonlinear regression. The thermophysical properties estimates at 100 °C were k = 0.501 W/m °C, Cp = 3600 J/kg and the kinetic parameters were k _{110 °C}= 0.0607/min and E_a = 65.32 kJ/mol. The 95% confidence intervals for the parameters and the confidence bands and prediction bands for anthocyanin retention were plotted. These methods are useful for thermal processing design for nutraceutical products.