DETERMINACION DE ISOTERMAS DE SORCION EN HARINA DE MAIZ A TRES TEMPERATURAS Y SU AJUSTE A MODELOS MATEMATICOS

Abstract

Nixtamalised corn flour is produced, stored and distributed in big volumes in Mexico by world class companies, as well as by millers in local stockpile centres all over the country. Despite the economic importance of this product, valuable information derived from knowing the thermodynamic parameters and the water monolayer is lacking. Such parameters describe flour performance during distribution and storage, as well as safe moisture content values for successful storage. This is why this research provides such data, using as raw material flour nixtamalised by big companies (industrial), as well as that obtained from artisanal producers (traditional). The process included determining the sorption isotherms at three temperatures (24, 30 and 35°C) in a water activity interval of 0.074 - 0.970 using the Proximity Equilibrium Cell (PEC) method. The resulting isotherms for both samples show a type II shape, from the five shape clasification by Brunauer. The monolayer value was determined using the GAB and BET equations resulting in values between 0.06-0.14 (grams H2O per gram of dry solid) for industrial nixtamalised flour and 0.11-0.15 (grams H2O per gram of dry solid) for traditional nixtamalised flour. Safe storage moisture content values (Xs) were also determined using the Caurie equation. Such values indicate moisture content levels at which maximum stability would be reached during flour storage. The resulting value was 0.03 - 0.04 (grams H2O per gram of dry solid). Finally, the isosteric heat of sorption required by nixtamalised corn flour to reach equilibrium was determined using the Clausius – Clapeyron equation. Values of Gibbs free energy are -386±3.61 for industrial nixtamalised flour and -161±1.22 for traditional nixtamalised flour. Hence: the process is spontaneous; enthalpy values indicate that attraction forces exist in the sorption process with values of -16.9±1.2 for industrial nixtamalised flour and 15.06±0.6 for traditional nixtamalised flour; and entropy values are 13.15±4.6 for industrial nixtamalised flour and 5.09±1.3 for traditional nixtamalised flour. These results imply that the sorption process in the water-solute interface is unstructured or highly disordered.