Modeling calcium chloride uptake in strawberries

ZORRILLA, SE; VERDINI, RA; RUBIOLO, AC

Río de Janeiro

Congreso; 2nd Mercosur Congress on Chemical Engineering and 4th Mercosur Congress on Process Systems Engineering; 2005

Universidade Federal do Rio de Janeiro y otros

Quality of strawberries decreases fast after harvest due to high softening rate and sensitivity to fungal attack. Calcium contributes to maintain firmness in strawberry because Ca2+ forms crosslinks between negatively charged carboxyl groups of pectins. One of the techniques used to increase calcium content after harvest is immersion of strawberries in calcium solutions. Our objectives were to develop a mathematical model to predict calcium uptake in strawberries during immersion in calcium chloride solutions and to analyze the mass transfer phenomena considering experimental values. The mathematical model was based on mass transfer in strawberries by diffusion involving a heterogeneous fluid-solid reaction. Strawberries were supposed to be spherical and physical properties of the solid were considered constant, as well as the concentration of the immersion solution. An isothermal system, Fick?s law for diffusion, binding sites uniformly distributed, and immersion solution well agitated were also assumed. The set of partial differential equations was solved by a finite difference method. Experimental tests were carried out dipping fresh strawberries in 1% and 10% (w/v) CaCl2 solutions at different temperatures and determining total and bound calcium contents at different times. Calcium was analyzed by atomic absorption spectrophotometry after acid digestion of strawberry samples and alcohol-insoluble solids. Some physical properties used in calculations were obtained from literature. No information is available neither on the diffusion coefficient of CaCl2 in strawberries nor on kinetics constant. A diffusion coefficient in the order of 1´10-11 m2/s was found by fitting the model to experimental data of total calcium content. It was observed that a pure diffusional model could be considered when the concentration of calcium is higher compared to the available binding sites. The experimental bound calcium contents were similar to the total calcium contents. This behavior was related to high binding site availability. The mathematical model allows to estimate calcium uptake in strawberries and also to understand the mass transfer phenomena that take place.