Using infrared (IR) thermography in the freeze-drying process of model solutions

Abstract

Among the drying methods, freeze-drying is used to obtain the high quality of the products. Different gel matrices have been employed to simulate fruit pulp assays, especially the gelatin. It facilitates the freeze-drying process, since its collapse temperature is high and increases the thermal stability of the product. The objectives of the present work were to apply the infrared thermography technique to study the heat transfer processes during the freeze-drying of model solutions added with carrier agents (sucrose, ethanol and maltodextrin). Also to determine the rheological and microstructural parameters of the model systems, establishing possible correlations amongst them. A rheological characterization of the model solutions was performed through shear tests at steady state for determining the viscosity at different temperatures (0 °C to 40 °C). An equation similar to the Arrhenius model was used to evaluate the effect of temperature on the apparent viscosity. In addition, microstructural analyses were carried out using a scanning electron microscope. The infrared thermography was useful to evaluate the heat transfer during the freeze-drying process, allowing to obtain the temperature history. The Newton Law and Power Law models have provided the best statistical adjustments to the experimental data. The treatments using sucrose and sucrose combined with ethanol have shown to be more viscous, while the treatment with ethanol alone presented lower apparent viscosity. There was also a decrease in viscosity as a function of temperature from 10 °C to 40 °C. The treatments with ethanol and sucrose and those with maltodextrin and sucrose contributed to a higher crystallization of the powders, respectively. The addition of ethanol and ethanol combined with sucrose promoted a smaller shrinkage of the sample compared to the other treatments. In this context, it was observed that only in these treatments there was collapse, which is responsible for the shrinkage that affects the quality of the product. In general, the addition of sucrose combined with ethanol was more efficient in the freeze-drying process. The study of parameters involving the food matrix is essential for a better understanding and optimization of processes such as freeze-drying.