Study of the melting point and solubility of lactose crystals in sweetened condensed milk using optical microscopy and Raman spectroscopy

Abstract

The study of physical-chemical parameters such as the melting point and lactose crystals solubility in sweetened condensed milk is justified by process stage demand of this product in industries, as well as the lack of these data in the scientific literature. The undesirable presence of large lactose crystals in sweetened condensed milk compromises the sensory quality of the final product. The aim of this study was to determine the melting point of the lactose crystals and the adjusted proportion of the reprocessing solution, made up of a mixture of sweetened condensed milk/diluent. The study provided two alternative methodologies to enhance the quality of the final product. A temperature control system coupled to the optical microscope was used to monitor the heating ramp until the effective melting of the lactose crystals in the samples of sweetened condensed milk, reprocessing solutions, and diluent solution. The average melting temperature of crystals in sweetened condensed milk was 83.3 °C. Microscopic analyzes were performed on the samples, and from micrographs of the reprocessing solutions, it was concluded that the T₂₀ sample is the most suitable to correct the problem of sandiness. The spectroscopic profile of all samples was observed by Raman spectroscopy and attributed to the characteristic bands of sugar crystallization. Therefore, it was possible to determine the temperature and dilution parameters for the reprocessing of sweetened condensed milk in order to minimize the problem of sandiness.