Influence of pH and soy protein isolate addition on the physicochemical properties of functional grape pectin confections

High-methoxyl pectin was combined with grape juice to generate a series of gel confections. The objectives of this work were to test the hypothesis that the release of phenolic compounds present in grape juice from pectin confections may be modulated by altering their microstructure by means of soy protein isolate addition and pH changes. To this purpose the influence of pH and addition of soy protein isolates (SPI) on texture, glass transition temperature and release of phenolic compounds of confections was investigated using dynamic rheometry, creep tests, confocal laser scanning microscopy, differential scanning calorimetry and dissolution tests. Confections had a soluble solids content of 73% and pH values ranging from 2.5 to 3.3. Decrease of pH by acidification resulted in an increases in the mechanical strength of confections due to the formation of denser gels promoted by the increased interactions between pectin molecules at lower pH. Such gels released phenolic compounds at a lower rate. In contrast, addition of up to 3.4% of SPI did not influence the dissolution behavior of the confections despite the fact that the microstructure was visibly altered, i.e. soy proteins were homogeneously interspersed in the pectin network in the form of particles. The glass transition temperature of the system was −59 ± 1 °C, and was independent of pH or SPI addition. Our results suggest that predominately alterations in the microstructure of the continuous matrix of confections affect mass transport processes.