Water distribution in maize starch-pea protein gels as determined by a novel confocal laser scanning microscopy image analysis method and its effect on structural and mechanical properties of composite gels

The objective of this study was to determine the water distribution in maize starch (MS) and pea protein isolate (PPI) composite gels and understand how the separate contributions of MS and PPI affect the mechanical properties of composite gels with varying MS/PPI ratios. A series of MS gels with increasing PPI contents and a series of PPI gels with increasing MS contents were prepared. The water distribution was evaluated with a novel image analysis method using confocal laser scanning microscopy (CLSM). This method was used to quantify the volume fraction of each phase, from which the water distribution between the starch phase and the protein phase within the gels was estimated. Depending on the MS/PPI ratios, either a dispersed or a bicontinuous gel was formed. The estimated stiffness of each phase was used to estimate the gel stiffness using models for either dispersed or bicontinuous structures. These predictions were shown to be in good agreement with experimental data obtained from compression tests. The determination of the properties of each phase separately allowed us to determine which phase dominated the properties of the composite gel. In filled gels, the MS network dominated the gel properties, unless the stiffness of PPI fillers was remarkably higher than that of the MS network. In bicontinuous gels, the stiffness of the composite gels was determined by the network with the highest stiffness. Dispersing pea protein aggregates (PPA) with different size into MS gels resulted in minor differences in mechanical properties compared to PPI-MS gel. These results show that this new analysis method can be used to predict water distribution within gels, and how this leads to changes in the stiffness, hardness and brittleness of the gels.