The effect of sugars on agar fluid gels and the stabilisation of their foams

The rising demand to reduce sugar in foods has resulted in the need to better understand its structuring properties and contribution to the overall microstructure of products. The effect of sugars on the microstructure of agar fluid gels and their novel foams, including stability, has therefore been studied. Gelation kinetics and material properties of agar fluid gels with added glucose, fructose and sucrose have been explored. The addition of sugar did not notably affect the temperature of ordering during gelation however, it did cause a reduction in fluid gel particle size through changes in solution viscosity during gelation. At high concentrations of sugar (above 50%), the conversion of an overall brittle agar gel network to a more rubbery-like structure was inferred through rheology, as the glass transition of sugar was approached. Below 50% sugar, shear viscosity and G′ increased with concentration due to an increase in particle gel strength (observed through initial Young's modulus). These changes in material properties were overall observed to be independent of sugar type. All systems showed good foaming properties where foam half-life increased with sugar concentration as a result of increased fluid gel yield stress. In order to increase understanding of the foam stability mechanism, fluid gel particle size was altered. When the continuous phase consisted of larger particles, the initial liquid drainage was considerably reduced as particles were more effective at accumulating in the foam channels preventing the flow of the bulk phase. However, foam half-life was ultimately dependent on fluid gel yield stress. This research has provided fundamental understanding of the effect of sugars on the microstructure of agar fluid gels and their use in foam stabilisation, allowing the development of an approach that can potentially be tailored to specific product requirements.