Control of calcium-protein interactions in designing casein-based food structures with novel functionality

AbstractThis thesis evaluated the basis of calcium-protein interactions in casein systems in order to develop novel casein-based food matrices with different structures and functionalities. Initially, in Chapter 2 the effects of calcium chelating salts (CCS) (disodium phosphate and trisodium citrate) on calcium-ion activity (ACa++), calcium distribution and protein solubility were investigated in model CaCl2 solutions (50 mmol L-1) and rennet casein dispersions (12 % w/w). In both model systems, adding trisodium citrate either alone or as part of a mixed chelating salt system resulted in high levels of dispersed “chelated” calcium; conversely, disodium phosphate addition resulted in lower levels, while the ACa++ decreased with increasing concentration of both CCS. Addition of either salt resulted in only a modest increase in soluble protein. Hence, the results suggested that CCS may play a more subtle role in modulating hydration during manufacture of casein-based matrices than simply solubilising calcium or protein. Therefore, despite the fact that the rennet casein dispersion used was relatively concentrated, an even more concentrated protein system was required to clarify the role of calcium chelators in casein hydration and matrix formation during manufacture of concentrated casein-based food matrices e.g. processed and analogue cheese.Hence, this work was further built upon in Chapter 3, where a small-scale manufacturing protocol for concentrated casein-based food matrices was developed using a Thermomix. Manufacture was stopped at various time-points, and the matrix formed and any free liquid still remaining were collected. The dispersed and insoluble phases were separated by centrifugation and the calcium (total calcium and ACa++) and protein contents of the dispersed phase analysed. The levels of calcium in the dispersed phase increased and ACa++ decreased during manufacture of casein matrices formulated with CCS; ~23 % of the total calcium was solubilised by the end of manufacture. In the absence of CCS, the levels of calcium solubilised were significantly (P 0.05) detected between matrices containing half or reduced fat levels for any of these attributes.In Chapter 6 CCS-free casein-based matrices prepared with different types of lipid (i.e. milk fat or rapeseed oil) were formulated with high (774 mg.100 g-1) or low (357 mg.100 g-1) calcium levels again by blending rennet and acid casein. Their physico-chemical characteristics (i.e. composition, texture, microstructure and water mobility) and in vitro digestibility were compared to conventionally formulated high-calcium (723 mg.100 g-1) casein matrices made from rennet casein with CCS. The CCS-free, high-calcium matrices were significantly (P < 0.05) softer than those with low calcium levels and showed the highest rates of disintegration during simulated gastric digestion. Despite having a higher moisture-to -protein ratio, the high-calcium matrices containing CCS had broadly similar hardness values to those of high-calcium concentration prepared without CCS, but had higher cohesiveness. The high-calcium matrices containing CCS had quite a different microstructure and increased water mobility compared to those made without CCS and showed the slowest rate (P < 0.05) of disintegration in the gastric environment. Gastric resistance was shown to be unaffected by the type of lipid phase present in the matrix. Conversely, fatty acid release was similar for all casein matrices prepared from milk fat, however, high-calcium matrices (CCS-free) prepared from rapeseed oil showed higher lipolysis. The results demonstrated that modifying the composition (i.e. calcium concentration, moisture to protein ratio, inclusion of CCS) and consequently the physical characteristics (e.g. texture, microstructure and water mobility) of casein-based food matrices affects their behavior during in vitro digestion. In particular, the results obtained showed that the physical properties of such matrices could be modified to alter resistance to gastric degradation which may have consequences for the kinetics of nutrient release and delivery of bioactives sentitive to the gastric environment