Rheological, melting, microstructural, and oil droplets-interfacial properties of model processed cheese made with calcium caseinate and trisodium citrate or sodium pyrophosphate

Abstract

Emulsifying salts play an important role in processed cheese manufacture, contributing to create a smooth and stable product. The aim of this thesis was to develop a fundamental understanding of the relationship between milk protein ingredients, specifically calcium caseinates, the different types of emulsifying salts, oil droplets and their interfacial properties, in a simplified model processed cheese. In this model processed cheese, which were manufactured on a lab scale using trisodium citrate (TSC) or tetrasodium pyrophosphate (TSPP), shear, protein and oil concentrations, emulsion stabilisers were varied. Rheology, particle size, melting measurements and microscopic observations were used to probe these systems. This study showed that increasing shear rates, up to a critical value, during processed cheese processing decreased the size of the oil droplets up to ~0.2 μm and promoted oil droplet emulsification. Other than that, processed cheese made with 20% of protein (w/w) and 20% oil (w/w) using TSC presented optimal results for particle size (~0.4 μm), rheology (G*= ~14 kPa) and melting properties (78%). Increasing oil concentration up to 30%, increased the firmness (G*= ~13.5 kPa) and oil droplet particle size (~0.6 μm) but decreased the meltability (69%) of processed cheese samples made with TSC due to stronger oil-protein and protein-water interactions. The results of the model processed cheese made with TSPP demonstrated similar trends with the results of processed cheese made with TSC in terms of rheology, particle size, microstructure and melting properties as a function of protein and oil concentrations. Comparison between model processed cheeses made with TSC and TSPP shows that optimal processed cheese are obtained when 20% protein and 20% oil are used. The type of interactions that occurs between the oil droplets and the protein matrix depends on the nature of the surface active material stabilising the oil droplets. Model processed cheese containing oil droplets stabilised by calcium caseinate or whey protein isolate (WPI) emulsified either by TSC or TSPP demonstrated the higher G* values, smaller oil droplets, a more compact structure and lower melting properties. This is due to the fact that both WPI and calcium caseinate are active fillers, interacting positively with the casein network matrix producing more elastic processed cheeses. Model processed cheese containing oil droplets stabilised by lecithin either emulsified by TSC or TSPP exhibited the lowest G* values, larger oil droplet particle size, loose structure and highest melting properties because oil droplets stabilised by lecithin may act as inert filler, not interacting with the casein network. A preliminary study was done to investigate the casein micelle structures of skim milk (10% w/w) containing four different types of emulsifying salts which are sodium phosphate (SP), TSPP, sodium tripolyphosphate (STPP) and sodium hexametaphosphate (SHMP) using small angle X-ray scattering (SAXS). SAXS data shows that the size of casein micelles decreased with the increase of the phosphate chain length according to the following order: control ≈ SP > TSPP > STPP > SHMP. The addition of TSPP or STPP or SHMP leads to alteration of the sub-micelles and the dissolution of the colloidal calcium phosphate nanoparticles (CPN). The dissociation of the casein micelle started to occur when the concentration of the emulsifying salts were: ≥1% SP, ≥0.5% TSPP, ≥0.25% STPP or ≥0.25% SHMP. Heat treatment only affected milk samples emulsified with SP (at 1% and 2% concentration).