The influence of oil droplets on the phase separation of protein–polysaccharide mixtures

A model system consisting of sodium caseinate (SC) + xanthan ± a low volume fraction of oil-in-water emulsion droplets was studied. The phase separation behaviour of xanthan + sodium caseinate was investigated as function of these two variables, followed by experiments on the same systems where oil droplets were introduced. More than 20 mM [Ca2+] was needed to induce phase separation at pH 6.4 and 5.9, but at pH 5.4, phase separation occurred at as low as 5 mM [Ca2+] and a lower concentration of SC or xanthan was required to induce phase separation. An increase in size of sodium caseinate aggregates is proposed as the main factor promoting phase separation. When oil droplets stabilized by sodium caseinate were added to systems containing 0.05–0.1 wt.% xanthan at pH 6.4 this appeared to inhibit significantly the phase separation of the mixtures at [Ca2+] = 22 mM. Simple calculations showed that this effect cannot reasonably be due to excessive accumulation of protein at the droplet surfaces, which is then carried away by the droplets due to creaming. Consequently, a possible mechanism of the inhibition is accumulation of droplets at and strengthening of, the water–water interface of the caseinate–xanthan phase separating entities.