In vitro digestibility of highly concentrated methylcellulose O/W emulsions: rheological and structural changes.

The changes in structure during the digestion of highly concentrated methyl cellulose (MC) O/W emulsions and of hydrated MC were investigated. The effect of human saliva and in vitro stomach digestion was attributed to a dilution effect, rather than to pH or pepsin activity. After in vitro intestine incubation, a decrease in viscoelasticity and an increase in fat globule size were observed. The fat released after the digestion of the MC emulsion was 49.8% of the initial fat, indicating the existence of a big physical impediment. In comparison with an O/W whey protein emulsion with fat content equal to the fat released during the MC emulsion digestion, a 12% reduction in free fatty acid formation was found, which indicates that the decrease in fat bioaccessibility in the MC emulsion should be attributed not only to a physical effect against fat release but also to a further impediment related to the fat digestion process. Fat released quantification informs about the physical retention of fat in the emulsion matrix structure. Enzymes may not act if fat is not released and solubilized. Free fatty acid quantification is the real indicator of fat digestion, but contrary to the total fat released, it is affected by a wide variety of enzymatic factors, which should be considered for the correct comparison of systems of different properties, for example systems where the amount of fat release during the digestion may be different or initially unknown.