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Combined effects of trisodium citrate (TSC), transglutaminase (Tgase) and heating were studied with relation to the mechanical properties of acid milk gels. Acidification of skimmed microfiltered milk samples was performed at 30 °C by addition of glucono – δ lactone to achieve final pH of 4.6 in 4 h. Increasing levels of TSC (0, 10 and 20 mM) were added to the samples, followed by addition of a constant level of Tgase (3 U/g of protein), and subsequent heating to 85 °C for 30 min. Prior to acidification, suspensions were analyzed in terms of amount of sedimentable material, hydration, hydrodynamic diameter (Dh) and ζ-potential (ζ) of the particles. Acid-induced gelation was studied using small deformation rheology followed by large deformation test. The water holding capacity of the gels (WHC) was performed at pH 4.6. Gels microstructure were investigated by confocal laser scanning microscopy at pH 4.6 and determination of mean pore size was performed by 2D-correlation analysis. Results showed that combined Tgase and TSC treatments had significant effect on rheological properties, WHC and gel microstructure. Higher stiffness of the gels was observed with Tgase and TSC at 10 mM, whereas a homogenous network distribution with smaller pore size was observed in the presence of Tgase and TSC at 20 mM.

The effect of seeding whey protein isolate (WPI) solution with pre-made whey protein soluble aggregates (WPSAs) and stirring on the yield and structural and chemical attributes of whey protein microgels (WPMs) was investigated. Fluorimetry indicated that the heat-exposed hydrophobic amino acids were buried again by WPM formation. Full substitution of WPI with WPSA under non-stirred conditions generated smaller WPMs of ∼200 nm diameter, imaged by atomic force microscopy. Furthermore, full substitution yielded WPMs of a lower percentage proportion of β-sheet structure and a higher proportion of random structures than those of the sample prepared from the 50/50 mix of WPI and WPSA. Stirring increased the hydrodynamic size of WPMs at every ratio of WPI/WPSA. Microgelation under stirring at a WPI/WPSA ratio of 25/75 gave the highest yield (80%) of WPM. Gel electrophoresis indicated that stirring and full substitution of WPI with WPSA caused formation of WPMs rich in β-lactoglobulin monomer.

A novel time-lapse synchrotron deep-UV microscopy methodology was developed that made use of the natural tryptophan fluorescence of proteins. It enabled the monitoring in situ of the microstructural changes of protein gels during simulated gastric digestion. Two dairy gels with an identical composition, but differing by the coagulation mode, were submitted to static in vitro gastric digestion. The kinetics of gel particle breakdown were quantified by image analysis and physico-chemical analyses of digesta. The results confirm the tendency of rennet gels, but not acid gels, to form compact protein aggregates under acidic conditions of the stomach. Consequently, the kinetics of proteolysis were much slower for the rennet gel, confirming the hypothesis of a reduced pepsin accessibility to its substrate. The particle shapes remained unchanged and the disintegration kinetics followed an exponential trend, suggesting that erosion was the predominant mechanism of the enzymatic breakdown of dairy gels in these experimental conditions.

The gastric digestion is the result of physical disintegration and enzymatic reactions leading to the release of nutrients from the food matrix to be absorbed in the upper intestinal tract. Recent works have shown on mini-pig model that the digestion process may be modulated by the initial structure of dairy matrix and suggested that the gastric phase is the critical step. There is a lack of information concerning the microscopic scale of digestion process and the hypothesis most often invoked is that the proteolysis would be limited by the diffusion of gastric fluid into the food, especially pepsin. Nevertheless, fundamental knowledge of gastric digestion had only focused on acid diffusion from the gastric fluid, but no data are currently available for pepsin diffusion. Using fluorescence recovery after photobleaching technique, diffusion coefficients of inactive isothiocyanate (FITC)-pepsin were measured in rennet gels across a range of casein concentrations, modulating different structures of protein aggregates networks. To investigate the microstructural parameters of native gels, electron microscopy image analysis were performed, and then they were qualitatively related to diffusion behavior of FITC-pepsin in these dairy gels. The diffusion coefficients of FITC-pepsin was reduced by a factor 2.4 as the casein concentration of the rennet gels increased from 32.5 to 130 g/kg. Models of polymer science were successfully applied, allowing the reliable prediction of diffusion coefficients calculated from known size of diffusing solutes and physicochemical properties of the food product, e.g. volume fraction, particle size of the gel network. In a more holistic view of food behavior in the human stomach, the diffusion of pepsin should be now confronted with its activity in acidic environment, in order to be able to define more precisely the food particle disintegration mechanisms.