Acid-induced gelation behavior of casein/whey protein solutions assessed by oscillatory rheology

Salt induced slowdown of kinetics and dynamics during thermal gelation of egg-yolk

We investigated the effect of the NaCl concentration (0.3-2M) on the structure and dynamics of hen egg yolk at room temperature and during thermal gelation at temperatures in the range of 66-90 °C utilizing low-dose x-ray photon correlation spectroscopy in ultra-small angle x-ray scattering geometry. With an increase in the salt concentration, we observe progressive structural and dynamic changes at room temperature, indicating the disruption of yolk components such as yolk-granules and yolk-plasma proteins. Temperature- and salt-dependent structural and dynamic investigations suggest a delay in the gel formation and aggregation of yolk low-density lipoproteins with increasing ionic strength. However, the time-temperature superposition relationship observed in all samples suggests an identical mechanism underlying protein aggregation-gelation with a temperature-dependent reaction rate. The sol-gel transition time extracted from kinetic and dynamic information follows Arrhenius's behavior, and the activation energy (460 kJ/mol) is found to be independent of the salt concentration.

Effect of milk pH at heating on protein complex formation and ultimate gel properties of free-fat yoghurt

The effect of milk pH before heating on casein-whey protein interactions and ultimate gel properties of the free-fat yoghurt was investigated. Reconstituted skim milk at different pH values (6.4, 6.8 and 7.2) was heated at 80 °C for 30 min. The type of protein and size of casein micelle in milk were determined. The storage modulus (G'), loss tangent (tan δ), flow behaviour as well as microstructure, firmness and water holding capacity of the yoghurt samples were measured. Heating milk at pH 7.2 formed mostly soluble protein complexes whereas at pH 6.4 micelle bound complexes was dominant. However, heating milk at pH 6.8 resulted in a relatively compact protein network due to a balanced contribution from both soluble protein/κ-casein complexes and whey protein-casein micelle associated complexes. Yoghurt prepared with milk heated at pH 6.8 showed significantly higher G' values, shorter gelation times, higher water holding capacity, firmness and more compact protein network compared to those at pH 6.4, 7.2 and unheated milk. The obtained results demonstrated that milk pH adjustment before heating could be an important factor governing uniform quality yoghurt production.

Syneresis investigations of lacto-fermented sodium caseinate in a mixed model system

BACKGROUND

The textural characteristics of fermented dairy products are important quality parameters that play a major role in their stability and consumer's acceptance. The aim of this study was to investigate the influence of sodium caseinate, starch, lactose and lactic acid bacteria as ferment on the syneresis in a mixed model system, and to evaluate their impact on the acid gel formation throughout pH and zeta potential monitoring. Accordingly, a protocol was designed to perform an experimental design by using a mixture of the selected factors.

RESULTS

A significant decrease of syneresis was detected in all mixtures at 8% of sodium caseinate, ranging between a minimum of 1.8% and a maximum of 20.6% compared to the mixtures at 3% of sodium caseinate in which the syneresis decrease had ranged between a minimum of 22.2% and a maximum of 47.8%. The addition of starch had a significant impact on the acidification profile and on the syneresis of the fermented mixed model. Moreover, the monitoring of pH and zeta potential during the lacto-fermentation process has also led to a better understanding of the acid gelation and the syneresis variations.

CONCLUSION

Syneresis varies very closely with sodium caseinate concentration, starch concentration and also with their association, regardless of the concentrations of lactose and ferment. In fact syneresis could be reduced to an optimum level if a sodium caseinate-starch mixed system is employed: Less syneresis gels could be obtained at a sodium caseinate concentration above 5% if starch is used above 1%.

Rheological and physical properties of camel and cow milk gels enriched with phosphate and calcium during acid-induced gelation

The rheological properties of acid-induced coagulation of camel and cow milk gels following the addition of calcium chloride (CaCl₂) and hydrogen phosphate dehydrate (Na₂HPO₄*2H₂O) were investigated using a dynamic low amplitude oscillatory rheology. For a considered condition, the final values of storage modulus (G') and loss modulus (G″) of camel milk gels were significantly lower than those of cow milk gels. The increase of the added CaCl₂ levels improved significantly the gelation properties of camel and cow milk gels, since a reduction in the gelation time and an increase in the gel firmness were observed. Following the addition of Na₂HPO₄*2H₂O at 10 and 20 mM, no significant effect on the gelation rate and the firmness of camel milk gels was observed, while, a significant decrease in the gelation rate and firmness were observed for cow milk gels.