Addition of calcium and magnesium chlorides as simple means of varying bound and precipitated minerals in casein micelle: Effect on enzymatic coagulation.

In casein micelle (CM), Ca is either precipitated in the colloidal calcium phosphate (CCP) stabilized by clusters of phosphoserine (SEP) residues, or is directly bound to SEP (or glutamic and aspartic acids) of caseins without inorganic phosphate. However, it is currently not possible to titrate separately the different micellar Ca forms, making it difficult to assess their respective importance for CM properties and behavior. Both Ca2+ and Mg2+ have the same binding constants with SEP. Moreover, MgHPO4 is more soluble than CaHPO4, and its natural concentration in milk is lower. Thus, upon addition of MgCl2, Mg is mainly exchanged with CM in the bound form, whereas upon addition of CaCl2, Ca is mainly exchanged in the precipitated form. Our objective was to assess the role of the 2 forms of micellar cations (bound and precipitated) during the enzymatic coagulation of cow milk. Magnesium chloride, CaCl2, or KCl (10 mM) were added to milk and pH was adjusted to 6.6 after overnight equilibration. The KCl-supplemented milk was a positive control to assess the effect of the increased ionic strength after MgCl2 and CaCl2 addition. Mineral partition between soluble and colloidal phases after salt addition was assessed both experimentally and by using computer simulation. Enzymatic coagulation was proceeded at 30°C. Hydrolysis of κ-casein was followed by the quantitative determination of caseinomacropeptide released by RP-HPLC, aggregation of para-κ-casein micelles was measured through the evolution of backscattering properties of milk and the gel time and gel firming kinetics were determined using a Chymograph (Chr. Hansen, Horsholm, Denmark). The KCl addition did not affect mineral partition. As expected, CaCl2 addition mainly increased the CCP content, whereas the addition of MgCl2 mainly increased the bound divalent cations content. The kinetics of κ-casein hydrolysis was slowed down after CaCl2 and MgCl2 addition, and was negatively correlated with the concentration of divalent cations in the soluble phase of milk. Aggregation and gel firming curves plotted versus the progress of κ-casein hydrolysis were similar for both CaCl2- and MgCl2-supplemented milk. In view of the dual-binding model for CM assembly, this means that both Ca forms reduce electronegative repulsions between para-micelles by specific charge shielding. Inclusion of 2 Ca forms in structural models for CM allows a more detailed comprehension of how mineral equilibria affect CM properties.

Bolus quality and food comfortability of model cheeses for the elderly as influenced by their texture.

The aim of this study was to examine the influence of the texture of dairy products on bolus properties and food comfortability as perceived by elderly persons. Four cheese models identical in terms of their nutritional composition but different by the type of texture (Soft, Hard, Processed and Whipped) were developed to resemble market cheese. Study was performed with a panel of 38 elderly persons without dental impairment but with variable rate of saliva flow. The time of chewing required to form the bolus was measured. The rheological properties of the bolus obtained were studied as well as the quantity of saliva incorporated. Food comfortability was assessed by means of a questionnaire asking about in-mouth comfort, bolus formation, pain, and perceived texture and flavor. Results showed that food comfortability was positively correlated to the ease of forming the bolus which depended on the food texture. Thus, the bolus formed with the Whipped product which required more saliva, was harder than the other three products and so judged the least comfortable. This product was characterized by a dry and sticky texture. On the other hand, the Soft and Processed products led to a softer food bolus that was more easily formed, and thus were judged as more comfortable. Their textures were perceived as soft, fatty and melting. This work highlights the importance of considering food bolus formation and related properties when looking for the link between food product characteristics and the enjoyment of eating in the case of the elderly population.

Rheology and phase behavior of dense casein micelle dispersions.

Casein micelle dispersions have been concentrated through osmotic stress and examined through rheological experiments. In conditions where the casein micelles are separated from each other, i.e., below random-close packing, the dispersions have exactly the flow and dynamic properties of the polydisperse hard-sphere fluid, demonstrating that the micelles interact only through excluded volume effects in this regime. These interactions cause the viscosity and the elastic modulus to increase by three orders of magnitude approaching the concentration of random-close packing estimated at C(max) approximately 178 g/l. Above C(max), the dispersions progressively turn into "gels" (i.e., soft solids) as C increases, with elastic moduli G(') that are nearly frequency independent. In this second regime, the micelles deform and/or deswell as C increases, and the resistance to deformation results from the formation of bonds between micelles combined with the intrinsic mechanical resistance of the micelles. The variation in G(') with C is then very similar to that observed with concentrated emulsions where the resistance to deformation originates from a set of membranes that separate the droplets. As in the case of emulsions, the G(') values at high frequency are also nearly identical to the osmotic pressures required to compress the casein dispersions. The rheology of sodium caseinate dispersions in which the caseins are not structured into micelles is also reported. Such dispersions have the behavior of associative polymer solutions at all the concentrations investigated, further confirming the importance of structure in determining the rheological properties of casein micelle systems.

The formation of calcium lactate crystals is responsible for concentrated acid whey thickening.

The use of spray drying for dehydration of acid whey is generally limited by the appearance of uncontrolled thickening and solidifying of the whey mass during the lactose crystallization step. The origin of this physical change is still unknown and probably linked to complex interactions between physical properties and chemical composition of these products. To understand this phenomenon, we simulated the thickening of concentrated acid whey on a laboratory scale by measuring the flow resistance changes as a function of time and whey composition. The thickening process was characterized by an amplitude of torque and a lag time (induction time). Thickening of lactic acid whey concentrate occurred regardless of the presence of whey proteins or lactose crystals. Moreover, this work clearly demonstrated that the thickening process was due to the formation of filamentous structures corresponding to calcium lactate crystals and showed a large dependence on calcium and lactate contents, pH, and phosphate concentration.