Rennet-induced aggregation of homogenized milk: Impact of the presence of fat globules on the structure of casein gels

Abstract

-The present study investigated the renneting behaviour of casein micelles as influenced by the presence of fat globules. The gelation of skim milk, homogenized milk and homogenized milk with Tween 20 added was observed using diffusing wave spectroscopy and rheology. By adding Tween 20 to homogenized milk, it was possible to displace most of the milk protein from the oil/water interface and create a system with the same colloidal attributes as homogenized milk but with oil droplets with a very different interfacial composition, and relatively inert during rennetinduced aggregation. The primary phase of gelation was faster in homogenized milk compared to skim milk. Casein micelles were altered by the homogenization process which spreads them at the interface, making κ-casein more readily available. The addition of Tween 20 did not further affect the enzymatic activity on the micelles. However, the onset of gelation occurred earlier in homogenized milk with Tween 20 added, as the micelles aggregated at a lower level of proteolysis compared to homogenized milk. This work clearly showed that the gels formed in homogenized milk with and without Tween 20 have different physico-chemical properties, because of the different colloidal state of the filler particles.homogenized milk / polysorbate / rennet coagulation / diffusing wave spectroscopyRésumé -Agrégation induite par la présure de lait homogénéisé : impact de la présence de globules gras dans la structure des gels de caséine. Cette étude porte sur l'influence de la présence des globules gras sur le comportement des micelles de caséine après ajout de présure. Article published by EDP Sciences La gélification de lait écrémé, de lait homogénéisé et de lait homogénéisé additionné de Tween 20 a été observée par diffusion dynamique de la lumière et par rhéologie. En ajoutant du Tween 20 au lait homogénéisé, il était possible de déplacer la plupart des protéines adsorbées à l'interface huile/ eau et de créer un système avec les mêmes éléments colloïdaux que le lait homogénéisé mais avec des gouttelettes lipidiques ayant une composition à l'interface très différente et relativement inertes durant l'agrégation induite par la présure. La phase primaire de la gélification était plus rapide dans le lait homogénéisé en comparaison au lait écrémé. Les micelles de caséines étaient altérées par l'homogénéisation qui les disperse à l'interface, rendant les caséines kappa plus disponibles. L'addition de Tween 20 ne provoquait pas de modification supplémentaire de l'activité enzymatique sur les micelles. Cependant, le démarrage de la gélification apparaissait plus précocement dans le lait homogénéisé avec Tween 20, puisque les micelles s'agrégeaient à un niveau plus faible de protéolyse que celui du lait homogénéisé. Ces travaux montrent que les gels formés dans du lait homogénéisé additionné ou non de Tween 20 ont des propriétés physico-chimiques différentes, en raison de l'état colloïdal différent des particules de remplissage.lait homogénéisé / polysorbate / coagulation ...

Acid coagulation behavior of homogenized milk: effect of interacting and non-interacting droplets observed by rheology and diffusing wave spectroscopy

“…Light scattering experiments are able to detect the dynamics of these "semi-free" droplets encased in the gel pores which are unaffected by the developing and further rearrangement of the network. A similar behavior was reported during renneting of homogenized milk without and with Tween 20 (Ion Titapiccolo et al 2010a) confirming the different role of "active" and "inert" fat globules in a case in network.…”

“…The surfactant displaces the casein micelles from the fat/water interface; this results in the removal of most of the proteins from the surface of the globules, which become again free into solution. The competitive displacement of Tween 20 in dairy-based emulsions has been previously described (Goff and Jordan 1989;Ion Titapiccolo et al 2010a;Mackie et al 1999;Pugnaloni et al 2004). The acid coagulation of homogenized milk with and without addition of 2% Tween 20 was monitored using small oscillatory rheology and DWS.…”

“…On the other hand, native fat globules behave as "inert" fillers because their membrane is not able to interact directly with the destabilized casein micelles (Michalski et al 2002;van Vliet 1988;van Vliet and Dentener-Kikkert 1982). Tween has been employed before in studies with milk, showing that Tween 20-stabilized emulsions formed droplets which did not play an active role in the structure of the casein gels (Ion Titapiccolo et al 2010a), while Fox and Hearn (Fox and Hearn 1978) showed that Tween 80 had no effect on the heat stabilitypH curve of casein micelles. In recombined milk, while casein-stabilized oil droplets behave as structure-forming, Tween 20-stabilized emulsions behave as inert fillers.…”

“…The two systems of study were homogenized milk and homogenized milk containing Tween 20, as small molecular weight surfactants are known to be able to displace proteins from the surface of oil droplets in protein emulsions (Mackie et al 1999). This methodology will ensure two systems containing fat globules with similar size distribution but different interacting interfaces (Ion Titapiccolo et al 2010a). When the different milks are subjected to acid coagulation, it will be possible to determine if the development of the gel-forming ability of the casein micelles is affected by the presence of the fat globules in the system and whether the different active interfaces on the droplets play a role in the development and final structure of the gels.…”

Effects of Size and Stability of Native Fat Globules on the Formation of Milk Gel Induced by Rennet

“…Diffusing wave spectroscopy (DWS) has recently been successfully applied to follow the dynamics of oil droplets in mixed systems with casein micelles (Gaygadzhiev and others ; Ion Titapiccolo and others ; Corredig and others ). Due to the dominated scattering effect of fat globules compared to caseins micelles, the noninvasive nature of the DWS light scattering technique therefore allows one to monitor the mobility of fat globules during initial stages of formation of the protein network.…”