Characterization Of Cohesive Forces Involved Between Casein Micelles In A Concentrated State

In membrane filtration of milk (UF and MF), casein micelles are accumulatedonto the surface of membrane until forming, under certain conditions, an irreversiblefouling layer (we can consider it as a ‘gel’) that limits performances of the operation.So it is reasonable to believe that the cohesive forces lead to the irreversibility ofthe ‘gel’. In order to understand this phenomenon more clearly, the laboratory-UMR STLO of the french institute INRA in Rennes, commenced to study on it. Theprevious study has shown that, in a concentrated state, the cohesive forces betweencasein micelles originated from the micellar surfaces. The objective of this work isto identify and characterize further the cohesive forces between the micellar surfacesin a concentrated state.Moreover, the main strategy is as following: firstly casein dispersions areprepared; and then the dispesions are highly concentrated by the method ofultracentrifugation to get ‘gels’ enough concentrated to form cohesive forcesbetween micelles; finally, we identify the nature of theses cohesive forces throughbreaking or enhancing certain cohesive forces by dipping the concentrated casein‘gels’ in aqueous phase under different physical and chemical conditions. But insome conditions, disintegration of native micelles makes it impossible to know theirreversibility. In this case, we can study on the cross-linked micelles that couldresist against the disintegration. The results of this study are:1. Concentration of casein dispersion by ultracentrifugation to get the ‘gels’enough concentrated, the optimal technological parameters:20℃,5g/L natural orcross-linked casein dispersion,10g, rotate speed150000g,3h.2. Identification of the nature of cohesive forces in the concentrated state bycomparing the irreversibility of gel (through a redispersed fraction, note FR) under aphysical and chemical condition (FRx) to that under the reference condition (FR0,0.15g of natural or cross-linked casein concentrated gel dipped in UF~20mL after16h). The different physical and chemical conditions are:①Distilled water: FRx≈FR0②EDTA (30mmol/L): FRx>FR0③Urea (6mol/L): FRx≈FR0④Alkaline distilled water (pH~8.5): FRx>FR0⑤Temperature(6℃,20℃,50℃): FR50℃>FR20℃>FR6℃These main results show that the cohesive forces between the micellar surfacesin a concentrated state are in equilibrium between two types of forces: attractiveforces as hydrogen bonds (more important) and ‘‘phosphocalcic bonds’’, and repulsive forces which are the electrostatic repulsions between the caseins. However,hydrophobic interactions do not play a significant role in the cohesive forcesbetween the micelles.