The determination of the zeta potential of casein micelles

Effects of pressure, shear, temperature, and their interactions on selected milk quality attributes

The effects of pressure, temperature, shear, and their interactions on selected quality attributes and stability of milk during ultra-shear technology (UST) were investigated. The UST experiments include pressure (400 MPa) treatment of the milk sample preconditioned at 2 different initial temperatures (25°C and 15°C) and subsequently depressurizing it via a shear valve at 2 flow rates (low: 0.15-0.36 g/s; high: 1.11-1.22 g/s). Raw milk, high-pressure processed (HPP; 400 MPa, ~40°C for 0 and 3 min) and thermal treated (72°C for 15 s) milk samples served as the controls. The effect of different process parameters on milk quality attributes were evaluated using particle size, zeta potential, viscosity, pH, creaming, lipase activity, and protein profile. The HPP treatment did not cause apparent particle size reduction but increased the sample viscosity up to 3.08 mPa·s compared with 2.68 mPa·s for raw milk. Moreover, it produced varied effects on creaming and lipase activity depending on hold time. Thermal treatment induced slight reduction in particle size and creaming as compared with raw milk. The UST treatment at 35°C reduced the effective diameter of sample particles from 3,511.76 nm (raw milk) to 291.45 nm. This treatment also showed minimum relative lipase activity (29.93%) and kept milk stable by preventing creaming. The differential effects of pressure, shear, temperature, and their interactions were evident, which would be useful information for equipment developers and food processors interested in developing improved food processes for dairy beverages.

Micelas de caseína: dos monômeros à estrutura supramolecular

Resumo A importância primária das micelas de caseína reside no fato de que os processos empregados na transformação do leite em quaisquer de seus derivados dependem, direta ou indiretamente, de sua estabilidade ou de sua desestabilização controlada. Assim, o objetivo do presente trabalho é apresentar uma revisão atualizada sobre a organização estrutural das micelas de caseína. Em termos físico-químicos, as micelas de caseína podem ser definidas como agregados supramoleculares esféricos e porosos, altamente hidratados, carregados negativamente, com diâmetro médio de 200 nm, e que apresentam aproximadamente 104 cadeias polipeptídicas. Além de água, as micelas são constituídas por quatro tipos de caseínas, chamadas de αS1, αS2, β, e κ-caseínas, que estão unidas por meio de interações hidrofóbicas e eletrostáticas, e pela presença de minerais, sobretudo sais de fosfato de cálcio, os quais são os principais responsáveis pela manutenção da estrutura micelar. A estabilidade das micelas de caseína é atribuída à presença de uma camada externa difusa, formada basicamente por κ-caseína. Apesar de as propriedades coloidais das micelas de caseína serem conhecidas, ainda não há consenso sobre como as moléculas de caseína estão estruturadas em seu interior. Portanto, os principais modelos que descrevem a organização interna das micelas de caseína são apresentados na parte final do artigo.

Initial stage of cheese production: a molecular modeling study of bovine and camel chymosin complexed with peptides from the chymosin-sensitive region of κ-casein

Bovine chymosin has long been the preferred enzyme used to coagulate cow's milk, in the initial stage of cheese production, during which it cleaves a specific bond in the milk protein κ-casein. Recently, camel chymosin has been shown to have a 70% higher clotting activity toward cow's milk and, moreover, to cleave κ-casein more selectively. Bovine chymosin, on the other hand, is a poor clotting agent toward camel's milk. This paper reports a molecular modeling study aimed at understanding this disparity, based on homology modeling and molecular dynamics simulations using peptide fragments of κ-casein from cow and camel in both bovine and camel chymosin. The results show that the complex between bovine chymosin and the fragment of camel κ-casein is indeed less stable in the binding pocket. The results also indicate that this in part may be due to charge repulsion between a lysine residue in bovine chymosin and an arginine residue in the P4 position of camel κ-casein.

Measurement of electrophoretic mobilities and zeta-potentials of particles from milk using laser Doppler electrophoresis

The use of a commercial laser Doppler electrophoresis apparatus to measure electrophoretic mobilities of milk constituents is demonstrated, and calculations of apparent ζ-potentials from these mobilities are given. Systems studied were: native and renneted casein micelles suspended in milk ultrafiltrate and different buffers containing Ca2+, particles from homogenized milk before and after renneting, fat globules, and homogenized fat/caseinate systems. The results confirmed previous estimates of the ζ-potentials of these materials, but also extended to cover the effect of changing the concentrations of Ca2+ and other salts on the electrokinetic properties of the particles. Increasing concentration of Ca2+ caused considerable decrease in the ζ-potentials of native and renneted casein micelles, and temperature increase caused the apparent ζ-potential to increase in both cases. Particles from homogenized milk showed ζ-potentials very similar to those of native casein micelles, and renneting of the homogenized milk gave behaviour similar to renneted casein micelles. The ζ-potential of the homogenized materials could be distinguished clearly from those of native fat globules. Likewise, milk fat which had been homogenized using whey protein showed distinct differences in its surface properties from particles based on caseinate or casein micelles.

Rennet coagulation of milk in the presence of sucrose

The aim of this work was to test the diffusion-controlled hypothesis of milk coagulation kinetics by reducing the diffusion coefficient of casein micelles. This has been achieved by increasing the solvent viscosity of milk through sucrose addition. Milk was reconstituted from skim milk powder and sucrose added at 100–300 g kg–1. Hydrolysis and coagulation were followed by chromatographic determination of caseinomacropeptide content and optical, thermal and viscoelastic measurements. Soluble and ionic calcium were determined by atomic absorption spectrophotometry and ionometry and micelle size was measured by dynamic light scattering. Addition of sucrose resulted in a substantial retardation of both enzymic and aggregation steps, a re-equilibration of calcium because of water reduction, and a micelle size increase. The enzymic rate constant was inversely proportional to the viscosity, according to a diffusion-controlled model, and the lag or characteristic times for the aggregation reaction were inversely proportional to the viscosity. These results are consistent with the involvement of diffusion-controlled steps in the sequence of reactions.

Precision conductometry in milk renneting

On renneting, the electrical conductivity of milk decreased as viscosity increased. The sigmoidal time course of the decrease resembled the time course of shear modulus, but was more rapid. The total amount of change was independent of the amount of rennet and proportional to milk conductivity and its casein content. The conductivity change was interpreted as a change in the way casein micelles obstructed the path of the charge-carrying ions.

Estimation of casein micelles' surface energy by means of contact angle measurements

The surface energies of highly hydrated casein micelle layers isolated from variously pretreated skim milks have been determined by means of contact angle measurements. The long range Lifshitz-Van der Waals (LW) and the short range hydrogen bonding (SR) components of surface energy were determined using α-bromonaphthalene and water for contact angle measurements. Casein micelles isolated from untreated and heat treated milks showed similar surface energy values of about 63·5 mJ.m-2 with an LW component of 19·2 mJ.m-2 and an SR component of 44·3 mJ.m-2. The calculated attraction potential energy was − 0·7 mJ.m-2. Casein micelles isolated from renneted milk showed a surface energy of 33·0 mJ.m-2 with an LW component of 30·7 mJ.m-2 and an SR component of 2·3 mJ.m-2. The attraction potential energy of renneted micelles was nearly two orders of magnitude higher than those of micelles from other milks ( − 63·3 mJ.m-2). The SR component of interfacial energy accounted for 98% of this attraction potential. The importance of attractive forces in relation to casein micelle stability is discussed.

Electroacoustic determination of size and zeta potential of casein micelles in skim milk

Measurements of the zeta potential and particle size of casein micelles in skim milk suspensions at natural and lower pH have been made using the technique of electroacoustics. This technique requires no dilution or change of environment of the casein micelles. The zeta potential obtained at natural pH for a commercial skim milk suspension was −18 mV; it became less negative with decreasing pH. The median particle size observed at natural pH for a commercial skim milk suspension was 0·2 εm, in good agreement with previously reported values. The particle size increased as the pH was decreased.