Abstract
During skim milk microfiltration (nominal pore size of 0.1 µm) at 10 °C, the whey protein purity in the permeate is reduced by an enhanced serum casein permeation, primarily of β-casein. To decrease casein permeation, the possibility of a pre-heating step under pasteurization conditions before the filtration step was investigated, so as to shift the equilibrium from soluble serum casein monomers to impermeable micellar casein. Immediately after the pre-heating step, low temperature microfiltration at 10 °C was conducted before the casein monomers could diffuse into the serum. The hypothesis was that the dissociation of β-casein into the serum as a result of a decreasing temperature takes more time than the duration of the microfiltration process. It was found that pre-heating reduced the β-casein permeation during microfiltration without significantly affecting the flux and whey protein permeation, compared with a microfiltration at 10 °C without the pre-heating step. Furthermore, the addition of calcium (5 and 10 mM) not only reduced the casein permeation and thus increased the permeate purity, defined as a high whey protein-to-casein (g L−1/g L−1) ratio, but also decreased the filtration performance, possibly due to the structural alteration of the deposited casein micelle layer, rendering the deposit more compact and more retentive. Therefore, the possible combination of the addition of calcium and pre-heating prior to microfiltration was also investigated in order to evidence the potential increase of whey protein (WP) purity in the permeate in the case of Ca2+ addition prior to microfiltration. This study shows that pre-heating very close to low temperature microfiltration results in an increased purity of the whey protein fraction obtained in the permeate.
Highlights
Milk protein fractionation by microfiltration (MF) is often conducted at low temperatures of around 10 ◦C [1,2,3]
Apart from reasons related to microbial growth, a point of difference between these temperatures is that the purity of the whey protein fraction in the permeate, as well as the casein yield in the retentate, are reduced at a low temperature [6] because of the enhanced migration of soluble casein, primarily β-casein, from the casein micelle into the serum [7]
The pasteurization of skim milk, which is a mandatory step in milk processing, could be such a pre-heating step
Summary
Milk protein fractionation by microfiltration (MF) is often conducted at low temperatures of around 10 ◦C [1,2,3]. Manipulation of the chemical composition of skim milk prior to microfiltration can negatively affect the filtration performance because of the enhanced interactions between the casein micelles deposited in a more compact fouling layer at the membrane surface [12]. When the 5–10 mM of added calcium was exceeded, the whey protein permeation decreased because of the additional retention effect of a dense fouling layer of casein micelles. As shown by Liu et al [15] by ultracentrifugal separation, the kinetics to reach the equilibrium state between serum casein and micellar-bound casein are more rapid when heating milk in a short period of time compared with cooling in a temperature range of 10–40 ◦C, because the equilibration is diffusion-driven, which is faster at higher temperatures. The time allowing equilibrium regarding casein diffusion at a low temperature is sufficiently high to perform microfiltration immediately after this dedicated heat treatment
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