Microfiltration

Microfiltration is a process where a liquid flowing tangentially over a semipermeable membrane is split into two fluxes, one that passes though the membrane (the permeate) and the other one that contains components/bodies that exceed the membrane pore size and are thus retained (the retentate).

It has been known for a long time that a water solution can turn germ-free by filtering through a membrane filter with a pore size of approximatly 0.2 micron.

In milk microfiltration, it must be taken into account that most of the fat globules and some of the proteins are as large as, or larger than, the bacteria. This results in the filter fouling very quickly when membranes of such a small pore size are chosen.

It is thus preferable first to separate the milk into skim and cream, and then pass the skim-milk phase through the microfilter, while the cream needed for standardization of the fat content is heat-treated separately (at least highly pasteurized). The two fluxes are mixed together in the desired proportion before being pasteurized, cooled, and packed.

A microfiltration system, including an indirect sterilization unit for combined sterilization of an adequate volume of cream for fat standardization and of the retentate from the filtration unit, is illustrated in Figure 1.1.2.1 (TetraPak Engineering, 2014).

The MF plant is provided with two loops working in parallel, each one handling up to 5,000 l/hr of skim milk, for a throughput capacity of approximately 10,000 l/hr. Capacity can be increased by adding loops.

The raw milk entering the plant is preheated to a suitable separation temperature, typically about 60° to 63°C, at which it is separated into skim milk and cream. A preset amount of cream, enough to obtain the desired fat content in the milk, is routed by a standardization device to the sterilization plant.

In the meantime, the skim milk is cooled to 50°C, the normal microfiltration temperature, before entering the MF plant, where the flow of milk is divided into two equal flows, each of which enters a loop where it is fractionated into a bacteria-rich concentrate (retentate), comprising about 5% of the flow, and a bacteria-reduced phase (permeate). The retentates from both loops are then united and mixed with the cream

Milk treatment including double-loop microfilter and sterilization of bacteria concentrate together with the cream needed for fat standardization of the cheese milk

Figure 1.1.2.1 Milk treatment including double-loop microfilter and sterilization of bacteria concentrate together with the cream needed for fat standardization of the cheese milk.

  • 1) Pasteuriser
  • 2) Centrifugal separator
  • 3) Automatic standardization system
  • 4) Double loop microfiltration plant
  • 5) Sterilization plant

intended for standardization before entering the sterilizer. Following sterilization at 120°-130°C for a few seconds, the mixture is cooled to about 70°C before being remixed with the MF permeate. Subsequently, the total flow is pasteurized at 72°C for about 15 seconds and cooled to filling temperature, typically 4°C.

Others configurations are possible, where, for exemple, two MF plants operate in series, the second being fed with the retentate of the first one, the two permeates being mixed together, with cream added according to the fat content required before being homogeneized, pasteurized, and filled into bottles/cartons. In this case, the retentate of the second MF plant (0.5% to 1.0% of the original milk volume) is discarded for animal feed use. Fluxes obtained industrially are in the order of 500 L/m2hr during 10 hours.

According to reached VCF (20 for the first step, 200 for the second), the observed permeation rates for proteins are 99% and 99.4%. Average observed bacterial decimal reduction (DR) is above 3.5 for developed dairy countries and higher than 6 in milk with poor bacteriological quality collected in emerging countries (De Carvalho & Maubois, 2009).

MF efficiency in bacterial removal is reported by other authors to be around 3 logs: 99.7% to 99.9% vegetative cells; 98.2% to 99.8% anaerobic spores; and 99.7% aerobic spores reduction, considering MF permeate VS skim milk (APV Invensys, 1995). When coupled to cream high pasteurization and final pasteurization, the bacterial removal efficiency (MF pasteurized VS raw milk) increases for vegetative cells to almost 100%, whereas it is lower for bacterial spores (98.2% to 98.7% aerobic; 93.0% to 95.2% anaerobic).

Use of MF membranes with smaller pore size (0.8 mcm) may prove advantageous: The flux at 50°C is reduced at 400 l/m2hr but the observed DR has been observed to be higher than 13 on Clostridium botulinum, a value meaning product sterility (AFSSA, 2002). Mixed with UHT cream for fat standardization, homogeneized at 80°C, heat- treated at 95°C for 6 seconds, and immediately conditioned and packed aseptically, the result is a commercially sterile milk, stable for 62 days at 40°C and more than 8 months at room temperature. This process, called Ultima Process by Tetra Laval, was not commercially developed for unknown reasons.

 
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