Milk Concentration by Microfiltration
MF can be used to remove bacteria from milk or concentrating casein depending on the type of membranes used. Filtering skim milk on small pore membranes (0.1 p average pore size) allows retention of native micellar casein: as the process proceeds, the reten- tate becomes more and more rich in micellar phosphocaseinate (MPC), whereas the permeate contains all other milk compounds. As previously reported for UF, milk concentration by microfiltration can also be performed for standardization purposes for the production of retentates. The main difference with respect to UF, is that MF retentates have the whey proteins/casein ratio modified—the extent by which the ratio decreases depends on the operative conditions.
In the case of application of a diafiltration step, the process can be used for producing purified MPC, almost depleted of whey proteins, which can be stored and sold as a protein fortification agent. From MPC it is also possible to manufacture pure p-casein or caseinomacropeptides (CMP) by further processing (Maubois and Ollivier, 1997). Independently from the way used for fortification, milk enriched in MPC has interesting technological properties, such as shorter coagulation time, and increased curd firmness. Furthermore, it also exhibits increased heat stability since the reduction of the ratio whey proteins/caseins significantly reduces the detrimental effects of heat treatment on rennet coagulability of milk. The technical aspects from which economic benefits of milk concentration by MF derive are almost the same than UF, but usefulness in enhancing cheese yield is still under question.
Neocleous et al. (2002) found that using MF retentates with a low concentration factor (1.26 to 1.82) only increased actual cheese yield (kg of cheese per kg of milk), but not the yield efficiency. According to Guinee et al. (2006), the yield efficiency could be improved in cheddar cheese making if the milk protein content is raised by MF to 4%, but UF remained more effective. A study on mozzarella made from milk concentrated by high concentration factor (about 8) indicated that some additional cheese yield can be obtained (Brandsma & Rizvi, 2001). In this case, the mechanism that is responsible of this result is probably optimization of the coagulating properties, which leads to better retention of milk solids. MPC can also be recombined with cream for the production of highly standardized cheese.
A second aspect that has to be considered in evaluating the economical productivity of a dairy MF plants is the permeate. The value of permeate from milk microfiltrate is higher compared to that of classical whey: it contains all milk compounds except casein and is sterile (both as to bacteria and viruses). It is considered a valuable co-product and is also called ideal whey or virgin whey since it is an excellent starting substrates for fractionation and isolation of whey proteins. It has the same pH as milk, unlike cheese whey, which is always more acidic. Concentration of ideal whey by ultrafiltration gives rise to native whey protein concentrate (WPC) or isolate (WPI): the former can be dried and used in applications in which excellent solubility, foaming, and gel-forming properties are required (Maubois et al., 2001; 0stergaard, 2003). A second valuable difference with respect to classical whey is absence of glicomacropeptide (GMP) deriving from k- casein hydrolysis. This characteristic improves quality of both WPC and WPI if they are destined to be used as ingredients in baby foods. In fact, GMP is thought to cause hyperthreoninemia in preterm infants as it contains 20% more threonine than human milk (Boehm et al., 1998).
On the whole, the economic benefit of microfiltration in cheese manufacture does not appear clearly defined. It strongly depends on individual skill of the system management, the type, and local price of the product—thus, the net effect of microfiltration can vary substantially.