Ultrafiltration of cheesemilk

16 Why is ultrafiltration used for cheesemaking and how is it applied?

Ultrafiltration (UF) is a membrane separation process that selectively concentrates milk protein and fat. The technology is used in cheesemaking to increase cheese yield [48] by the incorporation of whey proteins. UF can also be used to improve manufacturing efficiency by enhancing the casein content of the milk to optimise gel formation, the recovery of casein and fat during cheesemaking, and to maximise plant throughput. UF is classified into the three categories: low-concentration (LC), medium-concentration (MC) and high-concentration (HC) factor UF, depending on the extent of concentration.

In traditional cheese manufacture, the fat and casein in milk are concentrated by removal of moisture [34]. Moisture released during cheesemaking contains water-soluble components which include the whey proteins, lactose and minerals. The whey proteins account for approximately 20% of the total milk protein. The caseinomacropeptide, which is released from K-casein during renneting [24], is also found in the whey and accounts for approximately 4% of milk protein. Membrane separation of milk by UF can be used to incorporate these proteins into cheese. UF of milk produces a permeate (also called ultra-filtrate) containing water, lactose, soluble minerals, non-protein nitrogen and water-soluble vitamins, and a retentate in which the casein, whey protein, fat and colloidal salts are increased in proportion to the amount of permeate removed. As much of the water is removed prior to cheese manufacture, the level of syneresis required is reduced, and whey proteins are entrapped in the curd during cheesemaking.

The most widely used application of ultrafiltration in cheese manufacture is in the production of low concentration factor retentates to facilitate uniformity in milk composition by elimination of seasonal variation [3, 9] ('protein standardisation'). In LC UF the milk is concentrated approximately 1.5-fold. At this level of concentration it is possible to apply conventional cheese manufacturing techniques and the only investment required is UF equipment to concentrate the milk. Milk protein content is standardised to levels ranging from 3.7 to 4.5% prior to cheese manufacture. Low concentration factor retentates are used in the production of a variety of cheeses including Camembert [128], Cheddar [100] and Mozzarella [146]. The advantages of cheese-making using low-concentration factor retentates are uniformity in milk composition, production of a firm coagulum which encourages lower losses of casein in whey, increased cheese yield (approximately 6% on a protein basis), improved cheesemaking efficiency in terms of a higher throughput per vat, and no requirement for additional cheesemaking equipment with the exception of the ultrafiltration unit.

The increase in cheese yield using a low-concentration retentate is attributable to improved casein and fat retention due to improved curd firmness, and the retention of a small proportion of whey proteins. For Cheddar cheese, concentration of milk 1.6 or 1.7-fold is common. At higher levels of concentration, the rennet coagulum is extremely firm and difficult to handle and as a consequence fat losses in the whey may be high.

In MC UF a concentration factor of 2-6-fold is used to achieve the final solids content of the cheese without the need for whey expulsion. This approach effectively increases cheese yield through incorporation of whey proteins. Additional benefit is derived when the milk is heat treated to denature whey proteins prior to UF and the denatured whey proteins carry additional moisture into the curd.

MC UF has been particularly successful in the production of high-moisture unripened cheeses such as Quarg and Cream cheese [170] and of cheeses which are not heavily dependent on proteolysis for flavour development, for example Feta [164]. In the commercial production of Feta using UF the milk is concentrated 5-fold. The ultrafiltrated whole milk is homogenised [31], blended with lactic starter [18], salt and a lipase-rennet mixture [27], and poured into 18 kg tins, where the curd, which includes whey proteins, is formed. The curd is then covered with 6% salt or brine and is held for ripening.

Many reports have been published on the use of UF concentration to attain the final dry matter level of soft or semi-hard rennet-curd cheeses, including Camembert, Blue cheese [137], Havarti and Mozzarella. However, the use of UF technology for production of these cheeses has been limited owing to problems with flavour, texture and functionality. These problems are partly associated with changes in the buffering capacity of milk [22] on concentration by UF which impacts on critical cheesemaking parameters. UF of milk at its normal pH of 6.7 results in an increase in buffering capacity of the retenate. The increase in buffering capacity results from concentration of the colloidal calcium phosphate

[4] which is bound to the casein micelles and is concentrated to the same extent as the caseins. Critical factors influencing cheese quality, flavour and texture development are altered owing to the enhanced buffering capacity. These include the rate and extent of acidification by the lactic acid bacteria, the rennet coagulation time [30], the rheological properties of the curd, the activity of ripening enzymes [23, 28], the lysis of mesophilic lactic acid bacteria and the water-holding capacity of the cheese.

To avoid undesirable effects on cheese quality due to the increased buffering capacity, e.g. excessive acid taste, crumbly texture or abnormal ripening, the mineral content of UF retentates must be lowered. The extent to which minerals should be reduced is specific to the variety of cheese being produced. Lowering the mineral content of UF retentates can be achieved by solubilisation of the colloidal calcium phosphate by reducing the pH of milk prior to or during ultrafiltration so that soluble minerals pass into the permeate.

The maximum achievable concentration by ultrafiltration is about 7:1 for whole milk, and this is not sufficient to achieve the dry matter levels required for hard cheeses such as Cheddar. Following coagulation of the retentate, whey must be expelled through syneresis to attain the desired solids content in the final cheese, but the high concentration of curd solids prohibits the use of conventional cheese manufacturing equipment. HC UF is therefore used in conjunction with specially designed equipment to produce high-solids curds.

The APV Sirocurd process developed for Cheddar was an example of this technology. The process involved the continuous rennet coagulation of milk ultrafiltered to 40-45% total solids. A small proportion of the ultrafiltered milk was prefermented with lactic acid bacteria and used as bulk starter at the level of 10-12%. The continually formed coagulum was cut with specially designed wire knives and cubed curd pieces were transferred into a rotating drum where syneresis took place during heating to 38 0C over a 30-40 min period. Automated cheddaring occurred at the optimum pH, followed by milling and salting of the curd. Yield increases [48] of 6-8% were claimed with this process. However, after several years of operation this process is no longer in use because of technical difficulties and poor economics.

The most successful commercial applications of UF in cheese manufacture have been in the production of Feta-type cheeses and fresh acid-curd varieties such as Quarg, Ricotta and Cream cheeses, where substantial improvements in yield are attainable.

However, successful manufacture of all cheese varieties by UF technology will require careful consideration of the properties of the protein-enriched concentrates as they determine the quality of the end products, in addition to evaluation of the economics of use of membrane technology.

Further reading

FOX, P.F., GUINEE, T.P., COGAN, T.M. and McSweeney, P.L.H. (2000). Fundamentals of Cheese Science, Aspen, Gaithersburg, MD.

GUINEE, T.P., O'KENNEDY, B.T. and KELLY, P.M. (2006). Effect of milk protein standardization using different methods on the composition and yield of Cheddar cheese. J. Dairy Sci. 89, 468-482.

MISTRY, v.v. and MAUBOIS, J.L. (2004). Application of membrane separation technology to cheese production, in Cheese: Chemistry, Physics and Microbiology Volume 1, 3rd edn, P.F. Fox, P.L.H. McSweeney, T.M. Cogan and T.P Guinee (eds), Elsevier Academic Press, Amsterdam, pp. 265-275.

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