Ultrafiltration is a low-pressure membrane process used to separate high molecular weight compounds from a feed stream. Ultrafiltration has larger pores than nanofiltration and reverse osmosis and is therefore the least costly of the three to operate.
- Dairy applications: with the development of improved membranes, ultrafiltration has become a major separation process in the dairy industry. The two largest areas of application are the pre-concentration of milk for cheese manufacture and the production of protein concentrates from whey. Both applications are related to cheese-making.
Whole cow’s milk contains typically 3.5% protein, 4% fat, 5% lactose and 0.7% inorganic salts (ash). The balance (about 87%) is water. In the traditional cheese-making process, milk (usually, but not always, after adjustment of fat content and pasteurization) is coagulated through a combination of lactic acid fermentation and enzymatic reactions. After cutting and temperature adjustment, the coagulated mass separates into solid particles (curd) suspended in
liquid (whey). The curd contains the major milk protein – casein, and most of the fat. The whey is essentially a dilute aqueous solution of lactose, mineral salts and non-casein milk proteins (e.g. lactalbumins and lactoglobulins). The curd is subjected to various methods of treatment and curing, resulting in the vast variety of different cheeses. The whey is usually a problematic waste, due to its high volume (often 90% of the mass of milk used) and its high cost of disposal due to its extremely high biological oxygen demand (BOD).
Ultrafiltration of milk retains the fat globules and the proteins. The inorganic salts and the lactose, along with some of the water are partially removed as permeate. The resulting retentate is a partially concentrated milk with reduced lactose and mineral content. The concentration ratio is often 3- to 5-fold. If further reduction of lactose and mineral content is desired, the retentate is diluted with water and ultrafiltered once again. This kind of ultrafiltration is sometimes called ‘diafiltration’. The pre-concentrated milk is mainly used in the manufacture of many types of cheeses (most commonly soft cheeses). The advantages of using UF pre-concentrated milk for making cheese are (Cheryan, 1986 ):
- Increased yield, probably because of the inclusion of some of the non-casein proteins in the curd
- Lower energy consuption
- Reduced volume of whey.
The use of UF in cheese-making was developed by Maubois, Macquot and Vassal in 1969. The method is therefore known as the MMV Process (Zeman and Zydney, 1996). Plants for the UF pre-concentration of milk operate at a TMPD range of 200 to 500 kPa.
The second major application of UF in the dairy industry is the fractionation of whey. As stated earlier, whey contains the major part of the non-casein proteins, together with low-molecular weight solutes (lactose, mineral salts).Ultrafiltration of whey produces a valuable whey protein concentrate as the retentate and a protein- free permeate containing mainly lactose and minerals. The retentate is usually concentrated further by evaporation and spray-dried. Whey proteins find extensive use in the manufacture of cheese and a considerable array of food products and health food specialities.
- Other applications: the use of UF for the concentration and fractionation of plant protein extracts has been suggested ( Hojilla-Evangelista et al., 2004 ). Ultrafiltration is potentially useful in the process of isolated soybean protein production ( Lawhon et al., 1975, 1979 ; Omosaiye and Cheryan, 1979 ; Cheryan, 1983). In this process, defatted soy fl our is extracted with water at high pH. The aqueous extract is then concentrated by UF. The sugars and other low molecular weight solutes may be partially removed by repeated steps of dilution and ultrafi ltration (diafi ltration), producing a purifi ed and concentrated soy protein extract for further processing. Ultrafi ltration is used commercially as an alternative to evaporative concentration in the production of gelatin.
Lewis, M.J.1996, Ultrafiltration, in Separation Processes in the Food and Biotechnology Industries, ed. A.S. Grandison, M.J. Lewis, Woodhead Publishing, Cambridge, pp 97–154
Youravong, W., Lewis M.J., Grandison, A. S.2003, Critical Flux in Ultrafiltration of Skim Milk,Trans. Inst. Chem. Eng.81, 303–308
Renner, E., El-Salam, M.H.A.1991, Application of Ultrafiltration in the Dairy Industry, Elsevier Applied Science, London.
Guu, Y. K., Zall, R.R.1992, Nanofiltration Concentration on the Efficiency of Lactose Crystallisation,J. Food Sci. 57,735–739.
Premaratne, R. J., Cousin, M.A.1991, Changes in the Chemical Composition During Ultrafiltration of Skim Milk,J. Dairy Sci.74, 788–795.
de Boer, R., Koenraads, J. P.J. M.1991, Incorporation of Liquid Ultrafiltration –Whey Retentates in Dairy Desserts and Yoghurts, inNew Applications in Membrane Processes(International Dairy Federation Special Issue 9201), International Dairy Federation, Brussels.
Written by .
Syed Mudabbar Hussain Shah
The Author is final year student of
B.Sc (Hons.) in Food Engineering
Department of Food Engineering
University of Agriculture, Faisalabad