Milk fat: Physical, chemical and enzymatic modification

Abstract

Milk is an oil-in-water emulsion, in which the milk fat exists within the natural milk fat globule membrane. Comprehensive reviews are available on the structure and composition of milk fat (Christie, 1994; Jensen 2002; Chapters 1 and 2). The properties of milk fat products are influenced primarily by the fatty acid composition and the position of the fatty acids in the triacylglycerol molecule. Triacylglycerols account for 98% of the milk fat. The remaining minor lipids in milk fat comprise monoacylglycerol, diacylglycerol, free fatty acids, phospholipids, glycolipids and sterols, which although present in only small quantities, can also influence the properties of the milk fat. The main milk fat products are butter, which is produced by churning sweet or ripened cream, and anhydrous milk fat, prepared by separation of the milk fat from melted butter or high-fat cream. Milk fat is used traditionally as an ingredient in many food applications, including bakery products, ice cream, chocolate and confectionery. Milk fat ingredients are now Wnding applications in ready-to-eat short shelf-life products such as dips, dressings and ready meals (Burgess, 2001). Milk fat is valued for its pleasant Xavor but its melting and rheological properties often need to be modified to make it more suitable for many food applications. The uses of milk fat can be increased by the application of various processing interventions such as fractionation, selective blending and texturization, and chemical or enzymic processes to produce speciality milk fat ingredients (Kaylegian, 1999). Most of these modification procedures are concerned with changing the composition of the triacylglycerols. These treatments alter the physical and nutritional properties of the fat, enabling the tailoring of the milk fat for specific end-uses as well as expanding the range of applications. There are also other processing interventions aimed speciWcally at making the milk fat ingredient more attractive to the user. These are usually targeted at the minor components; one that has attracted interest over the years has been the application of processing treatments to remove cholesterol. More recently, the interest has shifted to treatments to increase the level of desirable components (e.g., conjugated linoleic acid) There is inherently signiWcant natural variation in the composition of milk fat, because it is aVected by many factors such as cow breed, stage of lactation, season and diet of the cow (Palmquist et al., 1993; Ashes et al., 1997; Chapters 1 and 2). Although on-farm approaches can be used to alter the composition of milk fat to improve its nutritional composition and physical characteristics (Banks et al., 1980; Banks and Christie, 1980; Hawke and Taylor, 1994; Baer, 2001; Fearon, 2001; Jensen 2002), these will not be covered here (see Chapter 2). This chapter is focussed on the post-farm modiWcation of milk fat by physical, chemical or enzymic means. The use and control of these processes for diVerentiation of milk fat to widen its application range or tailor it for specific end-applications (Mortensen, 1983; Mogensen, 1985; Boudreau and Arul, 1991; Rajah and Burgess, 1991) will be discussed. The eVects of the modification processes and minor lipid components on the texture and crystallization behavior of milk fat are covered. The potential for applying modification processes to improve the nutritional quality of milk fat is also considered.