Elevated free fatty acid (FFA) levels in milk can impair the flavour quality and shelf life of milk and milk products and thus have implications for the dairy industry. An increase in milk FFA is induced when the protective fat globule membrane is disrupted and the fat exposed to the action of lipoprotein lipase (EC 3.1.1.34). Various aspects of FFA development have been reviewed (Deeth & Fitz-Gerald, 1976; Fleming, 1979; International Dairy Federation, 1980). Mechanical factors that contribute to FFA development are directly related to poor milking systems and procedures (O'Halloran et al. 1975; Judge et al. 1977; Fleming, 1991). Mechanical abuse of milk in many modern milking systems arises from air admission to the milking system, height of milklines and pump operation. Surveys and experience (Fleming et al. 1996) showed that the quality of the components used and the overall maintenance of many milking machines was inadequate. In continental Europe low-level milking systems are generally recommended in preference to higher lines. However, low-level systems are considerably more expensive as units have to be doubled up compared with the mid-level system.

Some of the machine factors contributing to excessive FFA development in milk can be eliminated by proper design, installation, maintenance and operation; however, the benefits of design changes remain a source of debate. The purpose of this investigation was to determine the relative effects on milk FFA of various components of a modern mid-level milking system (a pipeline milking system with the milkline 1·2–1·5m above cow standing level) and then to compare the optimal mid-level system with direct-to-line and low-level milking systems.