Partial coalescence occurs when two or more partly crystalline oil droplets come into contact and form an irregularly shaped aggregate (Figure 7.21). The aggregate partly retains the shape of the droplets from which it was formed because the fat crystal network within the droplets prevents them from completely merging together (Mulder and Walstra 1974, Boode 1992, Dickinson and McClements 1995, Walstra 1996a). Partial coalescence is particularly important in dairy products, because milk fat globules are partly crystalline over a fairly wide range of temperatures (Mulder and Walstra 1974, Walstra and van Beresteyn 1975). The applica-
tion of shear forces or temperature cycling to cream containing partly crystalline milk fat globules can cause partial coalescence, which leads to a marked increase in viscosity ("thickening") and subsequent phase separation (Van Boekel and Walstra 1981, Boode et al. 1991, Boode 1992). Partial coalescence is an essential process in the production of ice cream, whipped toppings, butter, and margarine (Dickinson and Stainsby 1982; Goff et al. 1987; Barford and Krog 1987; Barford et al. 1987, 1991; Moran 1994). Oil-in-water emulsions are cooled to a temperature where the droplets are partly crystalline and a shear force is applied which leads to droplet aggregation via partial coalescence (Mulder and Walstra 1974). In butter and margarine, aggregation results in phase inversion (Moran 1994), whereas in ice cream and whipped cream, the aggregated fat droplets form a network that surrounds the air cells and provides the necessary mechanical strength required to produce good stability and texture (Barford et al. 1987, Goff 1993).
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