The relation between the bacteriological quality of raw milk and the quality of low‐and high‐heat powder made from it has been investigated. Powder quality is largely unrelated to raw milk quality and is determined by the heat treatment applied during manufacture. Nevertheless, evidence is presented which suggests that raw milk with bacterial counts greater than 2 × 106 colony forming units per ml should not be used for the manufacture of powders to be used for recombination.

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Within the modern dairy industry it is necessary to assess the potential heat stability of milk concentrates to be sterilized in cans or by continuous processes. The usual commercial practice is largely empirical and consists of testing pilot samples in the normal full scale processing plant. Stability is assessed as the grain point, i.e. the point at which coagulation is visible when a thin film of sterilized concentrate is closely examined. In contrast, almost all current research on the heat stability of milk employs test conditions similar to those devised by Davies & White (1966) and subsequently refined by Sweetsur & White (1975a), in which a sample of milk or milk concentrate enclosed in a glass tube is gently rocked in a temperature-controlled oil bath. Heat stability is defined as the coagulation time (HCT)-the time from immersion of the tube in the heating medium until the appearance of visible clots. Milk pH, headspace gas, assay temperature and agitation rate all influence HCT through complex interactions (Davies & White, 1966; Sweetsur & White, 1975 a; Hyslop & Fox, 1981). Consequently, precisely standardized conditions must be used if comparisons are to be made between the results of different workers, and indeed, a significant degree of standardization exists. The rocking tube method, by its nature a subjective assay, is less precise than the objective technique reported by White & Davies (1966). This method provides additional information on the nature of the coagulation process and can distinguish single from multiple step coagulation processes (Sweetsur & White, 1975a; Muir & Sweetsur, 1978). However, objective assessment of heat stability is exceedingly time consuming and is unsuitable for routine repetitive use. Muller (1963) and Newstead et al. (1975) have recognized the difficulties in comparing laboratory values of HCT with commercial indices of heat stability and have proposed objective tests more closely related to manufacturing practice. This note describes the relation between heat stability assessments carried out by the subjective method of Davies & White (1966) and by the methods currently used in commerce for the evaluation of stability of full-cream evaporated milk.

Food grade lecithin derived from soya beans can promote large increases in the initial heat stability of full cream evaporated milk. By means of lecithin incorporation, heat stable concentrate can be manufactured without addition of inorganic phosphate and can be processed at higher than usual homogenization pressures. As a result, the efficiency of reduction of fat globule size is enhanced. Limited storage trials have indicated no deleterious effects associated with the use of lecithin.