SummaryThe heat stability of the milk supply to manufacturing creameries in south-west Scotland was examined over 15 months from Nov. 1975 to Jan. 1977. For all but 2 months of this period the heat stability of the milk was very highly significantly correlated with the naturally occurring level of urea. Between 72 and 90% of the variation of coagulation time (CT), measured at the natural milk pH, was accounted for by changes in milk urea alone. For a short period, in May and June, the CT of the milk at natural pH fell within the minimum of the CT–pH profile, but insufficient data were available to allow the occurrence of this phenomenon to be related to changes in milk composition.

Coagulation time-pH curves with a coagulation-time minimum around pH 6-8 (type A curve) could progressively become type B (no minimum) as the heating temperature was decreased from 150 to 130 °C. The short coagulation time that most milks have when pH is around 6-8 was found to be the result of a 'premature' coagulation, probably caused by calcium phosphate deposition on the larger caseinate micelles. This is followed by a second coagulation, not visually detected, that coincides with the coagulation time that would be expected if no coagulation-time minimum existed on the coagulation time-pH curve. The coagulation time of milks giving type A and type B curves may therefore not be comparable.Forewarming milk for 30 min at 80 °C can introduce or accentuate a coagulationtime minimum when the milk is subsequently heated at a higher temperature. The effects of adding /?-lactoglobulin, copper and iV-ethylmaleimide on the heat stability of milk were examined and explanations proposed for these effects.In their studies on the stability of milk protein to heat, Davies & White (1966a, b) and White & Davies (1966) found that the coagulation time of milk as determined by their subjective heat-stability test could vary with the proportion of headspace O 2 to volume of milk and the age of the milk. Their objective heat-stability test revealed that 'normal' or 'abnormal' protein coagulation could occur in different milks, the latter apparently being a 2-stage process and often found with milk from cows with sub-clinical mastitis. The object of the present series of investigations was to elucidate further factors thought to govern the heat stability of milk, paying special attention to the well-known high sensitivity of coagulation time to pH (Rose, 1961a, b) EXPERIMENTAL Samples of whole milk were obtained from the Hannah Institute herd of Ayrshire cows, usually from individual cows but sometimes from the herd bulk milk, and

The addition of simple aldehydes brought about large increases in the heat stability of both skim-milk and concentrated skim-milk over a comparatively wide milk-pH range. The coagulation time-pH minima of type A milks were completely removed by aldehyde treatment. Some sugars, which react readily as aldehydes on heating, were also shown to stabilize concentrated milk to prolonged heat treatment at 120 C C.

SummaryAdditions of urea progressively increased the heat stability of milk outside of its coagulation time (CT)–pH minimum. In the region of the CT–pH minimum larger amounts of urea were required before an increase in heat stability occurred. The effect of urea was observed over the temperature range 125–140 °Cfornaturalmilk, milk which had been dialysed against synthetic sera, and milk to which a sulphydrylblocking agent had been added. Urea additions did not affect the activation energy of the heat coagulation reaction or the stability of milk to rennet or ethanol.

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