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S. Lamb rennet pastes were prepared by the procedure most commonly used by Idiazabal cheese manufacturers. We studied the effects on their coagulating and lipolytic activities of the state of the stomach at the time of death (full of milk or empty), the amount of NaCl added, the origin of the lambs and paste storage time. Coagulating activities were generally between 155 and 363 units\g tissue. Pastes prepared from stomachs of lambs from slaughterhouse flocks had significantly higher coagulating activities than those of lambs from separate flocks. No significant decrease in coagulating activity was observed after 1 year storage at 4 mC. Chymosin represented 75-80 % of the total coagulating activity with the remainder being pepsin. Rennet paste extracts with pH 4n7 did not have increased coagulating activities when their pH was lowered to 2n0, while those with pH 5n2 had activities 1n5-fold those before treatment. Lipase activity was higher in extracts of rennet pastes prepared using the stomachs of lambs that arrived at the slaughterhouse in the morning just prior to slaughter than in those prepared with the stomachs of lambs that had arrived on the previous evening. However, the reverse was the case for esterase activity. Activating the coagulating activity by pH cycling completely destroyed both lipolytic activities. Storage at 4 mC for 1 year did not affect esterase activity but lipase activity decreased substantially after 4-5 months. Lipase, but not esterase, activity was responsible for the liberation of short-chain free fatty acids from ovine milk fat.

Two methods were compared for the determination of free fatty acids (FFA) from acetic to long-chain acids in samples with a large excess of triacylglycerols (TG) (1[ratio ]200, w/w), such as cheese and other dairy products. In method 1, after fat extraction, FFA were separated from TG by aminopropyl-bonded phase chromatography, injecting the fraction containing FFA directly into the gas chromatograph. In method 2, extracted fat was treated with tetramethylammonium hydroxide, the methyl ester derivatives being formed in the injector. Cheese samples and standard mixtures of FFA and TG in different proportions were analysed by both methods. The cheese sample contained 2·4 times more FFA when analysed by method 2 as compared with the result obtained with method 1. The composition of the standard mixtures analysed by method 1 closely reflected that of the original mixture and gave 90–100% recovery of FFA, regardless of their chain length and the ratio of FFA[ratio ]TG (1[ratio ]1 or 1[ratio ]200, w/w). The composition of samples with a FFA[ratio ]TG ratio of 1[ratio ]200 (w/v) was severely distorted (as compared with the original composition of the sample) when analysed by method 2. Varying recoveries of FFA were also obtained, the largest differences being found for the shorter-chain components. We conclude that the FFA fraction should be separated from the TG fraction before derivatization and chromatographic analysis, particularly for samples in which the FFA represent a minor fraction of the TG.

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Alkaline phosphatase activity in raw, industrial ewe's milk increased steadily >2-fold between January [1.7 units (U)/mL] and June (3.75 U/mL), whereas acid phosphatase increased 4-fold in January and February (17 mU/mL) and then remained constant until the end of lactation. By contrast, lipoprotein lipase exhibited a downward trend and lactoperoxidase decreased 2-fold during lactation. When assayed at cheese-ripening temperatures, acid phosphatase retained 16% of its activity at 37 °C, whereas lactoperoxidase retained between 30 and 45% of its activity at 20 °C. The rate of hydrolysis of model triacylglycerols by lipoprotein lipase was highest for tricaprylin. Although alkaline phosphatase in raw milk cheeses was variable from 1 to 180 days of ripening, no apparent reactivation was observed. The activity of acid phosphatase increased 2-fold during the 180 days of ripening in the cheeses made in summer, whereas in winter and spring much smaller increases were observed. Both raw milk cheeses made in summer and all pasteurized milk cheeses had very low levels of lactoperoxidase throughout ripening. Keywords: Alkaline phosphatase; acid phosphatase; lactoperoxidase; lipoprotein lipase; ewe's milk; ewe's milk cheese; ovine cheese; lactation period; cheese ripening.

-Lipolysis was studied in cheeses manufactured with pasteurized and raw ovine milk with a starter culture added in winter, spring and summer, up to 180 d of ripening. Pasteurized milk cheeses had significantly lower levels of lipolysis than raw milk cheeses in winter after 180 d of ripening and in spring both after 90 and 180 d of ripening. The relative amounts of individual FFA after 180 d of ripening changed from winter to summer, both in pasteurized and in raw milk cheeses. In pasteurized milk cheeses made in winter the predominant FFA were C18:1 (2152 ± 386 µmol . kg -1 ), C4 (1954 ± 354 µmol . kg -1 ), C16 (1541 ± 406 µmol . kg -1 ) and C10 (1452 ± 188 µmol . kg -1 ). In contrast, in pasteurized milk cheeses made in summer C16 (2860 ± 1305 µmol . kg -1 ) and C18:1 (2677 ± 973 µmol . kg -1 ) were the major FFA. The percent FFA composition of both types of cheeses changed during ripening: short-chain (C4 to C10) FFA increased from approximately 25% to approximately 40 to 45%, whereas long chain (≥ C16:0) FFA decreased from approximately 55% to approximately 40 to 45% in winter and spring. However, in both types of cheese made in summer long chain FFA represented 52% and volatile FFA represented approximately 28 to 32% of the total after 180 ripening days. Milk pasteurization reduced the levels of acetic acid by 99% after 90 ripening days at the three times of the year studied.