Evaluation of spectrophotometric and fluorometric methods for alkaline phosphatase activity determination in ewe's milk

The alkaline phosphatase (ALP) activity test has been used since 1935 to assess the effectiveness of pasteurization. Different analytical methods exist for detecting ALP in milk. Unfortunately, there is little information about ALP activity in ewe's milk. The aim of this study was to assess and compare the official European method (spectrophotometric method) and the Fluorophos method (fluorometric method) regarding their use in ewe's milk. Bulk ewe's milk samples were taken from a flock and from three different dairies. A portion of the original sample was pasteurized at 63 degrees C for 30 min in a circulating bath; another portion was heated to and kept at 95 degrees C for about 2 min, and 0.1% (vol/vol) of raw milk was added. The samples obtained were analyzed in duplicate using the spectrophotometric and fluorometric methods. The relation between ALP activity determined by the two methods was characterized by the following equation: Y = 1.34 + 0.0039X (where Y = ALP in microg of phenol per ml of milk and X = ALP in mU/liter; R2 = 91.5%). Precision parameters (repeatability [r], standard deviation of repeatability [s(r)], and relative standard deviation of repeatability [RSDr]) for both methods were calculated. The values of RSDr for the Fluorophos method were 4.30 for pasteurized milk and 2.96 for 0.1% raw milk, close to the value indicated by Rocco in whole cow's milk (RSDr = 4.4). The repeatability for the official method (r = 2.16) was close to that indicated for whole cow's milk (r = 2).