Gamma-Glutamyltransferase as a Marker for the Pasteurization of Raw Milk

Abstract

The degree and rate of inactivation of gamma-glutamyltransferase in raw cow's milk by heating at 50, 60, 70, and 80 degrees C for 1, 2, 3, 5, 10, 15, 20, 25, and 30 min were measured to evaluate the suitability of this enzyme as a marker for the pasteurization of milk. The enzymes alkaline phosphatase and lactate dehydrogenase were also measured under similar conditions for comparison. The patterns of heat inactivation of gamma-glutamyltransferase and alkaline phosphatase were similar, with only a minimal inactivation of the enzymes at 50 degrees C. The rate of inactivation increased as a result of increasing temperatures and time. A complete inactivation of both enzymes was seen at 70 degrees C after 10 min and at 80 degrees C after 1 min. Lactate dehydrogenase showed a higher heat resistance with almost complete inactivation at 70 degrees C for 30 min, and compete inactivation at 80 degrees C for 3 min. No activities of these enzymes were found in commercially pasteurized or heat-treated milk. The levels of gamma-glutamyltransferase in raw milk were between 8 and 10% higher than those of alkaline phosphatase and lactate dehydrogenase, making it more sensitive and accurate as a testing marker. It seems that gamma-glutamyltransferase may serve as a good pasteurization marker. Furthermore, the simplicity of testing and the availability of commercial kits for testing by both wet and dry chemistry make it an attractive choice, especially because dry chemistry procedures overcome the difficulties originating from the turbidity of milk, which interferes with spectrophotometric procedures.

Studies on the thermal sensitivity of ?-glutamyl transpeptidase measured with a modified test procedure and compared with that of alkaline phosphatase and lactoperoxidase in milk

“…ALP was the most heat-sensitive enzyme; it retained less than 10% activity after 5 min heat treatment at 65 o C. Complete inactivation was obtained by heating at 69 o C for 15 min (Fig. 3A) which is in good γ -Glutamyl transpeptidase measurement 561 agreement with Dos Anjos [5] who reports a complete loss of activity at 70 o C for 10 min. γ -GT was more heat-stable than ALP: it retained nearly 50% of enzyme activity after heating at 65 o C for 5 min, and 5% of residual activity after 10 min at 69 o C. Complete inactivation was obtained after 10 min at 71 o C or 1 min at 75 o C (Fig.…”

“…3B). Dos Anjos [5] reports a complete inactivation of γ -GT after 15 min heating at 70 o C. McKellar et al [16] and Zehetner et al [31] obtain a complete loss in γ -GT activity after 16 s at 77 o C or 15 s at 78-80 o C without any reactivation or variation in the activity after a storage period longer than 3 d at 4 o C. LPO was the most heat-stable enzyme. Complete inactivation was obtained after 1 min of holding time at 80 o C (Fig.…”

“…The reactivation of ALP, that occurs in pasteurised milk stored at a temperature higher than 20 o C, or during melting of butter to obtain serum for analysis, has led McKellar et al [16] to propose γ -GT assay as an alternative method for the control of milk pasteurisation. Furthermore, γ -GT is more abundant in milk than ALP, its concentration being about 700% higher [5]. Thus, γ -GT residual activity could allow better estimation of heat load than ALP and LPO activities.…”

“…Enzyme assays, though limited to heat treatment up to the mildest of ultra high temperature (UHT) treatment, have a potential advantage in providing screening methods that are faster, simpler, and often less expensive than chemical analysis [30]. Measurement of peroxidase, catalase and alkaline phosphatase activities have been widely used to evaluate the degree and efficiency of mild heat treatments [5].…”

Historical Background of the Invention of EIA and ELISA

“…MCT-based formula is recommended in cholestatic infants because these patients show malabsorption of fatty acids, and MCTs are absorbed directly by intestinal mucosa, without requiring emulsification by bile acids (6 ). GGT is absent in milk formulas as a consequence of the usual pasteurization process (7 ). No GGT activity was detectable in a sample of the MCT formula used with our patients, and none of an age-and sex-matched control group of 10 noncholestatic infants fed with an MCT-based formula for malabsorption had hyperGGT.…”