Use of residual acid phosphatase activity in heat-processed atlantic cod (Gadus morhua) for estimating thermal load

Farmed Atlantic cod muscle tissue was heated isothermally at temperatures of 56 to 68 degrees C for 15 s to 25.5 min. Extracts from the heat-treated samples were prepared by mixing with Triton X-100 (1:9, wt/wt) and subsequent centrifugation. Residual acid phosphatase (ACP) activity was measured, and the inactivation was modeled in two phases. Mean (+/-standard error) kinetic parameters of thermal inactivation were determined as D(60 degrees C) = 34.93 +/- 2.02 min and z = 22.01 +/- 4.10 degrees C for short heating times and D(60 degrees C) = 3.19 +/- 0.11 min and z = 6.31 +/- 0.51 degrees C for heating times longer than 100 s. The data support the use of residual ACP activity for modeling a 6-log inactivation of Listeria monocytogenes at 60 degrees C but only a 4-log inactivation at 70 degrees C. Extracts prepared from raw muscle and subsequently heated isothermally at temperatures of 54 to 70 degrees C for 2 to 51 min were used to obtain kinetic parameters D(60 degrees C) = 7.98 +/- 1.11 min and z = 6.92 +/- 0.07 degrees C. A short initial drop in ACP activity was observed in raw cod muscle during freezing and cold storage. Subsequently, the activity was stable for 24 months. ACP activity in raw cod muscle did not seem to be influenced by gender, season, or brining with different levels of salt and phosphates. No other factors that could inactivate ACP were found, and usual activity of 2.54 +/- 0.02 Abs/(min*g sample) can be expected in frozen and thawed farmed cod muscle.