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.

Effects of phosphates and salt in ground raw and cooked farmed cod (Gadus morhua) muscle studied by the water holding capacity (WHC), and supported by P-NMR measurements.

A model system consisting of ground farmed cod muscle (80%, w/w) and added brine (20%, w/w) with different content and combinations of salt (0% and 3% in brine) and phosphorus compounds (mono-, di-, tri- and hexametaphosphates; 0% and 3% in brine) was used to simulate industrial brining of muscle foods. Individual phosphorus component concentrations and breakdown as function of time (0, 23 h) were analyzed using (31)P-NMR spectroscopy. The effects of salt and phosphate on water holding capacity (WHC) were measured at similar sampling times, and interrelations between phosphorous components determined by NMR and WHC were established. Addition of salt led to a significant increase (+18%) in WHC, and the combined effect of salt and phosphates was even more pronounced (+29%). The positive effect of triphosphate and salt on WHC was also seen after cooking (+36% in raw and +41% in cooked cod muscle, relative to control), although NMR analysis showed a rapid breakdown of di- and triphosphates.