Effect of Heat on Biuret-Positive Water-Extractable Porcine Muscle Proteins

Ground pork longissimus was heated in glass tubes in a controlled temperature bath at 2 (control), 20, 40, 45, 50, 55, 60, 62.5, 65, 67.5, 70, 75, or 80°C for 15 min after the sample reached the desired temperature, removed and chilled (2°C) immediately. Treated samples were homogenized with deionized water at a ratio of 1:3.3 (w/v) muscle to water. The resulting water-extractable proteins were determined by the biuret method. Eight ml of clear extract from each treatment was reheated for 15 min at 70°C, removed, and chilled (2°C) immediately. Coagulated proteins were removed by filtration (0.45 μm). Soluble protein was used as an index of heat denaturation. Water-extractable biuret-positive protein losses were 5.7% from 2 to 45°C, 69.7% from 50 to 67.5°C and 4.3% from 70 to 80°C. Reheating each treatment extract to 70°C yielded 20.2% baseline biuret-positive soluble materials. The ratios of soluble proteins at each treatment temperature with the baseline critical value of 70°C were 5.1, 5.1, and 4.9 from 2 to 45°C; 4.5, 3.9, 2.7, 2.2, 1.5, and 1.2 from 50 to 67.5°C and 1.1, 1.0, and 1.0 from 70 to 80°C. This indicates that coagulation of water-extractable biuret-positive compounds is nearly constant at about 70°C. These results suggest that a ratio of water-extractable biuret-positive proteins from heat treated porcine muscle may be useful in determining the temperature to which pork has been heat processed.