2.2. High pressure processing
Pressure treatments were performed using a 0.3 L capacity 850 Mini FoodLab high pressure vessel (Stansted Fluid Power Ltd., Stansted, UK) connected to a circulating water heater (PolyScience 9702, Niles, USA) set so that the temperature of the compression fluid in the chamber could be adjusted between 60 and 76 °C. The compression fluid used in the sample chamber consisted of 30% propyl- ene glycol in water (v/v). The inherent ramp rate was 20 MPa/s, so the time to reach 200 MPa was approximately 10 s. A decompression pro- cedure over a period of 45 s was used, consisting of ‘open’ 5 s and ‘closed’ 2 s. Following release of pressure, all samples were held in ice water for 20 min and then stored at 5 °C until required for analysis. Pressure treatment causes a temperature increase of the compression fluid and the meat samples as a result of adiabatic heating (Ting, Balasubramaniam, & Raghubeer, 2002). Under the conditions used here (200 MPa) it can be estimated that adiabatic heating resulted in an immediate increase in temperature of about 4–6 °C above the set temperature. As soon as the maximum pressure had been established, this additional heat was rapidly lost back into the system and the sam- ples remained at the pre-determined set temperature for the majority of the processing time (data not shown).