The Effects of High-Pressure Processing at Low and Subzero Temperatures on Inactivation of Microorganisms in Frozen and Unfrozen Beef Mince Inoculated with Escherichia coli Strain ATCC 25922

Frozen and unfrozen beef mince inoculated with Escherichia coli strain ATCC 25922 were exposed to a pressure of 300 MPa for 5 min at different temperatures (-10, -5, 0, 10 and 20 A degrees C). A maximum reduction of 1.5 log in total aerobic count (TAC) was obtained in unfrozen samples at -5 A degrees C, whereas at 20 A degrees C, the reduction was only 0.6 log. Microbial inactivation in beef mince was enhanced by freezing the beef mince prior to pressurization. An average log reduction of 3.0 (SD = 0.2) in both E. coli and TAC was obtained in frozen beef mince treated at -5 A degrees C. The highest bacterial reductions were observed in frozen samples. The extent of bacterial injury was substantially less in frozen samples than unfrozen samples, indicating that the damage inflicted on microorganisms in frozen beef mince by high pressure was irreversible. Lightness (L (*)), redness (a (*)) and yellowness (b (*)) values measured in accordance of Commission Internationale de l'Eclairage (CIE) for all the pressure-treated frozen and unfrozen samples differed slightly from unfrozen control samples (average total colour change, Delta E = 6.1, SD = 1.1). Water-holding capacity (WHC) measured by "high-pressure expressed moisture", a new method proposed in this study, showed that freezing the samples prior to pressurization could increase WHC of minced beef. The results suggest that high-pressure processing could be used to make safer traditional raw minced meat products, such as steak tartare and cig kofte, a traditional Turkish dish made with minced beef, bulgur and spices.