R-phycoerythrin as a fluorescent time-temperature integrator for monitoring thermal processing of beef products

Time-temperature integrators (TTIs) are a rapid and accurate method to verify thermal processing in foods because they can predict the lethality of a target attribute when the two systems are subjected to the same thermal process. The objective of this study was to characterize and verify the applicability of R-phycoerythrin (R-PE) to be used as the detector component of a TTI for monitoring of thermal processing in beef products.; R-PE, a protein which fluoresces in the visible range, was purified from algal tissues of Porphyra yezoensis by precipitation with ammonium sulfate and gel filtration chromatography. The isolated R-PE had a purity greater than 5 as determined by A565/A280. Isothermal experiments were conducted to determine the thermal inactivation parameters of R-PE calculated on the basis of fluorescence loss under different conditions of pH and concentrations of sucrose, NaCl, sodium-dodecyl sulfate (SDS), urea and beta-mercaptoethanol (ME). R-PE was most thermostable at pH between 5.0 and 9.0, but became more heat sensitive at pH 4.0 and 10.0. Sucrose and ME had a thermostabilizing effect, while SDS, NaCl and urea decreased thermal stability of R-PE. The variations obtained in the thermal inactivation parameters showed that the kinetics of R-PE can be modified by altering the solution composition.; When R-PE was placed in borate buffer, pH 9.0, the resulting z value (5.99°C) fell within the range of z values reported for Salmonella in ground beef (5.6--6.2°C). Salmonella is the target microorganism in thermal processing of beef products.; In non-isothermal experiments, R-PE fluorescence could distinguish between adequate and inadequate heat processes according to USDA requirements for hamburger and roast beef. The results suggested that thermal inactivation kinetics of R-PE do not follow a first-order reaction. A non-linear mathematical model was developed to fit thermal inactivation kinetics of R-PE and S. senftenberg. The reaction orders (n) calculated for R-PE and S. senftenberg were 0.55 and 1.2, respectively. The activation energies (EA) were estimated using the Arrhenius model. The calculated E A for R-PE and S. senftenberg were 347.4 and 220.84 kJ mole-1, respectively. The lethalities calculated for R-PE and S. senftenberg assuming non-linear kinetics were highly predictable (R2 of 0.98 for R-PE and 0.93 for S. senftenberg ). Results strongly suggest that, despite the differences in thermal inactivation parameters, the lethality of Salmonella can be predicted from R-PE fluorescence.