Prediction of Injured and Dead Inactivated Escherichia coli O157:H7 Cells after Heat and Pulsed Electric Field Treatment with Attenuated Total Reflectance Infrared Microspectroscopy Combined with Multivariate Analysis Technique

Attenuated total reflectance infrared microspectroscopy combined with soft independent modeling of class analogy (SIMCA) multivariate technique was used to differentiate between alive, dead, and injured Escherichia coli O157:H7 cells. E. coli O157:H7 cells were suspended in citrate phosphate buffer at pH 4.0 and 7.0 and treated by heat at 54 °C for 5, 10, 20, and 90 min or pulsed electric field (PEF) at 35 kV/cm for 10, 25, 50, and 60 pulses. The SIMCA analysis confirmed that major contribution to the discrimination of the untreated and treated E. coli cells were (1) the amide I band at 1,638 and 1,618 cm−1 corresponding to changes in β-pleated secondary protein structure (heat-treated cells at pH 7.0 and 4.0, and PEF-treated cells at pH 4.0), and (2) the bands at 1,078 and 993 cm−1 corresponding to changes in P = O (PO 2 − ) stretching of phosphodiesters or lipopolysaccharides and C–O–C or C–O stretching of different polysaccharides (PEF-treated cells at pH 7.0). The use of partial least squares regression analysis allowed for correctly predicting the survivors of the thermal treatment. Injured cells could be estimated from the comparison of cell counts predicted in nonselective and selective plating media with sodium chloride and bile salts. The prediction results yielded inactivation values with a coefficient of determination (R 2) of 0.83 or higher and a standard error of cross validation between 0.11 and 0.37 log cycles of inactivation.