Inactivation of Alicyclobacillus acidoterrestris in apple juice using pulsed light

We aimed to investigate the effect of pulsed light (PL) on inactivating Alicyclobacillus acidoterrestris in apple juice, the mechanism behind the inactivation, and the resultant effects on the quality of the juice. Our results showed that PL had a good inactivation effect on both spores and vegetative cells of A. acidoterrestris in apple juice, with a reduction of 3.5 +/- 0.1 and 2.1 +/- 0.1 log CFU/mL, respectively, following 3 s of treatment. This effect was better when lower concentrations of apple juice and lower liquid layer depths were used. Transmission electron microscopy (TEM) showed that the morphology and structure of A. acidoterrestris spores did not change significantly before and after treatment. When we compared the dipicolinic acid (DPA) release rate and death rate of the spores, we hypothesized that PL might have damaged the cortical proteins on A. acidoterrestris that are associated with the resistance of the spores, thus leading to their death. However, TEM revealed that the cell membranes of vegetative cells of A. acidoterrestris were intact and smooth before the treatment, but significantly wrinkled afterward. Raman spectroscopy revealed that the main targets of PL on A. acidoterrestris vegetative cells were: amide I on the proteins, nucleic acids [adenine(A) and guanine (G), in particular] and DNA (O-P-O- stretching) structures, and lipids (C-H-2 deformation). The PL treatment increased the sugar content significantly, while having little to no effect on the other basic physicochemical parameters of apple juice.