Inactivation of foodborne pathogens on fruit juice with different solid contents by using pilot-scale 915 MHz microwave heating system

Fruit juices are often distributed in the form of concentrate for the sake of quality conservation and cost-reduction during distribution. The objective of this study is to investigate the effect of solid content on heating rate, dielectric properties, and foodborne pathogen inactivation in fruit juices when subjected to 915 MHz microwave heating. 50 mL of fruit juice with different solid contents were exposed to 915 MHz microwave at 3.0 kW for up to 60 s. Solid content was found to be an important factor for heating rate. The fastest heating rate was observed when concentrates were diluted by 50% (36 $°$ Brix apple and 30 $°\text{Brix}$ orange juice). Though dielectric constant decreased along with increase in solid contents, different trends were observed for the dielectric loss factor. Especially, the fruit juices with higher magnitudes of the dielectric loss factor presented faster heating rate. Inactivation of the pathogens inoculated in the fruit juices was investigated by using E. coli O157:H7 and L. monocytogenes. In case of apple juice, 915 MHz microwave heating was most effective for 72 $°\text{Brix}$ as 20 s operation resulted in over 5-log reduction for both pathogens. However, in orange juice, 30 $°\text{Brix}$ was found to be most effective for 915 MHz microwave heating. In the inactivation mechanism analysis, cell membrane damage was observed when cells were exposed to higher solid contents due to low water activity. 915 MHz microwave heating did not include significant quality changes in terms color and total phenolic contents, except for 60 $°$ Brix of orange juice. The obtained results highlight the potential application of 915 MHz microwave heating for fruit juice processing