Challenges of dry hazelnut shell surface for radio frequency pasteurization of inshell hazelnuts

Abstract

Thermal inactivation kinetics (D and z values) of Salmonella and Enterococcus faecium in fresh hazelnut shell and kernel were determined. Aluminum pouches containing either 1.5 g hazelnut shell powder or 2 g of ground kernel inoculated with either bacterium at their original water activities (aw-shell = 0.91 ± 0.01, aw-kernel = 0.93 ± 0.01) were isothermally treated in a thermal death time (TDT) sandwich at 55, 60, 65, 70 and 75 °C for determining D values. A single layer inshell hazelnuts with one inoculated nut located at the center (cold spot) of a tray was subjected to hot air assisted radio frequency (HARF) heating in a 6-kW 27.12-MHz RF system with hot air temperature at ~75 °C for 10, 15 and 20 min. The log-linear model fits survival data of both Salmonella and E. faecium in hazelnut shell and kernel well. The D-values of both bacteria in hazelnut kernel were higher than those in hazelnut shell. Higher D-values of E. faecium demonstrated that it is a suitable surrogate for Salmonella in both hazelnut shell and kernel. There was a large variation between calculated (based on F-value) and experimental log reductions of Salmonella. Only 1.0 ± 0.1 log reduction of Salmonella was achieved in inshell hazelnut after 20 min of HARF heating. Because the RF pasteurization is a simultaneous drying process, the F-value methodology could not give an accurate and reliable lethality prediction due to the increased thermal resistance of Salmonella caused by moisture loss. Around one log increase in Salmonella inactivation was achieved after spraying 0.1% buffered peptone water (BPW) on inshell hazelnuts for the same RF treatment condition. While the water activity of the ground shell was 0.91 ± 0.01, the water activity of top surface layer of the shell where bacteria were inoculated for the RF treatment was 0.17 ± 0.01 before RF treatment. This very dry shell surface was the major factor for the deviation of calculated F-value from the observed microbial reduction in this RF microbial study. The dry shell surface creates a challenge for thermal inactivation of pathogens on inshell hazelnuts and other nuts with a similar shell structure. An efficient pasteurization protocol for inshell hazelnuts needs to be developed and optimized by combining surface disinfectants/salt solution and RF process conditions to enhance microbial food safety and quality of hazelnuts. This study provides valuable information and direction to the hazelnut industry for improving safety and quality of hazelnuts.