Biological Systems Engineering, Department of


First Advisor

David Jones

Second Advisor

Jeyamkondan Subbiah

Date of this Version

Summer 6-4-2020

Document Type



Chen, Long, "Improving Microbiological Safety of Low Moisture Food Products Using Radio Frequency and Ethylene Oxide" (2020).


A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Biological Engineering (Agricultural and Biological Systems Engineering), Under the Supervision of Professors David Jones and Jeyamkondan Subbiah. Lincoln, Nebraska: June, 2020

Copyright 2020 Long Chen


Recent foodborne illness outbreaks in US associated with consumption of low-moisture foods (LMF) have heightened concerns of their microbial safety. Salmonella is a pathogen of major concern in LMF due to its ability to persist in low water activity (aw) environments. The disadvantages of existing decontamination technologies for LMF call for novel and efficient intervention technologies. Radio frequency (RF) and ethylene oxide (EtO) were evaluated in this dissertation for improving microbial food safety and quality of LMF. Cumin seeds and inshell hazelnuts were selected as model foods.

It took < 2 min of stationary RF heating to achieve > 5 log reductions of Salmonella in cumin seeds without significant quality deterioration in terms of total phenolics, antioxidant activity, volatile compounds, and color. The continuous RF pasteurization of cumin seeds was also demonstrated to simulate the industrial application. Even though more quality loss was observed for continuous RF pasteurization than stationary RF heating possibly due to the longer processing time, the cumin quality is still acceptable. RF heating is a promising intervention technology for the industrial pasteurization of spices. RF preferentially heated hazelnut kernels than the shell. Therefore, RF was combined with hot air to dry inshell hazelnuts effectively with minimal nut quality (lipid oxidation and cracking ratio) deterioration. The thermal inactivation kinetics (D and z values) of Salmonella and Enterococcus faecium NRRL B-2354 were determined in hazelnut shell and kernel. The outside surface of hazelnut shell was very dry (aw=0.17), which creates a challenge for RF pasteurization of inshell hazelnuts. Spraying water on the inshell hazelnuts before RF treatment increased inactivation of Salmonella. The EtO inactivation of Salmonella and E. faecium in cumin seeds increased with the increasing relative humidity (RH) and temperature. RH is a critical factor for the successful EtO sterilization. Enterococcus faecium was demonstrated to be a suitable Salmonella surrogate in cumin seeds for RF and EtO treatments and in hazelnut shell and kernel for thermal treatment. The evaluated intervention technologies and generated results in this dissertation could provide valuable information and additional options for the food industry to improve microbial food safety and quality of LMF.

Advisors: David Jones and Jeyamkondan Subbiah