Salt content dependent dielectric properties of pistachios relevant to radio-frequency pasteurization

Abstract

This study was conducted to investigate the effect of salt content during radio-frequency (RF) heating on rate of temperature increase, dielectric properties (DPs), and reduction of pathogens in pistachios. Also, the effect of RF heating on pistachio quality of varying salt content was determined. Pistachios of different salt content (0, 100, and 330 mg sodium/serving) were inoculated with Salmonella enterica and treated in a 27.12-MHz RF heater. The RF heating rate increased when salt content was in the range of 0–100 mg sodium/serving, but there were no significant (P > 0.05) differences in the rate of temperature rise after salt content reached to 100 mg sodium/serving. Both dielectric constant and dielectric loss factor of pistachios increased with rising salt content. Along with increased salt content, RF treatment time required to reduce this pathogen by 4 log CFU/g decreased first and then remained the same above an upper limit of salt content corresponding to the peak heating rate. RF treatment did not significantly (P > 0.05) cause changes in the color and level of lipid oxidation of pistachios. The results of the current study imply that RF heating may be a potential intervention for inactivating foodborne pathogens in pistachios and that the effect of pasteurization is influenced by dielectric loss factor relative to salt content.

Effects of Moisture, Temperature, and Salt Content on the Dielectric Properties of Pecan Kernels during Microwave and Radio Frequency Drying Processes

“…The nonsignificant effect of salt on ε′ is in accordance with the observations presented by other researchers for various food materials [24,34,35]. The nonsignificant effect may be because the added salts can decrease the water activity and diminish the polarization characteristics [25]. In this research, the addition of an electrolyte (NaCl) did not considerably affect ε′; however, the addition did have a marked effect on ε″.…”

“…The addition of salt to the material could trigger large-scale electrophoretic migration when placed in an electric field, which would promote an increase in ε″ by ionic conduction. The dielectric response of salts is closely connected with the effective nuclear charge and depends on the volume and charge of dissolved salt ions [25]. Ionic conduction mainly occurs in the RF range, and dipole rotation is the main functional mechanism of dielectric heating in the MW range [36].…”

“…Ling et al [24] simulated quadratic polynomial equations for the temperature, moisture content, and frequency of non-salted pistachio nuts, according to the DP data measured from 10–4500 MHz at 25 to 85 °C. Jeong et al [25] found that RF heating can potentially inactivate foodborne Salmonella enterica in pistachios and that the inhibitory effect is controlled by the dielectric loss factor relative to the salt content. The dielectric heating of pecans can effectively prevent the attack of weevil [26], and the dielectric heating does not make the color of the epidermis darker when stored later than steam [27].…”

Inactivation of Escherichia coli , Salmonella enterica serovar Typhimurium, and Bacillus cereus in roasted grain powder by radio frequency heating

“…In this study, E. coli, S. Typhimurium and B. cereus (vegetative cell) inoculated on RGP were inactivated effectively by RF heating. Previous studies also reported that RF heating can be applied effectively to control foodborne pathogens in low water activity foods such as red and black pepper (Kim et al 2012b;Jeong and Kang 2014), raw shelled almonds (Jeong et al 2017), pistachios (Jeong et al 2019) and cumin seeds (Chen et al 2019). For example, Kim et al (2012b) reported that 50 s RF heating resulted in maximum 4Á29 log reduction of E. coli O157:H7 or S. Typhimurium in black pepper and> 5 log reduction in red pepper without colour change.…”

“…Radio frequency heating was conducted with a previously described (Jeong et al . 2019) RF heating system (Seoul National University, Seoul, South Korea; Dong Young Engineering CO. Ltd, Gyeongnam, South Korea). An RF electric field with a frequency of 27·12 MHz was generated between two parallel‐plate electrodes (30·0 × 35·0 cm; 0·6 cm thick) spaced 8 cm apart.…”