Non-thermal processing of liquid food products by using Radio Frequency Electric Fields (RFEF) technology

Abstract

Although conventional thermal processing ensures the microbial safety of liquid foods, it adversely affects the nutritional and organoleptic properties of these food products. Hence, the application of radio frequency electric fields (RFEF) processing, as a non-thermal alternative, has been investigated in the past few decades for liquid foods. This research was conducted to study the effect of RFEF processing on liquid foods in three main aspects. In the engineering aspect, Computational Fluid Dynamics (CFD) models were utilised to assess the effect of inserting stainless steel mesh in a co-linear chamber on process homogeneity, in comparison to a no-mesh co-linear configuration. The results indicated that the insertion of a stainless steel mesh in a co-linear configuration enhanced the homogeneity of the process by improving the electric field, temperature, and velocity profiles within the chamber. In the microbiological aspect, the effect of various RFEF processing parameters, such as electric field, temperature, frequency, and treatment time, on microbial inactivation was studied. In cranberry juice, an E.coli inactivation level of 6.57±0.02 logCFU mL-1 was achieved by multiple-stage RFEF processing with an electric field, outlet temperature, and treatment time of 13.2 kV cm-1, 40 °C, and 3240 μs, respectively. Also, based on inactivation results, a kinetic inactivation model as a function of the electric field, temperature, and treatment time was developed. In orange juice, multiple-stage RFEF processing with an electric field, temperature, treatment time of 11.7 kV cm-1, 42 °C, and 1.17×10-3 seconds, respectively, extended the shelf-life compared to the control sample. Lowering the frequency from 20 to 10 kHz increased the inactivation of E.coli in saline water with a peak at 12.5 kHz, and a further decrease in frequencies below 10 kHz reduced the inactivation levels due to electrolysis. A synergistic effect on E.coli inactivation was achieved when low frequency (LF) electric fields processing of above 9.6 kV cm-1 was combined with high frequency (HF) processing with outlet temperature and power above 55 °C and 739 W, respectively. In the nutritional aspect, a comparison study between multiple-stage RFEF processing with an electric field, temperature, and treatment time of 11.7 kV cm-1, 42 °C, and 1.17×10-3 s, and a heat processing with similar inactivation level referred to as thermal processing (81.5 °C for 10 s), and an industrial level heat processing, referred to as pasteurisation processing (90 °C for 30 s), during 45 days of storage at 4 °C, was conducted. The results obtained after the storage indicated that although a higher antioxidant capacity and vitamin C content were achieved in orange juice processed by RFEF compared to pasteurisation processing, the thermal processing resulted in the highest level of antioxidant capacity and vitamin C content in orange juice. Furthermore, regarding total phenolic compounds (TPC), orange juice processed by RFEF has the lowest content of TPC compared to both heat treatments. In conclusion, the RFEF processing demonstrated considerable potential as an alternative to conventional heat treatment, especially in the microbiological aspect. However, further investigations on the effect of RFEF processing parameters, such as electric field and temperature, on nutritional and organoleptic properties of liquid foods are required. These investigations can be complemented by cost analysis studies, which can help to advance RFEF processing towards commercialisation.