Effects of radio frequency heating on microbial populations and physicochemical properties of buckwheat.

Microbial contamination is a persistent problem for grain industry. Many studies have shown that radio frequency (RF) heating can effectively reduce pathogens populations in low moisture foods, but there is a lack on the efficacy to decontaminate natural microbiome. The main objectives of this study were to investigate the effects of different RF heating conditions on natural microbial populations and physicochemical properties of buckwheat. In this study, 30 buckwheat samples collected from 10 different Provinces in China were analyzed for their microbial loads, and the samples with the highest microbial populations were used for further study to select the suitable RF heating conditions. The results showed that microbial loads in tested buckwheat kernels were in the range of 3.4-6.2 log CFU/g. Samples from Shanxi (SX-3) had significantly higher microbial counts than other samples. The selected four temperature-time combinations: 75 °C-20 min, 80 °C-10 min, 85 °C-5 min, and 90 °C-0 min of RF heating could reduce microbial counts to <3.0 log CFU/g in buckwheat kernels at 16.5% w.b. moisture content. Furthermore, the reduction populations of the inoculated pathogens (Salmonella Typhimurium, Escherichia coli, Cronobacter sakazakii, and Bacillus cereus) reached 4.0 log CFU/g under the above conditions, and almost 5.0 log CFU/g especially at high temperature-short holding time combinations (85 °C-5 min and 90 °C-0 min). Besides, physicochemical properties evaluation also showed the insignificant color changes and nutrients loss after RF treatment at 90 °C-0 min. Therefore, the RF heating at 90 °C-0 min holds greater potential than the other lower temperature-longer holding time combinations for applications in buckwheat pasteurization.

Oregano Oil, Epsilon-Polylysine and Citric Acid Assisted Inactivation of Salmonella in Two Kinds of Tahini during Thermal Treatment and Storage.

Tahini and tahini-based products are popular with consumers due to their special flavor and high nutritional values, but often have been linked to Salmonella outbreaks. The objective of this study was to compare effects of different kinds of natural antimicrobials on Salmonella inactivation in undiluted and diluted tahini during thermal treatment and storage. Results showed that the Weibull model was more suitable to describe the thermal inactivation behavior of S. montevideo CICC21588 in two kinds of tahini than the first-order model. Inactivation curves were concave-upward in undiluted tahini but concave-downward in diluted tahini. During storage of undiluted tahini, 3% oregano oil caused extra 1.44 or 0.80 log CFU/g reductions after 7 days at 25 °C or 4 °C compared to the control and 0.5% citric acid caused an extra reduction of 0.75 log CFU/g after 7 d at 4 °C. For diluted tahini, 2-3% oregano oil and 0.4-0.5% ε-polylysine reduced more populations compared to undiluted tahini. These antimicrobials all inhibited the growth of S. montevideo during 24 h at 25 °C and ε-polylysine had the best effect. Furthermore, these antimicrobials enhanced the Salmonella inactivation in diluted tahini during thermal treatment, and there was less of a synergistic effect of thermal and antimicrobials in undiluted tahini due to less sublethal injured cells caused by heat. This study may provide useful information for Salmonella inactivation in tahini.