The effect of emulsifier type and oil fraction on Salmonella Typhimurium growth and thermal inactivation in oil‐in‐water emulsion

Abstract

High water activity oil‐in‐water emulsions can promote survival and growth of Salmonella Typhimurium. Nevertheless, the precise effect of emulsifier type and oil content on bacterial growth and inactivation is not fully understood. Here, emulsions were prepared using different emulsifiers (Tween 20, Tween 80, and Triton X‐100) and different oil fractions (20%, 40%, and 60% (v/v)). TSB (control), emulsifier solutions, and emulsions were inoculated with S. Typhimurium. Bacterial growth rate was measured at 7, 22, and 37°C, whereas thermal inactivation was performed at 55°C. Growth and inactivation data was fitted into Logistic and Weibull models, respectively. At an incubation temperature of 37°C, the presence of high amount of oil (60%) in Tween 20 and Triton X stabilized emulsions extended the lag phase (5.83 ± 2.20 and 9.43 ± 1.07 h, respectively, compared to 2.28 ± 1.54 h for TSB, p < 0.05), whereas individual emulsifiers had no effect on growth behavior compared to TSB. This effect was also prevalent but attenuated at 22°C, whereas no growth was observed at 7°C. In thermal inactivation, we observed protective effect in Tween 80 and Triton X‐100 solutions, where time required for five‐log reduction was 1914.70 ± 706.35 min and 795.34 ± 420.09 min, respectively, compared to 203.89 ± 10.18 min for TSB (p < 0.05). Interestingly, the presence of high amount of oil did not offer protective effect during thermal inactivation. We hypothesize that oleic acid in Tween 80 and lower hydrophobicity value of Triton X‐100 help maintain membrane integrity and improve the resistance of bacteria to heat inactivation.

The effect of emulsifier type and oil inclusion on stress‐related gene expression of Salmonella typhimurium in oil‐in‐water emulsion

“…The potential mechanism of the protective effect of emulsifiers after thermal treatment observed in a previous study remains to be fully elucidated (Tsai & Tikekar, 2023). Although bacterial movement to oil droplets is unlikely to be the cause of the effect, other mechanisms may be at play.…”

“…A comparable pattern was observed in environments incorporating Tween 80. The 5D value for S. typhimurium count was observed to be more extended in the solution (1914.70 ± 706.35 min) relative to the emulsion (136.45 ± 30.56 min) ( p < 0.05) (Tsai & Tikekar, 2023). Notably, higher expression levels of fabA and ompC genes were recorded in S. typhimurium cultured in Tween 80 solution as opposed to those cultured in Tween 80 emulsion ( p < 0.05).…”

“…For instance, our previous study found that emulsifiers with positive charge, such as whey protein isolate with positive charge, had a higher 5D value compared to negatively charged whey protein isolate. This result implies that the interaction between the positively charged emulsifier and cell membrane may be stronger, potentially providing greater protection against thermal treatment (Tsai & Tikekar, 2023). Another possible explanation is the stress caused by the high temperature itself, which can cause changes in bacterial membrane permeability and fluidity, and disrupt cellular functions, ultimately leading to the effect on bacterial inactivation (Cebrián et al., 2019; Guillén et al., 2021).…”

“…Incorporating 2% (v/v) Triton X‐100 as an emulsifier resulted in an upregulation of stress‐related genes, indicating the instigation of a harsh environment despite the addition of oil. Interestingly, the time required to achieve a 5‐log reduction in S. typhimurium count, also known as the 5D value, was markedly longer in the Triton X‐100 solution (795.34 ± 420.09 min) compared to the Triton X‐100 stabilized emulsion (207.23 ± 60.03 min) ( p < 0.05) (Tsai & Tikekar, 2023). This raised the question as to the cause of this significant difference, prompting us to conduct a comparative analysis of gene expression between Triton X‐100 solution and emulsion following thermal treatment in this study.…”

“…Considering the need to control the proliferation of this pathogen in food, particularly in oil‐in‐water emulsion products like tahini with 59% oil content, it is vital to study how different emulsifiers and the presence of oil influence the stress‐induced RNA expression of Salmonella in such emulsions (Szpinak et al., 2022). Our previous research has shown that a high oil fraction in emulsion system can extend the lag phase of Salmonella typhimurium growth; however, emulsifier solutions alone (Tween 80 and Triton X‐100), rather than emulsions, have been found to be protective against thermal inactivation (Tsai & Tikekar, 2023). To build on this knowledge, this study aims to investigate the effects of different emulsion conditions on stress‐related gene expression in Salmonella .…”