AUTHOR=Chuang Shihyu , Ghoshal Mrinalini , McLandsborough Lynne 

TITLE=Efficacy of acidified water-in-oil emulsions against desiccated Salmonella as a function of acid carbon chain-length and membrane viscosity

JOURNAL=Frontiers in Microbiology

VOLUME=14

YEAR=2023

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2023.1197473

DOI=10.3389/fmicb.2023.1197473

ISSN=1664-302X

ABSTRACT=<p>Sanitizing low-moisture food (LMF) processing equipment is challenging due to the increased heat resistance of <italic>Salmonella</italic> spp. in low-water activity (a<sub>w</sub>) environments. Food-grade oils mixed with acetic acid have been shown effective against desiccated <italic>Salmonella</italic>. In this study, different hydrocarbon chain-length (C<sub>n</sub>) organic acids were tested against desiccated <italic>Salmonella</italic> by using 1% v/v water-in-oil (W/O) emulsion as the delivery system for 200 mM acid. Fluorescence lifetime imaging microscopy (FLIM) was utilized with a BODIPY-based molecular rotor to evaluate membrane viscosity under environmental conditions such as desiccation and temperature elevation. Drying hydrated <italic>Salmonella</italic> cells to 75% equilibrium relative humidity (ERH) increased the membrane viscosity from 1,199 to 1,309 mPa·s (cP) at 22°C. Heating to 45°C decreased the membrane viscosity of hydrated cells from 1,199 to 1,082 mPa·s, and decreased that of the desiccated cells from 1,309 to 1,245 mPa·s. At both 22°C and 45°C, desiccated <italic>Salmonella</italic> was highly susceptible (&gt;6.5 microbial log reduction (MLR) per stainless-steel coupon) to a 30-min treatment with the W/O emulsions formulated with short carbon chain acids (C<sub>1-3</sub>). By comparison, the emulsion formulations with longer carbon chain acids (C<sub>4-12</sub>) showed little to no MLR at 22°C, but achieved &gt;6.5 MLR at 45°C. Based upon the decreased <italic>Salmonella</italic> membrane viscosity and the increased antimicrobial efficacy of C<sub>4-12</sub> W/O emulsions with increasing temperature, we propose that heating can make the membrane more fluid which may allow the longer carbon chain acids (C<sub>4-12</sub>) to permeate or disrupt membrane structures.</p>