We hypothesized that essential oils combined with milder heat treatments can accelerate Salmonella Typhimurium inactivation in low water activity (aw) environment. Although both trans-cinnamaldehyde (CA, 1000 ppm) and eugenol (EG, 1000 ppm) accelerated thermal inactivation of Salmonella in water and food matrices (peanut oil, corn starch and whey protein) with 0.93 aw, a similar effect was not observed at lower (0.35-0.45) aw. Both altered bacterial metabolic activity and membrane properties were hypothesized to contribute to higher resistance. Metabolic activity was measured with resazurin assay where fluorescence produced from metabolism of resazurin was monitored. Faster the sample reaches peak fluorescence intensity (lower tpeak), higher the metabolic activity. Regardless of aw (0.4/ 0.9), metabolic activity following 55 °C treatment in oil (tpeak=95±5/44±0 min), starch (tpeak=293±7/539±13 min), or protein (tpeak=156±14/375±24 min) was lower than at 22 °C (oil tpeak=40±3/17±2 min, starch tpeak=257±7/363±8 min, protein tpeak=117±2/316±12 min) (P &lt; 0.05). Addition of CA/ EG further reduced metabolic activity (P &lt; 0.05) at both aw compared to 55 °C treatment alone in oil and starch, but not in protein. These results explain bacterial response to thermal treatments at two aw values, but not the response in different matrices at low aw. Membrane fluidity was measured with 1,6-diphenyl-1,3,5-hexatriene, a membrane probe whose rotational movement is restricted by membrane lipid interactions. Its fluorescence polarization value (mPf) reflects membrane fluidity, where a larger value indicates lower fluidity. We also analyzed membrane fatty acid profile using gas chromatography. Bacteria at 0.4 aw had lower (P &lt; 0.05) membrane fluidity (mPf=116.3±6.5, Ru/f=0.31±0.01) and unsaturated to saturated fatty acids ratio (Ru/f ) than at 0.93 aw (mPf=96.2±1.6, Ru/f=0.34±0.01) or in fresh broth (mPf=97.0±0.8, Ru/f=0.37±0.01), suggesting that bacteria at low aw changed their membrane compositions to increase rigidity and reduce permeability. This explains increased resistance against both heat treatment and essential oils.

Resistance mechanism of salmonella typhimurium at low water activity against heat treatment and added trans-cinnamaldehyde or eugenol

ACS Spring 2022

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