Factors affecting contamination and infiltration of Escherichia coli K12 into spinach leaves during vacuum cooling

While it has been understood that vacuum cooling can result in bacterial infiltration into plant leaves, the details of the contamination and infiltration pathways could be explored further. We investigated contamination/infiltration pathways via experimental and modeling work wherein 1) microorganisms (Escherichia coli strain K12) external to the vacuum cooling chamber moved into it during repressurization and deposited on surfaces; 2) moist organic matter (e.g. mud or bird or animal feces) brought in with the produce could boil during depressurization, aerosolizing microorganisms and spreading them to produce; and 3) microorganisms present on surface water infiltrated into the leaves during repressurization. For external contamination, it was not possible to draw microorganisms in aerosolized form from an adjacent chamber; however, dried cultures deposited in the inlet readily contaminated the interior of the vacuum chamber. Boiling of moist organic matter within the chamber resulted in significant redistribution and contamination of chamber surfaces. For the infiltration pathway, the quantity of microorganisms entering into spinach tissue depended on vacuum level and rate of repressurization. Experimental data and model predictions show that infiltration is greatest when repressurization is slow (3.1 log CFU/g); under rapid repressurization, little (2.4 log CFU/g) or no infiltration occurs. Over the range studied (600–900 Pa), experimental data show that the pressure level during vacuum cooling did not significantly affect infiltration rate. Model predictions are in agreement on general trends, showing a slight influence of pressure level on infiltration. The contamination and infiltration pathways investigated here can be minimized in practice, improving microbial safety of vacuum cooling processes.