The Effectiveness of Solid-Phase Antimicrobials Against Gram-Positive Bacteria in Food Processing

The dairy industry is searching for additional methods to eliminate or mitigate bacterial growth, most prominently that of Listeria monocytogenes. Listeria monocytogenes (LM) is a ubiquitous gram-positive bacterium first identified as a food born pathogen in 1983. Listeriosis, caused by LM, possesses an alarmingly high mortality rate between 20-30% for infected individuals. While pasteurization effectively eliminates LM during milk processing, Listeriosis outbreaks remain prevalent in the dairy industry due to post pasteurization contamination. Consequently, technologies that kill or mitigate growth of LM have exceptional value.

Solid-phase antimicrobial surfaces represent one such method of targeting and destroying these bacteria. Membranes are utilized throughout food processing, especially within the dairy industry. Various polymeric membranes for microfiltration, ultrafiltration, nanofiltration, or reverse osmosis applications are packaged in spiral wound membrane elements. The polypropylene (PP) feed spacer mesh within the spiral wound membrane element is an ideal surface for functionalization with an antimicrobial agent. The feed spacer mesh is in intimate contact with the process stream and endures frequent cleaning. The cleaning in place (CIP) procedure consists of various rounds of acid, caustic, and enzyme treatments and is typically applied every 24 hours.

Quaternary ammonium compounds (QACs) are frequently incorporated into polymers and applied to surfaces to combat pathogenic bacteria. In the food and dairy industry, solid-phase covalent attachment of QACs ensures no leaching of chemical compounds into the final product. The QAC functionalized feed spacer combats bacterial growth through adsorption. The negatively charged phosphate head of the lipid bilayer of the bacteria cell wall is electrostatically attracted to the positively charged nitrogen head of the QAC. Once adsorbed, the lengthy hydrophobic tail of the QAC intercalates into the cell wall disrupting membrane integrity to induce cell lysis and death. Adsorbed bacteria are also killed when CIP is applied to the feed spacer mesh.

In this work, we functionalized PP by covalently grafting polymer chains with quaternary ammonium groups to the mesh surface through photoinitiated free radical reactions. The resultant quaternary ammonium poly(vinylbenzyl chloride) functionalized polypropylene mesh (QA-PVBC-PP) was structurally verified through Raman spectroscopy and chemical dyes. The QA-PVBC-PP was also found to be non-leaching. The QA-PVBC-PP was tested against Lactobacillus plantarum and Listeria innocua in solution. Gram positive L. plantarum and L. innocua were chosen due to similar antimicrobial resistances as other pathogenic bacteria, including L. monocytogenes, making them effective and safe surrogates. The QA-PVBC-PP demonstrated significant antimicrobial activity and bacterial adsorption over 3 hours compared to non-functionalized PP. Furthermore, the QA-PVBC-PP retained bacterial adsorption capabilities following CIP procedure demonstrating chemical stability and potential for repeated use.