Inhibitory activity of propolis against Listeria monocytogenes in milk stored under refrigeration

Propolis is a natural bee-product with documented antimicrobial properties in vitro. The objective of this study was to develop a protocol for adding propolis into milk and to determine whether the addition of propolis can confer anti-listerial activity during the storage of milk under optimal or improper refrigeration conditions. Upon dissolving propolis ethanolic extract (PEE) into glycerol, the PEE-glycerol mixture contained no visible insoluble particles and could be dispersed evenly into milk, without leaving any insoluble material. PEE, with or without glycerol, was added into extended shelf-life milk, artificially contaminated with Listeria monocytogenes. The addition of PEE dissolved into glycerol resulted in a pronounced and dose-dependent anti-listerial effect in milk stored at 4 °C, with the higher concentration tested (4 mg of dry PEE per mL of milk) resulting in complete inhibition of L. monocytogenes growth throughout 30 days of storage. The combination of PEE with glycerol was also effective in significantly reducing the growth rate of the pathogen in milk stored under improper refrigeration (10 °C). Based on a patented PEE-deodorization protocol, the addition of deodorized PEE into milk resulted in a product with average consumer acceptability. However, the PEE deodorization process resulted in reduction or even complete removal of propolis constituents with known antibacterial activity, with a concomitant significant reduction in its anti-listerial effect. Nonetheless, the data presented in this manuscript highlight the strong anti-listerial potential of propolis in milk and suggest that, upon further research on its deodorization and standardization, there may be room for the application of propolis as a natural preservative in dairy beverages.