THE BEHAVIOR OF Listeria monocytogenes IN CHOCOLATE MILK AT 4 AND 10°C WITH ADDED PROPOLIS EXTRACT

摘要

Propolis is a resinous substance collected by honeybees. Propolis is used as a natural sealant and protective material against contamination spots in the hive. Currently propolis is being used in many non-food applications, mostly cosmetics and Pharmaceuticals, due to its broad antimicrobial properties. The post-pasteurization contamination of chocolate milk with Listeria monocytogenes has caused a severe food-borne outbreak in the past. In the present study the behavior of L monocytogenes was investigated in Extended Shelf Life (ESL) chocolate milk with the addition of propolis extract (PE). Chocolate milk was placed in sterile glass vials containing PE (0 or 4 mg/mL of milk) and inoculated with a five-strain mixture of L. monocytogenes (ca. 10~3 cfu/mL). The vials containing PE had been prepared in two different ways. One set of vials contained ethanolic PE, whereas the other contained a mixture of ethanolic PE in glycerol. Prior to the addition of milk and Listeria, ethanol was removed by evaporation under vacuum. The behavior of L monocytogenes was investigated during storage of milk at 4 and 10"C. At 10°C the generation time of L monocytogenes in samples containing PE was significantly longer compared to that observed in the respective control samples (9.0 vs. 7.1 h; p = 0.016). However, after 10 days of storage at 10°C, the average populations of L monocytogenes in samples containing PE was essentially identical to those of the control samples. At 4°C the generation time of L. monocytogenes in samples containing PE was significantly longer compared to that observed in the respective control samples (56.6 vs. 27.4 h; p = 0.025). At 10°C the generation time of L. monocytogenes in samples containing PE diluted in glycerol was significantly longer compared to that observed in the respective control samples (59.4 vs. 9.3 h; p = 0.014) and the average pathogen populations at the 10th day of storage were lower by 4.8 log cfu/mL. At 4°C the addition of PE diluted in glycerol completely inhibited L monocytogenes growth, leading to average pathogen counts that were 4.4 log cfu/mL lower at the 20th day of storage compared to the respective L. monocytogenes populations observed in the absence of PE. Therefore, a significant anti-listerial effect was exhibited upon addition of PE in refrigerated chocolate milk and a mild anti-listerial effect was also observed during storage of contaminated milk at 10°C. However, when PE was diluted in glycerol, a substantial anti-listerial effect was also shown during storage at 10°C. Because of its antimicrobial effect, propolis could possibly serve as a novel natural food antimicrobial in chocolate milk.