The antimicrobial activity of chitosan and gelatin coatings containing pomegranate peel extract against <i>Listeria monocytogenes</i> on Medjool dates

Listeria monocytogenes was isolated from several low water activity (aw) foods including palm dates, with potentially high survival rates. The objectives of this study were to (i) assess the in vitro antimicrobial activity of pomegranate peel extract (PPE) by determining the minimum inhibitory (MIC) and minimum bactericidal concentrations (MBC) of PPE against 5 strains of L. monocytogenes in Mueller-Hinton broth at 37 °C; (ii) investigate the antimicrobial activity of 2% chitosan or 9% gelatin coatings containing 2.5, 5.0, or 10.0% PPE against L. monocytogenes, total mesophilic bacteria (TMB) and yeasts and moulds on Medjool dates stored at 4 or 24 °C for 56 d; (iii) determine the phytochemical compounds present in PPE using gas chromatography-mass spectrometry (GC-MS). The MIC and MBC of PPE ranged from 3.91–7.81 and 15.63–31.25 mg/ml, respectively, against L. monocytogenes at 37 °C. L. monocytogenes in Medjool dates coated with either gelatin or chitosan with 2.5, 5.0, or 10.0% PPE at 24 °C were eliminated and L. monocytogenes were not detected by enrichment. On the other hand at 4 °C, when Medjool dates were treated with gelatin or chitosan coatings containing 2.5–10.0% PPE, L. monocytogenes were reduced by 1.8–2.9 log CFU/g by the end of 56 d storage. Additionally, yeasts and moulds on Medjool dates treated with gelatin or chitosan containing 2.5, 5.0, or 10.0% PPE were reduced below detection levels by 42 d at both 4 and 24 °C. Twelve compounds were detected in methanolic PPE with 5-Hydroxymethylfurfural (5-HMF) being the most predominant compound. The pH and aw values of Medjool dates were slightly changed by the coating type or the PPE concentration. These results indicate that chitosan and gelatin coatings incorporated with PPE could be applied as a natural alternative strategy with a considerable potential to overcome the survival of L. monocytogenes on Medjool dates.