Inhibitory Activity of Plant-derived Antimicrobials against Lactobacillus brevis on Organic Leafy Greens

Introduction: Fresh-produce has become an important part of the regular diet, with an increased trend towards organic produce consumption, including leafy greens. However, controlling the growth of spoilage microorganisms on organic greens during transportation and storage is challenging. Lactobacillus brevis is a food-spoilage microorganism resulting in economic losses to the fresh-produce industry. Antimicrobials are routinely used during flume-washing, however, synthetic decontaminants are prohibited for organic fresh-produce. Plant-derived antimicrobials are therefore being considered as alternatives. 

Purpose: Evaluate plant-derived antimicrobials during flume-tank-washing of organic leafy greens to reduce Lactobacillus brevis. 

Methods: Carvacrol, cinnamaldehyde, and oregano and cinnamon essential oils (EO) were tested at 0.5% concentration. Hydrogen-peroxide, water and phosphate buffered saline (PBS) were used as controls. Organic baby spinach and romaine and iceberg lettuce were inoculated with Lactobacillus brevis (10⁶ CFU/g), washed with antimicrobials for 2 minutes, and stored at 4°C for 3 days. Surviving bacteria were enumerated on MRSA on day 0, 1, and 3. Wash waters were enumerated for L. brevis, and their pH recorded before and after washing leaves. Reduction in bacterial populations (as log CFU/g) were compared using ANOVA (P < 0.05). 

Results: Carvacrol and oregano EO were the most effective (P < 0.05) on all leafy greens, reducing L. brevis populations to undetectable levels immediately after washing on day 0. Cinnamon EO reduced populations by 1.0 - 1.5 logs by day 3 in all leafy greens. On leafy greens washed with cinnamaldehyde and hydrogen-peroxide, majority of reduction was seen on day 0 and 1 but by day 3, an increase in bacterial populations was observed, except in baby spinach where a 1.0-log reduction was observed on all three days. No growth was detected from wash waters for any of the treatments except PBS and cinnamon EO.

Significance: Plant-derived compounds and essential oils have the potential to inhibit spoilage microorganisms such as L. brevis on organic leafy greens.