But the study from a group of Spanish scientists concluded the main factor inhibiting growth of bacteria and moulds was related not to the total amount of active chemical released but instead to when these compounds reached the surface of material – known as the critical time.
The type of packaging material used also played an important role in the efficacy of the antimicrobial performance.
Based on a growing use of natural antimicrobials of plant origin, the team selected cinnamon and oregano as previous research had suggested the essential oils contained compounds that made them the most effective among these substances.
The antimicobial constituent of cinnamon is the compound cinnamaldahyde – which is believed to have the highest anti-fungal activity among aliphatic aldenhydes. Carvacrol and thymol are the relevant compounds in oregano, said the researchers led by Christina Nerin. The work was published in the journal Foodbourne Pathogens and Disease.
The substances were used to measure the effectiveness of inhibiting growth of Gram-positive Listeria monocytogenes, Gram-negative Salmonella chloreraesuis, the yeast Candida albicans and the mould aspergillus flavus.
The inhibition tests saw plates of solidified agar media inoculated in triplicate in a petri dish. The cover of the dish was replaced with active films and stored in appropriate conditions for the incubation period. After this time the number of colonies formed on the polypropylene (PP), and PE/EVOH materials.
The team fund that moulds and yeasts were more sensitive to the substances than the bacteria – with the cinnamon EO showing stronger activity against than oregano.
Regarding the plastics, a stronger effected was noted for PP than PE/EVOH.
The results showed a correlation between concentration of the compounds and the extent of inhibition. But the team also found that “the time when concentration is achieved is critical”. This process is “key to understanding the different behaviours of the films, it is necessary to reach a minimum concentration of active compounds in the agar during the lag phase of the microorganism”, added the research.
Release was found to be quicker in PP than PE/EVOH, so the initial concentration in PP could be lower in PE/EVOH to get the same results.
“Thus a critical point in the design of active packaging is the material used, and not only the active compound selected”, concluded the paper.
The researchers also found that an increase in the amount of compound used is “not directly related with an increase in antimicrobial effect”.
No adverse effects on the sensory perception of food were detected using these essential oils in packaging.
Source: Foodbourne Pathogens and Disease
Published online ahead of print: DOI:10.1089/fpd.2009.0516
Title: New Approach to Study the Mechanism of Antimicrobial Protection of an Active Packaging by Authors: C. Nerin et al