Incorporation of natural compounds into polymeric matrix: development of antimicrobial films

Antonia NOSTRO, University of Messina

Microbial food contamination is a global problem that causes serious health and hygiene issues. The survival and growth of common spoilage and pathogenic microorganisms such as Escherichia coli, Listeria monocytogenes, Salmonella spp., Staphylococcus aureus, Bacillus cereus, Clostridium perfringens, Aspergillus niger and Saccharomyces cerevisiae represent a risk for the foodborne disease transmission and/or food deterioration. The severity of the problem is further potentiated by a high prevalence of biofilm-producing bacteria, able to adhere on different surfaces.

Various traditional preservation techniques and different antimicrobial synthetic agents are commonly used. However, in recent years, an increase in consumer demand for safe, fresh, minimally processed foods with reduced content of chemical additives has led to the development of new preservation strategies. An interesting approach is represented by the use of natural substances such as essential oils and closely related components. Essential oils are mixtures of volatile substances consisting of terpene hydrocarbons, aromatic compounds and terpenoids. Their antimicrobial activity against pathogenic and spoilage microorganisms and their efficacy in food matrices such as fruits, vegetables, meat and fish suggest a possible role in food preservation. Despite this, the direct addition in foods still encounters limitations due to their high volatilization and hydrophobicity and to the impact on the organoleptic characteristics of the products. New technologies are aimed to their incorporation into polymeric materials to develop active, systems able to control microbial growth by confining the antimicrobial agent away from the food.

In this context, the chemico-physical properties of essential oils allow them to dissolve, diffuse, and permeate into the most common polymers. Several studies have demonstrated the efficacy of essential oil incorporated into synthetic polymer such as polypropylene (PP), polyethylene (PE)/ethylene vinyl alcohol copolymers and low-density polyethylene (LPDE) or natural polymer such as chitosan, starch, pectin, polylactic acid (PLGA).

Our recent studies show the effectiveness of of carvacrol and cinnamaldehyde incorporated into polymeric films of ethylene-vinyl-acetate (EVA) by melt mixing. Specifically, the films able to release the active components were effective in inhibiting the growth and the formation of the biofilm of Escherichia coli, Listeria monocytogenes, Staphylococcus aureus and Staphylococcus epidermidis. In particular, it was found a reduction of viable bacterial cells of about 2- 4 log units and a reduction of the biomass of about 60-90% in E. coli and S. aureus compared to the control. Interestingly, increasing the incubation time from 24 to 168 h, carvacrol and cinnamaldehyde maintained their high inhibitory efficacy. The analysis of the surface characteristics demonstrated that essential oil constituents lowered the contact angle values without causing any remarkable variation of the surface roughness. Carvacrol and cinnamaldehyde act as plasticzers which reduce the intermolecular forces of polymer chains, thus improving the flexibility and iextensibility of the film.

The perspectives of essential oils and their components incorporated into polymer systems open up new horizons in the development of food packaging aimed at the improvement of food safety, shelf-life and conservation.

 
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