Résumé
In the aim to reduce the total amount of preservatives added in food, edible coating is used in the present study as surface retention of active agents to maintain a local high effective preservative concentration where microorganisms are intended to contaminate and/or grow, i.e. on coating surface. A food/anti-microbial coating system with sorbic acid as the active compound, agar gel as model food and wheat gluten (WG) or beeswax (BW) film as edible coatings was studied. A mathematical model able to describe the release kinetics of the anti-microbial agents from the edible coating into food products was developed and validated. It was used for estimating the local surface concentration in sorbic acid of coated model food. This surface concentration is an essential value for predicting microorganism growth but cannot be evaluated by experiments. In the case of WG coating, the surface concentration drops below 10% of the initial value after 1 h whereas in the case of the BW coating, the surface concentration remains above 75% even after one week of contact. Simulation realized using the estimated surface concentration and a simplified equation for the growth inhibition kinetics of Saccharomyces cerevisiae were compared to microbiological efficacy assessment. The calculated amount of sorbic acid required to maintain a 0.2% surface concentration during 23 days was 100 times lower when introducing the additive in a beeswax thin layer than directly in the core of the high moisture food or in a hydrophilic film such as wheat gluten.Industrial relevance: The methodology presented in this study based on experimental measurements and mathematical predictions is of great interest for the rational design of anti-microbial coatings and could be used in industrial applications. Edible coatings are already used in commercial practice for their barrier properties (water barrier property for example to avoid remoistening or drying of cereal-based products, gas barrier properties for the coating of fresh fruits and vegetable, etc) or for food appearance improvement (brilliance of apples for examples could be improved using polysaccharide-based film). Edible coatings have proved to be suitable as vector of preservatives such as anti-microbial or anti-oxidants. By using such retention matrices, very small amount of additives is required since the preservative is concentrated at the product surface. The benefit of using anti-microbial edible coating for consumer health is consequently non-negligible and this aspect is more and more taken into account by food manufacturers. The use of active edible coatings at an industrial scale is expected to grow, in Europe, due to the European framework regulation (EC 2004/1935) which authorizes the concept of active packaging with intentional active agents' release.The efficacy of anti-microbial edible coating could be assessed through time-consuming experimental tests. But most of the time. the couple edible matrix/active compound suitable for one applicable won't be anymore suitable for another food product and empirical tests should be undertaken once more by the food manufacturer. By using an integrated approach such as the one presented in this work, based on mathematical model for predicting additive release kinetics, numerous experiments may be avoided, since once the preservative diffusivity values in the coating and the food are known, the numerical tool could be used to optimize the initial quantity of preservative to add in the coating, to predict the food shelf life as a function of coating thickness or coating concentration in preservative, etc. The data shown in this paper concerning wheat gluten- and beeswax-based films could be also added in data bases of industrial relevance for further commercial applications.The approach used in this study could be considered as an assistant and prediction tool that should (i) optimize food preservation and (ii) help manufacturers in elaboration of new food product and packaging.