Intrinsic antioxidant polylactic acid for food preservation

Active packaging, used in food and pharmaceutical sector, can be tailored on different purposes: one of its most common embodiments is the enhancement of the shelf-life of products, slowing down the kinetic of degradation. An important issues related to active packaging is that if active substances migrate into food they can affect its properties. In the present work, the “active” substrate was covalently linked to a bio-based polymer in order to prevent any kind of migration. We used poly(lactic acid) (PLA) due to its biodegradability, to its well-known industrial synthesis and processability with the classical methods used for common plastics processing. Active molecules used for the reaction with PLA are Tyrosol (TS) and Vanillyl Alcohol (VA) The phenolic moiety gives the antioxidant feature to these molecules and, on the other side, the aliphatic hydroxyl group makes them able to react with lactide, the precursor for the synthesis of PLA: in this way it is possible to obtain a polymer with high molecular weight and an antioxidant moiety directly linked to the polymeric chains. The rheological behavior of the material shows that the presence of functional molecules has the tendency to decrease melt viscosities of materials due to the effect as chain initiator, a detrimental feature that can be overcome modifying the molecular architecture. DPPH assay, a standard test used to evaluate the radical scavenging activity of a substance, was used to verify the antioxidant activity of the polymers. Pure vanillyl alcohol has a good reducing activity (about 92% of scavenging) that is retained when it is linked to the polymer whereas tyrosol shows a low activity (about 6%). Pure PLA and PLA with tyrosol do not have show significant scavenging activity, therefore the polymer with VA was selected to perform shelf life studies: the effect of PLA+VA on an industrial salami was evaluated. This food was chosen due to its high fat level. Salami was sliced and packed under vacuum with active PLA as interlayer between two slices. Typical parameters as water activity, pH of salami, color and TBARS were collected during the assay and also the radical scavenging activity of interlayer films was checked. Results show that the PLA with VA slows down the kinetic of oxidation: TBARS values, in fact, increase significantly only after 20 days whereas in the salami without interlayer TBARS value increase in the first 7 days of storage. Radical scavenging activity of PLA+VA is retained during all the assay: this means that VA cannot migrate into food and the material does not loose its antioxidant moiety.