Thermal, mechanical and water vapor barrier properties of sodium caseinate films containing antimicrobials and their inhibitory action on Listeria monocytogenes

Antimicrobial films were prepared by incorporating different concentrations of sodium lactate (Na lactate), potassium sorbate (K sorbate) and nisin into sorbitol-plasticized sodium caseinate (SC) films. The impact of antimicrobial compounds on the water sorption, water vapor barrier properties and thermo-mechanical behavior of SC films was investigated. Furthermore, their antimicrobial effectiveness against Listeria monocytogenes was studied as a function of antimicrobial concentration. The water content and water vapor permeability (WVP) of SC films increased with increasing Na lactate and K sorbate concentration, with Na lactate-containing SC films showing higher capacity to absorb moisture and greater WVP values than the films containing K sorbate. On the other hand, nisin addition did not cause significant changes in the water uptake and WVP of SC films. The incorporation of Na lactate and K sorbate affected in different way the glass transition temperature (Tg defined as tan δ peak) of antimicrobial SC films, with Na lactate increasing and the K sorbate depressing the Tg of the system. The addition of increasing Na lactate and K sorbate concentration resulted in reduction of Young modulus (E) and maximum tensile strength (σmax) and increasing of elongation at break (% EB) suggesting that both antimicrobials acted as plasticizers for the SC films. Nisin-containing SC films were the most effective in reducing growth of L. monocytogenes, followed by K sorbate-impregnated SC films, whereas films containing Na lactate were slightly effective in this respect and only at the higher concentration (40% w/w film dry basis). The results indicated that for effective applications of antimicrobial coatings in foods, sufficient knowledge is required not only on the independent properties of the coating film and the antimicrobial compound but also on their interactions.