Characterization and in vitro antimicrobial study of soy protein isolate films incorporating carvacrol

Soy protein, which is a by-product of soybean oil extraction, has been used as the carrier of essential oils in active packaging to inhibit the growth of spoilage microorganisms on food. In our study, the effects of incorporation of carvacrol in soy protein isolate (SPI) films on film characteristics, including mechanical properties, opacity, water vapor permeability (WVP), and antimicrobial activities, were evaluated. In addition, Fourier transform infrared (FTIR) spectroscopic characterization of the films was employed to examine the effects of the carvacrol on protein secondary structure and to study the release kinetics of carvacrol in the vapor phase. The results showed the mechanical properties, opacity and WVP of these SPI films were influenced by the concentrations of glycerol and carvacrol in the film-forming solution. The emulsified carvacrol acted as a plasticizer like glycerol to reduce resistance and rigidity of SPI films and increase their extensibility. In contrast, whereas films prepared with the higher concentrations of glycerol had significantly increased WVP, the effect of carvacrol concentration on WVP was not significant. FTIR spectroscopic analysis of protein secondary structure in the films revealed some transformation of β-sheet to α-helical structure with addition of carvacrol, consistent with a plasticizing effect. The release of carvacrol from films was monitored by gas-phase FTIR spectroscopy, and the release kinetics were well fit by a Weibull model. The results obtained by this FTIR methodology indicated that glycerol in the films promoted the release of carvacrol. Films prepared with ≥2% carvacrol exhibited high antimicrobial activities against Listeria grayi in vapor phase and could be employed in active packaging to improve the safety of food products.