Investigation and characterization of plasticizing effects of choline chloride and organic acid based deep eutectic solvents on biodegradable films

Today, with the rapid increase in urbanization and the working population, the consumption of packaged ready to eat food products has also increased. However, in recent years, the use of natural antimicrobial substances as protective agents in the food industry has been increasing, as consumers have turned to products consisting of natural components. One of the natural antimicrobial substances that can be used in food packaging systems is organic acids. Organic acids can enter into the structure of deep eutectic solvents (DES), which are defined as green solvents. DESs are inexpensive and environmentally friendly media that can be used as plasticizers in the packaging industry. In this study, DESs consisting of natural components with antimicrobial and biodegradable properties, which can be used instead of conventional plasticisers, were developed and the possibilities of their use as plasticisers and antimicrobial agents in food packaging were investigated. In the study, firstly, choline chloride and organic acid based (acetic acid (A), glycolic acid (Gli), glutaric acid (Glu), lactic acid (La), levulinic acid (Le), malic acid (M), pyruvic acid (P) and citric acid (S)) DESs were prepared in 1:1/1:2 molar ratios. DESs forming clear and homogeneous mixture were characterised in terms of antimicrobial properties by pH, volatility, density, viscosity and structure determination analyses. Three Gram negative bacteria (Escherichia coli, Salmonella Typhimurium and Escherichia coli O157), three Gram positive bacteria (Staphylococcus aureus, Listeria monocytogenes and Bacillus cereus), two yeasts (Candida albicans and Candida parapsilosis) and two molds (Aspergillus flavus and Penicillium chrysogenum) were used to determine antimicrobial properties. Then, DESs showing both antibacterial and antifungal effects were added to the polylactic acid (PLA) solution at the determined ratios and packaging films were produced under laboratory conditions by casting method from the solution. These films were characterised in terms of thickness, mechanical, optical, structure determination analyses and antimicrobial properties. When the FTIR spectra of DESs were analysed, characteristic peaks specific to choline chloride and organic acids were observed. All DESs showed strong acid character (pH<1) as expected due to the organic acids in their structure. It was observed that volatility values were between 2.01%-53.63%, density values were between 1.12-1.40 g/ml, viscosity values were between 36.17-437333.33 mPa.s. However, while all DESs showed antibacterial effect, antifungal effect was observed only in DESs containing acetic acid, levulinic acid and pyruvic acid. DESs showing antibacterial and antifungal effects were added to PLA films at 10%, 20% and 30% and evaluated primarily in terms of tensile strength and elongation at break. Then, considering their possible antimicrobial activity, it was decided that 20% DES added PLA films were the most suitable films and other analyses were performed. The tensile strength values of 20% DES added films were determined between 16.19-18.41 MPa, elongation at break values between 91.34%-123.22%, puncture strength values between 36.09-42.62 N, puncture elongation values between 4.72-5.81 mm, thicknesses between 0.14-0.16 mm, ∆E values between 1.59-2.82 and opacity values between 1.62-2.24. When the FTIR spectra were examined, characteristic peaks specific to DESs were also observed in the films. As a result of antimicrobial tests on the films, antibacterial and antifungal activity was not observed, probably due to the hydrophobic structure of PLA.