Incorporation of organic acids turns classically brittle zein films into flexible antimicrobial packaging materials

Abstract

This study aimed to turn classically brittle zein films into flexible antimicrobial ones by the use of lactic (LA), malic (MA) and tartaric acids (TA). The most effective plasticizer was LA (400% elongation at break [EB] at 4%), while MA (189% EB at 4.5%) and TA (68% EB at 5%) showed moderate and limited plasticizing effects, respectively.The LA-and MA-loaded films maintained their flexibility during 30-day storage at 4 C or 25 C. Fourier transform infrared (FTIR) analysis suggested that the plasticization of LA and MA could be related to secondary structural changes in zein such as increased α-helix and random coils (mainly by MA) and spaced/modified intermolecular (only by LA) and intramolecular (mainly by MA) β-sheets. Atomic force and scanning electron microscopy showed that LA and MA gave more homogenous and smoother films than TA. Films with LA showed the highest water vapour permeability followed by those of control, MA-and TA-loaded films. Films with 3%-4% LA or MA formed clear zones on Listeria innocua and Klebsiella pneumonia, but only films with LA formed clear zones on Escherichia coli. All OA-loaded films gave unclear zones on Staphylococcus aureus in disc-diffusion tests, but this bacterium was inactivated rapidly in antimicrobial tests based on surface inoculation tests. LA is the best OA to develop flexible antimicrobial films from zein, an industrial by-product that films could not have been utilized as a widespread packaging material due to their brittleness.

Development of antimicrobial composite materials for food packaging applications

“…The tensile strength of packaging materials are not greatly affected when only small quantities of antimicrobial agents are incorporated or when the molecular weight of the antimicrobial agent is less than that of polymer materials (Bastarrachea et al, 2011). For instance, the addition of 2.00-2.50% (w/w) of malic acid did not significantly affect the tensile strength of zein films (Sözbilen, Çavdaroğlu, & Yemenicioğlu, 2022). Similar results were obtained when low concentrations of cinnamon, thyme or lemon EO was incorporated into biodegradable films (Peng & Li, 2014;Syafiq et al, 2021).…”

“…Zein films containing organic acids, such as lactic acid, malic acid and tartaric acid, exhibited greater elongation at break than control films due to their plasticizing effect (Sözbilen et al, 2022). Khaneghah et al (2018) and Varghese et al (2020) suggested that the incorporation of most EO and plant extracts to polymer materials typically reduces the tensile strength and improves the elongation at break of food packaging films.…”

“…It was reported that the addition of 3.5% (w/w) of organic acid, such as lactic acid, increased the WVP of plasticized zein films, whereas the incorporation of malic acid and tartaric acid reduced their WVP (Sözbilen et al, 2022). Even though zein films are hydrophobic, their high WVP is primarily due to the weak chain interaction and the presence of voids in the structure.…”

“…and Clostridium botulinum, at a lower level of oxygen (Abdul Khalil et al, 2018). Also, an increased oxygen level in the food packaging may lead to nutritional value and quality losses, oxidative rancidity and changes in color, texture, aroma and flavor of food products containing oil (e.g., nuts) (Sözbilen et al, 2022). All these unfavorable effects dramatically reduce the quality and shelf life of food products.…”

“…In recent years, researchers have reported the antimicrobial activity of packaging materials incorporated with synthetic antimicrobial agents, such as weak organic acid and its salts, chelating agents and many others. For instance, zein films incorporated with organic acids, such as lactic acid and malic acid, were very active against E. coli, Klebsiella pneumoniae, Listeria innocua and S. aureus (Sözbilen et al, 2022). Other weak organic acids, such as citric acid and tartaric acid (Suganthi, Vignesh, Sundar, & Raj, 2020), have also been added into food packaging materials to improve their antimicrobial activity.…”