Biodegradable bilayer ﬁlms prepared from individual ﬁlms of different proteins Daiane Nogueira , Vilásia Guimarães Martins School of Chemistry and Food, Federal University of Rio Grande, 96203-900, Rio Grande RS, Brazil Correspondence to: V. Guimarães Martins (E-mail: vilasiamartins@gmail.com) ABSTRACT: Bilayer ﬁlms propose to combine the characteristics of different individual polymer sources in a single structure, aiming to improve the properties of the ﬁlms obtained. The objective of this study was to develop and characterize bilayer ﬁlms prepared from individual ﬁlms of zein, wheat gluten, and protein isolate from hake (Cynoscion guatacupa), and evaluate the biodegradability of these polymers. Through the casting technique, individual ﬁlms and bilayers were prepared. The hake proteins isolate ﬁlms, as well as the bila- yers hake protein isolate/zein and hake protein isolate/wheat gluten (BI PI/WG) showed higher tensile strength, while wheat gluten ﬁlms presented greater elongation at rupture and lower solubility. In addition to having good tensile strength, BI PI/WG presented the best elongation. All ﬁlms and bilayers reached melting and glass transition temperatures above 100  C. Electromicrographs of the ﬁlms pre- sented an irregular surface with pores and grooves, being less pronounced in BI PI/WG. In the bilayers, it was not necessary to use any type of adhesive to form a single structure. Films and bilayers showed total biodegradability in 40 days, with the exception of the indi- vidual zein ﬁlm. BI PI/WG proved to be the most suitable for the development of food packaging materials. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46721. KEYWORDS: biodegradable ﬁlm; food packaging; gluten; hake protein isolate; zein Received 12 June 2017; accepted 15 May 2018 DOI: 10.1002/app.46721 INTRODUCTION The development of bilayer ﬁlms which can be used as food pack- aging, from different renewable sources, has been a promising alter- native in terms of minimizing the environmental impacts caused by packaging derived from synthetic materials. The bilayer ﬁlms could improve the properties of individual ﬁlms being more useful in the market. 1 Films can be made from proteins, such as wheat gluten, 2–5 corn zein, 6–8 ﬁsh, 9–12 and also from polysaccharides such as rice starch, 13 potato starch, cassava, and rice starch. 14 The appropriate combination of these matrices may lead to improvement in the properties of those ﬁlms. 15,16 These polymers have advantages in their commercial use, such as non-toxicity, biocompatibility, low cost, and being easy to obtain, with the exception of zein that could have a higher cost comparing with the other polymers. Elaboration of these ﬁlms involves, besides the main matrix (pro- tein, polysaccharide, among other polymers), the use of solvents and plasticizers which increase the ﬂexibility of the polymer net- work. 17 The casting method is among the most used to obtain biodegradable ﬁlms, mainly at laboratory scale. This method is based on mixing the components in a solution, with subsequent molding on ﬂat and inert surfaces, followed by drying under con- trolled conditions for solvent removal and ﬁlm formation. 16 Proteins are one of the main group of compounds in agricultural raw materials, meat and ﬁsh products. They are an important source of energy, nitrogen, and essential amino acids. 18 Accord- ing to Haque et al. 19 and Sforza and Tedeschi, 18 the proteins are composed of amino acids as building blocks, which indepen- dently of the origin (animal, vegetable, or microbial) have 20 amino acids, forming a highly complex structure. Amino acids are organic compounds formed by an amino group (NH 2 ), a car- boxyl group (COOH), and a side chain (R), which distinguishes one amino acid from the other. When many amino acids are chemically linked, a polypeptide is formed, which gives rise to a unit of protein when numerous polypeptide chains are attached. Although the main function of proteins is linked to the nutri- tional and structural function of plants and animals, 20 different proteins, such as ﬁsh myoﬁbrillar protein, 21 keratin chicken feathers, 22 soy protein, 23 whey protein, 24 porcine meat, and bone meal protein ﬁlms, 25 peanut protein, 26 zein, 27 wheat gluten 2 have been used in the development of biodegradable ﬁlms. Proteins may undergo a variety of intermolecular interactions and then it can be used in the preparation of biodegradable ﬁlms. 28 However, the low water vapor resistance of protein ﬁlms and their lower mechanical resistance compared to the synthetic © 2018 Wiley Periodicals, Inc. 46721 (1 of 13) J. APPL. POLYM. SCI. 2018, DOI: 10.1002/APP.46721