Partially Carboxymethylated Feather Keratins. 2. Thermal and Mechanical Properties of Films Peter M. M. Schrooyen, † Pieter J. Dijkstra, Radulf C. Oberthu ¨ r, ‡ Adriaan Bantjes, and Jan Feijen* Department of Chemical Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands Free cysteine thiol groups of keratin extracted from chicken feathers were partially carboxymethy- lated with iodoacetic acid (25-76% cysteine modification). Stable dispersions were used for the preparation of films by solution casting. Glycerol was used as a plasticizer (0.05-0.47 g/g of keratin), and films were stored at a constant relative humidity (20, 30, 50, 70, or 90%). The degree of crystallinity in the films was higher when more cysteine residues were carboxymethylated. The films displayed an optimum in mechanical properties at ∼50% cysteine carboxymethylation. The tensile strength at this optimum was 25 MPa, the E modulus, 350 MPa, and the elongation at break, 50%. Probably, this optimum was the result of both a decreasing amount of disulfide bonds and an increasing degree of crystallinity for higher degrees of cysteine modification. The influences of a higher amount of glycerol and of different storage conditions on the mechanical properties of films from keratin with a defined degree of cysteine modification were also investigated. Keywords: Feather; keratins; films; chemical modification; thermal properties; mechanical properties INTRODUCTION Over the past 15 years, poultry production in the European Union increased by 5% annually, which led to a growing waste stream of feathers. In 1996, >770000 tons of chicken feathers was available as a byproduct of the poultry industry. Feathers are mainly composed of the structural proteins keratins and are generally transformed into hydrolyzed feather meal. Feather meal has little added value and is used as an organic fertilizer or as an additive to animal feed. The function of feathers as a tough, insoluble, fibrous material that provides a protective outer covering indicates the potential of the feather keratin molecule for applications in which these properties are desirable. Water insolubility and me- chanical strength are mainly due to the occurrence of a large amount of hydrophobic amino acids and cysteine residues, which are mainly present as the disulfide bonded, dimeric amino acid cystine, and to the struc- tural organization of the keratin molecules in the feather. Recently, there has been an increased interest in the use of proteins as a renewable resource for the development of biodegradable films, for example, for compostable packaging, agricultural film, or edible film applications (1-6). Only limited attention has been given to keratin in this field (7-11). Feather keratins are composed of ∼20 proteins, which differ by only a few amino acids. These proteins have approximately the same molecular weight of 10.4 kDa (12). The distribution of amino acids is highly nonuni- form, with the basic and acidic residues and the cysteine residues concentrated in the N- and C-terminal regions. The central portion is rich in hydrophobic residues and has a crystalline -sheet conformation (13). There are essentially two types of keratin, tradition- ally classified as either “soft” or “hard” (14). The soft keratins, with a low content of disulfide bonds, are found in the stratum corneum and callus, whereas the hard keratins are found in epidermal appendages such as feathers, hair, nails, and hoofs and have a high disulfide content. Apparently, the amount of disulfide bonds determines largely whether a keratinous material is soft, flexible, and extensible, like the epidermis, or hard, tough, and inextensible, like hair or feathers (15). For film preparation by solution casting, stable solu- tions or dispersions are needed. For feather keratins it has been demonstrated previously that such solutions or dispersions can be prepared by partially carboxy- methylating the cysteine residues (16). To obtain films with a high E modulus and tensile strength, it appears to be appropriate to decrease the amount of intermo- lecular cross-links in keratin by partially modifying the cysteine residues, leaving the remaining cysteine free to oxidize during the film-forming process. In this study we report on the preparation of solution cast films from partially carboxymethylated feather keratin dispersions. The effect of the degree of cysteine modification, added glycerol as a plasticizer, and water on thermal and mechanical properties of these films was investigated. MATERIALS AND METHODS White body feathers from broilers, 70 days old, were supplied by Hago Rijssen (The Netherlands). All chemicals were of analytical grade and were purchased from Merck (Darmstadt, Germany) except for 5,5′-dithiobis(2-nitrobenzoic acid) (DTNB), which was obtained from Sigma (St. Louis, MO). * Corresponding author (telephone +31 53-489 29 68; fax +31 53-489 38 23; e-mail j.feijen@ct.utwente.nl). † Present address: NIZO Food Research, P.O. Box 20, 6710 BA Ede, The Netherlands. ‡ Present address: Labor Dr. Oberthu ¨ r GmbH, Bruchweg 10, 49844 Bawinkel, Germany. 221 J. Agric. Food Chem. 2001, 49, 221-230 10.1021/jf0004154 CCC: $20.00 © 2001 American Chemical Society Published on Web 12/16/2000