Impact of Thiocyanate Salts on Physical, Thermal, and Rheological Properties of Zein Films

Cereal Chem. 90(3):204–210 A new class of zein additives was investigated, thiocyanate salts. Ammonium, potassium, guanidine (GTC), and magnesium thiocyanate salts were added to solutions of zein in with various amounts of tri(ethylene glycol) (TEG), cast as films, and then tested to determine the impact that each salt had on properties. The presence of these salts affected solution rheology and intrinsic viscosity, demonstrating that the salts interacted with the protein. It was found that these salts acted as plasticizers, as they lowered the glass transition temperature of zein when evaluated with differential scanning calorimetry. In zein films in which TEG was present, these salts increased elongation and reduced tensile strength. However, unlike traditional plasticizers (such as TEG), when the salts were used as the only additive, elongation was not increased and tensile strength was not decreased. Of the salts tested, GTC in combination with TEG was found to increase elongation the most. The impact of salts on elongation was greatly affected by the relative humidity in which the samples were stored. With the production of ethanol from corn, whether production is done with dryor wet-milling processes, a great deal of proteinaceous coproduct is generated (Lawton 2002). The dominant protein present in the dryor wet-milling coproduct is zein. Developing improved end uses for zein will improve the economics of the ethanol production industry. In the early 1930s, zein was used in the textile fiber market as a wool alternative (Swallen 1939; Croston et al 1945; Evans and Croston 1949). For use in this market, the zein was treated with formaldehyde to impart improved physical properties and solvent durability. With the introduction of textile fibers based on petroleum feeds, the use of zein in the textile fiber market disappeared. To replace products that use nonrenewable materials, zein products need improved elongation and improved solvent resistance. Various nonformaldehyde routes have been investigated to improve the solvent resistance of zein, including glyoxal (Selling et al 2012), glutaraldehyde (Sessa et al 2008), multivalent (e.g., citric) acids (Reddy et al 2009; Jiang and Yang 2011), and diisocyanates (Wu et al 2003; Yao et al 2007). These reagents typically lead to branching and/or cross-linking and lower elongation. Techniques have been developed to improve the elongation of zein, albeit with reduced tensile strength, but these techniques typically involve the use of plasticizers such as diols, triols, and long-chain carboxylic acids (Padua and Santosa 1999; Paramawati et al 2001; Lawton 2004; Gao et al 2006; Ghanbarzadeh et al 2007; Gillgren and Stading 2008; Gillgren et al 2009; Wu et al 2010). These additives suffer from moisture sensitivity, migration, and loss of plasticizer resulting from deposit formation or volatility. There is a need for developing additional methods to improve the elongation of zein. The Hofmeister series (abbreviated listing in Fig. 1) is a set of guidelines that can be used to determine if a particular salt will increase protein solubility in water (salt in, chaotropes, to the right in Fig. 1) or cause a protein to decrease in solubility in water (salt out, kosmotropes, to the left in Fig. 1) (Whitford 2005; Cho et al 2008; Rembert et al 2012). Inorganic salts are known to interact with proteins, altering the protein structure or changing their solubility characteristics (Baldwin 1996). Salts such as calcium chloride and sulfate have been shown to alter the physical properties of soy protein films through the cation by apparently forming calcium bridges between proteins (Park et al 2001). However, calcium cations and chloride anions are known to increase its solubility, whereas sulfate anions will decrease its solubility. Sodium lactate was shown to reduce tensile strength and increase elongation of whey protein films (Zinoviadou et al 2010); however, in that study only the one salt was evaluated, and both the sodium and lactate ions would be in the middle of the Hofmeister series in terms of their impact on a protein. Salts have also been used to modify the properties of zein. Alkali salts typically having long-chain fatty acid anions have been shown to alter the solubility of zein (James 1945). However, these long-chain fatty acids have been shown to have compatibility issues with zein (Santosa and Padua 2000). Other researchers have used salts such as sodium chloride, sodium sulfate, zinc sulfate, ammonium sulfate, ammonium phosphate, and so on to modify the properties of zein cross-linked with formaldehyde (Cline 1949; Croston and Evans 1949; Jenkins and Magee 1958). In these studies, the impact of salts was not detailed in a controlled fashion, nor were the singular impacts of the salts on zein properties determined. Ammonium thiocyanate (ATC) has been used in conjunction with p-toluenesulfonic acid, monochloroacetic acid, and formaldehyde to provide durable zein fibers (Huppert 1946). It was proposed in a patent by Huppert (1946) that the purpose of the ATC was to convert the globular zein protein into a fibrous structure; however, no data was provided to support this claim. A systematic study with salts that fit the series has not been undertaken. The effect that Hofmeister salts have on proteins has almost exclusively been studied in water. With zein not being soluble in water, it is not clear what effect these salts will have on zein. The salts in this series have been evaluated in conjunction with polymers other than proteins (Heuvingh et al 2005), albeit in water solution. Thiocyanate salts have been used in zein systems (Huppert 1946), and based on the Hofmeister series, the thiocya1 Plant Polymer Research Unit, National Center for Agricultural Utilization Research, USDA–Agricultural Research Service, 1815 N. University St., Peoria, IL 61604. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the USDA to the exclusion of others that may also be suitable. USDA is an equal opportunity provider and employer. 2 Corresponding author. Phone: (309) 681-6337. Fax: (309) 681-6691. E-mail: [email protected] 3 USDA–Agricultural Research Service CGAHR, 1515 College Ave., Manhattan,