Effects of different treatment methods on properties of potato starch-lauric acid complex and potato starch-based films

Highlights

• The DSH, UT and PD methods could increase the amylose content in gelatinized starch.

• PD method would significantly contribute to amylose-lipid complex formation.

• Debranching treatment prior to film-making could improve the properties of the films.

Abstract

The effects of different treatment methods on the physicochemical properties of potato starch (PS) and PS-based films were studied. The complexing indices of PS-lauric acid (LA) complexes followed the order: pullulanase debranching (PD) > ultrasound treatment (UT) > dimethyl sulfoxide heating (DSH) > Control. Light microscopy showed that PS-LA complexes exhibited irregularly shaped fragments. X-ray diffraction indicated that the diffraction intensity of the PD sample was stronger than that of the other samples. The melting enthalpies (△H) of the DSH, UT and PD samples were higher than that of the Control. PD sample had the lowest enzymatic-hydrolysis rate among all of the tested samples. PS-LA composite films showed higher tensile strength, lower elongation at break, and lower moisture permeability than native starch-based film, and the films prepared by PD method had the highest tensile strength and lowest water vapour permeability among all of the tested films.

Introduction

Eco-environment protection is a growing concern. Replacement of plastic products with biopolymer materials could reduce environmental pollution, which has generated an increased interest in developing biopolymer films in the packaging industry [1,2]. Starch, which is one of the most important natural biopolymer materials, consists of two polymeric substances, i.e., 20–30% of linear amylose and 70–80% of branched amylopectin [3].

Amylose molecules are known to form helical complexes with various molecules, aromatic compounds, emulsifiers and lipids, which are called V-type amylose complexes [[4], [5], [6]]. Usually the guest molecules are embedded in the helical interior of the starch by hydrogen bonding and occasionally, the guest molecules are trapped between the helices [7]. Fatty acids are embedded in the amylose hydrophobic cavity to form a single helical complex, which is stabilized through hydrophobic interactions [8,9]. In recent papers, lauric acid has been incorporated in starch for preparing the amylose-lipid inclusion complexes [10,11]. Lauric acid was more strongly bound with amylose than other fatty acids with longer carbon chain length [12]. Meng et al. [13] found that the corn starch with long carbon chain fatty acids had lower complexing index values than corn starch with lauric acid due to their poor dispersivity in gelatinized starch. According to Kawai, et al. [14], the melting enthalpies (the amount of inclusion complexes) decreased with an increase in the number of carbon atoms in the saturated fatty acids containing lauric acid C12:0, myristic acid C14:0, palmitic acid C16:0, stearic acid C18:0.

In previous studies on amylose-lipid complex formation, amylose (chain length and content) [[14], [15], [16], [17]], lipid characteristics (lipid type, unsaturation and chain length) [[17], [18], [19]], and complexation temperature [20] have been extensively investigated. The amylose content is an important factor that influences the formation of complexes. Garcia, et al. [15] reported that the amylose content had a positive relationship with the quantity of amylose-lipid inclusion complexes, specifically long amylose chains were beneficial for forming more stable V-type complexes. Exarhopoulos & Raphaelides [21] suggested that the characteristics and degree of crystallinity of inclusion complexes were dependent on the amylose content, fatty acid chain length and different heating methods.

In recent years, many studies have been published reporting the effect of the amylose content in different types of starch on inclusion complex formation. However, studies using different treatment methods to increase the amylose content in starch and to evaluate the effect of different treatment methods on V-type complexes formation and the properties of potato starch (PS)-lauric acid (LA) composite films have not been performed. Dimethyl sulfoxide (DMSO) is a polar solvent that can dissolve starch and release amylose, which in turn, interacts with fatty acids and other ligands to generate inclusion complexes [16,22]. Lalush et al. [23], investigating the effect of water/DMSO and KOH/HCL complexation methods on inclusion complexes formation, suggested that the amylose-conjugated linoleic acid complexes prepared by water/DMSO method showed superior protection to conjugated linoleic acid oxidation. Ultrasonication is considered to be a new treatment for the preparation of inclusion complexes. Previous literature reported that ultrasonic treatment preferentially degraded starch amorphous region and intruded into amylose more easily than amylopectin [24]. According to Liu, Wang, Kang, Cui, and Yu [25], ultrasonication disintegrates starch granules to release more amylose molecules and further facilitates the formation of V-type inclusion complex. In contrast to isoamylases, pullulanase decomposes the smallest unit of starch branched chains to increase the amylose content in the system [26]. Wongprayoon et al. [27] and Zhang et al. [11] suggested that pullulanase debranching treatment promotes the release of linear starch chains, which increasing the contact opportunity between amylose and fatty acids in the system.

The aims of this study were to systematically examine the effects of different treatment methods on the formation of PS-LA complexes and investigate the mechanical and moisture barrier properties of PS-based film.