Mechanical and Barrier Properties of Potato Protein Isolate-Based Films
Potato protein isolate (PPI) was studied as a source for bio-based polymer films. The objective of this study was the determination of the packaging-relevant properties, including the mechanical properties and barrier performance, of casted potato protein films. Furthermore, the films were analyzed for cross-linking properties depending on the plasticizer concentration, and compared with whey protein isolate (WPI)-based films. Swelling tests and water sorption isotherm measurements were
... ements were performed to determine the degree of swelling, the degree of cross-linking, and the cross-linking density using the Flory-Rehner approach. The effects of different plasticizer types and contents on compatibility with potato protein were studied. Glycerol was the most compatible plasticizer, as it was the only plasticizer providing flexible standalone films in the investigated concentration range after three weeks of storage. Results indicated that increasing glycerol content led to decreasing cross-linking, which correlated in an inversely proportional manner to the swelling behavior. A correlation between cross-linking and functional properties was also reflected in mechanical and barrier characterization. An increasing number of cross-links resulted in higher tensile strength and Young's modulus, whereas elongation was unexpectedly not affected. Similarly, barrier performance was significantly improved with increasing cross-linking. The overall superior functional properties of whey protein-based films were mainly ascribed to their higher percentage of cross-links. This was primarily attributed to a lower total cysteine content of PPI (1.6 g/16 g·N) compared to WPI (2.8 g/16 g·N), and the significant lower solubility of potato protein isolate in water at pH 7.0 (48.1%), which was half that of whey protein isolate (96%). Comparing on an identical glycerol level (66.7% (w/w protein)), the performance of potato protein isolate was about 80% that of whey protein isolate regarding cross-linking, as well as mechanical and barrier properties. of the industrial potato starch industry. It is released in large quantities, and is mainly used as animal feed or fertilizer, thus providing only low economic value  . Therefore, valorizing this by-product into a high value raw material could contribute to developing sustainable value chains of environmental and economical relevance. PFJ contains 30%-41% of proteins in total solids, and is therefore an interesting source for obtaining potato protein isolates (PPI)  . The conventional industrial technique for producing PPI is a combination of heat coagulation at temperatures of 75 • C to 120 • C, and acid precipitation at pH 3.5-5.5, followed by spray drying  . This treatment results in high protein yields, and a low price of 1.4 €/kg to 1.5 €/kg  . However, it also often leads to an extensive loss of functional properties due to protein denaturation  . In order to maintain the functionality of the proteins, other extraction techniques have been investigated, including metal salts (FeCl 3 , ZnCl 2 , MnCl 2 ) [7,10], ethanol , membrane separation (especially ultrafiltration) [8, 11] , ion-exchange chromatography  or expanded bed adsorption (EBA) chromatography [11, 13] . Compared to proteins from other cereal and vegetable sources, potato proteins are regarded to be of high nutritional quality, as they contain a balanced amino acid composition, and moreover, a high percentage of lysine (~8%)  , which is often deficient in these crops  . Along with their health-promoting qualities, potato proteins also exhibit good functional properties, such as high foaming  and emulsifying capacities  . Potato proteins are commonly classified into three groups: patatin (~40%), protease inhibitors (20%-30%), and other high molecular weight proteins (20%-30%)  . The patatin fraction comprises a family of glycoproteins existing as an 88-kDa dimer consisting of two 40 kDa to 43 kDa isoforms [12, 17] . In contrast, protease inhibitors are a distinctly heterogeneous group of proteins, with molecular weights ranging from 4.3 kDa to 25 kDa  and solubility through a wide pH range, whereas patatin has its solubility minimum at pH 4.5  . Regarding the preparation of film-forming solutions for coating applications, a pH >7 therefore provides a high solubility for all of the potato protein fractions, thus ensuring stable protein dispersion and good network formation as a consequence [2, 10] . For biopolymer processing, commonly, both wet-i.e., film casting or the coating of aqueous protein solutions-as well as dry processing technologies, including extrusion, compression, and injection molding, are employed . In both methodologies, the formation of protein films is based on chemical or thermal denaturation during processing. During denaturation, the proteins' molecular structure unfolds, thus resulting in the exposure of initially buried functional groups and sections. These are then capable of forming new intermolecular chain-to-chain interactions, such as disulphide and hydrogen bonds [9, 19] . With increasing protein-protein interactions, the mechanical strength and barrier properties of the polymer are enhanced  . However, in order to improve processability and durability, as well as alter the properties of the required final structure, plasticizers (e.g., glycerol) have to be applied as a formulation constituent  . Since various proteins have demonstrated appropriate oxygen and CO 2 barrier performances, protein-based materials are primarily of interest as gas barrier films in packaging systems. However, their mechanical properties are mostly inferior competitors with fossil-based polymers. The functional properties of films based on proteins from different sources have been analyzed in several studies. However, information about the use of potato protein as a source for bioplastic materials is rare. The objective of the present study was to investigate the suitability of potato protein isolate as a new source for bio-based films with regard to mechanical and barrier performance. Furthermore, this study offers a brief overview of the compatibility of potato protein with different commonly used plasticizers in terms of film-forming properties. It also aimed to determine the cross-linking parameters of the protein films, depending on the plasticizer concentration, and relate these parameters to structure-dependent properties, including oxygen and water vapor permeation, as well as mechanical properties. For this, cross-linking properties were investigated with swelling tests using the Flory-Rehner approach and water sorption isotherm measurements. Test samples were produced using a process developed for whey protein isolate-based films whose capability of a high oxygen barrier was shown in previous studies [21, 22] . To the authors' knowledge, Coatings 2018, 8, 58 3 of 16 no fundamental investigations of the barrier and mechanical properties of potato protein-based films have been carried out in previous studies.