Based on the growing environmental concerns of industry and consumers alike, biodegradable and bio-based packaging systems have received increasing interest in recent decades [1
]. Among other biopolymers, various proteins from vegetal or animal sources have been investigated as substitutes for synthetic petroleum-based polymers. In particular, soy, wheat gluten, corn zein, and whey have been commonly studied [2
]. Thereby, the high barrier properties against the oxygen and CO2
of protein-based coatings were of particular interest. However, their low moisture barrier performances and relatively poor mechanical characteristics as compared to fossil-based materials limit their use in packaging applications [2
]. In terms of environmental benefits, the use of co-products from industrial processes is of interest for the development of bio-based materials. Potato fruit juice (PFJ), a by-product 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 [7
]. 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) [8
]. 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 [9
]. This treatment results in high protein yields, and a low price of 1.4 €/kg to 1.5 €/kg [9
]. However, it also often leads to an extensive loss of functional properties due to protein denaturation [7
]. In order to maintain the functionality of the proteins, other extraction techniques have been investigated, including metal salts (FeCl3
], ethanol [7
], membrane separation (especially ultrafiltration) [8
], ion-exchange chromatography [12
] or expanded bed adsorption (EBA) chromatography [11
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%) [14
], which is often deficient in these crops [7
]. Along with their health-promoting qualities, potato proteins also exhibit good functional properties, such as high foaming [15
] and emulsifying capacities [16
Potato proteins are commonly classified into three groups: patatin (~40%), protease inhibitors (20%–30%), and other high molecular weight proteins (20%–30%) [17
]. The patatin fraction comprises a family of glycoproteins existing as an 88-kDa dimer consisting of two 40 kDa to 43 kDa isoforms [12
]. In contrast, protease inhibitors are a distinctly heterogeneous group of proteins, with molecular weights ranging from 4.3 kDa to 25 kDa [17
] and solubility through a wide pH range, whereas patatin has its solubility minimum at pH 4.5 [18
]. 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
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 [2
]. 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
]. With increasing protein–protein interactions, the mechanical strength and barrier properties of the polymer are enhanced [20
]. 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 [2
]. Since various proteins have demonstrated appropriate oxygen and CO2
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
]. To the authors’ knowledge, no fundamental investigations of the barrier and mechanical properties of potato protein-based films have been carried out in previous studies.
Potato protein isolate-based biopolymers, plasticized with different polyols, were prepared. After investigating the compatibility of various plasticizers, glycerol was the only plasticizer that was able to overcome brittleness and thus produce flexible standalone films. The results of swelling tests indicated a relation between the ability of glycerol to reduce intermolecular bonding and the amount of water that is accommodated during swelling. The degree of cross-linking and cross-linking density showed an inversely proportional correlation to the degree of swelling. Comparing potato protein and whey protein-based films on the same glycerol level (66.7% (w/w protein)), the cross-linking density of potato protein is approximately 80% that of whey protein. 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), as well as the significantly lower solubility of potato protein isolate in water at pH 7.0 (48.1%), which was half the value of whey protein isolate (96%).
An increasing number of cross-links might play a central role for increasing tensile strength and Young’s modulus, whereas elongation at break was unexpectedly not affected; this was a different behavior as compared to previous studies on protein-based polymers. Barrier performance also was significantly improved with increasing cross-linking, with the whey protein standard showing a considerably higher barrier against oxygen. Comparing on an identical glycerol level (66.7% (w/w protein)), the performance of potato protein isolate-based films was about 80% that of whey protein isolate films regarding cross-linking, mechanical, and barrier properties, suggesting a correlation between the cross-linking density and techno-functional properties.
In summary, the determination of barrier and mechanical properties considering quantitative analysis of cross-linking in PPI-based films was carried out for the first time. These results can further be used to evaluate the structure of the protein network in potato protein films, and also allow a comparison with other packaging materials. Therefore, the deployment of the industrial by-product potato protein and the development of PPI-based films and coatings contribute to expanding the field of biodegradable and sustainable packaging materials as an alternative to fossil-based systems.