Abstract
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 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.
Highlights
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]
This study offers a brief overview of the compatibility of potato protein with different commonly used plasticizers in terms of film-forming properties
As a prerequisite for the formation of flexible coherent films, plasticizers must be compatible with the polymer and permanent in the polymer matrix to achieve the requested film properties [35]
Summary
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]. Various proteins from vegetal or animal sources have been investigated as substitutes for synthetic petroleum-based polymers. Thereby, the high barrier properties against the oxygen and CO2 of protein-based coatings were of particular interest. 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.
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