Mechanical and Barrier Properties of Potato Protein Isolate-Based Films

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.

Effects of glycerol and sorbitol on optical, mechanical, and gas barrier properties of potato peel‐based films

“…After a few weeks, crystallization and phase separation occurred in all p-peel-based films plasticized with sorbitol. Sorbitol crystallization was also found in other biopolymer films, like maize starch- 27 potato protein isolate (PPI)-, 12 and potato starch-based films 11 For p-peel-based films, Kang and Min also reported wet and sticky films, when plasticized with more than 33% w/w glycerol and stored at 23 C and 52% RH. In contrast to these findings, p-peel-based films in this study, containing glycerol concentrations of 50% to 70%, were neither sticky nor wet when stored at 23 C and 50% RH.…”

“…30,31 The absence of increase or decrease of EMC with increasing plasticizer concentration could be explained by inability to incorporate/release further water molecules into/from the polymer matrix. 12 However, moisture contents of p-peel-based films plasticized with glycerol are much higher than those plasticized with sorbitol, confirming the higher ability of glycerol to attract and retain water. Comparing equivalent plasticizer contents, Schäfer et al found similar results for PPI-based films and Shaw et al for WPI-based films.…”

“…Comparing equivalent plasticizer contents, Schäfer et al found similar results for PPI-based films and Shaw et al for WPI-based films. 12,32 The higher water binding capability and plasticizing effect of glycerol could be explained by its low molecular weight. Glycerol has a groups.…”

“…The degree of proteincrosslinking also determines the mechanical properties of the film. 12 Rommi et al demonstrated that starch-free p-peel, which is rich in pectin and cellulose, can be also used for the development of films with similar properties to those of starch rich p-peel-based films. 7 In p-peel-based films, a combination of the film-forming mechanisms of those different materials occur.…”

“…), whey protein isolate (WPI), potato protein isolate, gelatin, or alginate. 12,[15][16][17][18][19][20][21] To gain further fundamental knowledge of p-peel-based films, the influence of high glycerol and sorbitol concentrations on characteristic film properties like water vapor and oxygen barrier, as well as mechanical properties of p-peel-based films, were examined in this study. Furthermore, equilibrium moisture content (ECM), color, light transmittance, and surface energy of p-peel-based films were analyzed.…”