Fully Biodegradable Edible Packaging Foils on the Basis of Potato Starch–Lipid–Protein Ternary Complexes

Abstract

Fully biodegradable foils were prepared from potato starch, egg albumin, and either stearic or oleic acid. Foils prepared with oleic acid have higher tensile strength, relative elongation, thermal stability, and a more uniform macrostructure. Foils produced with stearic acid were characterized by a higher index of crystallinity than foils made with oleic acid. Functional properties of the foils can be modulated involving a sequence of blending of their components. The simultaneous blending of starch (10 weight parts of 5% aq. gel), albumin (1 weight part of liquid composed of 1 g of albumin in 7 mL of water), and stearic acid (5 weight parts of powder) provided the foil with the highest tensile strength (64.91 MPa/mm). Independently of the method of preparation, foils were white with a greenish-yellow shade. Analysis of the ATR-FTIR spectra showed that the macrostructure of the foils is built involving interactions between all three components.

1. Introduction

Apart from political and economic factors, the exploitation of petro- and carbo-chemical resources evokes a task of ecologists accompanying environmental pollution, particularly in coal processing. These factors have induced a growing concern about the utilization of natural, versatile, renewable resources such as polysaccharides, proteins, and lipids.

Ancient people utilized biodegradable materials such as wood, ladder and animal entrails, cane, straw, wool, and cotton. Papyrus, parchment, and paper were already known well before Christ ages. Goodyear’s technology of rubber production started a run for caoutchouc known already in the Middle Ages. Crosslinking caoutchouc (vulcanization) with sulfur provided either rubber or ebonite. Their biodegradability was poor. On the XIX/XX century break, materials of formaldehyde-crosslinked fiber became available. Then biodegradable plastics produced from milk acid casein (galalith) and products of nitrocellulose–cellophane and celluloid met a high appreciation [1,2]. The applicability and usefulness of biodegradable plastics depend on their strength and thermal resistance, barrier properties, rate of decomposition, and decomposition products. Because none of the materials listed above provided anticipated properties and the cost of their manufacture was relatively high, in the 1980s, the introduction of disposable products was attempted. Starch-made shapings and extrudates for making biodegradable tableware, casts, and packages are available [3,4,5]. Since their mechanical strength and stability were too low, the starch base was supplemented with some natural [6] and synthetic [7] biodegradable fiber, providing slowly biodegradable polymers of the Mater-Bi type [8,9]. Biodegradable starch-free polymers contained poly(lactic acid) and polylactides [10,11,12] and other poly[hydroxyalkanoic acids] [13,14], their low- and high-molecular copolymers [15,16], polyester–poly(caprolactone) [17], and polyamide-poly(caprolactam) called nylon 6 [18]. So-called green polymers have also been introduced. They are composites manufactured of, for instance, starch (up to 40%) and vinyl polymers such as polyethylene. Such materials satisfy the aesthetic but not the ecological demands of consumers. Starch is fairly readily decomposed into carbon dioxide and water, leaving synthetic polymer non-degraded and dispersed, invisible upon first sight of the residue [19]. There are known synthetic polymers from polymerized vinyl monomers, which fully decompose to CO₂ and H₂O (d2w polymers). They contain certain catalysts. Their production is expensive and highly energy-consuming [20]. Starch–lipid complexes have been studied, and their effects on the functional properties and nutritional value of starch have been characterized [21,22]. The study showed that the formation of starch–lipid complexes in food systems reduces the swelling power and solubility of starch, retards starch gelatinization and retrogradation, and slows down the rate of its enzymatic digestion [23]. Starch–lipid complexes are of particular interest in microscale technology, where they are used for the encapsulation of protecting bioactive 𝜔-3 and 𝜔-6 unsaturated fatty acids (FAs) and isoflavone genistein against autoxidation and degradation and stimulating their efficacy [24]. The complexation of starch to lipids has been intensively studied over the past 50 years, whereas the study of starch–lipid–protein complexes is a relatively new field of study [25,26,27]. In ternary systems of simple molecules, two of them usually aggregate with one another involving efficient intermolecular interactions, whereas the third molecule either stays separate from that aggregate or utilizes simple physical interactions with that pair [28]. This report presents foils prepared from potato starch, which is an anionic polysaccharide [29,30], lipid acids, and egg albumin. This research follows UE directives for applications of versatile renewable resources [31]. Because all components are accounted for as agricultural products, the results can be in concert with the agricultural policy of Poland and other countries with advanced agriculture. The development of agriculture in those countries is limited by European Union restrictions on the level of agricultural production for nutritional purposes. These limits do not work when production for non-nutritional purposes is considered, for instance, for energy production. Thus, the production of biodegradable materials of fully agricultural origin is in line with the attempted revitalization of agriculture in the European Union [32]. These arguments, as well as the presentation of a new generation of fully biodegradable ternary component plastics, constitute the objectives of this paper. In other words, the presented studies check whether foils of useful functional properties made from common agricultural products would be available. The presented procedures take into account the utilization of the cheapest, most common agricultural products and possibly the lowest-cost generation of the foils.

The ternary complexes under study were composed exclusively of natural products of agricultural origin, that is, potato starch, selected lipid acids, and albumin. The physicochemical properties of those ternary complexes provide evidence that all three components are involved in mutual interactions to build the investigated structures. The nature of such interactions was recently exhaustively reviewed [27]. The results of those interactions in a liquid phase are chiefly dependent on the thermodynamic compatibility of interacting components. In case of limited compatibility, two components can form a liquid two-phase water-in-water emulsion. When the interactions of both components involve electrostatic forces in a two-phase system, a complex coacervation takes place. Both components, which either do not interact or form soluble complexes, provide homogeneous stable solutions. Potato starch (P) in an aqueous solution forms a paste. Because amylopectin is randomly esterified with phosphoric acid, P takes on an anionic character [29,30]. Albumins (A) are constituted by a group of proteins residing in almost all fluids and tissues of plants and animals. They are low-molecular-weight, readily water-soluble, heat-coagulated substances. In their composition, aspartic and glutamic acids, leucine, and isoleucine are the most essential components [33]. Egg white contains approximately 90% water. Its remaining 10% content contains around 149 proteins, trace minerals, fats, vitamins, and glucose [34]. In this study, lipid components of the ternary complexes are represented by stearic acid (S) with the C17 saturated carbon chain, and oleic acid (O), the ω-9 monounsaturated acid with C17 carbon chain of the cis-conformation. S is a solid of m.p. 69.3 °C, practically insoluble in water. Hence, under experimental conditions, it remained non-dissociated [35] O is liquid at room temperature and is immiscible with water. It decomposes at 360 °C under atmospheric pressure [36].

2. Results and Discussion

2.1. Physical Properties of Foils

2.1.1. Water Content, Solubility, and Swelling Degree of Foils

All foils had a similar water content (

Table 1. Water content, solubility, and swelling degree of foils.

Foil *	Water Content [%]	Solubility [%]	Swelling Degree [%]
(P+A+S)	8.75 ± 0.06 c	53.26 ± 0.09 d	179.25 ± 0.62 b
(P+A)+S	9.19 ± 0.03 a	54.28 ± 0.11 c	119.99 ± 0.98 d
(S+A)+P	9.07 ± 0.05 b	61.59 ± 0.12 a	188.97 ± 0.75 a
(S+P)+A	8.53 ± 0.07 d	57.06 ± 0.17 b	160.81 ± 1.12 c
(P+A+O)	8.62 ± 0.06 d	41.26 ± 0.08 g	81.15 ± 0.68 h
(O+A)+P	8.78 ± 0.03 c	54.03 ± 0.16 c	88.85 ± 1.17 g
(P+A)+O	9.27 ± 0.07 a	47.06 ± 0.13 e	102.22 ± 1.06 e
(O +P)+A	9.06 ± 0.03 b	45.02 ± 0.14 f	98.01 ± 0.85 f

^a–h—different letters in the same column indicate significant differences (Fisher test. p ≤ 0.05). * P—potato starch, A—albumin, S—stearic acid, O—oleic acid.

). The foils prepared with stearic acid (S) exhibited a higher swelling degree and solubility than foils prepared with oleic acid (O).

The (S+A)+P and (P+A+O) foils were the most and least water soluble, respectively. Galus et al. [37] proved that the incorporation of oil into the whey protein structure can reduce the swelling index. Both almond oil and walnut oil affected the water retention of whey protein films with their contribution, but it was the former oil that showed a statistically significant impact. The authors explained this behavior of the hydrophobic character of lipids, which causes an intermolecular interaction between protein matrix and oils, resulted in low swelling properties. A similar tendency was noticed for other hydrocolloid films modified with hydrophobic substances [38]. In our work, the effect of the type of lipid on the solubility and swelling degree of the obtained films was also observed. Films containing oleic acid (O) showed higher water resistance compared to films containing stearic acid (S). The differences may be due to the different strengths of interactions between the polymers in the system. Films involving stearic acid likely blocked starch and albumin from water retention and solubility to a lesser extent than oleic acid. In turn, Faraye et al. [39], upon examining the effect of various fatty acids on the parameters of maize starch films, found that the presence of lauric and caprylic acids increased the solubility of the biopolymer films, which was likely influenced by the degree of solubility of the fatty acids in water. On the other hand, no significant difference was seen in the water solubility of the biopolymer films prepared with palmitic, myristic, capric, and caproic acids [39]. According to Kester and Fennem [40], the advantage of starch films prepared with the addition of lipids is the blocking of moisture transport due to the high hydrophobicity of lipids, as well as the reduction in abrasion of the food surface during handling and transport. As claimed by Gontard et al. [41], the retention and barrier abilities of biopolymer films enriched with lipids depend on the hydrophobic/hydrophilic ratio provided by their components, as well as the polarity, unsaturation number, and degree of branching presented by the polymer chains of the matrix used.

Solubility and the degree of swelling are important foil parameters that affect its further use in the industry. Natural and biodegradable raw materials are challenged in the design of films, as their high solubility makes them suitable for use under specific conditions. One example is the use of such composites in biomedical applications as coatings or carriers for medicinal substances applied orally or through the skin. In such cases, the rapid or controlled release of the drug substance depends on the ability of the coating to dissolve in different mediums [42,43].

2.1.2. Color Parameters of Foils

Table 2. Color parameters of foils.

Foil	L*(D65)	a*(D65)	b*(D65)	C*	h*
(P+A+S)	86.12 ± 0.23 d	−0.61 ± 0.04 a	7.75 ± 0.43 d	7.77 ± 0.43 d	94.52 ± 0.32 d
(P+A)+S	89.19 ± 0.67 a	−0.70 ± 0.04 b	8.95 ± 0.43 b	8.97 ± 0.44 b	94.50 ± 0.08 d
(S+A)+P	87.12 ± 0.16 c	−0.64 ± 0.05 a	8.19 ± 0.43 cd	8.22 ± 0.43 cd	94.50 ± 0.16 d
(S+P)+A	87.93 ± 0.23 b	−0.93 ± 0.05 c	10.21 ± 0.42 a	10.25 ± 0.42 a	95.23 ± 0.23 c
(P+A+O)	83.12 ± 0.21 f	−0.88 ± 0.02 c	8.72 ± 0.18 b	8.76 ± 0.18 b	95.76 ± 0.21 b
(O+A)+P	86.89 ± 0.26 c	−0.73 ± 0.02 b	8.64 ± 0.33 bc	8.67 ± 0.32 bc	94.86 ± 0.27 c,d
(P+A)+O	88.94 ± 0.19 a	−1.00 ± 0.04 d	8.80 ± 0.36 b	8.86 ± 0.36 b	96.51 ± 0.32 a
(O+P)+A	84.63 ± 0.42 e	−0.60 ± 0.10 a	7.73 ± 0.32 d	7.76 ± 0.31 d	94.49 ± 0.88 d

^a–f—different letters in the same column indicate significant differences (Fisher test. p ≤ 0.05).

shows the results of measuring the color of the film, which is determined by three components. The component L* describes the brightness (luminance) of the color in the range from 0 to 100, where 100 represents the brightest color. The a* component represents the proportion of green or red in the color being analyzed. The b* component, on the other hand, represents the proportion of blue or yellow [44,45].

The tested films were characterized by high brightness, as the parameter L* was in the range of 83.12–89.19 and was brighter than other biodegradable films reported in the literature—films based on chia seeds mucilage [46], amaranth protein–lipid film [47], sodium alginate, and chitosan reinforced with graphene oxide and carbon nanotubes films [43]. Interestingly, the obtained films showed a darker surface than chitosan-alginate films with the addition of turmeric extract [43], starch/chitosan films modified by graphene oxide [42] or quantum dots [48], and wheat starch and whey-protein isolate-based films [49].

All of the studied foils were characterized by the parameter a*< 0, thus indicating a clear dominance of the red color. On the other hand, in the case of parameter b, the proportion of yellow dominated over blue (b* > 0). The color of the packaging plays an important role in protecting the packaged product. This is especially important when packaging labile substances/materials—sensitive to light. In this case, the film covering the product has a protective function [43].

The values of the C* parameter of the tested films were at similar levels. Only in the case of the (S+P)+A sample was a slightly higher value noted, that is, the color was perceived as brighter or more intense with the observer adapted to white [50].

The hue angle (h*) allows one to determine the difference in a given color with reference to a grey color of the same lightness. An angle of 0°or 360°reflects the red hue, while angles of 90, 180°, and 270° represent the shades of yellow, green, and blue, respectively [51]. The values of all the films tested were at similar levels and ranged between 94.49 and 96.51°, representing a yellow hue.

2.1.3. Thickness and Mechanical Properties of Foils

The method of preparation of the foils, that is, the sequence of blending their components, appeared crucial for their mechanical properties, that is, tensile strength and elongation (

Table 3. Thickness and mechanical parameters of freshly prepared foils.

Foil *	Thickness [mm]	Tensile Strength [MPa]	Relative Elongation [%]
(P+A+S)	0.438 ± 0.003 e	28.43 ± 0.05 c	13.14 ± 0.01 e
(P+A)+S	0.483 ± 0.001d	16.80 ± 0.52 e	13.86 ± 0.05 d
(P+S)+A	0.575 ± 0.001 b	12.61 ± 0.13 f	8.29 ± 0.02 g
(A+S)+P	0.643 ± 0.003 a	10.25 ± 0.10 g	5.93 ± 0.02 h
(P+A+O)	0.408 ± 0.004 f	6.84 ± 0.04 h	8.73 ± 0.01 f
(P+O)+A	0.520 ± 0.003 c	29.61 ± 0.12 b	30.67 ± 0.02 b
(P+A)+O	0.570 ± 0.020 b	25.58 ± 0.37 d	15.61 ± 0.04 c
(A+O)+P	0.533 ± 0.050 c	31.70 ± 0.10 a	39.86 ± 0.02 a

* P—potato starch, A—albumin, S—stearic acid, O—oleic acid, ^a–h—different letters in the same column indicate significant differences (Fisher test. p ≤ 0.05).

). Tensile strength (TS) is described in the literature as the force needed to break a length of film of a certain size. The higher the tensile strength of the material being tested, the greater its usefulness [44].

In terms of the tensile strength recalculated per the foil thickness, the foil from P simultaneously blended with A and S [(P+A+S)] offered the highest tensile strength. In order to prepare foil of the highest tensile strength using O instead of S, O should be first combined with A following the subsequent addition of P [(A+O)+P]. The tensile strength of that foil was slightly lower than that of the (P+A+S) foil. Nevertheless, the TS values of this and most of the foils obtained were higher than those of low-density polyethylene (LDPE) (16.5 MPa) [52]. The (P+O)+A and (A+O)+P sequence of blending offered foil with remarkable elongation, higher than that of polyester (PE) (18%) and polyvinylidene chloride (PVDC) (19%) [52]. Generally, foils prepared with O offered higher elongation than those prepared with S. Biopolymer films have low extensibility, but by introducing glycerol into the system, it is possible to improve elongation. This is because plasticizers modify the functional properties of biopolymer films by reducing intermolecular forces and increasing the mobility of polymer chains [53]. Ternary systems that include starch, fatty acids, and protein form a complex where the hydrophobic tail of the fatty acids interacts with the starch to form starch–fatty acid inclusion complexes, and the negatively charged carboxyl group of the fatty acid interacts with the protein [54,55]. Depending on how the individual components are combined with each other, the strength of the interactions between the polymers in such ternary systems can vary. As a result, this affects differences in the mechanical properties of the films.

2.2. Thermogravimetry

Thermogravimetric studies (TG) provided data on the thermal stability of those complexes (

Table 4. TG/DTG characteristics of foils.

Foil	Weight Loss
	Step I			Step II
	Temp. of Beginning [°C]	[mg]	[%]	Temp. of Beginning [°C]	[mg]	[%]
(P+A)+S	145	3.3697	52.2310	277	1.4805	22.9899
(P+S)+A	108	1.2479	54.4928	280	0.5540	24.1908
(S+A)+P	104	3.8324	53.9017	276	1.8304	25.7436
(P+A+S)	100	1.7385	47.6299	245	1.0286	28.1807
(P+A)+O	120	1.7509	47.9691	258	0.9526	26.0982
(P+O)+A	165	3.8784	47.0680	281	2.2496	27.3008
(O+A)+P	115	2.7618	38.5148	236	2.5306	35.2943
(P+O+A)	110	0.4886	29.9743	220	0.6530	40.0590

).

Clearly, the thermal stability of the foils depended on the method of their preparation. The (P+O)+A and (P+A)+S foils were the most temperature resistant. Likely, the interactions of P with O and P with A in binary systems were relatively weak, and the admixing of the third partner considerably changed the macrostructure of those complexes.

Decomposition of foils proceeded in two steps In the first step, (P+A)+S, (P+S)A, and (S+A)+P foils lost over 50% of their original weight, (P+A+S), (P+A)+O, and (P+O)+A foils lost 47 to 48% of their initial weight, and (O+A)+P and (P+O+A) foils lost hardly 38.5 and 30% of their weights, respectively. These differences had no relation to the water content of those foils, thus they depended on the structure of matrices in which weak intermolecular was preferred. The second step decomposition of foils took place between 220 and 280 °C. The (P+O+A) foil decomposed at 220 °C and lost 40% of its weight (Figure 1 and Figure 2).

2.3. Scanning Electron Microscopy (SEM)

Figure 3 demonstrates SEM images of foils with O and S. They all present smooth surfaces without breaks and separated particular components. The received films differ on their surface depending on the order of mixing the ingredients. Mixing all the ingredients at once results in obtaining a film with the most uniform surface.

Scanning Electron Microscopy (SEM) showed that independently of the composition and sequence of blending their components, every foil contained two fractions, although their proportions varied from one foil to another (Figure 3). Generally, foils prepared with O were more uniform because the proportion of the first low-temperature fraction was lower than that in the foils with A. Based on that criterion, among O-A-P foils, (O+A+P) and (P+A+O were the most uniform. For the same reason, among P-A-S foils, the (S+A)+P foil offers the highest uniformity. A comparison of these results with the results presented in Table 1 showing the functional properties of those foils revealed that the functional properties not only depended on their uniformity but also on non-covalent intermolecular interactions in the major fraction of the foils.

2.4. X-Ray Diffraction

Powder X-ray diffractometric (XRD) studies delivered information on the internal structure of the foils.

The XRD pattern for the P+S+A sample (Figure 4) contains diffraction peaks (2θ) at 5.59°, 15.12°, 17.21°, 19.54°, 22.40°, 23.90°, and 26.27°, which correspond to the crystalline structure of starch [56].

For the original, non-gelatinized P sample, the crystallinity index was 0.35, while for other samples with P gelatinized prior to complexation, it significantly decreased (Figure 3). Evidently, in complexes, P retained its amorphous character. The values of Xc for all samples were as follows in descending order of the crystallinity index (Xc): (P+A+S) 0.28, (P+S)+A 0.15, (P+A)+S 0.14, (S+A) +P 0.10, (P+A+O) 0.15, (P+O)+A and (O+A)+P 0.11, (P+A)+O 0.09.

The study of the P-S-A and P-O-A ternary component systems showed that as a result of the polymerization process, starch forms a polymer matrix. This is evidenced by a sharp decrease in the index of crystallinity. Simultaneous combining all three components provided foils of the highest crystallinity index in their groups, i.e., P+S+A and P+O+A. The components are evenly distributed in the polymer matrix. The X-ray pattern of all complexes, regardless of their composition and method of preparation, closely resembled one another. However, O in its complexes provided slightly lower values of the crystallinity index than S did. This could be associated with the certain rigidity of the carbon chain of the O molecule caused by the double bond and the resulting cis-conformation influencing the possibility of the formation of helical complexes of amylose with that acid as the guest molecule.

The sequence of decreasing intensity of the diffractograms in the series of foils with either S or P did not follow the sequence of corresponding tensile/thickness and elongation.

2.5. Fourier Transformation Infrared Spectroscopy Attenuated Total Reflectance (FTIR-ATR)

Analysis of the results of Fourier Transformation Infrared Spectroscopy Attenuated Total Reflectance (FTIR-ATR) measurements should deliver information on the nature of interactions between components of ternary complexes under study. Figure 5 presents relevant spectra of particular components of the complexes.

In the spectrum of S (Figure 5), two sharp bands at 2916 and 2850 cm⁻¹ were assigned to the asymmetric and symmetric CH₂ stretching modes, respectively, and the absorption peak around 1705 cm⁻¹ belongs to the carboxylic acid C=O stretching vibrations. The O–H in-plane and out-of-plane bands appeared at 1467 and 936 cm⁻¹, respectively. The bands at 1300 and 724 cm⁻¹ could be assigned to the CH₂ bending and wagging vibrations, respectively.

In the FTIR spectrum of the pure O (Figure 5), the band at 1409 cm⁻¹ corresponded to the CH₃ umbrella mode. The intense peak at 1706 cm⁻¹ belongs to the C=O stretching modes, and the band at 1284 cm⁻¹ exhibited the presence of the C–O stretching modes. The O–H in-plane and out-of-plane bands appeared at 1464 and 936 cm⁻¹, respectively. The weak band at 3006 cm⁻¹ points to the cis-configuration of that acid.

In the FTIR spectrum of pulverized egg albumin (Figure 5), characteristic peaks were observed at 1653 cm⁻¹ (–C=O stretching) due to amide I band and 1539 cm⁻¹ (C-N stretch with N-H bending mode) due to amide II band.

Based on literature data [57,58,59,60] in the spectrum of P (Figure 5), a band at 3321 cm⁻¹ is attributed to the stretching vibrations of the hydroxyl group (O–H), a reduced band at 2924 cm⁻¹ is assigned to the C–H stretching vibrations, and the peaks at 1021 and 1151 cm⁻¹ belong to stretching vibrations of C–O–C. The 1464 cm⁻¹ band is assigned to –CH₂ bending and –COO stretch, 1084 cm⁻¹ and 1022 cm⁻¹ are assigned to the crystalline and amorphous regions of starch, respectively, and 1162 cm⁻¹ is assigned to vibrations of the glucosidic C-O-C bond and the whole glucose ring, which can present different modes of vibrations and bending conformations. The bond at 928 cm⁻¹ was assigned to the skeletal mode vibrations of α 1 → 4 skeletal glycoside bonds, 780 cm⁻¹ was assigned to the C-C stretch, and 575 cm⁻¹ was assigned to the skeletal modes of the pyranose ring.

FTIR-ATR spectra of binary component complexes are given in Figure 6.

Generally, the spectra of the complexes contain peaks of both components. Their position is shifted with respect to those in the spectra of single particular components. These shifts resulted from the involvement of interactions of the hydrogen bond type, van der Waals, and dispersion forces between those components.

In the spectra of P with S and P with O, the band belonging to the COOH group could be seen, and it was evidence that there are eventually weak interactions of the COOH group of those lipid acids with starch. That band also remained in the spectrum of S with A and ceased in the spectrum of O with A, pointing to the hydrogen bond-type interaction of the COOH group with the basic center of A. That effect could result from the formation of a specific emulsion with liquid O, whereas the formation of the corresponding emulsion with solid S was obscured.

The spectra of those ternary complexes (Figure 7) fairly closely resembled one another. In the spectra with S, the band of the COOH group above 1700 cm⁻¹ resided in a residual form, whereas in the corresponding spectra with A, it completely ceased except for the spectrum of (P+O+A). That pointed to an equilibrium established in that ternary system.

2.6. Staling of Foils

The functional properties of foils only slightly change over time. After 3 months of their storage, regardless of the composition and method of preparation, the thickness of the foils decreased by 9.1 to 9.7%, tensile strength decreased by 8.7–9.7%, and relative elongation declined by 7.1 to 8.7%. Within 3 months, the water content, solubility, and swelling usually declined by 9.3 to 9.6%, and only for the (P+A+O) did the swelling exceed 10% (10.1%) (

Table 5. Thickness and mechanical parameters of foils after 3 months *.

Foil **	Thickness [mm]	Tensile Strength [MPa]	Relative Elongation [%]
(P+A+S)	0.407 ± 0.002 (−9.3%) f	27.23 ± 0.04 (−9.2%) c	11.20 ± 0.01 (−8.7) d
(P+A)+S	0.452 ± 0.001 (−9.35%) e	15.43± 0.13 (−9.2%) e	10.74 ± 0.05 (−7.7%) e
(P+S)+A	0.531 ± 0.001 (−9.2%) b	11.13 ± 0.11 (−8.6%) f	6.94 ± 0.02 (−8.4%) f
(A+S)+P	0.587 ± 0.001 (−9.1%) a	9.59 ± 0.08 (−9.4%) g	4.63 ± 0.02 (- 7.8%) h
(P+A+O)	0.369 ± 0.002 (−9.0%) g	6.42 ± 0.03 (−9.3%) h	6.23 ± 0.01 (−7.1%) g
(P+O)+A	0.492 ± 0.004 (−9.5%) d	28.69 ± 0.05 (−9.7%) b	24.74 ± 0.02 (- 8.1%) b
(P+A)+O	0.528 ± 0.007 (−9.2%) b	24.82 ± 0.05 (−9.8) d	12.32 ± 0.04 (−7.9%) c
(A+O)+P	0.518 ± 0.009 (−9.7%) c	30.82 ± 0.06 (−9.7) a	33.42 ± 0.02 (−8.4%) a

* Percentual change in the value of given parameter with respect to those measured for freshly prepared foils. ** P—potato starch, A—albumin, S—stearic acid, O—oleic acid. ^a–h—different letters in the same column indicate significant differences (Fisher test. p ≤ 0.05).

and

Table 6. Water content, solubility, and swelling degree of the foils after 3 months *,**.

Foil	Water Content [%]	Solubility [%]	Swelling Degree [%]
(P+A+S)	8.32 ± 0.03 (−9.5%) c,d	49.52 ± 0.06 (−9.3%) e	173.11 ± 0.32 (−9.7%) b
(P+A)+S	8.54 ± 0.05 (−9.3%) b	51.11 ± 0.09 (−9.4%) d	117.45 ± 0.68 (−9.8%) d
(S+A)+P	8.31 ± 0.03 (−9.2%) c,d	58.94 ± 0.05 (−9.6%) a	183.36 ± 0.37 (−9.7%) a
(S+P)+A	8.25 ± 0.02 (−9.6%) c,d,e	53.24 ± 0.08 (−9.3%) b	154.52 ± 0.65 (−9.6%) c
(P+A+O)	8.19 ± 0.04 (−9.6%) e	38.28 ± 0.09 (−9.3%) h	82.62 ± 0.33 (−10.1%) h
(O+A)+P	8.22 ± 0.06 (−9.4%) d,e	52.18 ± 0.08 (−9.5%) c	84.35 ± 0.26 (−9.5%) g
(P+A)+O	8,73 ± 0.05 (−9.4%) a	46.23 ± 0.06 (−9.8%) f	96.11 ± 0.66 (−9.4%) e
(O+P)+A	8.35 ± 0.08 (−9.2%) c	44.22 ± 0.09 (−9.8%) g	92.21 ± 0.49 (−9.4%) f

* P—potato starch, A –albumin, S –stearic acid, O –oleic acid. ** Percentual change in the value of given parameter with respect to these measured for freshly prepared foils. ^a–h—different letters in the same column indicate significant differences (Fisher test. p ≤ 0.05).

).

Over the course of time, the color parameters of the foils changed to an insignificant extent (

Table 7. Color parameters of foils after 3 months.

Foil	L*(D65)	a*(D65)	b*(D65)	C*	Hue Angle
(P+A+S)	87.02 ± 0.21 c	−0.56 ± 0.03 a	8.12 ± 0.32 e	8.14 ± 0.32 e	93.91 ± 0.06 e
(P+A)+S	89.23 ± 0.45 a	−0.63 ± 0.02 c,d	9.23 ± 0.25 b	9.25 ± 0.25 b	93.92 ± 0.11 e
(S+A)+P	87.18 ± 0.11 c	−0.61 ± 0.05 b,c	8.34 ± 0.36 d,e	8.36 ± 0.36 d,e	94.20 ± 0.18 d,e
(S+P)+A	88.03 ± 0.21 b	−0.87 ± 0.04 f	10.68 ± 0.48 a	10.71 ± 0.48 a	94.74 ± 0.32 c
(P+A+O)	83.22 ± 0.20 e	−0.81 ± 0.05 e	8.61 ± 0.26 c,d	8.64 ± 0.26 c,d	95.35 ± 0.17 b
(O+A)+P	87.09 ± 0.21 c	−0.68 ± 0.03 d	8.84 ± 0.28 c	8.87 ± 0.28 c	94.38 ± 0.31 d
(P+A)+O	89.04 ± 0.15 a	−0.97 ± 0.02 g	9.23 ± 0.18 b	9.28 ± 0.17 b	95.99 ± 0.06 a
(O+P)+A	85.02 ± 0.36 d	−0.57 ± 0.05 a,b	7.96 ± 0.11 e	7.98 ± 0.11 e	94.13 ± 0.36 de

^a–g—different letters in the same row indicate significant differences (Fisher test. p ≤ 0.05).

).

3. Materials and Methods

3.1. Materials

All of the following substances were analytically pure: Potato starch (catalogue number S4251-2KG) of 18–21% moisture content, pH of a 2% suspension 5.8–7.8, and amylopectin: amylose ratio of 27:73, stearic and oleic acids and egg albumin (catalogue number A5253-1KG) were purchased from Sigma-Aldrich (St. Louis, MO, USA).

3.2. Methods

Preparation of Foils

The synthesis involved the combination of the following components:

• Potato starch (P) (10 weight parts of 5% aq., gel), stearic acid (S) (5 weight parts of powder), and albumin (A) (1 weight part of liquid composed of 1 g albumin in 7 mL water).
• Potato starch (P) (10 weight parts of 5% aq., gel), oleic acid (O) (5 weight parts of liquid), and albumin (A) (1 weight part of liquid composed of 1 g aq. albumin in 7 mL water).
• Potato starch (P), lipid acid (S or O), and albumin (A) taken in the proportions above were blended simultaneously.

The components were connected in the following order:

(1)

(polysaccharide+protein)+lipid in the ratio 10:5:1 (polysaccharide:protein:lipid)

(2)

(polysaccharide+lipid)+protein in the ratio 10:5:1 (polysaccharide:lipid:protein)

(3)

(lipid+protein)+polysaccharide in the ratio 10:5:1 (lipid:protein:polysaccharide)

Where starch was a 5% gel solution, stearic acid was a powder, oleic acid was a liquid, and albumin was dissolved in water in a ratio of 1 g albumin/7 mL water at a water temperature of 20 °C.

The systems were obtained by mechanically mixing the components and heating the individual substrates to 95 °C in the right order for 20 min each. Samples with a volume of 30 mL were poured onto Teflon discs with a diameter of 11 cm. The mixtures were then placed in an oven set at 25 °C and 25% humidity for 24 h.

3.3. Physical Properties of Foils

3.3.1. Water Content, Solubility, and Swelling Degree

The water content, solubility, and swelling degree of foils were determined according to Souza et al. [61]. Composites were cut into rectangular (2 × 2 cm) specimens and weighed in an analytical balance (with 10⁻⁴g precision) providing the initial weight of the sample (M1). Then specimens were dried for 24 h at 70 °C in an oven and the initial dry mass (M2) was repeatedly weighed. The samples were then dissolved in 30 mL of distilled water and stored for 24 h at 25 °C. Insoluble portions were superficially dried using filter paper and weighed again (M3). These residual film samples were dried at 70 °C for 24 h in an oven and the final dry mass was determined (M4). The analysis was performed in four replications. The water content, solubility, and swelling degree of films were calculated according to the following equations:

Water content (%) = (M1 − M2)/M1 * 100

(1)

Solubility (%) = (M2 − M4)/M2 * 100

(2)

Swelling degree (%) = (M3 − M2)/M2 * 100

(3)

3.3.2. Surface Color Measurements

The measurement of the surface color was carried out with Konica MINOLTA CM-3500d equipment (Konica Minolta Inc., Tokyo, Japan) with a 30 mm diameter window. A D65 illuminant/10° observer served as the reference. The results were expressed using the CIELab system. The following parameters were determined: L* (L* = 0 black, L* = 100 white), a*—share of the green color (a* < 0) or red (a* > 0), b*- share of blue (b* < 0) or yellow (b* > 0). In addition, the following C* and h* parameters were calculated. Chroma (C*) Hue angle (h*) is a quantitative attribute of an object’s color that represents the difference in hue in comparison to a grey color of the same brightness. C* saturation takes values from 0 (at the center of the coordinate system) and increases as you move away from the center. This parameter can be calculated by the following equation [62]:

$${\mathbf{C}}^{\mathbf{*}}=\sqrt{{{\mathbf{a}}^{\mathbf{*}}}^2+{{\mathbf{b}}^{\mathbf{*}}}^2}$$

(4)

Hue angle (h*) is the degree value that corresponds to the three-dimensional color diagram (i.e., 0 for red, 90 for yellow, 180 for green, and 270 for blue) as seen by the human eye [51,63] and can be calculated by the following equation:

$${\mathbf{h}}^{\mathrm{*}}={\mathbf{t}\mathbf{a}\mathbf{n}}^{-1}\left(\frac{{\mathbf{b}}^{\mathrm{*}}}{{\mathbf{a}}^{\mathrm{*}}}\right)$$

(5)

The measurements were taken on a white background standard. The estimations were run in 5 repetitions.

3.3.3. Thickness Measurement

The thickness of ternary blends was measured with a micrometer, catalogue no. 805.1301 (Sylvac SA, Crissier, Switzerland), with a 10⁻³ mm resolution. It was performed in five repetitions at various points of the foil within the gauge length area.

3.3.4. Mechanical Properties

For 48 h prior to analyses, dry ternary blends were conditioned in desiccators at 25 °C and 52% relative humidity (RH) maintained by saturated solutions of magnesium nitrate-6-hydrate. The samples for textural analyses were prepared according to ISO Standards [64]. Analyses were performed with the TA-XT plus texture analyzer (Stable Micro Systems, Haslemere, UK). Films were cut into 35 × 6 mm strips and fixed into holders. The initial grip separation between holders was 20 mm and the rate of grip separation was 2 mm/min.

3.4. Thermogravimetry

Thermal analyses were performed using the Mettler Tolledo, simultanic thermal analyzer TGA/DSC (Mettler Tolledo, Greifensee, Switzerland). Samples were heated in argon in the temperature range of 30–500 °C with a temperature rate increase of 10 °C/min. The estimations were duplicated.

3.5. Scanning Electron Microscopy (SEM)

The structure of foils was determined by means of a TESCAN VEGA3 electron microscope (Tescan, Brno, Czech Republic) equipped with an Oxford Instruments energy dispersive X-ray analyzer, Aztec ONE system, High Wycombe, UK.

3.6. X-ray Diffraction

The reaction product was analyzed by means of X-ray powder diffraction (XRD) using the Rigaku MiniFlex D 600 powder diffractometer (Curadiation), (Rigaku, Tokyo, Japan) employing the D/teX Ultra silicon strip detector, Cu-Kα radiation). To assess the crystallinity, the method described by Hulleman et al. [65] was used. The values of the crystallinity index (Xc) for all samples were obtained using the following Equation (6):

Xc = Hc/(Hc + Ha)

(6)

where Hc and Ha are the intensities for the crystalline and amorphous profiles with typical diffraction reflex (121) at a value of 2θ between 17° and 18° as shown in Figure 3. Ha and Hc are the amorphous and crystalline profile heights, respectively.

3.7. Fourier Transformation Infrared Spectroscopy Attenuated Total Reflectance (FTIR-ATR)

The FTIR-ATR spectra of the films were recorded in the range of 4000–400 cm⁻¹ at a 4 cm⁻¹ resolution using a Mattson 3000 FT-IR (Madison, WI, USA) spectrophotometer. That instrument was equipped with a 30SPEC 30° reflectance adapter fitted with the MIRacle ATR accessory from PIKE Technologies Inc., Madison, WI, USA.

3.8. Staling of Foils

The aging test procedure for the biodegradable material included several steps aimed at assessing the material’s ability to degrade in the aging chamber SmartPro KK115 Pol-eko Apparatus:

• Sample preparation: Biodegradable material samples were carefully prepared, placed on Teflon discs, and marked.
• Sample exposure: Samples were exposed to natural aging conditions such as temperature of 25 °C, humidity of 25%, and ventilation. The samples were exposed in the aging chamber SmartPro KK115 Pol-eko Apparatus. The exposure time was 3 months [66,67] so that changes in the material could be observed.
• Monitoring changes: After a specified period of time (3 months), changes in the materials were monitored by taking measurements of physical properties, such as thickness and mechanical, water content, solubility and swelling degree, and color parameters.

After carrying out the above steps, the biodegradable material’s potential for degradation in the aging chamber was assessed.

3.9. Statistics

Statistical analysis involved Statistica 12.5 (StatSoft, Tulsa, OK, USA) software employing analysis of variance with the Fisher test for measuring the significance of the differences at p < 0.05.

4. Conclusions

Functional properties of the three-component potato starch–egg albumin–lipid acid foil can be modulated by the selection of the lipid component. Generally, oleic acid offers foils of higher tensile strength, relative elongation, thermal stability, and, as shown by SEM micrographs, a more uniform macrostructure. Foils produced with stearic acid were characterized by a higher index of crystallinity than foils made with oleic acid. Functional properties of the foils under study can be also modulated by a sequence of blending of the components. The simultaneous blending of starch, albumin, and stearic acid provided the foil with the highest tensile strength, whereas the parameters of the foil prepared by simultaneous blending of starch, albumin, and oleic acid were poor. The composition and the method of preparation had only a small effect on color parameters of the foils. They were white with a greenish-yellow shade. FTIR ATR spectra showed that the macrostructure of the foils is built involving interactions between all three components. During storage, all foils showed slight changes in their mechanical parameters, water content solubility, and swelling. Usually, the values of those parameters decline by no more than 10% of the values estimated for freshly prepared foils. The color parameters of foils did not change during their storage of 3 months.