Thickness, moisture content, colour parameters and opacity are shown in Table
2. To form stand-alone structures edible films should have the proper thickness (usually up to 0.25 mm) [
31,
32]. The film thickness results showed some significant differences among treatments. Pure sodium alginate edible films had the highest thickness, while the incorporation of apple puree resulted in the lowest values of thickness. The addition of vegetable oils caused a slight increase in the values of thickness compared to sodium alginate–apple puree films. These results indicated that the addition of apple puree and vegetable oils altered the thickness and microstructure of the films. However, this was not later confirmed by microscopic analysis and may be explained by a very non-uniform structure of the films caused by the presence of thickened areas in the matrix as well as large particles of apple puree.
Table 2Physical properties of sodium alginate–apple puree edible films
SA | 160.47 ± 19.10b | 18.18 ± 0.71b | 93.69 ± 0.58c | 1.25 ± 0.06cd | 1.74 ± 1.32a | 6.29 ± 1.67a | 0.67 ± 0.16a |
AP | 122.78 ± 4.00a | 17.51 ± 0.88b | 92.47 ± 0.25bc | 1.76 ± 0.09d | 5.96 ± 0.37b | 11.15 ± 0.44ab | 5.53 ± 0.64bc |
RO | 132.91 ± 6.29a | 19.18 ± 1.21b | 86.97 ± 0.73ab | − 0.90 ± 0.15ab | 15.35 ± 1.20d | 20.08 ± 1.39d | 6.14 ± 0.29cd |
CO | 128.06 ± 18.80a | 15.49 ± 0.18a | 89.16 ± 0.11b | − 2.43 ± 0.22a | 15.53 ± 0.96d | 12.64 ± 0.98bcd | 6.79 ± 0.81d |
HO | 137.65 ± 9.90ab | 17.18 ± 0.11ab | 85.30 ± 0.53a | − 0.42 ± 0.16bc | 8.68 ± 0.73c | 11.82 ± 0.72bc | 5.37 ± 0.20ab |
The moisture content of the researched films ranged from 15.49 ± 0.18 to 19.18 ± 1.21% for the films with coconut oil and rapeseed oil, respectively. Nonetheless, only the values of moisture content for sodium alginate–apple puree edible films with coconut oil differed significantly in comparison with the rest of the samples.
General appearance and consumer acceptance are greatly influenced by the colour of food packaging [
33]. For the developed edible films, the colour was closely related to the colorimetric parameters of apple puree, while the addition of vegetable oils was responsible for their milky appearance. The colour parameter
\( L^{*} \) provides a measure of lightness. The value of this parameter ranges from 0 to 100 (dark–light). A positive
\( a^{*} \) value is a measure of redness, and a negative value of greenness, while a positive
\( b^{*} \) value is a measure of yellowness, and a negative value of blueness [
19]. Not only colour but also film transparency is an important factor influencing the overall appearance and quality of edible packaging. Lower film opacity was considered to be a desirable feature for food packaging films and coatings due to the fact that consumers prefer to see the foods inside a package [
34]. Surface colour and opacity of the films were greatly influenced by blending with apple puree and vegetable oils as shown in Table
2. The pure sodium alginate film was clear and transparent with a high lightness value of 93.69 ± 0.58. In general with the addition of apple puree, there were insignificant changes in
\( L^{*} \) and
\( a^{*} \) values. The addition of apple puree caused only statistically significant increase in the value of
\( b^{*} \) (the films were more yellow). Vegetable oils were in charge of the significant changes in the values of all colorimetric parameters causing a decrease in the lightness
\( L^{*} \) and greenness
\( a^{*} \) and increase in yellowness
\( b^{*} \), indicating the selected vegetable oils had significant influence on producing opaque milky films. The hazelnut oil appeared to have the biggest influence on the lightness, while coconut oil on the values of
\( a^{*} \) and
\( b^{*} \) resulting in yellowish and greenish films. The addition of fruit puree as well as vegetable oils caused a statistically significant increase in the values of opacity, which was compatible with visual observations. Consequently, the total colour difference (Δ
E) of the films with apple puree as well as with apple puree and the selected vegetable oils increased significantly compared with the pure sodium alginate film. Orsuwan et al. [
35] explained the increase in the values of
\( b^{*} \) agar–banana powder blend films by the presence of the phenolic and terpenoids (
β-carotene) compounds in the banana powder. Similar observations related to the effect on vegetable oils of colorimetric parameters of edible films based on whey protein, were reported by Galus and Kadzińska [
36,
37] while rapeseed oil, almond oil and walnut oil incorporation. All the obtained films were characterized by a decrease in the value of lightness
\( L^{*} \) in comparison with the control sample without vegetable oils. Nonetheless, equivocal changes in the values of
\( a^{*} \) and
\( b^{*} \) were observed when nut oils were added. In general, the films showed positive values of parameter
\( a^{*} \) (up to 0.98 ± 0.05) and negative values of parameter
\( b^{*} \) (up to − 0.79 ± 0.08). More consistent with the results obtained in this research were the results for whey protein isolate film with rapeseed oil resulting in more yellowish appearance. Moreover, the emulsified films were much more opaque than control whey films. Pereda et al. [
38] explained this phenomenon by the fact that droplets dispersed in the matrix affect the transparency by preventing light transmission through the film.