Effect of Glycerol Concentration on the Properties of Semolina- and Farina-Based Biodegradable Films

Abstract

This study investigates the properties of biopolymer films derived from semolina and farina, focusing on the effect of varying concentrations of glycerol as a plasticizer. The research fills a gap in the study of grains such as semolina and farina, which have the potential to expand the range of biodegradable materials. Mechanical tests revealed significant differences between the two film types. Farina-based films were notably more ductile, exhibiting an elongation at break of up to two times their original length, but with a low tensile strength of only 1–2 MPa. In contrast, semolina-based films were significantly stiffer, with a maximum elongation at break of 10%. A notable exception was the semolina film with a 25% glycerol concentration, which displayed an exceptionally high tensile strength of 17 MPa. This is a significant improvement over the typical potato starch-based film tested, which breaks at 5 MPa under static tearing. Furthermore, the study examined the films’ morphology, color, SFE, and surface roughness. Free surface energy ranged from 40 to 60 mJ/m² in the tests, where the influence of the plasticizer was significant. Color tests clearly show yellow discoloration.

1. Introduction

1.1. Background on Biodegradable Materials

Adopting biodegradable materials over traditional non-biodegradable materials offers significant environmental benefits, primarily through reducing pollution and reliance on fossil fuels. Biodegradable materials, often derived from renewable resources such as plant-based biopolymers and natural fibers, decompose into natural elements like water, carbon dioxide, and biomass. This process minimizes environmental impact and waste accumulation [1,2,3]. These materials are particularly advantageous in applications such as food packaging, agriculture, and single-use items, where they can replace conventional plastics that contribute to persistent pollution [2,4,5,6]. Furthermore, they can be engineered to enhance mechanical and barrier properties, addressing some limitations compared to traditional plastics [1,5]. The integration of biodegradable materials into existing waste management systems, such as composting, further enhances their environmental benefits by ensuring proper degradation and reducing landfill use [7]. Despite challenges such as higher costs and inferior performance in certain applications, ongoing research and technological advancements are improving the functionality and cost-effectiveness. As a result, they are becoming a viable solution for sustainable development and pollution mitigation [3,8]. Overall, this transition represents a crucial step in tackling global challenges linked to plastic waste and pollution [9]. Semolina and farina, both derived from wheat, possess unique properties that make them suitable for use in biodegradable cereal-based materials. Semolina, a hard wheat flour, and farina, a soft wheat flour, exhibit distinct sorption properties, which are crucial for their application in biodegradable materials. The sorption isotherms of these flours indicate that they have a significant capacity for water absorption, with semolina having a monolayer capacity of 5.53 g per 100 g and farina 5.88 g per 100 g, and specific surface areas of 194.2 m²/g and 206.6 m²/g, respectively [10]. The high amylose content in these flours contributes to the formation of biodegradable materials with good resilience and compressibility, which are desirable traits for packaging applications [11]. Furthermore, the gluten proteins in wheat flour, such as gliadin and glutenin, provide extensibility, elasticity, and gas retention, enhancing the mechanical properties of the resulting materials [12].

1.2. Overview of Semolina and Farina

The rheological properties of semolina and farina significantly influence their film-forming capabilities, as these properties determine the mechanical and barrier characteristics of the films. Semolina, derived from durum wheat, exhibits unique rheological properties due to its high protein content and gluten strength. These factors are crucial for film formation. The viscoelastic properties of semolina doughs, influenced by factors such as water content, salt, and ingredient ratios, affect the gluten network’s strength and extensibility, thereby impacting the film’s mechanical properties and processability [13,14]. The addition of plasticizers like sorbitol and glycerol to semolina-based films can modify their physico-mechanical properties, such as tensile strength and elasticity, by altering the film’s moisture content and solubility, which are critical for film flexibility and durability [15].

In contrast, farina, typically made from softer wheat, may exhibit different rheological behaviors due to its lower protein content, affecting its film-forming potential. The presence of gluten in wheat-based films, including those made from semolina, can decrease inter-chain hydrogen bonding, reducing viscosity and enhancing the film’s processability, although it may compromise surface smoothness and barrier properties [16]. The rheological behavior of semolina is also influenced by the presence of other ingredients, such as tapioca flour, which can increase viscosity and improve the film’s thickness and stability [17]. Overall, the rheological properties of semolina and farina are pivotal in determining the quality and functionality of the films they form. Semolina’s strong gluten network provides a robust structure, while the addition of plasticizers and other ingredients can tailor the films’ properties for specific applications [15,18].

1.3. Applications of Biodegradable Films

Biodegradable films based on semolina and farina have a range of potential applications, particularly in the fields of agriculture, packaging, and food preservation. These films, which are primarily composed of starch and plant proteins, offer an eco-friendly alternative to conventional plastic films due to their biodegradability and renewable nature. In agriculture, such films can be used as mulch to enhance soil quality, reduce weed growth, and be biodegradable and environmentally safe [19,20]. In the packaging industry, these films can serve as sustainable solutions for food packaging, reducing fossil fuel reliance and lowering emissions. They can be produced using various techniques such as extrusion and solvent casting, and can be modified to improve their properties through processes like esterification and grafting [1].

Additionally, these films can be used in food and nutraceutical applications, where they can extend the shelf life of products by reducing moisture loss and lipid oxidation, and by acting as carriers for antimicrobial and antioxidant agents [21]. Incorporating specific additives, such as plasticizers, can enhance the thermoplastic nature of the films, making them more suitable for melt processing and improving their mechanical properties [22]. Furthermore, these films can be tailored to possess antibacterial properties, which are beneficial for food packaging applications, helping to inhibit microbial growth and enhance food safety [23]. Overall, biodegradable films based on semolina and farina present a versatile and sustainable option for various applications, aligning with the growing demand for environmentally friendly materials.

1.4. Objectives of the Study

In conclusion, exploring biodegradable films derived from semolina and farina highlights a pivotal advancement in the quest for sustainable materials that can effectively mitigate environmental challenges associated with plastic waste. The inherent properties of semolina and farina, coupled with their ability to be modified through the incorporation of plasticizers and other additives, position these materials as promising alternatives to traditional non-biodegradable films. There is a lack of research discussing the properties of film obtained from farina and semolina, which this article attempts to fill. The influence of plasticizers, the most common additive to all types of polymers, is also presented. As the global community increasingly seeks solutions to reduce pollution and reliance on fossil fuels, developing and applying biodegradable films in agriculture, packaging, and food preservation emerges as a critical strategy. However, there is limited understanding of how varying glycerol concentrations influence the mechanical, morphological, and optical properties of semolina- and farina-based films under identical processing conditions, representing a key gap in current knowledge. This study aims to systematically investigate these effects through controlled mechanical testing, surface characterization, and color analysis, thereby contributing to the knowledge necessary for advancing sustainability in material science. Clarifying these relationships will support optimized film formulations for targeted industrial applications. Through ongoing research and innovation, these biodegradable films have the potential to play a significant role in shaping a more environmentally responsible future. It should be noted that there are also other methods of strengthening materials, such as reinforcing with polymers like PVA, incorporating functional nanoparticles and inorganic fillers, and tailoring surface properties, providing a valuable framework for developing and strengthening biodegradable films based on materials like farina and semolina. By adapting these principles, films’ mechanical, barrier, and functional properties can be significantly improved for diverse applications [24]. The study also does not examine the environmental impact itself, which is often a more complex issue, ranging from CO₂ production during crop growth [25] to the disposal of materials by microorganisms that decompose materials artificially or naturally [26].

2. Materials and Methods

2.1. Materials Propotions/Samples

Six solutions were prepared: three contained food-grade farina as a polymer, and the other three contained food-grade semolina. Vegetable glycerin was added in different concentrations as a plasticizer and water as a solvent. These solutions weighed 400 g each.

Table 1. Composition of farina- or semolina-based solutions.

Entry	Farina or Semolina (g)	Glycerol (g)	Glycerol (%)
1	40	10	25
2	40	20	50
3	40	30	75

includes the formulation values of glycerol.

Semolina and farina, commercially available, were used to conduct the tests. Farina was additionally ground, using a food grinder, before use to increase the uniformity of its properties.

Origin of reagents:

• Glicerol
• By Biomus—Poland (Chemiczna 7, Lublin); class: pure; Nr CAS: 56-81-5; Nr EC 200-289-5

• Semolina
• By Radix-bis—Poland (Gerberowa 22, Rotmanka); From durum wheat from Austria

• Farina
• Made by Cenos Sp. z o. o.—Poland (Gen. Sikorskiego 22 Września); From common wheat from Poland

2.2. Film Preparation

To prepare the coating solution, first food-grade semolina or farina, vegetable glycerin, and water at a weight ratio described later was combined. Then the mixture to approximately 90 °C was heated with continuous stirring until it became a smooth, homogeneous solution. Mixing was performed using a magnetic stirrer, initially at maximum speed (1200 rpm; after exceeding 50 °C and viscosity increased, the speed was reduced to 200 rpm) for approximately 15–20 min.

The solution was then cooled to a temperature between 50 and 60 °C and applied to the substrate using a micrometer adjustable film applicator from RK Print Coat Instruments. This tool ensured a uniform initial film thickness of 3 ± 0.1 mm.

Finally, the coated plates were left to dry at room temperature for 10 days on a laminate surface before being tested for static peel strength. Drying conditions (average 18.4 °C in June) did not significantly influence the results and are provided for reproducibility.

2.3. Tensile Testing

Tensile testing was performed following the ISO 527-3 standard (PN-EN ISO 527-3:2019-01, Plastics—Determination of tensile properties—Part 3: Test conditions for films and plates, Warsaw, Poland, 2019). The tests adhered to the following specified parameters:

Sample width: 15 ± 0.2 mm

Sample length: 50 mm

Testing speed: 100 ± 10 mm/min

A Zwick-Roell Z010 tensile testing machine (Ulm, Germany), fitted with a 1 kN load cell, was employed to assess the mechanical performance of the fabricated films. Each sample group underwent testing in ten replicates, and the average of these measurements was recorded as the final result.

2.4. Surface Properties

The surface properties of both the starch-based film-forming solutions and the resulting starch films, including tension and wettability, were also analyzed. Water contact angle (WCA) assessments were conducted in accordance with ISO 15989 (ISO 15989:2004, Plastics—Film and sheeting—Measurement of water-contact angle of corona-treated films, Tokyo, Japan, 2004). These measurements were carried out using a Drop Shape Analysis System (DSA 30E, Krüss, Germany, Hamburg). The surface free energy was derived using the Owens–Wendt–Rabel–Kaelble (OWRK) approach (Owens and Wendt, 1969) [27], which is based on static contact angle readings obtained from water and diiodomethane.

The films were cut into rectangular samples measuring approximately 2.5 cm × 6 cm for these evaluations. Sessile droplets of water and diiodomethane were applied to the sample surfaces using 0.5 mm diameter dispensing needles. The static contact angles were measured 5 s after drop placement using the Tangent 2 method.

To further examine the surface morphology of the films, a Keyence VHX-950F digital optical microscope (Keyence, Belgium, Mechelen) equipped with a VH-Z100R zoom lens (100–1000× magnification) (Keyence, Belgium, Mechelen) was utilized. Standard surface micrographs were captured, and three-dimensional surface topography was reconstructed through image stacking at various focal depths, allowing for surface roughness analysis. Additionally, cross-sectional images were obtained by sectioning the samples using a punch and press technique.

The measured roughness parameters are: Sa (arithmetic mean height): the average absolute deviation of the surface from the mean plane. Sz (maximum height): the vertical distance between the highest peak and the lowest valley within the measured area. Sq (root mean square height): the root mean square (RMS) average of the height deviations from the mean plane. Ssk (skewness): the asymmetry of the height distribution. A positive value suggests a surface with more peaks than valleys. Sku (Kurtosis): describes the sharpness of the height distribution. A value close to 3 indicates a Gaussian distribution. Sp (maximum peak height): the height of the tallest peak above the mean plane. Sv (maximum valley depth): the depth of the deepest valley below the mean plane.

For more detailed insight into the surface characteristics, scanning electron microscopy (SEM) analysis was performed using a JOEL JCM-7000 system (Peabody, MA, USA), operated at an acceleration voltage of 15 kV. Prior to imaging, a thin layer of gold was sputter-coated onto the samples to enhance conductivity and image quality. The gold coating process took 1 min. Imaging locations were selected randomly at least 1 cm from the edge, to avoid edge effects from drying.

2.5. Colorimetric Properties

The colorimetric properties of the films were analyzed using an X-Rite eXact spectrophotometer (X-Rite Inc., Grand Rapids, MI, USA). This analysis was conducted under specific, controlled settings: a D50 illuminant, a 2° standard observer, and the M0 measurement mode. Measurements were taken over both a white and a black backing to thoroughly evaluate the films’ color characteristics. These substrates were chosen because of their nearly perfect light reflection and absorption capabilities. They established initial color readings for the substrates themselves. Afterward, the films were applied, and a second set of measurements was taken. The resulting color changes attributed to the films were then determined by comparing the substrates’ measurements to those with the films on top [28].

The measured color parameters are:

• L*: This represents the lightness of the color. Its value ranges from 0 (absolute black) to 100 (diffuse white).
• a*: This axis represents the green-red opponent colors. A negative a* value indicates a greener color, while a positive a* value indicates a redder color.
• b*: This axis represents the blue-yellow opponent colors. A negative b* value indicates a bluer color, while a positive b* value indicates a yellower color.
• D: Absorbance.

3. Results

3.1. Process for Obtaining Samples

The film produced from farina showed high adhesion, which increases as the glycerin content in the mixture increase.

In the case of semolina film with a 25% glycerin additive, exceptionally easy separation from the substrate was observed. This material exhibited some brittleness, resulting in crumbling during cutting. This film was also prone to breaking after a single double bend.

The semolina film with 50% glycerin added was characterized by very strong adhesion to the substrate and low cohesion. Attempts to separate it from the substrate resulted in the material tearing. It was noted that making an incision on the surface eased peeling, but minor tears still occurred.

In contrast, the Farina film with 75% glycerin content showed significantly stronger adhesion to the substrate than the variant with 50% content. Separation was challenging, and the material tore repeatedly. Strong re-adhesion to the substrate was also observed, even after initial detachment.

In addition, the farina film with 25% glycerin content showed severe structural defects, manifested by complete cracking (Figure 1), which may suggest the occurrence of substantial internal shrinkage.

3.2. Tensile Testing

Farina 25 was not tested due to the inability to obtain suitable test strips. The film obtained was cracked, as shown in Figure 1.

The tensile properties of the films, as detailed in

Table 2. Tensile properties of the developed farina and semolina films.

Sample	Direction	σM [MPa]	Uncertainty [MPa]	εtM [%]	Uncertainty [%]
Farina 50	Along	0.95	0.13	84	11
	Across	1.08	0.09	100	4.7
Farina 75	Along	0.73	0.73	110	110
	Across	0.73	0.03	95	5.8
Semolina 25	Along	17.1	0.7	5.1	0.4
	Across	13.9	0.774	7.5	1.1
Semolina 50	Along	1.04	0.097	89	6.8
	Across	1.81	0.69	72	9.7
Semolina 75	Along	0.226	0.0391	38	6.9
	Across	0.168	0.0448	38	7.8

, reveal significant differences based on their primary component. Films derived from semolina and farina exhibited lower tensile strength compared to those made from potato starch, yet they showed similar extensibility. A notable exception was the semolina 25 sample, which will be discussed separately.

As anticipated, an increase in glycerol reduced the breaking force and increased extensibility, consistent with the standard effect of plasticizers. A key distinction was observed between the two flour types: farina films demonstrated greater extensibility, achieving a twofold increase in length, whereas semolina films, even at the highest plasticizer concentration, only increased in length by 10%.

The semolina 25 sample proved to be an outlier. It displayed a tensile strength of approximately 17.1 MPa, which is considerably higher than the 5 MPa strength obtained in previous studies for standard potato starch films [29]. However, its elongation at break was only 1%, making it extremely non-stretchable.

The film thickness for semolina-based samples showed a positive correlation with increasing glycerol concentration. A similar, though less linear, trend was observed in the farina-based films. All thickness measurements have an associated uncertainty of approximately 10%. All results are presented in

Table 3. Thickness of developed films.

Sample	$\overline{\mathrm{x}}$ [mm]	s [mm]
Farina 25	0.2361	0.0204
Farina 50	0.1967	0.0415
Farina 75	0.4646	0.0201
Semolina 25	0.3767	0.0576
Semolina 50	0.3922	0.0457
Semolina 75	0.4466	0.0592

$\overline{\mathrm{x}}$—Average thickness. s—Standard deviation.

.

3.3. Surface Free Energy

The addition of glycerol significantly impacted the Surface Free Energy (SFE) of both farina- and semolina-based films. This effect was primarily due to a change in the polar component of the SFE, as shown in

Table 4. Surface Free Energy result for farina samples.

Side	Concentration	SFE [mJ/m2]	Dispersive [mJ/m2]	Polar [mJ/m2]
Up	25	38.07	36.34	1.73
	50	41.76	41.2	0.56
	75	46.88	42.34	4.54
Down	25	46.92	41.32	5.6
	50	63.44	40.1	23.34
	75	61.53	41.33	20.2

and

Table 5. Surface Free Energy result for semolina samples.

Side	Concentration	SFE [mJ/m2]	Dispersive [mJ/m2]	Polar [mJ/m2]
Up	25	39.82	39.78	0.04
	50	55.83	40.42	15.41
	75	49.01	36.41	12.6
Down	25	47.2	41.44	5.76
	50	65.08	37.77	27.31
	75	63.28	35.47	27.81

.

At a 25% glycerol concentration, the polar component was below 0.1 mJ/m² for both film types. However, at 50% and 75% concentrations, the polar component increased significantly, reaching its highest observed value in the tested range for these film bases. Specifically, it was ~20 mJ/m² for farina and ~27 mJ/m² for semolina. This suggests that the change in the polar component may occur either gradually between 25% and 50% glycerol concentration or discretely, due to molecular rearrangements.

It is also important to note that the average SFE values of 40 and 60 mJ/m² were observed. The 60 mJ/m² value, in particular, may require an activation such as corona treatment process to ensure compatibility with different types of printing inks.

3.4. Surface Microscopic Analysis

3.4.1. Optical Images

Farina’s optical images are shown in Figure 2. As the concentration of glycerol increases, increased structure formation can be observed. These structures are visible on both sides, although they are much more pronounced on the upper layer. Our previous studies showed that the drying surface has a greater impact on the bottom layer [29].

Semolina optical images are shown in Figure 3. In the case of semolina, there is also a clear increase in the number of structures, which, however, have a more rounded shape. The patterns are more visible from the bottom layer, while the top layer behaves more chaotically.

3.4.2. SEM Images

Figure 4 shows SEM images of both tested films. Only the highest concentrations were selected for SEM imaging, as they showed the most pronounced changes in structure. The remaining SEM images show similar changes, but to a lesser extent. For the sake of clarity, they have been moved to Appendix A (lower layer) and Appendix B (top layer). The patterns observed under an optical microscope are visible. The rounded semolina patterns formed by microcracks (Figure 4d) are visible, although their shape can be better defined. They may be related to crystallization, as similar macro patterns were observed when removing the film. The surface of farina (Figure 4a–c) is much more homogeneous.

3.4.3. Optical Roughness Measurement

The surface roughness parameters, which are presented in

Table 6. Surface roughness parameters of developed films.

Sample	Surface	Sa [µm]	Sz [µm]	Sq [µm]	Ssk [µm]	Sku [µm]	Sp [µm]	Sv [µm]	Area Size [µm2]
Farina 25	Bottom	1.15	8.32	1.43	−0.73	3.32	3.31	5.01	86,251.58
Farina 50	Bottom	4.15	26.27	5.02	0.15	2.35	14.06	12.21	219,435.57
Farina 75	Bottom	4.88	24.00	5.74	0.34	2.14	14.28	9.71	216,146.84
Semolina 25	Bottom	3.09	18.80	3.75	−0.59	2.65	6.77	12.02	236,273.00
Semolina 50	Bottom	4.09	19.44	4.76	0.68	2.23	12.05	7.39	190,221.24
Semolina 75	Bottom	15.61	67.65	18.36	−0.27	2.06	29.60	38.05	227,487.94
Farina 25	Top	6.34	34.65	7.52	−0.15	2.12	16.86	17.79	192,593.52
Farina 50	Top	5.59	29.85	6.84	0.74	2.76	19.14	10.71	222,282.77
Farina 75	Top	2.87	23.96	3.70	0.63	3.67	15.86	8.10	227,060.74
Semolina 25	Top	5.41	45.24	7.31	1.16	5.70	32.05	13.19	222,621.67
Semolina 50	Top	10.39	46.82	11.95	−0.17	1.82	22.88	23.93	195,662.68
Semolina 75	Top	11.00	47.39	12.56	0.55	2.06	27.51	19.88	214,698.17

, particularly Sa (average roughness) and Sq (root mean square roughness), vary significantly between the farina and semolina films and between the top and bottom surfaces.

For the farina films, the bottom surface generally has a lower average roughness (Sa ranging from 1.15 to 4.88 μm) compared to the top surface (Sa ranging from 2.87 to 6.34 μm). This suggests that the side of the film in contact with the casting surface is smoother. Interestingly, there is no clear, consistent trend of how roughness changes with increasing glycerol concentration (indicated by the numbers 25, 50, and 75).

The semolina films show a different pattern. The bottom surfaces have Sa values that increase significantly with glycerol concentration, from 3.09 μm for semolina 25 to a very high 15.61 μm for semolina 75. This indicates a dramatic increase in roughness as the plasticizer content rises. The top surfaces of the semolina films also become rougher with increasing glycerol, with Sa values ranging from 5.41 μm to 11.00 μm, suggesting that the high concentration of plasticizer might be promoting a less uniform, more textured surface structure on both sides of the film.

Comparing the two materials, semolina films at higher plasticizer concentrations (semolina 75) are substantially rougher than any of the farina films. The highest Sa value for semolina (15.61 μm) is significantly greater than the highest farina value (6.34 μm). The Sz parameter (maximum peak-to-valley height) also supports these findings, with semolina 75 (bottom) having an exceptionally high value of 67.65 μm, far exceeding any other sample.

3.5. Color

Comparing the two materials, farina films generally appear slightly less light than the semolina films when placed on a white background, with farina’s L* values clustering around 90–96 and semolina’s L* values being more consistently in the 96 range. When on a black background, the L* values are more similar, though farina 50 is the darkest at 45.734. The a* and b* values, which indicate the red-green and yellow-blue axes, respectively, show some minor variations. Farina films on a white background tend to have a slightly negative b* value, suggesting a bluish tint. In contrast, the semolina films on white have a slightly positive b* value, indicating a slight yellowish tint. All films on a black background show a positive b* value, confirming a yellowish hue, likely inherent to the raw materials.

The D value is a metric for transparency, with values closer to 1 indicating higher transparency. All films measured on a black background have high D values (ranging from 0.740 to 0.822), confirming their translucent nature. No clear, consistent trend shows a strong correlation between glycerol concentration (25%, 50%, 75%) and a significant change in color or transparency for either farina or semolina. The most significant finding is the general translucency of all film samples, which allows the underlying background to visibly alter their apparent color and lightness.

4. Discussion

The tensile strength results align with expected behavior for plasticized biopolymer films; increasing the concentration of glycerol consistently led to a decrease in tensile strength and a corresponding increase in extensibility. This is a well-documented phenomenon where the plasticizer molecules disrupt the polymer network, increasing chain mobility [30,31,32]. However, a significant difference was observed between the two flour types. Farina films showed a remarkable twofold increase in extensibility with higher plasticizer concentrations, while semolina films only showed a modest 10% increase. This suggests that the polymer structure of farina is more susceptible to plasticization, allowing for greater chain separation and movement.

The semolina 25 sample emerged as a notable outlier with an exceptionally high tensile strength of 17 MPa, far exceeding typical values for potato starch films (5 MPa [29]). High strength and its low elongation at break (1%) indicate that the film’s internal structure is rigid and brittle.

The thickness of the films also correlated with glycerol concentration, increasing with higher plasticizer content. This may be due to increased viscosity and reduced water content that could evaporate [33]. It should be noted that not all studies with varying glycerol concentrations report significant thickness changes; such variations may result from differences in drying behavior influenced by film composition [34].

The surface analysis revealed significant differences in roughness and wettability. The roughness data (Table 6) showed a clear distinction between the top and bottom surfaces, and between the two materials. For farina, the bottom surface was consistently smoother, which is likely a direct result of being cast and dried against a smooth substrate. The top surface, exposed to air, likely experienced more rapid evaporation, leading to a less uniform surface texture [34].

In contrast, semolina films exhibited a dramatic increase in roughness, particularly on the bottom surface, as glycerol concentration rose. The Sa and Sz values for semolina 75 were substantially higher than for any farina film. The increase in roughness may be linked to the structural changes observed under microscopy, such as the formation of distinct, rounded structures. These structures may result from phase separation or crystallization of specific components (e.g., starch or protein) within the semolina matrix, which is more pronounced at higher plasticizer concentrations [34,35].

The Surface Free Energy (SFE) data further supports the structural changes. A significant increase in the polar component of SFE was observed for both materials at 50% and 75% glycerol concentrations, while it remained near zero at 25%. This suggests a threshold effect where a certain concentration of glycerol triggers the reorganization of polar functional groups to the film’s surface. This change in SFE is critical for applications like printing, as an average SFE of 60 mJ/m² may require surface activation to ensure compatibility with certain inks (Table 4 and Table 5) [31].

The color and transparency analysis confirmed that all films are translucent. This is evidenced by the large difference in lightness (L*) values when the films are measured against a white versus a black background. This can be seen in

Table 7. Film color.

Sample	Substrate	L*	a*	b*	D
	White background	101.740	0.520	−7.420	−0.020
	Black Background	37.680	1.480	1.960	0.990
Farina 25	On white	90.724	−0.360	−0.454	0.112
Farina 50	On white	96.318	0.154	−2.240	0.042
Farina 75	On white	90.394	0.040	−0.944	0.128
Semolina 25	On white	96.376	−0.306	1.310	0.044
Semolina 50	On white	96.550	−0.310	−0.120	0.044
Semolina 75	On white	96.070	−0.410	−0.326	0.050
Farina 25	On black	51.116	−0.184	1.332	0.740
Farina 50	On black	45.734	0.472	0.834	0.822
Farina 75	On black	47.392	0.126	0.966	0.770
Semolina 25	On black	47.716	0.086	1.794	0.776
Semolina 50	On black	48.284	0.160	1.812	0.778
Semolina 75	On black	49.048	−0.136	1.354	0.776

, which shows the color measurement results on different backgrounds. The inherent yellowish tint of the raw materials, confirmed by positive b* values on a black background, is present in all samples. The lack of a clear trend between glycerol concentration and changes in color or transparency suggests that glycerol does not significantly alter the inherent optical properties of the base materials themselves. Instead, the films’ appearance is primarily determined by their translucent nature, with structural features observed under microscopy that are not dense enough to cause significant light scattering or opacity.

In conclusion, while glycerol acts as a plasticizer for both farina and semolina films, the degree to which it modifies mechanical and surface properties is highly material-dependent. Semolina films, especially at higher glycerol concentrations, develop a rougher, more structured surface (Table 6) with enhanced strength in a specific case (semolina 25), while farina films are more prone to increased extensibility (Table 2). These findings emphasize the importance of material selection and plasticizer concentration in tailoring film properties for industrial applications, such as food packaging and agricultural mulch films.

5. Conclusions

This study demonstrated that semolina- and farina-based biodegradable films respond differently to glycerol plasticization, with farina formulations showing greater extensibility and semolina formulations generally offering higher stiffness and strength. The semolina 25 formulation demonstrated the most promising overall properties, yet its low elasticity remains a critical limitation, making it unsuitable for applications requiring high stretchability. Surface roughness and SFE analyses confirmed that both material type and glycerol concentration strongly influence film wettability and compatibility with printing or coating processes.

To address this, future research should focus on a systematic investigation into plasticizer concentration effect, incorporating different additives, surface modification techniques to improve SFE, and optimizing semolina content to enhance the material’s mechanical properties. Such developments could enable targeted biodegradable films for specific uses in food packaging, agricultural mulching, and other sustainable applications, meeting both industrial and environmental requirements.