Development of a Flexible Film Based on Purple Yam Flour and Nanoparticles Obtained by Aqueous Counter Collision

Abstract

The utilization of biopolymers as raw materials for the development of sustainable materials has become one of the most promising strategies to minimize the negative impact of plastic pollution. Tubers such as purple yam are rich in starch, which serves as the main component for producing strong and durable bioplastics with properties comparable to conventional plastics. In this study, purple yam flour was used as a raw material to develop a biodegradable film through the casting method. Additionally, Flour Nanoparticles (FN) extracted via the Aqueous Counter Collision technique were incorporated to enhance the mechanical, morphological, and barrier properties of the films. The nanoparticles exhibited sizes below 100 nm, as determined by DLS analysis. The casting process was carried out using film solutions containing 2 wt% flour and 15 wt% glycerol, with FN concentrations of 5 wt%, 15 wt%, and 25 wt%. The main results showed that the films with 25 wt% FN displayed improved mechanical strength, increasing from 2.2 MPa (control) to 7.3 MPa, as well as enhanced thermal resistance, rising from 68 °C (control) to 102 °C. The films also exhibited a smoother morphology, indicating improved water vapor transmission (WVT). The incorporation of FN thus contributed to the development of films with reduced hydrophobicity.

1. Introduction

The worldwide consumption of conventional plastics has become one of the most critical environmental issues. Biodiversity including oceans, wildlife, and humans has been severely impacted, leading to health problems and a significant decline in flora and fauna. The Food and Agriculture Organization (FAO) reported statistical data on the amount of plastic in oceans, estimating that more than 5 billion pieces of plastic are floating and causing the death of thousands of animals [1]. Furthermore, the disintegration of plastics generates microplastics, which accumulate in fish consumed by humans. These microplastics enter the bloodstream, resulting in numerous health concerns.

Several alternatives have been proposed to mitigate this problem and preserve biodiversity for future generations. Among these, the development of bioplastics has gained popularity due to the use of natural resources. However, achieving a balance between utilizing food resources for non-food products requires extensive research, processes, and financial investment.

The development of biodegradable films based on starch represents a promising solution, as starch is one of the most abundant natural resources; it is inexpensive and widely available [2]. Starch can be obtained from various sources, including cereals, tubers, vegetables, and fruits. Tubers are the most common source for producing plastic films with properties similar to conventional plastics. These films can degrade within weeks, leaving no toxic residues [3]. Yam is a well-known tuber in Colombia, especially in the northern region, and contains more than 80% starch. However, purple yam is relatively uncommon, making it an innovative alternative [4].

Purple yam is notable for its vibrant coloration, which indicates the presence of anthocyanins—bioactive pigments with antioxidant properties. Anthocyanins are widely used in products for human consumption and are associated with reducing the risk of cardiovascular diseases, cancer, metabolic syndromes, and neurodegenerative disorders. They are also responsible for the orange, pink, red, and purple hues in plants and fruits.

In recent years, the development of biomaterials has advanced significantly, incorporating technologies such as nanotechnology. The addition of nanoparticles to biopolymer films has proven to enhance mechanical, morphological, and barrier properties. Nanoparticles derived from the same source material have been successfully integrated into these films [5]. In this study, nanoparticles from purple yam flour were extracted using Aqueous Counter Collision (ACC) technology. ACC operates with two nozzles through which the sample passes at high pressure, pulverizing the material. This technique has primarily been used for extracting cellulose nanofibrils, producing particles smaller than 100 nm through a nozzle diameter of 100 nm at pressures exceeding 270 MPa [6,7].

The most recent study utilizing starch or flour with ACC treatment was conducted in 2020, where starch-based film solutions were treated at 200 MPa using ACC, resulting in smoother and cleaner films [8]. To date, the use of flour as a source for nanoparticle extraction via ACC has not been reported. This study is the first to employ purple yam flour for nanoparticle extraction and incorporate these nanoparticles into purple yam flour-based films to improve their mechanical, morphological, and barrier properties.

2. Materials and Methods

2.1. Materials

Purple yam flour was obtained from a small farm located in the Department of Bolívar, Colombia. The flour composition was as follows: starch content 83.93%, protein 4.80%, moisture content 13.93%, and ash content 2.40%.

2.2. Production of Flour Nanoparticles (FN) from Purple Yam and Characterization

Nanoparticles were produced using Aqueous Counter Collision (ACC) technology (Starburst Mini HJP 25001, Sugino Machine, Japan). Flour concentrations of 1.5 wt%, 2.0 wt%, and 2.5 wt% were processed through the nozzles for 10, 15, and 20 cycles, indicating the number of times the flour–water solution passed through the equipment, following the methodology described in [6,8,9]. The pressure was maintained at 200 MPa throughout the procedure using two water jets at 95 °C.

2.3. Dynamic Light Scattering (DLS)

DLS analysis was performed according to [10], with modifications, to determine particle size. Measurements were conducted using a Zetasizer Nano ZS (Malvern Instruments, Malvern, UK, λ = 633 nm). The refractive index was set at 1.53 and the absorption index at 0.1. The scattering angle was 127°. Data were analyzed using ANOVA in JMP Pro 16.0.0 (NCSU, USA) with a significance level of p < 0.05.

2.4. Scanning Electron Microscopy

Morphological analysis was performed using a Scanning Electron Microscope (Analytical Instrumentation Facility). A 1.0% suspension drop was placed on the holder and dried for 24 h. Samples were mounted on carbon tape, coated with a gold layer, and examined under 5 kV voltage at a magnification greater than 12,000× [9].

2.5. Development of the Films

Film preparation followed [11] with modifications. Two grams of purple yam flour were mixed with 100 mL of water and 15 wt% glycerol. The solution was stirred at 90 °C for 5 min until starch gelatinization occurred. Subsequently, FN was incorporated at concentrations of 5 wt%, 15 wt%, and 25 wt%. The mixture was centrifuged at 5000 rpm for 1 min and dried at 60 °C for 24 h. Films were conditioned in a NaCl desiccator at 75% relative humidity for 24 h, then peeled and stored until analysis. Control films were prepared under identical conditions without nanoparticles. ANOVA and Tukey tests were performed at p < 0.05.

2.6. Mechanical Properties

Tensile strength (MPa) and elongation (%) were measured according to ASTM D88-02 [11,12] with modifications. Tests were conducted using an INSTRON 3400 machine (Department of Forest Biomaterials, North Carolina State University, Raleigh, NC. School of Natural Resources) at 22 °C and 5 N load. Films were cut to 25 mm width and 110 mm length and tested at a speed of 1 mm/s.

2.7. Water Vapor Transmission and Solubility

Water vapor transmission (WVT) was determined following ASTM E96-00. Films were placed on pre-weighed discs to prevent gas or pressure interference. Five grams of calcium chloride was added to create a moisture gradient, allowing vapor to pass through the films for 8 h, with measurements taken every 15 min. WVT was calculated using:

$$WVT ={\displaystyle \frac{G}{tA}}$$

where G is weight change, t is time, and A is area (g/m²·24 h). For solubility, 2 cm × 2 cm film samples were immersed in 50 mL of distilled water for 24 h, then dried at 100 °C [4].

2.8. Contact Angle

Contact angle measurements were performed using an optical contact angle meter (CAM 101, KVS Instruments, North Carolina State University, Raleigh, North Carolina, College of Natural resources). A drop of ultrapure water was placed on the film surface, and 100 images were captured over 60 s. Data were analyzed using KSV Software 2008 [13].

2.9. Differential Scanning Calorimetry (DSC)

Thermal properties (glass transition temperature and melting point) were analyzed using DSC (Q2000, TA Instruments, North Carolina State University, Raleigh, North Carolina, College of Natural resources, USA). Approximately 3 mg of sample was sealed in hermetic pans and equilibrated at 25 °C for 30 min before testing. Samples were heated from 20 °C to 200 °C at a rate of 10 °C/min. An empty pan was used as a reference [14].

3. Results and Discussion

3.1. Aqueous Counter Collision Treatment

The performance of Aqueous Counter Collision (ACC) depends on the use of small nozzle diameters, allowing the sample to pass through opposing water jets [7]. During this process, the sample is subjected to high pressures ranging from 20 to 270 MPa, resulting in rapid pulverization and the formation of nano-sized particles [6].

The development of nanoparticles from purple yam flour was challenging due to limited prior data. Nevertheless, nanoparticle extraction using this novel process for starches or flours was successfully achieved.

As shown in

Table 1. Effect of the concentration of flour in the size of nanoparticles through DLS based on concentration and pressure, turbidity and cycles.

	Concentration (wt%)	Cycles ACC	Pressure (MPa)	Initial Turbidity (NTU)	Final Turbidity (NTU)	DLS Size d. nm
1	1.5	15	200	162	120	162
2	1.5	15	200	-	-	156
3	1.5	15	200	-	-	166
4	2.0	15	200	175	156	-
5	2.0	15	200	-	-	-
6	2.0	15	200	-	-	-
7	2.5	15	200	234	136	236
8	2.5	15	200	-	-	242
9	2.5	15	200	-	-	233
10	1.5	10	200	183	107	102
11	1.5	10	200	-	-	98
12	1.5	10	200	-	-	95
13	2.0	10	200	179	160	-
14	2.0	10	200	-	-	-
15	2.0	10	200	-	-	-
16	2.5	10	200	229	132	109
17	2.5	10	200	-	-	107
18	2.5	10	200	-	-	102
19	1.5	20	200	162	133	201
20	1.5	20	200	-	-	197
21	1.5	20	200	-	-	196
22	2.0	20	200	175	115	126
23	2.0	20	200	-	-	128
24	2.0	20	200	-	-	125
25	2.5	20	200	234	136	120
26	2.5	20	200	-	-	120
27	2.5	20	200	-	-	125

, purple yam flour subjected to high-pressure mechanical treatment produced particle sizes ranging from 95 to 200 nm, representing a significant reduction compared to the native granule size. The smallest particle size obtained in this study was 95 nm (Table 1). Purple yam flour exhibits unique characteristics such as color, particle size variation, and starch granule morphology. According to [15], yams are an excellent source of protein and starch, with average contents of 9% and 80%, respectively. Additionally, purple yam owes its color to phenolic compounds, primarily anthocyanins, along with vitamins and minerals [16]. These compounds may have influenced the inability to achieve particle sizes smaller than 95 nm (T12). However, the ACC method, which applies high-pressure mechanical forces through repeated spray cycles without chemical alteration, enabled nanoparticle production from an impure source such as yam flour.

Higher flour concentrations in suspension resulted in greater particle aggregation, making pulverization more difficult. This is evident in treatments T8 and T9, where particle sizes exceeded 200 nm regardless of cycle number or pressure (Table 1). Conversely, treatments T25 and T26 demonstrated that longer processing cycles at higher concentrations produced particles around 120 nm, while shorter cycles at lower concentrations yielded particles below 100 nm (T11 and T12). Statistical analysis confirmed a significant effect of flour concentration and cycle number on particle size (p = 0.001). These findings suggest that optimizing ACC treatment with short cycles, 200 MPa pressure, and low flour concentrations can produce nanoparticles smaller than 100 nm, enhancing potential applications for purple yam flour. Furthermore, zeta potential measurements for T25 and T26 indicated excellent nanoparticle dispersion stability [16].

3.2. Scanning Electron Microscopy

SEM analysis was challenging due to the small nanoparticle size and the presence of residual compounds such as proteins, cellulose, and other impurities. However, as shown in Figure 1, Image C, nanoparticles around 200 nm exhibited intact, rounded granules, differing from the native granule morphology in Image A. This confirms that ACC treatment reduced particle size without structural destruction. After 20 cycles, significant morphological changes were observed (Image E), whereas 15 cycles resulted in less damage (Image C), suggesting that fewer cycles may preserve granule integrity [17].

Treatments such as 10 cycles (Image B) produced nanoparticles smaller than 100 nm (Table 1), confirming successful nanoparticle formation from purple yam flour. Optimization of this process is necessary, as lower flour concentrations and fewer cycles favored nanoparticle production. The altered granule shape compared to native starch likely resulted from increased exposure to high pressure during pulverization.

Narváez-Gómez et al. [12] reported that native yam starch granules are relatively large, making nanoparticle production below 95 nm challenging. Achieving this size represents a significant advancement in starch-based nanomaterial research. Fang et al. (2021) [16] noted that high-pressure treatments weaken starch granules by disrupting amylose and amylopectin chains, producing particles ranging from 16.7 to 2420 nm. Thus, yam starch is a promising source for nanoparticle development using mechanical, non-chemical methods.

3.3. Differential Scanning Calorimetry

For film development, treatments with the best reproducibility from ACC were selected, T12 (1.5% flour, 10 cycles) and T25 (2.5% flour, 20 cycles), corresponding to nanoparticle sizes of 95 nm and 120 nm, respectively (

Table 2. Concentration of flour nanoparticles (FN) extracted by ACC.

Film 2% PYF	FN %	Glycerol %
Control	0	15 wt%
Film 1	5 wt% (T12)	15 wt%
Film 2	15 wt% (T12)	15 wt%
Film 3	25 wt% (T12)	15 wt%
Film 4	5 wt% (T25)	15 wt%
Film 5	15 wt% (T25)	15 wt%
Film 6	25 wt% (T25)	15 wt%

). Thermal analysis assessed film resistance to high temperatures (

Table 3. Change on the transition temperature Tg and solubility with the addition of FN in the film solution.

Sample	Tg °C	Solubility %
Control	68.66	2 ± 0.004
Film 5 wt% (T12)	98.37	2 ± 0.012
Film 15 wt% (T12)	86.12	12 ± 0.001
Film 25 wt% (T12)	98.37	25 ± 0.015
Film 5 wt% (T25)	55.00	41 ± 0.001
Film 15 wt% (T25)	53.24	78 ± 0.005
Film 25 wt% (T25)	102.25	50 ± 0.001

). The glass transition temperature (Tg) of native starch is approximately 79.8 °C. Control films exhibited Tg at 68.66 °C, whereas films containing nanoparticles showed increased thermal resistance, reaching 98.37 °C, 86.12 °C, and 89.37 °C for 5%, 15%, and 25% FN concentrations with 10 cycles, respectively. These results indicate that nanoparticles enhanced thermal stability. However, films with 5%, 15%, and 25% FN concentrations processed with 20 cycles exhibited lower Tg values than the control, suggesting that excessive nanoparticle incorporation and processing may weaken granule structure, reducing thermal resistance [9]. Additionally, glycerol (15 wt%) may have contributed to reduced thermal stability due to its plasticizing effect and the complex composition of yam flour. Lower Tg values also indicate dehydration and disruption of amylopectin double helices under high temperatures [11].

Solubility depended on starch composition, as starches are generally insoluble in cold water [17,18]. Nanoparticle incorporation significantly increased solubility, possibly due to amylose-rich composition and structural changes induced by high-pressure treatment. Similar trends were reported with solubility in potato starch films increasing from 3.03% to 27.50% [9].

3.4. Mechanical Properties

The mechanical properties of the developed films are presented in

Table 4. Tensile and (%) elongation on purple-yam-based films.

Sample	Tensile (MPa)	% Elongation
Control	2.2 ± 0.01	17.41 ± 0.001
5% (T12)	3.3 ± 0.08	14.66 ± 0.60
5% (T25)	4.3 ± 0.07	13.73 ± 0.07
15% (T12)	6.5 ± 0.05	09.06 ± 0.10
15% (T25)	4.1 ± 0.06	13.09 ± 0.10
25% (T12)	7.1 ± 0.03	9.89 ± 0.04
25% (T25)	4.3 ± 0.04	13.73 ± 0.08

. Increasing the concentration of nanoparticles generally enhanced tensile strength. However, when nanoparticles extracted under treatment T12 were incorporated, tensile strength decreased from 7.1 MPa to 4.1 MPa. This reduction may be attributed to the high concentration of flour nanoparticles (FN) with a nano-structured surface, which could have weakened the material, causing it to fracture under lower force. For comparison, yam starch films can reach tensile strengths up to 36.63 MPa [19]. Also, refs. [11,20] reported values between 2.21–6.01 MPa and 2.56–4.06 MPa for yam starch-based films, which align with the results of this study.

According to Gujral et al. (2020) [9], incorporating nanoparticles from the same starch source should improve mechanical, biodegradability, and barrier properties, noticing that increasing nanoparticle concentration significantly enhances tensile strength. Ref. [21] further explained that crystallinity, influenced by amylopectin content, affects material strength; the higher the amylopectin concentration, the greater the strength due to a more organized structure. Although the flour used in this study did not exhibit tensile values as high as those reported by other authors, the results demonstrate the potential of using impure sources to develop materials with acceptable mechanical properties. Statistical analysis confirmed that nanoparticle concentration significantly affected tensile and elongation properties (p = 0.001). These findings suggest that higher nanoparticle concentrations can improve strength, but excessive processing beyond 10 cycles is unnecessary for achieving favorable particle characteristics.

Several studies have utilized starch nanoparticles to enhance the mechanical and barrier properties of starch-based films. Ref. [5] reported that incorporating up to 5 wt% waxy corn starch nanocrystals improved the mechanical and barrier properties of pea starch films. This improvement is attributed to the small particle size and strong interactions between nanoparticles and the polymeric matrix [22]. Flour typically contains not only starch but also proteins, lipids, fiber, minerals, and minor pigments, all of which can interfere with polymer chain packing and network formation during film casting. In contrast, using pure starch generally allows for better control of gelatinization, retrogradation, and intermolecular interactions between starch chains, which can result in a more continuous and homogeneous polymer network.

3.5. Contact Angle

Contact angle is a critical property for determining whether a material is hydrophilic or hydrophobic, as well as its water repellency. Ref. [23] stated that surfaces with contact angles below 90° are hydrophilic, while those above 90° are hydrophobic. Ref. [16] further noted that superhydrophobic surfaces exhibit angles greater than 150° [24].

As shown in

Table 5. Changes in the contact angle respect to the concentration of FN in the films.

Concentration of FN (%)	Mean—Contact Angle (°)
5%	80.54 ± 0.1
15%	99.74 ± 0.1
25%	85.06 ± 0.1

, contact angle increased significantly (p < 0.05) with higher concentrations of purple yam flour nanoparticles obtained via ACC. At a 15 wt% nanoparticle concentration, the contact angle reached 99.74°, classifying the material as hydrophobic (Figure 2). However, at a 25 wt% concentration, the angle decreased to 85.06°, reducing water repellency. Similar behavior was observed at a 5 wt% concentration. Ref. [25] reported that hydrophobicity in starch-based materials increases with starch concentration, as starch polymers inherently resist cold-water solubility. Ref. [26] emphasized that smaller particles enhance water resistance, and nano-scale structures produced by mechanical processes yield stronger, more hydrophobic materials. Ref. [16] also highlighted that particle size and granule shape significantly influence hydrophobicity, with sharp-edged polyhedral granules increasing contact angle values [24].

Although purple yam flour is not a pure material, contact angles of up to 99° were achieved. ACC treatment altered particle size and morphology, improving this property as previously noted by [8], Comparable results were reported by [27], with contact angles between 85.16° and 89.76° when chitosan was added at 0.5%.

3.6. Scanning Electron Microscopy of the Films

SEM analysis (Figure 3) revealed notable morphological changes upon incorporating flour nanoparticles extracted via ACC. Films with a 5% nanoparticle concentration (Image b) exhibited a more homogeneous surface compared to control films (Image a), although fibrous structures were still visible. This suggests that nanoparticles influenced matrix organization during starch gelatinization, producing a more compatible and uniform structure [25].

Films with 15% nanoparticle concentration (Image c,) showed further surface organization, with fewer visible starch granules. At a 25% concentration (Image d), the surface appeared smoother (Figure 4) and fibers disappeared, although some ungelatinized starch granules remained. This may be due to temperature variations, granule resistance, or impurities in the flour.

Films developed under T25 (5%, 15%, and 25% nanoparticle concentrations; Images e, f and g) displayed even smoother surfaces despite larger nanoparticle sizes (120 nm). This indicates that adding yam flour nanoparticles improves film compatibility and visual uniformity. However, occasional ungelatinized granules were observed, likely due to incomplete gelatinization or interference from flour impurities [28].

3.7. Water Vapor Transmission

Water vapor transmission (WVT) is a key barrier property for packaging materials. As shown in

Table 6. Water vapor transmission of films with 5, 15 and 25% of nanoparticles.

Film 2% PYF	Nanoparticles (wt%)	WVT (g/m2·24 h)
Control	0	15.85 ± 1.09
Film 1	5% (1.5% 10 cycles)	15.43 ± 2.08
Film 2	15% (1.5% 10 cycles)	13.35 ± 2.75
Film 3	25% (1.5% 10 cycles)	4.68 ± 1.09
Film 4	5% (2.5% 20 cycles)	8.01 ± 1.08
Film 5	15% (2.5% 20 cycles)	6.68 ± 2.35
Film 6	25% (2.5% 20 cycles)	5.60 ± 2.09

, films containing nanoparticles exhibited significant differences (p = 0.001) compared to controls. For example, Film 3 demonstrated reduced WVT, likely due to the higher nanoparticle concentration (25 wt%), which improved water resistance. Treatments with 15% nanoparticles achieved a WVT of 6.68 g/m²·24 h, indicating an enhanced barrier performance compared to the control. These improvements are consistent with previous findings that nanoparticle incorporation strengthens film structure and reduces permeability [28].

4. Conclusions

The developed films demonstrated significant improvements in mechanical, barrier, and morphological properties through the incorporation of nanoparticles produced via the Aqueous Counter Collision (ACC) mechanical process. Furthermore, combining purple yam flour nanoparticles within the same yam flour matrix resulted in improved compatibility, confirming the potential of this approach for developing sustainable and functional biopolymer materials. Notably, using flour instead of purified starch represents a key achievement by leveraging impure, low-cost agricultural byproducts while maintaining performance. Although ACC proves highly effective for nanoparticle extraction, its implementation remains financially challenging due to equipment costs and processing demands.