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Article

pH-Sensitive Starch-Based Packaging Films Enhanced with Wild Blackberry Extract

by
Kristina Cvetković
1,
Aleksandar Lazarević
1,
Sandra Stamenković Stojanović
1,
Simona Dordevic
2,
Dani Dordevic
2,
Ivana Karabegović
1 and
Bojana Danilović
1,*
1
Faculty of Technology, University of Niš, Bulevar Oslobođenja 124, 16000 Leskovac, Serbia
2
Department of Plant Origin Food Sciences, Faculty of Veterinary Hygiene and Ecology, University of Veterinary Sciences, 61242 Brno, Czech Republic
*
Author to whom correspondence should be addressed.
Processes 2025, 13(4), 1148; https://doi.org/10.3390/pr13041148
Submission received: 6 March 2025 / Revised: 2 April 2025 / Accepted: 6 April 2025 / Published: 10 April 2025
(This article belongs to the Special Issue Antimicrobial Food Packaging: Materials and Technologies)
Browse Figures
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Abstract

:
This study aims to develop and evaluate pH-sensitive food packaging films, based on starch and enriched with aqueous wild blackberry extract (Rubus sp.). The extract was selected for its high anthocyanin content due to color changes in different pH environments. Extract analysis revealed a dry matter content of 23 mg/mL and a polyphenol concentration of 21.10 mg GAE/g (dry extract), with high antioxidant activity, measured to be an 86.57% DPPH radical neutralizer. Films were produced with wild blackberry extract at concentrations of 0%, 5%, 10%, and 15%. The analyses determined the barrier, mechanical, physical, and intelligent properties of biodegradable films. The introduction of the extract resulted in a substantial rise in water content (9.6–21.36%), swelling capacity (35.27–43.06%), dissolution rate in water (288.05–459.89%), and permeability to water vapor (1.99–3.69 × 10−10 g/(Pa × m × s)). The bioactive compounds in the extract enhanced the films’ antimicrobial and antioxidant properties, with the highest effectiveness observed in the film containing 15% extract. These starch-based films, enriched with aqueous wild blackberry extract, demonstrated strong potential for packaging foods prone to pH changes during fermentation, such as fruits, dairy products, meat, and fish.

1. Introduction

Food packaging preserves food quality and microbiological integrity while also serving as a means of conveying product information. Various materials, including paper, glass, metal, and plastic, have long been utilized in food packaging [1]. At present, around half of all food packaging materials originate from fossil fuels [2]. According to the data from the Center for Environmental Improvement (2022), as much as 79% of plastic originating from fossil fuels ends up in landfills. As a consequence of the degradation of plastic, microplastic particles are created and can easily accumulate and enter the food chain [3]. Hence, it is imperative to prevent and reduce the disposal of plastic and preserve resources. One of the solutions is the replacement of plastic and the introduction of packaging materials made of natural biopolymers [4,5]. Edible films can enhance the shelf life of packaged food by offering protection against microorganisms and oxidation, while also boosting its nutritional content and sensory attributes [6]. Certain types of food packed in edible films can be consumed without removing the packaging, pointing out one more advantage of this packaging [7,8].
Starch is a widely available biopolymer in nature, making it a highly economical choice for producing edible films [1,9]. Thanks to characteristics such as transparency, the absence of smell and taste, and gelling ability, starch can compete with most polymers obtained from fossil fuels [10]. For the production of films, it can often be combined with other biopolymers, such as gelatin [11,12], chitosan [13], and carrageenan [14], to enhance mechanical and barrier characteristics. To improve the antioxidant and antimicrobial properties of edible films, different fortifying agents can be added, such as essential oils [15,16], probiotics and prebiotics [17], or plant extracts [18,19]. Intelligent packaging has the ability to monitor the integrity of food, detect changes and irregularities, indicate the condition of packaged food and thus warn of potential problems [20]. The advancement of intelligent starch-based packaging films is attracting considerable attention, especially in the design of pH-responsive films. These films can detect and signal pH changes in packaged food, which often result from spoilage [13,18,21,22].
Wild blackberry (Rubus sp.), a perennial species in the Rosaceae family, contains a diverse range of bioactive compounds, including essential vitamins, minerals, flavonoids, terpenes, tannins, a variety of acids, and lipids, all of which contribute to its nutritional and therapeutic value [23]. Among the phenolic compounds, anthocyanins are the most common, while ellagitannins, flavonols, and hydroxybenzoic acids are also present [24,25]. Anthocyanins are pigments responsible for the color of the wild blackberries, but can also be hydrogen donors and have the ability to neutralize monovalent oxygen. These properties contribute to the pronounced antioxidant activity of blackberries [26]. Apart from polyphenols, the antioxidant activity of blackberries mostly relies on the present ascorbic acid [27]. Additionally, polyphenols present in blackberry extract can have antimicrobial activity [27,28]. In addition to their antimicrobial and antioxidant potential, anthocyanins can to change color at different pH values, so they can be used as indicators [29]. Blackberry extract can be utilize in the pH-sensitive packaging production due to its high anthocyanin content, which is sensitive to pH fluctuations in the surrounding environment [30].
In response to the increasing demand for alternative food packaging solutions, this study focuses on developing and characterizing starch-based pH-sensitive packaging films that are enriched with different concentrations of wild blackberry extract (WBE) to improve the properties of the films.

2. Materials and Methods

2.1. Preparation of Wild Blackberry Extract

A water-based extract of wild blackberry (Rubus sp.) was obtained using a modified protocol adapted from Norajit et al. (2010) [31]. Fresh wild blackberry fruits were initially finely chopped and then subjected to extraction with distilled water at a 1:10 ratio. The extraction process was carried out using a SCILOGEX SCI280-Pro stirrer (Rocky Hill, CT, USA) at 50 °C for a duration of two hours. Following extraction, the mixture was passed through Whatman No. 3 filter paper to eliminate any remaining fruit residues. The purified extract was then stored in a refrigerator until it was needed for further analysis.

2.2. Determination of Dry Matter Content in the Wild Blackberry Extract

To determine the dry matter content, the wild blackberry extract (WBE) sample was dehydrated at 105 °C until a constant mass was reached. A 10 mL portion of WBE was transferred into a pre-weighed flask and subjected to drying. After the drying process, the sample was placed in a desiccator to cool, and its weight was subsequently recorded using an analytical balance (Joanlab, Ningbo, China). The dry matter content was determined based on dry residue content and expressed as mg/mL of extract.

2.3. Preparation of Starch-Based Films

Edible packaging films were produced following the methodology described by Al-Hashimi et al. (2020) [16]. The films were fabricated using the casting technique, where 4.5 g of starch (Centrohem doo, Stara Pazova, Serbia) was dissolved in 130 mL of distilled water. The solution was continuously stirred on a magnetic stirrer (SCILOGEX SCI280-Pro, Rocky Hill, CT, USA) at 75 °C and 400 rpm until it formed a gelatinous consistency. Subsequently, 1.1 mL of glycerol was introduced, and stirring continued for an additional 10 min. Wild blackberry extract (WBE) was incorporated into the starch matrix at concentrations of 0, 5, 10, and 15% w/w (expressed as grams of dry extract per gram of prepared film). The mixture was then homogenized using an ultrasonic homogenizer (Witeg, Wertheim, Germany) at 13,500 rpm for 3 min. Finally, the prepared solution was poured into Petri dishes (ϕ16 cm) and allowed to dry at room temperature for 48 h.

2.4. Determination pH Sensitive Properties of Starch-Based Films

The color-responsive behavior of starch-based films in different pH environments was analyzed using a modified approach based on Prietto et al. (2017) [32]. To evaluate pH sensitivity, a 2 cm × 2 cm film sample was exposed to a drop of either 0.1 M HCl solution, distilled water, or 0.1 M NaOH. The resulting color change was visually assessed by comparing the film’s appearance to its original color in contact with water. Each experiment was performed in triplicate to ensure reliability and accuracy.

2.5. Determination of Mechanical Properties of Starch-Based Films

The thickness of the films was measured using a thickness gauge (INSIZE 2364-10, Precision Measurement, Suzhou, China) following the ISO 4593:1993 standard [33]. Measurements were taken at five different locations to ensure accuracy [34].
Tensile strength (MPa) and elongation at break (%) were determined using a texture analyzer (TA.XT plus, Stable Micro Systems, Godalming, UK) according to the method described by Dordevic et al. (2023) [35]. Film strips measuring 15 cm × 1 cm were cut and tested following the ASTM D882-02 standard [36].

2.6. Assessment of Water Content, Swelling Behavior, and Solubility of Starch-Based Films

The films were analyzed for their water content, swelling behavior, and solubility using a modified procedure based on Souza et al. (2017) [37]. Film samples were cut into 2 cm × 2 cm squares and initially weighed on an analytical scale, with the recorded mass designated as W1. The samples were then dried in a laboratory dryer (Sutjeska, Belgrade, Serbia) at 105 °C for 2 h and reweighed (W2). Following this, they were submerged in 25 mL of water and allowed to soak at room temperature for 24 h. After the immersion period, the samples were carefully blotted dry using filter paper and weighed again (W3). Finally, the films underwent a second drying process at 105 °C for 24 h, after which their final weight (W4) was recorded. The analyses were performed in triplicate. The values were calculated according to Equations (1)–(3):
Water content (%) = [(W1 − W2)/W1] × 100
Solubility (%) = [(W2 − W4)/W2] × 100
Degree of swelling (%) = [(W3 − W2)/W2] × 100

2.7. Measurement of Water Vapor Permeability in Starch-Based Films

A modified gravimetric method was employed to assess the water vapor permeability of the films, following the procedure described by [38]. A vial was filled with silica gel, and its opening was sealed with the film sample under evaluation. The vial was then weighed using an analytical scale and subsequently placed in a desiccator containing distilled water at room temperature. During three days, the samples were weighted and the water vapor permeability was calculated as (Equation (4)):
WVP = (W × x)/(A × t × ΔP)
where W—increase in the sample weight (g), x—film thickness (m), t—elapsed time (s), A—surface of leakage area (m2), and P—partial pressure difference between pure water vapor and dry atmosphere (2339 Pa at 20 °C).
The experiment was performed in three replications and expressed as µg × m−1 × s−1 × Pa−1.

2.8. Determination of Antioxidant Activity of Extract and Starch-Based Films

The antioxidant activity was evaluated using the DPPH method [34]. To prepare the samples, starch-based films were crushed, and 0.1 g of each was mixed with 20 mL of ethanol. The same process was used to evaluate the antioxidant properties of WBE. The samples were subjected to sonication for 30 min and then filtered. A 3 mL aliquot of the resulting filtrate was combined with 1 mL of a 0.1 mM DPPH radical solution (Sigma Aldrich, St. Louis, MO, USA) in ethanol and left to incubate for 30 min in the dark. The absorbance at 517 nm was measured using a UV-VIS spectrophotometer (model UV-M51, Bel Engineering, Monza, Italy), with ethanol serving as the blank. The DPPH radical neutralization capacity was determined according to Equation (5):
DPPH scavenging activity [%] = [(AbsDPPH − Abs sample)/AbsDPPH] × 100
where AbsDPPH—DPPH solution absorbance, Abs sample—sample absorbance.

2.9. Determination of Total Polyphenols of Starch-Based Films

The total polyphenol content was measured using the method described by Dou et al. (2018) [39]. The samples were dissolved in distilled water at a ratio of 1:40. After 10 min, 5 mL of Folin–Ciocalteu reagent was added to 1 mL of the solution, followed by the addition of 4 mL of 7.5% Na2CO3. The mixture was then left in the dark for 30 min. Absorbance was recorded at 765 nm, with distilled water serving as the blank and gallic acid being used as the standard for quantification.

2.10. Measurement of Bioactive Component Migration from Starch-Based Films

The migration of bioactive components was assessed using the method outlined by Dordevic et al. (2021) [34]. Film samples measuring 1 cm × 1 cm were placed in pre-labeled tubes containing 2.5 mL of a 10% ethanol solution and incubated at 30 °C for 10 days. After this period, the solution was filtered to remove any film remnants. The analysis involved measuring both the total polyphenol content and the DPPH radical scavenging ability of the filtrate.

2.11. Assessment of Antimicrobial Properties of Extract and Starch-Based Films

The disk diffusion method, described in the guidelines of the European Committee for Antimicrobial Susceptibility Testing (EUCAST) was used for determination of antimicrobial activity. Disks, each 5 mm in diameter, were cut from the films and disinfected by exposure to UV light (260 nm) for 60 s. The effectiveness was tested against Escherichia coli ATCC 25922, Klebsiella pneumoniae ATCC 700603, Proteus vulgaris ATCC 8427, Pseudomonas aeruginosa ATCC 27853, Bacillus subtilis ATCC 6633, Staphylococcus aureus ATCC 25923, Candida albicans ATCC 2091, and Bacillus cereus. The inoculum concentration was approximately 1–2 × 108 CFU/mL, in line with the McFarland 0.5 turbidity standard. Using sterile swabs, the inoculum was spread across the surface of the substrates. For bacterial cultures, nutrient agar (Torlak, Belgrade, Serbia) was used, and for yeast, Sabouraud maltose agar (Torlak, Belgrade, Serbia). For the determination of the antimicrobial activity of the extract, 50 µL of WBE was placed on paper disks 5 mm in diameter, with water used as a control sample. The disks of films and paper disks with WBE were placed on the inoculated surfaces, and the plates were incubated at 35–37 °C for 24 h. Antimicrobial activity was assessed visually, and inhibition zones were expressed in mm.

2.12. FTIR Analysis of Starch-Based Films

Fourier transform infrared (FTIR) spectroscopy was used to analyze starch-based edible films containing blackberry extract. The FTIR spectra were obtained in the range of 400–4000 cm−1 [40]. To prepare the KBr pellet, 0.1 g of the film was finely ground and blended with 1.5 g of KBr. The measurements were performed using a BOMEM MB-100 spectrometer (Hartmann & Braun, Brampton, ON, Canada), which was equipped with a KBr detector and had a resolution of 4 cm−1.

2.13. Statistical Analysis

The data are expressed as mean values ± standard deviation. To determine statistically significant differences among the samples, a one-way ANOVA was conducted, followed by Tukey’s post hoc test. Statistical analysis was carried out using SPSS 21.0 software (IBM, New York, NY, USA), with a significance threshold set at p < 0.05.

3. Results and Discussion

3.1. Extract Characterization

Based on the results obtained (Table 1), the dry matter content was 23.0 mg/mL of extract. The polyphenol content in the extract was notably high, at 21.10 mg GAE/g of dry extract, leading to a significant DPPH radical scavenging activity of 86.57%. Analyses of DPPH radical neutralization capacity showed that blackberry extract has high antioxidant activity, which further indicates a high anthocyanin extract content [28].
Previous findings indicate that the content of polyphenols in blackberry extract range from 17 to 38 mg GAE/g dry extract [24,41] in accordance with the results presented in this paper. These results align with the findings of Albert et al. (2022) [42], who documented the polyphenol content of 21.43 mg GAE/g of dry extract in ethanolic blackberry extract. Their study also reported a DPPH radical scavenging capacity ranging from 47% to 76%, depending on the extraction solvent, which is lower than the antioxidant activity observed in the extract analyzed in this research.
The antimicrobial activity of wild blackberry extract (WBE) was evaluated against both Gram-positive and Gram-negative bacteria, as well as the yeast Candida albicans (Table 2). WBE exhibited strong antimicrobial effects against all tested bacteria, with inhibition zones ranging from 9.2 to 9.8 mm in diameter. Pseudomonas aeruginosa and Proteus vulgaris showed the greatest sensitivity to the extract’s antimicrobial properties. These results are in agreement with those of Gil-Martínez et al. (2023) [27], who demonstrated the antimicrobial efficacy of blackberry extract against a broad spectrum of bacterial strains, including Salmonella enterica CECT 7160, Bacillus cereus CECT 8168, Listeria monocytogenes CECT 4032, Escherichia coli CECT 405, Shigella sonnei CECT 457, Pseudomonas aeruginosa CECT 116, Staphylococcus aureus CECT 239, Zygosaccharomyces bailii CECT 11997, and Aspergillus niger CECT 2090. These results indicate the great potential of WBE in the production of edible films in terms of the microbial stability of the product. On the other hand, the inhibition was not recorded against C. albicans, indicating a weak antimycotic activity of the extract. Similarly, research conducted by Četojević-Simin et al. (2017) [43] indicated that blackberry extract did not show antimicrobial activity against yeasts. According to some research, polyphenols, primarily anthocyanins from blackberry extract, can have the ability to negatively affect the metabolic activity of microorganisms. The mechanisms of action are not clarified, but it is believed that the antimicrobial potential of blackberry extract is based on the presence of polyphenols, i.e., anthocyanins, which play a key role in preventing the metabolic activity of bacteria [44]. One of the assumptions is that the dissociation of phenols occurs on the membrane of the microorganism, which causes a sudden drop in the pH value inside the cell, which inhibits ATP synthesis [45]. Other polyphenols, such as phenolic acids and tannins, as well as organic acids present in the extract, can have an influence on the antimicrobial activity of blackberry extract. Organic acids have the ability to diffuse through the membrane, where dissociation occurs, causing changes in the pH value of the cytoplasm and the inhibition of enzymes and damage to the cellular structures of microorganisms, resulting in cell death [46].

3.2. Starch-Based Films Characterization

Incorporating WBE into starch-based films led to the development of a promising smart packaging material capable of changing color in response to pH variations. This extract was chosen for its high anthocyanin content, which contributes to this pH sensitivity and enhances its suitability for smart packaging applications. The characteristics of the pure extract, including its anthocyanin profile and antioxidant activity, are presented in Table 1.
The resulting films were not transparent. The control film was white in color, while the intensity of the color for other films varied depending on the amount of added extract (Figure 1). According to the visual appearance, it can be concluded that, with the change in the WBE content in the films, a change in color can be observed.

3.3. pH Sensitive Properties of Starch-Based Films

WBE is rich in anthocyanins that have the ability to change color in relation to the pH value of the environment [47]. Visual analysis revealed that the films change color depending on the pH value (Figure 2). The films acquired a blue-green color in the alkaline medium, the visually detected intensity of which depended on the concentration of the extract. In an acidic medium, the films had an intense pink-red color that intensified with increasing extract concentration. The color change is attributed to the anthocyanins present in the blackberry extract [30,47]. As shown in Figure 2, films without the added extract exhibited no color change under varying conditions. Similar findings were reported by Gutiérrez (2017) [48] for starch-based films containing blackberry pulp. The color shift in films exposed to environments with different pH values indicates that these films can be classified as intelligent packaging [47].

3.4. Mechanical Characteristics of Starch-Based Films

The evaluation of the films’ mechanical properties (Table 3) highlighted significant differences depending on their composition. Film thickness varied between 0.14 and 0.16 mm, as shown in Table 3. Statistical analysis revealed no significant differences in thickness between the different films (p > 0.05), indicating that the addition of various concentrations of WBE did not substantially impact the thickness of the starch-based films. These results are consistent with those reported by Sganzerl et al. (2021) [28], who found an average thickness of 0.16 mm for films made from carboxymethyl cellulose and blackberry extract.
The tensile strength values significantly decreased with the addition of WBE. The control film exhibited the highest tensile strength value (0.83 MPa), while the films with 10% and 15% extract had significantly lower values (0.20–0.19 MPa, respectively). This decrease can be attributed to the interactions between the extract components and the polymer matrix, leading to the weakening of molecular bonds within the film [49].
The inclusion of wild blackberry extract (WBE) led to a slight increase in the elongation at break of the films, with values ranging from 251% to 253%, compared to 235% for the control film without the extract. This suggests that the extract enhanced the films’ flexibility, likely due to the hydrophilic compounds acting as additional plasticizers [50,51]. The toughness of the films also improved with the addition of WBE, varying between 0.02 and 0.05 MJ/m3, whereas the control film exhibited the lowest toughness. Similar trends have been reported in previous studies. Jha et al. (2020) [50] observed that incorporating grape seed extract into starch–chitosan films reduced tensile strength while increasing elongation at break. Likewise, Yan et al. (2013) [52] found comparable results in alginate–starch films containing rosemary extract. Other research has highlighted how higher phenolic compound concentrations affect mechanical properties in biopolymer films. For instance, adding Lapacho tea extract to carrageenan-based films decreased tensile strength while enhancing elongation at break. Similarly, Kanmani et al. (2014) [53] found that grapefruit seed extract significantly reduced tensile strength while increasing elongation, likely due to interactions between phenolic compounds and polysaccharides [51].

3.5. Determination of Water Content, Degree of Swelling, Solubility, and Water Vapor Permeability of Starch-Based Films

The examination of the physical properties of starch-based films (Table 4) with the inclusion of WBE demonstrated that the water content varied between 4.92% and 21.36%. The starch-only films, without the addition of WBE, had the lowest water content. As the concentration of the extract increased, the water content also increased significantly (p < 0.05). This finding aligns with the research by Yong et al. (2019) [29], which indicated that higher concentrations of plant extracts rich in hydrophilic anthocyanins led to an increase in the moisture content of films. The obtained results align with the existing literature, which suggests that starch-based films generally have low water content. In the study by Nogueira et al. (2022) [54], starch-based films containing grape pomace extract at concentrations of 0%, 20%, 30%, and 40% exhibited water content ranging from 8.17% to 12.48%. Similarly, Nogueira et al. (2018) [55] reported that incorporating blackberry pulp into starch-based films resulted in increased water content. Taweechat et al. (2021) [56] also observed a rise in moisture content in starch-based films with higher concentrations of banana peel extract. Conversely, Sganzerla et al. (2021) [28] found that increasing the concentration of blackberry ethanol extract reduced moisture content in carboxymethyl cellulose-based films. These variations in findings may be attributed to differences in solvent selection during extract preparation.
The solubility of the starch-based film was lower compared to the other films analyzed; however, the incorporation of the extract resulted in a significant increase (p < 0.05) in the solubility, with values ranging from 30.59% to 43.06%. The degree of swelling in the starch-based films was high, ranging from 202.53% to 459.89%. Films containing 15% WBE exhibited the highest swelling degree, while the starch-based film without the extract had the lowest swelling degree.
The addition of WBE had a significant impact (p < 0.05) on the water vapor permeability of starch-based films. The permeability values varied between 0.61 and 3.69 × 10−10 g/(Pa × m × s), with the control film (without extract) exhibiting the most effective barrier properties. As the concentration of WBE increased, so did the water vapor permeability, with the highest permeability observed in the film containing 15% extract. These results align with findings by Kurek et al. (2019) [57], who reported an increase in water vapor permeability in films based on chitosan and carboxymethyl cellulose when blueberry and grape skin extracts were added. Similarly, Taweechat et al. (2021) [56] found that the water vapor permeability was lowest in films made from banana starch without added extract, but the addition of banana peel extract significantly enhanced permeability. Piñeros-Hernandez et al. (2017) [58] also observed that rosemary extract increased the water vapor permeability of starch films. These results, combined with the existing literature, suggest that incorporating plant extracts into starch-based films increases water vapor permeability. The hydrophilic nature of plant extracts, particularly the -OH groups, likely interacts with water molecules, promoting water vapor transport through the film [57]. Research by Yun et al. (2019) [59] further supports this by showing that higher concentrations of mountain peach extract cause the formation of aggregates that disrupt the biopolymer network, thereby enhancing water vapor movement.

3.6. Total Polyphenol Content and Antioxidant Activity of Starch-Based Films

The total content of polyphenols in starch-based films with the addition of WBE and their antioxidant activity is shown in Table 5.
Starch-based films without the addition of WBE exhibited a low polyphenol content (0.10 mg GAE/g of film). As anticipated, incorporating WBE led to a significant (p < 0.05) increase in polyphenol content. The Folin–Ciocalteu test measured polyphenol levels in films with added extract, ranging from 0.72 to 1.31 mg GAE/g of film. The high polyphenol content in the WBE (Table 1) directly contributed to the antioxidant capacity of the films. The results indicate that the degree of radical neutralization increased by almost 20% with the addition of 5% WBE, and further increasing the WBE concentration in the films led to an additional rise in DPPH radical neutralization. These findings demonstrate that the polyphenols in the starch matrix play a significant role in the antioxidant activity of the films [60]. These results align with those of Nogueira et al. (2018) [55], who found that both the polyphenol content and antioxidant potential of films were influenced by the concentration of blackberry pulp. Gutiérrez (2017) [48] also noted that starch-based films enriched with blackberry pulp had higher total polyphenol content and antioxidant capacity compared to control films. The literature supports the idea that the inclusion of plant extracts in films can notably enhance the antioxidant stability of food products [59,61,62].

3.7. Migration of Bioactive Components from Starch-Based Films

The migration of bioactive components from the film was studied to assess the extent to which active substances can transfer from the packaging to the food. The analysis of the migration of bioactive components was based on the monitoring of the total content of polyphenols and the degree of DPPH radical neutralization in filtrate obtained by immersing a film into a simulation medium (Table 6).
The results indicated that polyphenols were released from the starch-based films containing added WBE. The film with 15% WBE showed the highest level of polyphenol migration, reaching 0.05 mg GAE/g of film. This outcome was further supported by the degree of DPPH radical neutralization, which varied between 53.76% and 68.26%, depending on the concentration of the extract in the film samples. The measurements, taken after 10 days, suggest that the antioxidant activity of the films remained intact over time, likely due to the controlled release of polyphenols from the biopolymer matrix [48].

3.8. Antimicrobial Activity of Starch-Based Films

The examination of the antimicrobial activity of the films showed effectiveness against all tested bacteria (Table 7), including E. coli ATCC 25922, P. aeruginosa ATCC 27853, P. vulgaris ATCC 8427, S. aureus ATCC 25923, B. subtilis ATCC 6633, K. pneumoniae ATCC 700603, and B. cereus. These results were anticipated based on the antimicrobial activity of the extract (Table 2). The antimicrobial activity of the starch films with the added extract was significantly different (p > 0.05) from that of the starch film without the extract, confirming the positive impact of wild blackberry extract on the films’ antimicrobial properties. Statistical analysis indicated that increasing the concentration of wild blackberry extract up to 15% did not lead to significant differences in antimicrobial activity, suggesting a threshold beyond which additional extract concentration no longer significantly impacted bacterial growth inhibition. Furthermore, films with varying extract concentrations exhibited similar antimicrobial potential, indicating that increasing the extract concentration beyond a certain point did not enhance bacterial growth inhibition. Consistent with the extract itself, starch films with different concentrations of wild blackberry extract showed no antimicrobial activity against Candida albicans yeast (Table 5). These findings are in line with previous research by Gutiérrez (2017) [48], who reported that blackberry pulp did not notably enhance the antimicrobial activity of starch films against yeasts. Some studies suggest that the antimicrobial mechanism of these films involves damage to the microbial cell membrane [63]. Polyphenols are believed to be responsible for the antimicrobial activity [64], as they can interact with bacterial membranes, increasing membrane permeability, causing leakage of intracellular contents, and leading to cell death. Additionally, phenolic compounds may interfere with the bacterial metabolism by inhibiting essential enzyme activity and disrupting bacterial signaling, ultimately limiting their growth and reproduction [65].

3.9. FTIR Spectrums of Starch-Based Films

Figure 3 presents the FTIR spectra of starch-based films incorporating varying concentrations of wild blackberry extract. A prominent broad band at 3381 cm−1 appears in all samples, corresponding to OH groups involved in hydrogen bonding within the starch matrix. A band detected at 2952 cm−1 is associated with C-H stretching vibrations, with its intensity increasing as the extract concentration rises. The peak at 1644 cm−1 is linked to hydrogen bonding from water molecules absorbed in the polysaccharide structure. Additionally, a deformation vibration, commonly referred to as scissoring, is observed at 1458 cm−1. The presence of a band at 1042 cm−1 signifies C-O stretching vibrations. However, distinct peaks characteristic of blackberry extract are not easily discernible, as their signals often overlap with those of starch and other polysaccharides.

4. Conclusions

The incorporation of WBE into starch-based films significantly altered their mechanical, physical, barrier, and functional properties, making them suitable for use in packaging perishable products. The films exhibited pH-sensitive behavior, changing color in response to environmental pH variations, and demonstrated strong antimicrobial activity against a range of bacterial strains. Although the addition of WBE led to a decrease in tensile strength, it improved the films’ flexibility and toughness. Additionally, the increased water content and solubility suggest that the hydrophilic nature of the extract influenced the films’ water absorption properties. The films also displayed notable antioxidant activity, further enhancing their potential for packaging sensitive ingredients. In conclusion, these findings emphasize the potential of using WBE to create biodegradable and intelligent packaging materials with antimicrobial, pH-sensitive, and antioxidant properties.

Author Contributions

Conceptualization: B.D. and D.D.; methodology, K.C.; software, A.L.; validation, A.L.; formal analysis, S.S.S.; investigation, K.C.; resources, D.D. and S.D.; data curation, I.K.; writing—original draft preparation, K.C. and A.L.; writing—review and editing, B.D. and I.K.; visualization, S.S.S.; supervision, B.D. and I.K.; project administration, S.S.S.; funding acquisition, D.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Ministry of Science, Technological Development and Innovations of the Republic of Serbia grant numbers 451-03-137/2025-03/200133.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Processes 13 01148 g001
Figure 1. Appearance of films based on starch with the addition of WBE—s5%WBE, s10%WBE, s15%WBE—starch-based films with the addition of 5, 10, and 15% wild blackberry extract, respectively; s—starch-based film without added extract.
Figure 1. Appearance of films based on starch with the addition of WBE—s5%WBE, s10%WBE, s15%WBE—starch-based films with the addition of 5, 10, and 15% wild blackberry extract, respectively; s—starch-based film without added extract.
Processes 13 01148 g001
Processes 13 01148 g002
Figure 2. Color change in starch-based films with the addition 0, 5, 10, and 15% of WBE in acidic (a) and alkaline (b) environments.
Figure 2. Color change in starch-based films with the addition 0, 5, 10, and 15% of WBE in acidic (a) and alkaline (b) environments.
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Processes 13 01148 g003
Figure 3. FTIR spectrums of starch-based films with the addition of (a) 0%, (b) 5%, (c) 10%, and (d) 15% blackberry extract.
Figure 3. FTIR spectrums of starch-based films with the addition of (a) 0%, (b) 5%, (c) 10%, and (d) 15% blackberry extract.
Processes 13 01148 g003
Table 1. Characteristics of wild blackberry extract.
Table 1. Characteristics of wild blackberry extract.
Dry matter content (mg/mL)23.0 ± 0.01
Total polyphenol content (mg GAE/g dry extract)21.10 ± 0.4
DPPH radical neutralization capacity (%)86.57 ± 0.18
Table 2. Antimicrobial activity of aqueous wild blackberry extract.
Table 2. Antimicrobial activity of aqueous wild blackberry extract.
MicroorganismInhibition Zone (mm)
Escherichia coli ATCC 259229.2 ± 0.4
Pseudomonas aeruginosa ATCC 278539.8 ± 0.4
Proteus vulgaris ATCC 84279.8 ± 0.4
Staphylococcus aureus ATCC 259239.6 ± 0.5
Bacillus subtilis ATCC 66339.3 ± 0.5
Klebsiella pneumoniae ATCC 7006039.4 ± 0.5
Candida albicans ATCC 2091na
Listeria monocytogenes ATCC 153139.6 ± 0.5
Bacillus cereus9.2 ± 0.4
na—no activity.
Table 3. Mechanical characteristics of starch-based films prepared with different wild blackberry extract concentrations.
Table 3. Mechanical characteristics of starch-based films prepared with different wild blackberry extract concentrations.
Film Thickness (mm)Tensile Strength (MPa)Elongation to Break (%)Toughness (MJ/m3)
s0.14 ± 0.01 a0.83 ± 0.04 c235.47 ± 3.42 a0.02 ± 0.01 a
s5% WBE0.15 ± 0.01 a0.25 ± 0.02 b252.94 ± 0.25 b0.05 ± 0.00 c
s10% WBE0.15 ± 0.02 a0.20 ± 0.0 a251.73 ± 2.97 b0.05 ± 0.01 bc
s15% WBE0.16 ± 0.01 a0.19 ± 0.01 a252.27 ± 1.53 b0.04 ± 0.00 b
s5%WBE, s10%WBE, s15%WBE—starch-based films with the addition of 5, 10 and 15% wild blackberry extract, respectively; s—starch-based film without added extract; a–c—different letters refer to the statistically significantly different values in the same column (p < 0.05).
Table 4. Water content, solubility, and degree of swelling of starch-based films with the addition of wild blackberry extract.
Table 4. Water content, solubility, and degree of swelling of starch-based films with the addition of wild blackberry extract.
Water Content, %Solubility, %Degree of Swelling, %Water Vapor Permeability × 10−10, g/(Pa × m × s)
s4.92 ± 0.17 a30.59 ± 0.02 a202.53 ± 1.21 a0.61 ± 0.005 a
s5%WBE9.60 ± 0.02 b35.27 ± 0.08 b288.05 ± 0.33 b1.99 ± 0.003 b
s10%WBE12.77 ± 0.01 c39.63 ± 0.05 c360.69 ± 1.65 c2.71 ± 0.001 c
s15%WBE21.36 ± 0.12 d43.06 ± 0.06 d459.89 ± 0.87 d3.69 ± 0.002 d
s5%WBE, s10%WBE, s15%WBE—starch-based films with the addition of 5, 10 and 15% wild blackberry extract, respectively; s—starch-based film without added extract; a–d—different letters refer to the statistically significantly different values in the same column (p < 0.05).
Table 5. Total content of polyphenols and degree of neutralization of free radicals of films based on starch with the addition of wild blackberry extract.
Table 5. Total content of polyphenols and degree of neutralization of free radicals of films based on starch with the addition of wild blackberry extract.
Total Polyphenol Content (mg GAE/g)Degree of Neutralization of Free Radicals (%)
s0.10 ± 0.005 a59.01 ± 0.23 a
s5%WBE0.72 ± 0.01 b71.74 ± 0.09 b
s10%WBE1.12 ± 0.01 c73.94 ± 0.14 c
s15%WBE1.31 ± 0.01 d76.95 ± 0.14 d
s5%WBE, s10%WBE, s15%WBE—starch-based films with the addition of 5, 10 and 15% wild blackberry extract, respectively; s—starch-based film without added extract; a–d—different letters refer to the statistically significantly different values in the same column (p < 0.05).
Table 6. Migration of bioactive components from starch-based films with the addition of wild blackberry extract.
Table 6. Migration of bioactive components from starch-based films with the addition of wild blackberry extract.
Total Polyphenol Content of Filtrate (mg GAE/g)Degree of Neutralization of Free Radicals of Filtrate (%)
s5%WBE0.239 ± 0.0.01 a53.76 ± 0.18 a
s10%WBE0.310 ± 0.003 b59.63 ± 0.3 b
s15%WBE0.864± 0.005 c68.17 ± 0.09 c
s5%WBE, s10%WBE, s15%WBE—starch-based films with the addition of 5, 10 and 15% wild blackberry extract, respectively; s—starch-based film without added extract; a–c—different letters refer to the statistically significantly different values in the same column (p < 0.05).
Table 7. Antimicrobial activity of films based on starch with the addition of wild blackberry extract expressed as the diameter of inhibition zones (mm).
Table 7. Antimicrobial activity of films based on starch with the addition of wild blackberry extract expressed as the diameter of inhibition zones (mm).
Escherichia coli ATCC 25922Pseudomonas aeruginosa ATCC 27853Proteus vulgaris ATCC 8427Staphylococcus aureus ATCC 25923Bacillus subtilis ATCC 6633Klebsiella pneumoniae ATCC 700603Candida albicans ATCC 2091Listeria monocytogenes ATCC 15313Bacillus cereus
s7.8 ± 0.4 a8.0 ± 0.0 a8.0 ± 0.0 a7.3 ± 0.4 a8.0 ± 0.0 a7.1 ± 0.4 ana7.5 ± 0.5 a7.2 ± 0.4 a
s5%WBE8.3 ± 0.4 b8.8 ± 0.2 b8.7 ± 0.5 b8.8 ± 0.5 b8.8 ± 0.4 b8.0 ± 0.2 bna8.8 ± 0.4 b8.2 ± 0.4 b
s10%WBE8.8 ± 0.3 b8.8 ± 0.5 b8.5 ± 0.5 b8.5 ± 0.5 b8.8 ± 0.4 b8.1 ± 0.2 cna8.8 ± 0.2 b8.8 ± 0.4 c
s15%WBE8.6 ± 0.5 b8.7 ± 0.5 b8.5 ± 0.5 b8.5 ± 0.5 b8.7 ± 0.5 b8.1 ± 0.2 cna8.5 ± 0.5 b8.2 ± 0.4 b
s5%WBE, s10%WBE, s15%WBE—starch-based films with the addition of 5, 10 and 15% wild blackberry extract, respectively; s—starch-based film without added extract; a–c—different letters refer to the statistically significantly different values in the same column (p < 0.05); na—no activity.
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MDPI and ACS Style

Cvetković, K.; Lazarević, A.; Stamenković Stojanović, S.; Dordevic, S.; Dordevic, D.; Karabegović, I.; Danilović, B. pH-Sensitive Starch-Based Packaging Films Enhanced with Wild Blackberry Extract. Processes 2025, 13, 1148. https://doi.org/10.3390/pr13041148

AMA Style

Cvetković K, Lazarević A, Stamenković Stojanović S, Dordevic S, Dordevic D, Karabegović I, Danilović B. pH-Sensitive Starch-Based Packaging Films Enhanced with Wild Blackberry Extract. Processes. 2025; 13(4):1148. https://doi.org/10.3390/pr13041148

Chicago/Turabian Style

Cvetković, Kristina, Aleksandar Lazarević, Sandra Stamenković Stojanović, Simona Dordevic, Dani Dordevic, Ivana Karabegović, and Bojana Danilović. 2025. "pH-Sensitive Starch-Based Packaging Films Enhanced with Wild Blackberry Extract" Processes 13, no. 4: 1148. https://doi.org/10.3390/pr13041148

APA Style

Cvetković, K., Lazarević, A., Stamenković Stojanović, S., Dordevic, S., Dordevic, D., Karabegović, I., & Danilović, B. (2025). pH-Sensitive Starch-Based Packaging Films Enhanced with Wild Blackberry Extract. Processes, 13(4), 1148. https://doi.org/10.3390/pr13041148

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