Abstract
Barrier properties are very important in food packaging; the better the properties, the longer the product can stay fresh in the packaging, which prolongs the shelf life of the product. One way of improving barrier properties is to modifying the existing coatings with the addition of metal oxides, nanoparticles, or natural extracts. The aim of the study was to compare the barrier properties between paper coated with starch and paper coated with starch and black cherry extract mix. Prunus serotina extracts were prepared by ultrasonic extraction in 60°C water. The extracts were then filtrated and evaporated to obtain a constant mass. The reference coatings were prepared by dissolving starch in hot water to obtain a solution of 30%. The experimental coatings were prepared by substituting 10% starch for black cherry extract. Coatings were applied on the paper by laboratory coater in two thicknesses: c.a. 7 and 15 g/m2. The papers were characterised by grammage, coating weight, porosity, and roughness. Barrier properties were tested by following methods: Cobb-Unger, KIT, water vapour transmission rate, and heptane vapour transmission rate. Papers coated with formulations including extract had better barrier properties against oils and waxes, better results in KIT, Cobb-Unger, and heptane vapour transmission rate tests. The results of the water vapour transmission rate test as well as porosity and roughness were the same for papers coated with and without extract addition, they were influenced by the thickness of the coating, not the extract addition. The addition of black cherry extract influenced the barrier properties of the coated paper, and the extract improved the properties against oils and waxes.
1 Introduction
Paper-based packaging is an environmental friendly, biodegradable solution for protecting goods during transport or storage. However, there are some disadvantages of paper and cardboard packaging in comparison with plastic, glass, or metal. One of the biggest disadvantages are the barrier properties and moisture adsorption of packaging based on natural fibres [1]. Barrier properties are the most important factors influencing the condition of the product during storage. Good barrier properties protect packed goods from outside conditions, such as air, humidity, heat, and UV [2]. There are several methods to improve barrier properties of paper, most of them are focused on using different coating materials such as synthetic polymers, thin metal foil, starch, chitosan, etc. It is also possible to combine different products with the coating matrix.
Among coating materials, matrix starch is the most common in the paper industry, mostly due to its low price and high availability. Despite price and availability, starch has many properties that are excellent for application in the paper industry, starch is hydrophilic and creates dispersions in water, and it also creates hydrogen with cellulose and hemicellulose [3]. Due to the ability to make films, starch is frequently used as a matrix for biodegradable films. Starch films themselves have poor mechanical and moisture properties and moderate oxygen barrier properties. To enhance the mechanical and barrier properties of starch films, there were many attempts of incorporating fillers into starch matrix [4]. Mostly, fillers are metal particles, metal oxide nanoparticles, cellulose, and nanocellulose, and starch in different forms is also used as a filler, e.g. nanostarch, starch nanocrystals, starch derivatives. The other option to improve starch properties as coating agent is to blend it with polymers, preferably biopolymers [5].
In this research, starch was mixed with black cherry extract. Black cherry (Prunus serotina Ehrh.) is a widely distributed invasive alien plant species in Europe [6]. It has been reported as a species out of control in four European countries, Belgium, Denmark, Germany, and Poland [7]. Due to its invasive character, deferent control procedures are used to prevent its spreading in the European forest. The most popular control procedure is to remove black cherry by simply cutting it down [8]. Cutting leads to collecting a lot of biomasses with no determined method of utilisation other than burning it. One way of utilisation worth exploring is extraction. Extraction is a great method to obtain a highly valuable substance from biomass. Previous studies showed that P. serotina extracts are rich in phenolic compounds, especially flavonoids [9]. These extracts also show antifungal properties against wood decaying fungi [10].
The aim of this study was to increase the barrier properties of two types of base paper by coatings made of starch with the addition of black cherry extract.
The aim of the study was to compare the properties of paper starch coatings with and without the addition of Prunus serotina extract.
2 Materials and methods
Two base papers with different surface structures for the coatings were used. Both were made from virgin fibres. The first (B1) had a grammage of 43 g/m2, the second (B2) was precoated and had a more closely arranged surface structure, the grammage was 67 g/m2. The base papers were from Slovenian paper mil Papirnica Vevče, Slovenia. The main goal of using two different types of paper was to see the influence of the coating on the papers.
2.1 Extract preparation
The extract that was added to the coating was prepared from twigs with the leaf and the unmatured fruit of Prunus serotina. Twigs were collected from the area of Dąbrowa Forest District of Polish State Forests (location: latitude 53°29′45.74″N, longitude 18°31′0.72″E, 79 m altitude). The material was immediately dried at 60°C in the laboratory dryer, crushed, and stored in the dark at room temperature until further analysis. Extract was prepared as follows. 20 g of fresh twigs were placed in a beaker, 200 mL of distilled water was added. The extraction was carried out under the lid for 60 min at 40°C in an ultrasonic bath. After the extraction, the extract was evaporated in the vacuum rotary evaporator. The extraction yield was 16%.
2.2 Coating preparation
The experimental coatings were prepared by substitution of 10% of the starch with extract. The total solid content of the coating solution was 30%. The starch with extract and warm water was carefully mixed until fully dissolved, and the prepared coating had pH 6.64 and viscosity of 201.1 mPa s. The prepared coating was then applied to the base papers.
The reference coating was produced by preparing 30% solution of starch (Stabilys, Roquette, Lithuania) in hot water at 40–45°C, mixed until fully dissolved. The obtained coating had pH 7.81 and viscosity of 221.9 mPa s. The prepared coating was then applied to the base papers.
Coatings were applied with the use of lab coater machine K Control Coater (PrintCoat Instruments, Royston, UK). We used two application rods. Rod 3 and rod 6. These rods allowed to apply 6 and 14 g of coating per m2, respectively. In total, eight different types of coated paper samples were prepared.
2.3 Characterisation of coated papers
2.3.1 Basic properties
At the beginning of the characterisation process, grammage was checked in accordance with the ISO 536 standard. Grammage check allowed to determine the coating amount on all test samples. It is of great importance because the coating amount influences both the barrier and tensile properties. For grammage 20, the samples were cut into 10 cm × 10 cm size and weighed on a scale with precision to 0.001 g (Radwag, Radom, Poland).
2.3.2 Porosity and roughness
Air permeance of the tested paper was determined according to the standard method of ISO 5636-3, using the Frank PTI testing device (Frank PTI, Birkenau, Germany) for the determination of surface roughness and air permeance according to the Bendsten method.
2.4 Barrier properties
2.4.1 Water vapour transmission Rate (WVTR)
The WVTR was determined according to the ISO 2528 standard method, at 23.2% relative humidity (RH). The WVTR was then calculated in accordance with the following equation:
where A is the tested area in cm2, t is the time after 24 h testing, and
2.4.2 Oil resistance
The oil resistance of the tested paper was determined according to the TAPPI T559 standard method. Reagents with different rheology and surface energy were dropped onto the tested paper surface from a height of 13 mm and wiped with the cotton swab after 15 s. The testing solution was numbered from 1 to 12, and the solution with the highest number that did not change the colour of the tested sample was considered to pass the KIT number.
2.4.3 Heptane vapour transmission rate (HVTR)
The heptane permeance of the tested paper was determined in accordance with the modified method described by Gaudreault et al. [11]. It was performed with the use of a permeability cup with a sealable closure that was fixable to the screws. There is an open surface area in the closure that is sealed with the test material. The cup is filled with 9–10 mL of heptane and then weighed and placed under controlled experimental conditions, 23 ± 1°C and 50 ± 2% RH. The weighing is repeated every hour for 4 h. The results were then calculated as heptane vapour transmission per m2 per hour.
2.4.4 Oil absorbency of the paper
The oil absorbency of the paper was determined by the Cobb-Unger method according to the SCAN-P 37:77 method. In short, 250 mL of castor oil is poured into the cup of Cobb-Unger apparatus. Oil temperature must be 23 ± 0.5°C. The tested material must be weighed to the nearest 5 mg and then placed over the opening of the open cup, after which the lid should be closed. After closing, the cup was turned upside down for 25 s, then returned to the original position and the lid was released. The test paper was removed from the apparatus, placed on a blotting paper, and the excess oil was then wiped off. The oil wiping time was 2 s. The test samples were then again weighed. The absorbency of the Cobb-Unger oil was calculated as follows:
where X – Cobb-Unger oil absorbency in g/m2, G 1 – mass of test piece before the test, G 2 – mass of test piece after the test, and A – test area in m2.
2.5 Antibacterial properties
Each test sample was examined for antibacterial properties. The testing specimen was E. coli. The test was performed as follows. Sterilized agar was poured into sterile Petri dishes, after solidifying the previously cultivated agar, bacteria were transferred to the Petri dishes with the use of a Drigalski spatula. Then, on the bacterial film, 1 cm diameter discs were placed, with coated side towards the bacterial film. The discs were cut out of each coated paper. After placing the discs, the plates were incubated at 37°C for 48 h with the first check-up after 24 h. The experiment was carried out against negative control in 5 repetitions.
2.6 Statistics
Two analyses were carried out: one for the B1 paper and the other for the B2 paper. For each examined characteristic, the Kruskal–Wallis test showed that there are significant differences between at least one pair of groups, which were determined based on Dunn’s test with Holm’s correction. Statistic calculations were performed in the R software with the use of DescTools library.
3 Results
During the experiment four different coatings on two different base papers were tested. The results are presented in Tables 1–3. Coatings applied with rod 3 had weight from 5.9 to 8.4 g/m2. The weight of coating on base paper 1 was higher than the weight of coating on base paper 2. Rod 6 allowed to obtain a coating weight between 14.4 and 15.4 g/m2, again the coating weight of base paper 1 was higher.
Description of the tested papers
| B1 | Base paper with a grammage of 43 g/m2 |
| B2 | Base paper with a grammage of 67 g/m2 |
| 1. | B1 paper coated with 30% starch solution, with a coating weight of 7.8 g/m2 |
| 2. | B1 paper coated with 27% starch and 3% extract solution, with a coating weight of 8.4 g/m2 |
| 3. | B1 paper coated with 30% starch solution, with a coating weight of 15.0 g/m2 |
| 4. | B1 paper coated with 27% starch and 3% extract solution, with a coating weight of 15.4 g/m2 |
| 5. | B2 paper coated with 30% starch solution, with a coating weight of 6.7 g/m2 |
| 6. | B2 paper coated with 27% starch and 3% extract solution, with a coating weight of 5.9 g/m2 |
| 7. | B2 paper coated with 30% starch solution, with a coating weight of 14.4 g/m2 |
| 8. | B2 paper coated with 27% starch and 3% extract solution, with a coating weight of 14.4 g/m2 |
Basic and barrier properties of papers tested with base paper 1
| Base paper | Extract | Cobb-Unger (g/m2) | KIT number | WVTR (g/m2/day) | HVTR (g/m2/day) | Porosity (mL/min) | Roughness (mL/min) | |
|---|---|---|---|---|---|---|---|---|
| B1 | — | — | 14.6 | 0 | 710 | 3,184 | 251 | 490 |
| 1. | B1 | — | 0.6 | 5* | 390 | 645 | 1.58 | 368 |
| 2. | B1 | + | 0.9 | 5* | 260 | 343 | 0.93* | 350 |
| 3. | B1 | — | 0.4 | 5* | 240* | 121 | 0.55* | 645 |
| 4. | B1 | + | 0.3* | 5* | 310 | 78.7* | 0.49* | 477 |
*Significant better results according to Kruskal–Wallis test with Dunn’s test with Holm’s correction; + The results are for formulation with extract; − The results are for formulation without extract.
Basic and barrier properties of papers tested with base paper 2
| Base paper | Extract | Cobb-Unger (g/m2) | KIT number | WVTR (g/m2) | HVTR (g/m2) | Porosity (mL/min) | Roughness (mL/min) | |
|---|---|---|---|---|---|---|---|---|
| B2 | — | — | 2.4 | 1 | 480 | 1,651 | 0.97 | 56.6 |
| 5. | B2 | — | 0.2* | 8 | 190 | 27.6 | 0.02 | 39.7 |
| 6. | B2 | + | 0.3* | 10* | 150 | 16.6* | 0.15 | 74.4 |
| 7. | B2 | — | 0.4 | 8 | 120* | 28.6 | 0.00* | 62.1 |
| 8. | B2 | + | 0.5 | 9 | 120* | 16.6* | 0.00* | 45.9 |
*Significant better results according to Kruskal–Wallis test with Dunn’s test with Holm’s correction; + The results are for formulation with extract; − The results are for formulation without extract.
Oil absorbency tested with the Cobb-Unger method for B1 paper showed significantly better results with 14.4 g/m2 coating with 10% extracts, for B2 paper better results were obtained while using ±8 g/m2 of coating, with the same significance of the results with and without extract addition to the coating formulation.
The results of the KIT tests showed that for B1, all coating formulations gave the same results. For B2, the statistically significant best result gave formulation with 10% extract addition and coating weight of ±6 g/m2. Other formulations gave results without significant difference.
Thicker coatings gave better results in the water vapour test. With B1 paper, the best result was obtained with 15.0 g/m2 of coating without the addition of extract, for B2 paper best result was obtained with 14.4 g/m2 of coating, there were no significant differences between the formulations with and without extract.
In the heptane vapour test, all formulations helped with the reduction in heptane emission. B1 paper formulation with extract gave better results, while the best result was obtained with a thicker coating layer with the addition of 10% Prunus serotina extract, whereas for B2 paper, best results were obtained for both thicker layer coatings, with or without extract.
Porosity values dropped after each layer of coating. For B1 paper, best results were for three samples 2, 3, and 4, and the sample with thinner layer of coating without extract had the worst properties in this test. For B2 paper, best values (0.00 mL/min), were obtained for thicker coatings, results were the same with and without extract.
With regard to antibacterial properties, during antibacterial tests, there were no inhibition zones observed around any of the tested discs. There was no difference in the growth of E. coli colony around the disc made with base papers, papers coated with starch, and papers coated with a mixture of starch and extract.
4 Discussion
Resistance to oil and grease is an important property of cardboard and paper, especially if they are used in food packaging [12]. Uncoated paper has poor oil resistance. B1 paper absorbed 14.6 g/m2, while B2 paper absorbed 2.4 g/m2 of reference oil in the Cobb-Unger test. The coating of the paper allowed to reduce oil absorption of B1 paper by 96–98%, with the best results for high-coated paper with extract addition. B2 paper had much lower oil absorption from the beginning; however, the coating allowed to reduce oil absorbency by 80–92%, in contrary to B1 coated papers. B2 coated papers had the best reduction with a lower amount of applied coating with no significant difference between coating formulation with and without extract. The application of a starch-based coating layer allowed reducing oil uptake from 14.6–2.4 to 0.2–0.9 g/m2. A similar reduction in oil absorbency was reported in a previous work [13], with the starch coating, authors were able to reduce the oil absorbance from 25 to 2 g/m2. In the cited work researchers used high molecular anionic starch to obtain the best resistance to oil and grease.
The second test used to establish coating oil resistance was the KIT test; the test consists of 12 mixtures of castor oil, n-heptane, and toluene. Mixture no. 1 has the lowest surface energy, while mixture 12 has the highest [14]. Both base papers had poor oil resistance KIT 0 and 1. Despite similar Cobb-Unger values for all coated papers, the KIT test results are very different. All coated papers of paper B1 had KIT number 5, which is a moderate result that does not guarantee a good barrier property for oily and greasy products. Coatings made on paper B2 had better KIT test results. Results were ranging from 8 to 10, with the best results for the coating with 10% addition of Prunus serotina extract. Results between 8 and 10 are moderately good on a 12-point scale. Papers with this value may be used in the packaging of oily materials which are not very aggressive.
The results of the WVTR are shown in Figure 1. For the samples made of paper B1, the best result was for coating 3, 15.2 g/m2 of starch coating without extract addition, while for the samples made of paper B2, there was no significant difference between thick coating with and without extract. No clear difference in WVTR between coatings containing extract and those without extract may be caused by the hydrophilic character of both ingredients of the coating. Results of the WVTR test suggest that the tested coating formulation does not present good barrier properties against water; however, the coating allowed to limit WVTR by more than 40% for samples made of paper B1 and by more than 50% for the samples made of paper B2. Polysaccharide-based coatings have been reported as coatings with poor WVTR properties, which is mainly due to their chemical properties as they are made of hydrophilic material [15]. One of the ways of limiting this negative effect of polysaccharides is to mix them with proteins, long chain fatty acids, and waxes [16].
Results of the Water and Heptane Vapor Transmission Rate tests: (a) and (c) made on base paper 1, (b) and (d) made on base paper 2.
Results of the E. coli inhibition test.
On the other hand, HVTR results are clearly in favour of coating formulations with extract addition. In the series made of B1 paper HVTR results were better for formulations with 10% extract addition. Samples made of B2 paper had in general better HVTR results than samples made of B1 paper. In this series, it was also observed that the sample that contained the extract had better results. The reduction in HVTR for these samples was from 1,651 g/m2/day for the base paper 2 to 16.6 g/m2/day for papers 6 and 8. This means that papers coated with coating formulation with extract addition have improved barrier properties against nonpolar substances. The method is sufficient to determine migration of saturated hydrocarbons with molecular weight up to 400 g/mol, for instance, hydrocarbons in mineral oils [17].
Porosity and roughness are two properties of the paper that describe its structure. Porosity determines how porous is the structure of the tested paper. Base paper 1 was very porous with porosity value of 251 mL/min, while base paper 2 had porosity value of 0.97 mL/min. These results showed that base paper 1 was not sized, while the base paper 2 was sized. Each layer of coating reduced porosity of the tested papers, and porosity is lower for the papers with thicker layer of coating. There are no differences between coating formulations with and without extract. Roughness describes the structure of the paper surface. Base paper 1 had higher roughness values compared to base paper 2. Coatings made on both base papers did not improve the roughness of their surface. All coated papers presented roughness values similar to those of their base papers. There was no difference between coating with and without extract addition.
The antibacterial property tests showed that coating formulation with the addition of pure extract had a positive influence on the growth of bacterial colonies. The colonies of E. coli were observed to be greater around the paper circles with coatings containing the extract. This could indicate that the extract contains chemicals, e.g. sugars, which are promoters of growth of E. coli. To eliminate sugars, extracts were purified with the use of a separatory funnel and acetic acetate. The acetic acetate fraction was then evaporated, and a dry substance was used to impregnate the paper.
Paper after impregnation of acetic acetate fraction of the Prunus serotina extract was then again tested against E. coli. Results of this test are shown in Figure 2, which shows visible small inhibition zones around paper circles. The purification of the extract may be a way to obtain an extract with antibacterial properties from black cherry. The results presented are just a preliminary attempt to determine whether leaf Prunus serotina extracts can inhibit E. coli growth. Other studies show that Prunus serotina leaves show weak antibacterial activity whether it is water or organic solvent [18].
5 Conclusion
The addition of Prunus serotina extract to the starch base coating influences the properties of the coating. The paper coated with the extract formulation has improved barrier properties against oil and grease. The results of KIT, Cobb-Unger, and HVTRs are in favour of formulations with extract. The change in other properties was mostly influenced by the thickness of the coating layer. Paper coated with an extract not purified did not show antibacterial properties, and even promoted E. coli growth. The paper impregnated with purified extract showed a small inhibition zone. Further research is needed on this topic to prove the antibacterial properties of black cherry extract.
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Funding information: Research presented in this work were conducted during internship financed within the framework of Ministry of Science and Higher Education program “Regional Initiative of Excellence” in years 2019–2022, Project No. 005/RID/2018/19.
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Conflict of interest: The authors state no conflict of interest.
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Data availability statement: The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.
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