Development of Cork-Bamboo-Latex as An Alternative Composite for Bottles Stoppers

Introduction

Cork stoppers are produced from the outer part of cork oak, Quercus suber L. [1]. The cork oak bark removal must be done after the tree reaches 25 years. Its periodic extraction takes place every nine years, depending on the radial growth and the region [2].

Cork is an outstanding material widely used as a sealing bottle in the wine industry because it is a natural, renewable, and sustainable raw material [3]. Furthermore, its physic-chemical and mechanical properties, such as low permeability to liquid sand gases, low density, high compressibility and flexibility, and chemical and microbiological stability provide suitable characteristics that arouse the interest of the manufacture of cork stoppers [4].

Cork is a natural composite constituted by suberin, the major component (33-50 wt. %); lignin (13-29 wt. %), an aromatic polymer and responsible for resistance to compression; polysaccharides, as cellulose and hemicelluloses (6-25 wt. %) and it also contains other components, the extractives, soluble phases that represent 8.5-24% of the cork composition [5,6].

In the current market, cork is used to manufacture stoppers. Nonetheless, during cork processing, considerable amounts of residues are produced. Due to the low granulometry of the residues, the by-products obtained (about 20%) are not suitable for conventional cork applications and, for this reason, they are applied in other areas, such as cork pavement, insulation panels, composites, and as cork filler in agglomerated wine stoppers [7]. Nevertheless, it can be considered a sustainable material since its origin is a vegetable one, and its degradation does not generate microplastics [8].

Slow growth and development of the cork bark and the globalization wine trade encouraged the production of new types of stoppers [9]. Cork-based materials have been developed, as well as composite materials which combine cork and their by-products with adhesive and natural fibers [10]. These composites could be employed as wine stoppers, which could be considered eco-friendly materials [11].

Cork agglomerated stoppers are obtained from the corking process as granulated cork. As a natural fiber, bamboo stands out and it is used as reinforcement or filler in composite materials [12]. Bamboo fibers have a similar composition to a cork since it contains lignin, cellulose, hemicellulose, and extractives. They have been used in textile, paper, construction, package, and healthcare areas, due to advantages such as low density, low cost, high mechanical strength, stiffness, and anti-bacterial, anti-UV, and antistatic properties [13].

Regarding composites reinforced with bamboo fibers, it can be highlighted that polylactic acid (PLA) samples reinforced with kenaf, and bamboo fibers presented high tensile strength and stiffness compared to single-fiber composites [14].) Agglomerated polyurethane-bamboo fibers samples presented ductile behavior under compressive forces, and they presented low swelling/water absorption capacity; they are suitable for non-structural applications [15].

High-density polyethylene-sisal-cork samples were produced by extrusion, and it was observed that the best mechanical performance was obtained using 2% of maleic anhydride as a coupling agent. In addition, high-density polyethylene-coconut-cork extruded samples also presented improved mechanical properties with the addition of the coupling agent, but also, the presence of 10% of coconut fibers function as reinforcement in the composites [16].

Agglomerated materials based on cork, bamboo fibers, and an adhesive, such as latex, could be potential composite stoppers, besides being environmentally friendly. Thus, this work aims to develop a cork-bamboo-latex composite and to evaluate the interaction between three composite interfaces as well as their mechanical properties to produce an alternative material to wine stoppers.

Materials and Methods

Materials

The commercial agglomerated cork (RA) with an average particle size (of 1-2) mm was supplied by Logicork (Portugal). Raw bamboo (Bambusa vulgaris, FB) was obtained from the Forest Institute of the Rural Federal University of Rio de Janeiro (Seropédica, Brazil). It was peeled, pressed, and crushed. Pre-vulcanized NR Latex (LT), used as adhesive, was donated by the State University of Rio de Janeiro (Brazil). The wine (VV) employed at the immersion test was based on an Uruguayan wine produced from Tannat grapes (Concha Toro®, 2015). In addition, solid (RM) and agglomerated (RA) used wine stoppers, collected as discarded material from local commerce, were also employed and characterized to compare them to the agglomerated virgin commercial stoppers (RV), obtained in the local commerce, and to compare to the produced composite stoppers in this work.

Composite Manufacturing

The raw materials were dried at 70 ºC overnight before further treatments. They were mixed according to the respective amounts: 35% of cork, 35% of bamboo, and 30% of latex. The composite was mechanically pre-mixed (Fisatom) for 15 min and compression-molded manually in glass molds to produce cylindrical samples (19x24 mm). The stoppers produced (CMP) were then dried and crosslinked in the oven (equipment FANEM) at 110 ºC for 48h. The glass molds were manually discarded, and the samples were conditioned at room temperature in a desiccator.

Immersion Test

The commercial agglomerated cork (RA) and the cork-bamboo-latex composite produced (CMP) sizes were previously measured (average diameter and height dimensions) to determine their densities. Three samples of each type (RA 1-3 and CMP 4-6, respectively) were partially immersed in 10 mL of wine (corresponding to 5 mm of height), placed in a closed glass flask, and kept in the refrigerator for seven days. After the removal of the wine medium, the samples were weighed, and their dimensions were measured. The immersion test aims to observe the permeation of cork and bamboo residues to wine and the fungi’s proliferation.

Composite Characterization

The cork-bamboo-latex stoppers were analyzed by Fourier transform infrared spectroscopy (FTIR) using an ATR accessory. The spectra were collected on Spectrum 100 (PerkinElmer) in a range of 4000-600 cm−1 at 32 scans with a resolution of 4 cm−1. The compression tests on the commercial agglomerated cork and the produced composite stopper materials were performed until 60% strain in a universal testing machine UMC 300 (Contenco). The samples (19x24 mm) were radially compressed at a constant rate of 2 MPa.min-1 using a 20 t load cell. The compression tests were performed in triplicate and the stress-strain average curve was recorded to obtain Young’s modulus.

Microbial analysis of the composite

The cork-bamboo-latex composite produced was crushed and diluted (1:10) in an aqueous 0.1 % wt. peptone solution (peptone 1g.L-1 and sodium chloride 8.5 g.L-1 solution). An aliquot (1 mL) of this solution was inoculated onto the potato dextrose agar plates and incubated at 30 °C for 72 h for molds and yeasts growth. After the incubation period, colony counting was expressed as the number of viable colonies per weight of cork material in grams (CFU/g).

All analysis that was performed in triplicates were evaluated by ANOVA-1way analysis, (factor: composition; levels: CMP e RA), level of significance of 95%. Tukey test revealed the significant difference between pairs, level of significance of 95%.

Results and Discussions

The cork-bamboo-latex composite produced has a cylindrical shape, dimensions values typically for wine bottles, and brown coloration [17]. The weight, diameter, height, and density changes to stoppers before and after the immersion test are displayed in Table 1.

The cork-bamboo-latex composite produced is heavier and denser than commercial agglomerated cork stoppers. Probably, this higher density is due to the bamboo used in the preparation of the composites stoppers which presents an average density of (0.6-0.7) g/cm3 [18]. Different parts of the cork can also influence (increasing or decreasing, depending on the case) the density of the cork stoppers [19]. Corks from different manufacturers can present distinct densities. The incorporation of bamboo parts to cork granules physically adhered contributes to increased mechanical properties, though the light weight of bamboo could diminish the composite density. However, the added components (bamboo, latex) to cork increased the samples density [20]. However, the highest difference in the density value is found after the immersion test, which leads to the conclusion that the bamboo component is the factor that most contributes to the increase in the density of the cork stopper.

The results after the immersion test in wine showed that both materials (RA and CMP) presented weight gain. However, only the cork-bamboo-latex composite presented an increase in diameter, while commercial cork stoppers (an average density of 0.371 g/cm3) [21] retained their size after seven days of immersion in wine. This observation is reflected in the density data, which were raised mainly for cork-bamboo-latex samples. The composite stoppers produced (an average density of 0.653 g/cm3) exhibited a relative increase of 31.02% in density and a slight increment in their diameter, whereas the commercial cork stoppers presented an increase of 6.26% in density. Usually, commercial cork stoppers also present wine absorption, which usually stabilizes after months of contact with the liquid (3-24 months) and it would be the result of micro capillarity or evaporation-condensation phenomena [21].

Tukey test was used to verify if there is a difference statistically significant between both corks employed in this study adopting 95% of confidence. This test indicated that there is a significant difference between the wine absorption (swelling in the media (wine)) from RA stoppers and the CMP samples (p<0.05) [5]. This measurement suggests that there was a greater migration of wine to the cork-bamboo-latex composite produced in comparison with the commercial agglomerated cork. This may be related to the automatic compression-molded process of the commercial agglomerated stoppers, where there is improved cork packaging. On the other hand, the composites produced were compressed-molded manually which makes it difficult to control the particle packaging. Besides that, there are the intrinsic properties of cork and bamboo, where cork can be considered a hydrophobic material, where the presence of a hydrophilic material, such as bamboo residue, aggregated to it, could increase the absorption of fluids rich in -OH groups, such as wine [22]. Cork absorbs wine is related to the fluid diffusion trough the cork’s cell wall, where the driving force is the concentration gradient [21].

FTIR spectra for the solid (RM) and agglomerated (RA) cork stoppers collected from local commerce (cork stoppers used) compared with cork-bamboo-latex composite (CMP) produced and agglomerated virgin (RV) cork stopper is shown in Figure 1. This comparison aims to show that the commercially available corks have a similar profile, they present the same main FTIR bands of the cork-bamboo-latex composite produced, indicating that the cork phase is dominant. Figure 1 shows bands at 3340 cm-1 characteristics of OH bonding of the cork. which contain OH bonds in their structure [4]. Bands from 2960 to 2850 cm-1 can be attributed to the CH2 vibration of the carbon chain present in cork components [23].

However, only the cork-bamboo-latex composite stoppers showed a band at 2960 cm-1 and a slight displacement of the C-H bands to 2922 and 2853 cm-1. This evident band may be related to the C-H stretching vibration (CH3) of latex which is the only different component compared to the other samples. The shift can be associated with the composition since it presents cork, bamboo, and latex. Furthermore, it could be due to water absorption related to the manual manufacturing process, which contributed to bands broadening. Bands at 1736 cm-1 are related to the carbonyl group from suberin. Some bands (1617, 1511, and 1448 cm-1) were attributed to C=C aromatic ring stretches. The band at 1376 cm-1 is ascribed to O-H in-plane deformation in polyphenols mainly from lignin [24]. The 1150 cm-1 absorption band is associated with the stretching vibrations of C-O-C bonds characteristic of the polysaccharides with 1 → 4 glycosidic linkage (cellulose and hemicellulose compounds) [25]. Other bands (~1124 and ~1053 cm-1) are also attributed to the C-O-C bond, which could be present in the FTIR spectra of samples, however, due to the broadening band, these bands could be overlapped. The region from 840 to 720 cm-1 can be associated with ring vibrations (C-H out of the plane and in-plane bending) of the lignin [23]. The highest difference found in the composite-produced spectrum was the evident band at 2960 cm-1 and the low intensity of the C=O band at 1737 cm-1..

Figure 1.Figure 1. FTIR spectra of the commercial solid (RM), agglomerated (RA), agglomerated virgin (RV) cork, and cork-bamboo-latex composite (CMP) stoppers

To analyze the release of cork, bamboo, and latex residues from the composite produced to the wine after the immersion test, an FTIR analysis of each component (Figure 2 a) and wine before and after the immersion test (Figure 2 b) was performed. Figure 2a shows that the components of the composite produced present the same bands previously cited. A strong absorption band at 3357 cm-1 is ascribed to hydrogen-bonded O-H or N-H. It is worth noting that the latex spectrum exhibits a band in 2960 cm-1, the same one found in the cork-bamboo-latex composite. This band is characteristic of the asymmetric stretching vibration of the C-H bond included in the CH3 group of latex in the composite [26]. The band at 1640 cm-1 is assigned to the C=O symmetric stretching of amide I also present in the latex. 1450-1370 cm-1 region is related to the angular deformation of CH2 and CH3 groups, as well as the C-OH and N-CH2-N stretching vibration. This last group (N-CH2-N) could be confirmed by the 1160 cm-1 band [27]. FTIR spectra of the wine after the immersion test are displayed in Figure 2 b. The intense band at 3306 cm-1 is assigned to O-H groups from water and ethanol. Region 1800-900 cm-1 is considered a fingerprint region and provides valuable information about wine characteristics [28].

This region includes stretching vibration of C-O, C-N, C-H, C-C, N-H e C=O bonds which are typical of different molecules present in wine [29]. FTIR analysis showed similar bands profile for all samples even after the immersion test. There was no modification of the spectra, either by the presence or absence of a band or by band displacement. Apparently, there was no components release of the cork-bamboo-latex composite produced to the media (wine).

Figure 2.Figure 2. FTIR spectra of the: (a) individual components and cork-bamboo-latex (CMP) composite produced (as well as bamboo fibers – FB, latex – LT, cork - CP) and (b) wine after immersion test of RA_V (agglomerated cork) and CMP_V (cork-bamboo-latex composite cork); wine before the test - VV).

The mechanical performance of the cork stoppers was studied by compressive experiments, Figure 3 (1 curve of each triplicate is shown). The compressive behavior response of cork stoppers can be divided into three regions: the first region (up to 7.5% strain), which is an elastic area corresponding to the reversible bending of the material; the second region (up to 35% strain), where there is large plateau corresponding to elongation of the material; and, the last one, where there is a compaction region characterized by an abrupt increase in strain when critical stress is reached [30].

The cork-bamboo-latex composite stoppers display anincrease in Young’s modulus (from 0.033 MPa (RA) to 0.037 MPa (CMP)) when compared with commercial agglomerated cork (p<0.05). Higher elastic modulus indicates that the produced composite stoppers support a higher load to reach 60% of deformation leading to an increase in the stiffness. This increase in Young’s modulus could be also related to the higher density of the cork-bamboo-latex composite in comparison with the commercial agglomerate cork stopper. Cork might be physically anchored in the matrix, but it does not contribute considerably to the mechanical properties [31]. The increase in the maximum strength for CMP (from 4.11 g/cm³ (RA) to 4.72 g/cm³ (CMP)) could indicate that the matrix is well compacted and there was a good adhesion between the components of the composite produced [32]. However, the statistical analysis of data did not show significant difference between RA and CMP cork stoppers for maximum strength (p>0.05). the composite mechanical properties could interfere with the wine bottle seal negatively, making it difficult to place the stopper on the bottleneck, it could also contribute positively, preventing the permeation of the liquid out of the bottle.

Figure 3.Figure 3 - Compressive properties of the commercial agglomerated cork (RA) and cork-bamboo-latex composite stopper (CMP) until 60% deformation.

Microbial analysis of molds and yeasts in the composite presented 1.5 x 10² CFU/g (colony-forming unit per gram) indicating the microorganism’s presence, which is undesirable even in low concentrations. A possible reason for the development of molds can be related to humidity present in the composite due to poor efficiency in the drying process, regarding its time and temperature [5,32]. In addition, the bamboo hydrophilicity could contribute to microorganisms’ proliferation [33]. One of the treatments previously adopted to prevent the proliferation of fungi was chlorination. However, if the washing and sterilization process was not well performed, there was the formation of undesirable chlorophenol compounds. These compounds permeate the cork, causing flavor unpleasant and compromising the wine quality. For this reason, nowadays, this treatment is completely forbidden in cork-stopper manufacturing processes [34]. Unfortunately, the musty and moldy flavor in wine is associated with the cork stoppers, which would be associated with cork’s 2,4,6-trichloroanisole. Anisoles are related to at least 80% of cork taint [35]. A possible route to change microorganism growth could be related to surface treatment by plasma radiation [21]. Brazilian legislation do not include microbiological parameters for cork stoppers themselves, but the microbiological parameters should reach the ones for the foods in contact with the packaging, RDC n. 12/01.

Conclusions

It was possible to produce a cork-bamboo-latex composite stopper that exhibits properties similar to commercial agglomerate corks. It was observed that there was no migration of the components used in the composite's fabrication (cork, bamboo, or latex) for the wine, as well as there was no visible interaction between wine and composite observed by FTIR. It was also observed that the composite produced was physically agglomerated and exhibited a good adhesion of all components. Throughout the immersion test, it was possible to verify the presence of molds and yeasts in the composite, even in low amounts. The composite produced presented mechanical properties suitable for the application of wine stoppers. However stiffer than commercial stoppers, this feature can be positive in the sealing process. It is considered that the cork-bamboo-latex composite presents a potential as an alternative material to wine stoppers since the substitution of part of the cork with bamboo could reduce its use and the cost of the material, as well as stimulates the production of a sustainable material. It is worth mentioning that further development is required to avoid mold and yeast growth.

Acknowledgements

The authors acknowledge CETEM/UFRJ and CNPq. Este estudo foi financiado pela FAPERJ – Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro, Processo SEI E-26/201.381/2021 (260532). This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) – Finance Code 001.

References

1. Agatonovic-Kustrin S., Loescher Christine M.. Qualitative and quantitative high performance thin layer chromatography analysis of Calendula officinalis using high resolution plate imaging and artificial neural network data modelling. Analytica Chimica Acta. 2013; 798DOI
2. Agostini D.L.S, Constantino C.J.L., Job A.E.. Thermal degradation of both latex and latex cast films forming membranes. Journal of Thermal Analysis and Calorimetry. 2008; 91(3)DOI
3. Akbari Sweeta, Gupta Arun, Khan Tanveer Ahmed, Jamari Saidatul Shima, Ani Norlirabiatuladawiyah Binti Che, Poddar Pradeep. Synthesis and characterization of medium density fiber board by using mixture of natural rubber latex and starch as an adhesive. Journal of the Indian Academy of Wood Science. 2014; 11(2)DOI
4. APCOR Cork Information Bureau. Associação Portuguesa de Cortiça, 1(1):1–8, 2019. Associação Portuguesa de Cortiça, 1(1):1–8, 2019. 2023.
5. Azevedo Joana, Lopes Paulo, Mateus Nuno, Freitas Victor. Cork, a Natural Choice to Wine?. Foods. 2022; 11(17)DOI
6. Cravero Maria Carla. Musty and Moldy Taint in Wines: A Review. Beverages. 2020; 6(2)DOI
7. Dias Ana Cláudia, Boschmonart-Rives Jesús, González-García Sara, Demertzi Martha, Gabarrell Xavier, Arroja Luis. Analysis of raw cork production in Portugal and Catalonia using life cycle assessment. The International Journal of Life Cycle Assessment. 2014; 19(12)DOI
8. Berndsen Rodrigo Strzelecki, Klitzke Ricardo Jorge, Batista Djeison Cesar, Nascimento Eduardo Mauro, Ostapi Fabiano. RESISTÊNCIA À FLEXÃO ESTÁTICA E À COMPRESSÃO PARALELA DO BAMBU-MOSSÔ (Phyllostachys pubescens). FLORESTA. 2013; 43(3)DOI | PubMed
9. Fernandes Emanuel M., Aroso Ivo M., Mano João F., Covas José A., Reis Rui L.. Functionalized cork-polymer composites (CPC) by reactive extrusion using suberin and lignin from cork as coupling agents. Composites Part B: Engineering. 2014; 67DOI
10. Fernandes Emanuel M., Correlo Vitor M., Chagas José A.M., Mano João F., Reis Rui L.. Properties of new cork–polymer composites: Advantages and drawbacks as compared with commercially available fibreboard materials. Composite Structures. 2011; 93(12)DOI
11. Fernandes Emanuel M., Correlo Vitor M., Mano João F., Reis Rui L.. Cork–polymer biocomposites: Mechanical, structural and thermal properties. Materials & Design. 2015; 82DOI
12. Fernandes Emanuel M., Correlo Vitor M., Mano João F., Reis Rui L.. Polypropylene-based cork–polymer composites: Processing parameters and properties. Composites Part B: Engineering. 2014; 66DOI
13. Gil Luís. Cork Composites: A Review. Materials. 2009; 2(3)DOI
14. González-Adrados J.R., González-Hernández F., García de Ceca J.L, Cáceres-Esteban M.J., García-Vallejo M.C.. Wine absorption by cork stoppers. Spanish Journal of Agricultural Research. 2008; 6(4)DOI
15. Gonzalez-Adrados Jose R, Garcia-Vallejo Maria C., Caceres-Esteban Maria J., Garcia de Ceca Jose L., Gonzalez-Hernandez Florentino, Calvo-Haro Rosa. Control by ATR-FTIR of surface treatment of cork stoppers and its effect on their mechanical performance. Wood Science and Technology. 2012; 46(1)DOI
16. Jiang Liuyun, Li Ye, Xiong Chengdong, Su Shengpei. Preparation and characterization of a novel degradable nano-hydroxyapatite/poly(lactic-co-glycolic) composite reinforced with bamboo fiber. Materials Science and Engineering: C. 2017; 75DOI
17. Pradeep K. Kushwaha, Rakesh Kumar. Studies on Water Absorption of Bamboo-Polyester Composites: Effect of Silane Treatment of Mercerized Bamboo. Polymer-Plastics Technology and Engineering. 2009; 49(1)DOI
18. Lagorce-Tachon Aurélie, Karbowiak Thomas, Champion Dominique, Gougeon Régis D., Bellat Jean-Pierre. Mechanical properties of cork: Effect of hydration. Materials & Design. 2015; 82DOI
19. Li Xiaoyan, Lei Wencheng, Zhang Zhenhua, Zhang Weizhong, Li Neng, Yu Yuxiang, Li Chao, Chen Yuhe, Rao Fei. Fabrication and mechanical behavior of scalable lightweight high-strength cork–bamboo sandwich composites. Industrial Crops and Products. 2023; 192DOI
20. da Silva Sara P. Magalhães , Lima Paulo Silva, Lima Paulo Silva, Oliveira José Martinho. Rheological behaviour of cork-polymer composites for injection moulding. Composites Part B: Engineering. 2016; 90DOI
21. Oliveira Vanda, Pereira Helena. Cork and Cork Stoppers: Quality and Performance. In: Cosme Fernanda, Nunes Fernando M., Filipe-Ribeiro Luís. Chemistry and Biochemistry of Winemaking, Wine Stabilization and Aging. IntechOpen: IntechOpen; 2020. DOI
22. Pereira H.. Preface. In: Pereira Helena. Cork. Elsevier Science B.V.: Elsevier Science B.V.; 2007. DOI
23. Ríos-Reina Rocío, Callejón Raquel M., Oliver-Pozo Celia, Amigo José M., García-González Diego L.. ATR-FTIR as a potential tool for controlling high quality vinegar categories. Food Control. 2017; 78DOI
24. Sanchez-Saez S., Barbero E., Garcia-Castillo S.K., Ivañez I., Cirne J.. Experimental response of agglomerated cork under multi-impact loads. Materials Letters. 2015; 160DOI
25. Sánchez Martha L., Morales Luz Y., Caicedo Juan D.. Physical and mechanical properties of agglomerated panels made from bamboo fiber and vegetable resin. Construction and Building Materials. 2017; 156DOI
26. Santos Eduardo Braga Costa, Moreno Camila Gomes, Barros Janetty Jany Pereira, Moura Danusa Araújo de, Fim Fabiana de Carvalho, Ries Andreas, Wellen Renate Maria Ramos, Silva Lucineide Balbino da. Effect of Alkaline and Hot Water Treatments on the Structure and Morphology of Piassava Fibers. Materials Research. 2018; 21(2)DOI
27. Sierra-Pérez J. , López Forniés I. , Boschmonart-Rives J., Gabarrell X.. The Potential Applications Of Eco-materials In Improving Sustainability In Buildings: The Case Of Cork. WIT Transactions on The Built Environment. 2016; 161DOI
28. Silva S. P., Sabino M. A., Fernandes E. M., Correlo V. M., Boesel L. F., Reis R. L.. Cork: properties, capabilities and applications. International Materials Reviews. 2005; 50(6)DOI
29. Silva Sandra D., Feliciano Rodrigo P., Boas Luís V., Bronze Maria R.. Application of FTIR-ATR to Moscatel dessert wines for prediction of total phenolic and flavonoid contents and antioxidant capacity. Food Chemistry. 2014; 150DOI
30. Vasconcelos Gabriela Celso Melo Soares de, Carvalho Laura Hecker de, Barbosa Renata, Idalino Rita de Cássia de Lima, Alves Tatianny Soares. Effects of weathering on mechanical and morphological properties cork filled green polyethylene eco-composites. Polímeros. 2020; 30(1)DOI
31. Vilela Carla, Sousa Andreia F., Freire Carmen S.R., Silvestre Armando J.D.. Novel sustainable composites prepared from cork residues and biopolymers. Biomass and Bioenergy. 2013; 55DOI
32. Vlachos Panagiotis, Kampioti Adamantia, Kornaros Michael, Lyberatos Gerasimos. Development and evaluation of alternative processes for sterilization and deodorization of cork barks and natural cork stoppers. European Food Research and Technology. 2007; 225(5)DOI
33. Lixia Xi, Daochun Qin, Xin An, Ge Wang. Resistance of Natural Bamboo Fiber to Microorganisms and Factors that May Affect Such Resistance. BioResources. 2013; 8(4)
34. Yusoff Rosni Binti, Takagi Hitoshi, Nakagaito Antonio Norio. Tensile and flexural properties of polylactic acid-based hybrid green composites reinforced by kenaf, bamboo and coir fibers. Industrial Crops and Products. 2016; 94DOI
35. Zakikhani Parnia, Zahari R., Sultan M.T.H., Majid D.L.. Extraction and preparation of bamboo fibre-reinforced composites. Materials & Design. 2014; 63DOI