Effect of Vacuum Freeze-drying on Enhancing Liquid Permeability of Moso Bamboo
Jun Xu,a Sheng He,a Jingpeng Li,a Hui Yu,b Siqi Zhao,a Yuhe Chen,a,* and Lingfei Ma c,*
Permeability has been proven useful and important in the application and basic research of biomaterials, such as anti-mildew, dyeing, or other impregnated modification, especially in the study of bamboo. However, the traditional methods of improving bamboo’s permeability are chemical treatments and destructive physical treatments. This study proposed an innovative way to produce more porous bamboo with effective penetration via the vacuum freeze-dried method. The greatest advantage of this method is that the original form of bamboo was preserved according to the three-phase principle of water. From scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP), the pore characteristics and microstructure of moso bamboo were characterized, the porosity of bamboo increased to 73%, a microporous bamboo was formed, and the liquid penetration of bamboo improved noticeably. Meanwhile, the vacuum freeze-dried method turned bamboo into a mould-preservation biomaterial that effectively removed starch grains. Moreover, testing the mechanical properties showed that the vacuum freeze-dried method did not have a noticeable impact on bamboo’s mechanical properties, although it had a remarkable impact during later-stage processing and utilization. More importantly, this work provided a good example with which to expand high-value applications of bamboo resources.
Keywords: Permeability; Moso bamboo; Vacuum freeze-dried method; Mechanical properties
Contact information: a: China National Bamboo Research Center, Key Laboratory of High Efficient Processing of Bamboo of Zhejiang Province, Chinese Academy of Forestry, Hangzhou 310012, P. R. China; b: East China Forestry Investigation Planning and Design Institute, Chinese Ministry of Forestry, Hangzhou 310014, P. R. China; c: School of Engineering, Zhejiang Agricultural and Forestry University, Lin’an 311300, P. R. China;
* Corresponding authors: email@example.com; firstname.lastname@example.org
With the shortage of woody resources, non-wood resources are garnering increased attention. Bamboo, as a sustainable biological material, has been stated to be the best substitute for wood (Atanda 2015). In East Asia, the utilization of bamboo has been intensively investigated from its raw form to processing, in areas such as bamboo crafts and other building materials (Yan et al. 2017). However, bamboo is readily infected with mould fungi due to its high content of sugar, starch, and protein, which greatly limits the applications of bamboo in furniture and building materials (Feng et al. 2010). To improve the durability and value of bamboo products, chemical preservation and modification have become an indispensable part of bamboo processing (Chen et al. 2017). Due to the conspicuous heterogeneity of bamboo (Dixon and Gibson 2014), the distribution of the main tissue system (the vessel, sieve tube, and parenchyma cells) in bamboo is longitudinal, with no horizontal transmission system (Huang et al. 2015). Furthermore, it has low permeability compared with wood, and the relative poor volume of bamboo is also far less than that of wood (bamboo: 47%, wood: 67%) (He et al.2017). Hence, it is difficult for pharmaceuticals to be impregnated into bamboo. Therefore, improving permeability has become an important aspect in bamboo processing.
Previous studies have revealed that several methods are effective for improving wood’s permeability (Hill 2006; Jiang et al. 2006; He 2014; He et al. 2016). It is reasonable to consider that such methods can also be used to modify the permeability of bamboo, as bamboo is also a biologically porous material like wood. Microwave treatment is effective for enhancing the liquid permeability of wood as high steam pressure generated within wood cells would result in the destruction of its microstructure (Jiang et al. 2006; Torgovnikov and Vinden 2009; He et al. 2014, 2016). Solvent treatment with ethanol is used to improve bamboo permeability by dissolving corresponding components from bamboo (Sun et al. 2011). Superheated steam to improve permeability is also effective, but the pit membrane and basic organization of the wood are destroyed, which affects the mechanical properties (Bao and Zhou 2017). Moreover, drilling techniques and mechanical incising methods have been reported (Emaminasab et al. 2015; Taghiyari 2015). In summary, an improved permeability is accompanied by degradation in mechanical properties or weight loss resulting from extractive dissolving. Freeze-drying is a dehydration process typically used in food processing. It freezes the material and then reduces the surrounding pressure to allow the frozen water in the material to sublimate directly from the solid phase to the gas phase (Fig. 1). At the same time, extractives in the material may also evaporate along with water. Additionally, consecutive pore paths formed as evaporated water or extractives break through the originally closed pores. Thus, liquid flow in the treated material is facilitated, which means that the liquid permeability is improved. As Lu et al. (2005) had investigated, after freeze-drying treatment, the wood’s liquid permeability increased as low pit-aspiration ratio and cracks of pits membrane of some bordered pits were found. Such studies have verified that freeze drying is an important means to improve the permeability for both wood and bamboo.
During the process of vacuum freeze-drying, bamboo immersed with water would be frozen first, then water inside it would sublimate directly from solid phase under vacuum conditions. As a result, the dry bamboo had a high surface area and more porous structure (Fig. 2). Microstructural changes contribute to the improvement of liquid permeability for the freeze-dried wood or bamboo. A scanning electron microscope (SEM) was widely used to identify the microstructural changes after treatment. Furthermore, changes in the microstructure would result in the variation in porosity and pore diameter distribution. Methods for measuring the porous parameters of wood include mercury intrusion porosimetry (MIP) (Schneider 1982, 1983; Ding et al. 2008; Plötze and Niemz 2011), gas adsorption isotherms, solute exclusion (Berthold et al. 1997), as well as nuclear magnetic resonance (Furo and Daicic 1999). The MIP method can be used to measure macro- and mesopores in the range of 1.8 nm to 58,000 nm (Plötze and Niemz 2011). As the pore radii of different kinds of cells in bamboo are key in this range (the radius of a sieve tube is approximately 50 μm, the radius of parenchyma cells is approximately 47 μm, the radius of vessels is approximately 15 μm to 200 μm, and the radius of fibre cells is approximately 9.3 μm), the MIP method is suitable for examining the porosity and pore size distribution of bamboo before and after freeze drying.
Fig. 1. Three-phase diagram of water (Xu et al. 1994; Han 2007)
Fig. 2. Preparation of porous bamboo by vacuum freeze-drying
In the present study, the freeze-drying method was used to improve the liquid permeability of bamboo. Microstructure changes in the treated bamboo were verified by SEM examination. A comparison of pore parameters was performed with the MIP method to measure the porosity and pore size distribution of bamboo before and after freeze-drying. The mechanism of permeability improvement through freeze-dried bamboo was analysed. Moreover, the volume expansion rate and water absorption rate tests were conducted to verify the effects of freeze drying on liquid permeability improvement, while mechanical property tests of modulus of elasticity (MOE) and modulus of rupture (MOR) were conducted to evaluate the effects of freeze drying on the mechanical strength of bamboo.
Moso bamboo (Phyllostachys pubescens), aged 5 years, was obtained from Anji City, Zhejiang Province, China. From the internodes between 1.5 m and 2.0 m from the base of the bamboo culm, three types of bamboo specimens (S1, S2, and S3) were prepared for experimental treatment and testing (Fig. 3). These specimens were: S1, of 160 mm (length) 20 mm (width) 5 mm (thickness) for mechanical properties test; S2, of 50 mm (length) 20 mm (width) 5 mm (thickness) for the MIP and SEM analyses; and S3, of 40 mm (length) 20 mm (width) 8 mm (thickness) for other physical performance tests.
Fig. 3. Preparation of the specimens
Vacuum freeze-drying and control processing
The experimental processes are displayed in Fig. 4. Each type of specimen was immersed in water until the equilibrium of water absorption was attained; then the material was divided into two groups. One was pre-treated with the vacuum freeze-dried method, while the other was oven-dried as the control group (Fig. 4a). The vacuum freeze-dried method was performed using the vacuum freeze-drying machine (FD-5; JingFu, Shanghai, China) to dry the bamboo specimens. The pre-frozen bamboo temperature was -55 C, and the vacuum degree was 4 Pa. The oven-dried method was performed in accordance with the national standard (GB/T 15780 (1995). After treatment, all of the specimens were sealed in the vacuum environment to isolate air, especially water.
Evaluation of permeability
To evaluate the changes in permeability, samples (control and treated samples respectively) were soaked in water for 100 h, the volume expansion rate (VER) and water absorption rate (WAR) were calculated based on the equation displayed in Fig. 4b. V1 is the volume (mm3) before the specimens were immersed in water and V2 is the volume (mm3) after being treated; W1 is the weight (g) of the treated specimens, and W2 (g) is the weight of the treated specimens after water absorption.
Fig. 4. Overview of the experiment
Observation of bamboo’s microstructure
Scanning electron microscopy (Hitachi S3400, Fukushima, Japan) was used to observe the microstructure of bamboo (vessels, parenchyma cells, fibre cells, pits, etc.). The samples obtained from S2, as shown in Fig. 4c, were coated with gold prior to observation (Anji, Zhejiang, China).
MIP and main properties
The characterisation process was performed using an automated mercury porosimeter (AutoporeTM IV 9500; Norcross, GA, USA) to force mercury into the pores of the bamboo samples as shown in Fig. 4c. Measurements of total intrusion volume, total pore surface area, pore size, and distribution were all available. The pore distribution was determined with the Washburn equation (Peng et al. 2015). Furthermore, some studies (Ilic 1995) found that water, frozen inside the cell, would impart a compressive stress to the cell wall, which may have an influence on mechanical strength; thus, the mechanical properties of wood in timber research has been investigated. Therefore, the MOR and MOE were used to explore the effects of the freeze-dried method on the mechanical properties of bamboo (Fig. 4d) according to the national standard GB/T 15780 (1995). The data were analysed for each property, to determine the MOE effect and MOR effect separately through an analysis of variance (ANOVA), and further using the Least Significance test using SPSS Statistics 19.0 (SPSS Inc., Chicago, IL, USA).
RESULTS AND DISCUSSION
Verification of Permeability
The freeze-dried method is important for food storage (Valarmathi et al. 2017), because it can improve the rehydration of food (water absorption); therefore, the authors used the water absorption rate to characterize bamboo’s liquid penetration. The influence of VER and WAR of the bamboo samples, treated with vacuum freeze drying are shown in Figs. 5 and 6. As shown in Fig. 5, the volume expansion rate of the treated bamboo was positive, different from the negative growth of the control bamboo. Meanwhile, the water absorption rate of the control and treated bamboo increased when the soaking water time increased from 1 h to 100 h, but the water absorption rate of the treated bamboo was higher than that of the control bamboo. The rapid increase in the water absorption rate of the treated bamboo may have been due to multiple factors. Water, frozen inside the bamboo, leads to the expansion of bamboo, which was already verified with the volume expansion rate. When frozen water had sublimated, the original location of it formed the pores, thereby it raised the porosity of the bamboo. However, sublimation broke through part of the pits and holes under vacuum conditions, which was verified in the SEM analysis.
Fig. 5. Volume expansion rate of bamboo samples for C (control) and T (treated) bamboo
Fig. 6. Water absorption rate of bamboo samples for C (control) and T (treated) bamboo
Analysis of Microstructure
In the field of organic chemistry and materials science, there is a saying: Structure can reflect the properties of materials (Jain et al. 1992; Nogata et al. 1995). Bamboo belongs to the grass family Gramineae (Zhang et al. 2017). Vascular bundles and parenchyma ground tissues are the main structures of bamboo, with fibre cells encircling the vascular bundle vessels as shown in Fig. 7. The authors’ previous study revealed that bamboo was less permeable than normal wood because of some differences in the anatomical features, such as organizational structure and pore size (Emaminasab et al. 2016), which affect fluid flow paths. Figure 7T1 displays the difference with the control bamboo (Fig. 7C1). Vertical to the vessel’s wall, there were splits after the bamboo had been freeze-dried. Furthermore, the comparison of Figs. 7C2 and T2 showed another result: with vacuum freeze-drying, starch grains were removed from the parenchyma cells, which means that the treated bamboo may contain a certain degree of mold preservation. All the results reflected that water, frozen in bamboo, caused a pressure upon the vessel and cell walls, with the sublimation of ice inside the cell lumens, which caused starch grains to be removed and some pores to open.
Figure 8 shows the comparison of the vascular bundle vessels around the treated and control bamboo. The vessels in Figs. 8C1 and T1 were protoxylem vessels, and in Figs. 8C2 and T2 the vessels were primarily phloem vessels (sieve tube). There were more pits on the protoxylem vessel walls (Fig. 8a), in which the main type of pits was alternate pitting. As bamboo was treated, the pits were enlarged, and the vessel walls collapsed (Fig. 8b). Both oven-dried and vacuum freeze-dried methods had affected the structure of primary phloem vessels (Figs. 8C2 and T2), but the effect of vacuum freeze drying on the sieve tube was more obvious. The size of the sieve holes on the sieve tube walls increased, and the pore diameter of the sieve tube expanded to 70 μm (Fig. 8d), larger than that of the sieve tube in Fig. 8c. Moreover, the parenchyma cells around the vessels of the treated bamboo were extruded when water inside the vessels swelled (Fig. 8T1). All these phenomena proved that the structure of bamboo became looser and that porosity increased, which was consistent with the results of Fig. 9T.