Abstract
The viscoelastic behavior of Sapindus mukorossi pericarp (the covering of the Indian soapberry fruit) was investigated. Samples were subjected to creep-recovery and oscillatory shear tests. The moisture content had a strong effect on the dynamic moduli, whereas the temperature had little effect. Both the storage and loss moduli had a weak dependency on the frequency. In general, the storage and loss moduli decreased and slowly converged with an increase in the moisture content. The storage and loss moduli decreased with an increasing temperature. The creep-recovery experiment and dynamic shear tests indicated that the pericarps with high moisture contents were more prone to permanent deformation. The dynamic shear tests further demonstrated that a low moisture content, temperature, and frequency resulted in the pericarps having a more solid behavior and undergoing elastic deformation. Images of the microstructure showed that cells in the raw pericarps were round, of similar size, and neatly arranged. In the treated pericarps, the cells gradually became smaller and finally disappeared. It was inferred that drying with hot air caused the loss of intracellular substances, which in turn strengthened the dynamic characteristics of the pericarps. The completely dried pericarps thus behaved as strong viscoelastic solid in contrast to the raw pericarps.
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Viscoelastic Behaviors and Microstructure of Sapindus mukorossi Pericarp for Different Moisture Contents
Mengjiao Xu,a,b Cheng Tian,a and Daochun Xu a,b,*
The viscoelastic behavior of Sapindus mukorossi pericarp (the covering of the Indian soapberry fruit) was investigated. Samples were subjected to creep-recovery and oscillatory shear tests. The moisture content had a strong effect on the dynamic moduli, whereas the temperature had little effect. Both the storage and loss moduli had a weak dependency on the frequency. In general, the storage and loss moduli decreased and slowly converged with an increase in the moisture content. The storage and loss moduli decreased with an increasing temperature. The creep-recovery experiment and dynamic shear tests indicated that the pericarps with high moisture contents were more prone to permanent deformation. The dynamic shear tests further demonstrated that a low moisture content, temperature, and frequency resulted in the pericarps having a more solid behavior and undergoing elastic deformation. Images of the microstructure showed that cells in the raw pericarps were round, of similar size, and neatly arranged. In the treated pericarps, the cells gradually became smaller and finally disappeared. It was inferred that drying with hot air caused the loss of intracellular substances, which in turn strengthened the dynamic characteristics of the pericarps. The completely dried pericarps thus behaved as strong viscoelastic solid in contrast to the raw pericarps.
Keywords: Sapindus mukorossi; Viscoelastic behavior; Moisture content; Microstructure; Drying
Contact information: a: School of Technology, Beijing Forestry University, Beijing 100083, China; b: Key Lab of State Forestry Administration for Forestry Equipment and Automation, Beijing Forestry University, Beijing 100083, China; *Corresponding author: xudaochun@bjfu.edu.cn
INTRODUCTION
The genus Sapindus includes approximately 13 species and is widely distributed in the warmer regions of the Americas, Asia, and Oceania. There are four species and one variety in China that grow in the Yangtze River basin and various provinces in southern China. China has already started developing a large planting area of Sapindus, with the area that has been planted reaching nearly 100,000 h·m2. In Fujian Province, the planted area has reached more than 7000 h·m2. More than 1000 h·m2 of Sapindus were planted in southern Fujian in 2008, while more than 3000 h·m2 were planted in southern Fujian in 2009 (Jia and Sun 2012).
The literature has reported that Sapindus mukorossi pericarp is rich in saponins, with a total saponins yield of up to 33.4% (Wu et al. 2015). Saponins are natural surfactants and usually serve as commercial ingredients in cosmetics, washing soaps, and other cleaning products. Saponins are also a class of plant-derived biosurfactant. Biosurfactants are an ideal ecofriendly candidate for cleaning technologies because of their biodegradability and low toxicity (Pacwa-Płociniczak et al. 2011). Furthermore, the extracts of ethyl acetate show strong antibacterial activities against bacteria and dermatophytes (Wu et al. 2014).
Lovato et al. (2014) reported that Sapindus seeds are rich in oil, with the oil yield being approximately 40%. In the extracted oil, the most abundant fatty acid is oleic acid (Sun et al. 2017). The oil extracted from Sapindus is preferred in the biodiesel industry because of its low production costs. Chakraborty and Baruah (2013) investigated the properties of extracted oil to be used for biodiesel production and showed that S. mukorossi has favorable distillation characteristics.
The tyrosinase inhibition, free radical scavenging, anti-microbial, and anti-cancer properties of extracts from S. mukorossi seeds have been investigated (Chen et al. 2010). These bioactivities demonstrate the high potential of applying these extracts in the fields of medical cosmetology, food supplementation, and antibiotics and chemotherapy. Hu et al. (2018) analyzed the antifungal activities of triterpenoid saponins from S. mukorossi pulp. An amazing finding was that S. mukorossi stem bark can be used for disorders related to inflammation (Shah et al. 2017). Moreover, Sunanda et al. (2013) used Sapindus seed hulls for the preparation of activated carbon.
The viscoelastic properties of fruits and vegetables have been extensively studied in the fields of storage and processing to determine the relationship between processing (storage) and food texture (Lin and Pitt 1986; Jack et al. 1995; Jackman and Stanley 1995; Alvarez and Canet 1998; Nieto et al. 1998; Martínez et al. 2007; Wu and Guo 2010; Xu and Li 2015; Wang et al. 2016; Wu et al. 2016). Wu et al. (2016) investigated the effects of a pretreatment that uses a strong pulsed electric field on the dynamic viscoelasticity of fruits and vegetables. Xu and Li (2015) evaluated the correlations of the viscoelastic behavior with the water state and microstructure of carrots dried with hot air. Wu and Guo (2010) investigated the dynamic viscoelastic behavior and microstructural changes of Korla pears for various turgor levels. Martínez et al. (2005), Nieto et al. (1998), and Lin and Pitt (1986) investigated the texture of apples and potatoes that were processed in various ways. Alvarez and Canet (1998) studied the rheological properties of fresh and mature potato tissue. Wang et al. (2016) studied the rheological properties of different varieties of apple peels and established mathematical models for the relaxation and creep of apple peels. Jackman and Stanley (1995) studied the creep behavior of tomato pericarp tissue affected by ambient-temperature ripening and chilled storage.
Sapindus mukorossi pericarp is rich in saponins, the seed is rich in oil, and the value of S. mukorossi as a biomass-energy tree species has been widely studied. However, there are no published studies on the viscoelastic characteristics of S. mukorossi pericarp. The dynamic moduli of S. mukorossi pericarp indicate energy loss and energy storage when the pericarp is subjected to an external force, and this can directly reflect the resistance of S. mukorossi pericarp under different conditions. Furthermore, the results obtained in this work can provide some theoretical guidance for the shelling of the fruit.
In the present study, the viscoelastic behaviors of the pericarp under different conditions were investigated to gain a theoretical basis for the design of a shelling machine. The specific objectives of the research were to investigate the viscoelastic properties of S. mukorossi pericarp and analyze the resulting factors. The experimental results will provide valuable information for the design of a machine that is used for shelling and are thus economically important.
EXPERIMENTAL
Materials and Methods
Material preparation
Sapindus mukorossi fruit, as shown in Fig. 1, was harvested in October 2017 in Fujian, China. The fruit was fresh and undamaged, and it was largely mature and uniform in size. Samples were wrapped and then stored in a refrigerator at a temperature of 4 °C to 6 °C. The fruit was removed from storage at the time of the experiments, and the pericarp was separated from the S. mukorossi fruit. The pericarps were grouped and numbered. Three groups were placed in an electrical thermostatic drying oven (Shanghai Boxun Industry & Commerce Co. Ltd, Shanghai, China) for 1 h, 2 h, and 3 h at 80 °C, while a fourth group was dried at 80 °C for 6 h, and then removed for weighing (Yuyao Jinnuo Balance Instrument Co. Ltd, Zhejiang, China). The drying process was repeated until a constant weight was reached. Additionally, a control experiment was conducted where a fifth group of pericarps was not dried. Following these steps, the pericarps were peeled and two slices (thickness of 2.0 mm ± 0.5 mm and width of 10 mm ± 1.0 mm) were cut from the center of each pericarp.
Fig. 1. Fruit of Sapindus mukorossi
Moisture content
Complete fruit samples were refrigerated again, but the remaining pericarp was weighed, and the data were recorded as the initial mass (W1, g). The pericarps were then placed in the drying oven until a constant mass (W2, g) was obtained. The data reported were the mean values of at least three measurements. The moisture content was considered the same for each pericarp. The moisture content (Xw, %) was calculated with Eq. 1:
(1)
Evaluation of the Viscoelastic Properties
The storage modulus (G‘), loss modulus (G“), and loss tangent (tan δ) were measured in a shear configuration on a Dynamic Thermomechanical Analyzer DMA Q800 (TA Instruments, Wakefield, MA, USA). Each shear test required two identical slices. To meet the requirements of the shear test, square slices (thickness of 2.0 mm ± 0.5 mm, width of 10 mm ± 1.0 mm, and length of 10 mm ± 1.0 mm) were shaped from the middle of the pericarp using a sharp razor blade. After sample preparation, the size was again checked with Digimatic calipers to avoid large deviations. The frequency sweep test was performed at 0.1 Hz to 100 Hz and lasted 10 min. A room temperature of 22 °C to 23 °C was maintained. The temperature sweep test was performed at 10 °C to 90 °C and the heating process lasted 20 min. Before starting the test, a preliminary experiment was conducted to obtain the linear viscoelastic region of the pericarp and then select an appropriate amplitude that ensured the material was not damaged. The amplitude was finally set to 3 μm for the test.
In the creep-recovery tests, square samples (thickness of 2.0 mm ± 0.5 mm, width of 10 mm ± 1.0 mm, and length of 10 mm ± 1.0 mm) were compressed using a compression configuration. A constant compressive force of 2 N that lasted 180 s was applied in the tests. A previous force sweep range of 0 N to 2 N was used so that the selected force guaranteed the pericarp was in the linear viscoelastic region. After removal of the force, sample recovery lasted for an additional 180 s. These tests were executed using the DMA Q800.
Theoretical Basis of Viscoelasticity
In the dynamic shear tests, samples were subjected to a sinusoidal change in the stress or strain, while the dynamic mechanical properties of the material were measured. For an ideal elastomer, the strain response to the stress is instantaneous and the strain response is therefore a sinusoidal function that is in phase with the stress. For an ideal viscous body, the strain response of a material lags by π/2ω, where ω is the angular velocity. Biomaterials are mostly characterized by the viscoelasticity, and the strain response lags by a certain angle δ, which is within the range 0 to π/2. The storage modulus, loss modulus, and loss tangent are related according to the following equations,
where σ is the stress (Pa), ε is the strain of the material, G‘ is the storage modulus (Pa) that characterizes the energy storage of the material during deformation because of elastic deformation, G” is the loss modulus (Pa) that characterizes energy loss in the form of heat because of viscous deformation, and tan δ, tan δ is the mechanical loss, which is the ratio of the loss modulus to the storage modulus.
The data were reported as one measurement because one fruit is too small and can only be tested once.
Microstructure
Scanning electron microscopy (SEM) (Zeiss Merlin Fesem, Jena, Germany) was used to examine the S. mukorossi samples. After being dried with hot air, the samples had different moisture content and were then placed in a freeze dryer (Thermo Scientific Forma 900 Series, Massachusetts, USA). The freeze dryer was used in order to change the water in the pericarp directly from liquid state to gaseous state, thus to meet the requirements of SEM for the observation environment (that is, the sample does not contain water) and also to ensure that the microstructure was not destroyed by this step. The samples were then cut into small pieces (thickness of 1.5 mm, width of 3 mm, and length of 7 mm) using tweezers and a knife. The samples were fixed on a conductive plate with a conductive adhesive, and then immediately coated with Au (Fig. 2). The samples were firstly placed in the sample room and a vacuuming was performed. Then the operating voltage and working distance was determined. Finally, it was operated on a computer for the microscopy observation.
Fig. 2. Specimens for scanning electron microscopy
RESULTS AND DISCUSSIONS
Composition of the Pericarps
Slices of the pericarps were prepared and analyzed with SEM. It was observed that the surface of a pericarp is comprised of agglutinated irregular flakes. The energy spectrum showed that the elemental composition of the S. mukorossi pericarp contains carbon (C), oxygen (O), and potassium (K). Figure 3 and Table 1 show that there were large amounts of C and O, while the amount of K was comparatively low. These results were similar to the results published by Borah et al. (2017), who conducted an energy-dispersive X-ray analysis to determine the elemental composition of the shell powder from S. mukorossi.
Fig. 3. Energy disperse spectroscopy results
Table 1. Elemental Composition of the S. mukorossi Pericarp
Creep-recovery Response
The creep curves of the Sapindus pericarp samples for the different drying times are presented in Fig. 4. Drying with hot air significantly affected (p < 0.01, Microsoft Excel Analysis) both the creep strain at 180 s and residual strain after an additional 180 s of recovery. Figure 4 shows that the moisture content of the pericarp had an important effect on the instantaneous viscoelasticity, and there was obvious elastic deformation. With an increasing moisture content, the strain increased, which indicated that the pericarps with a high moisture content were more prone to permanent deformation. The minimum strain value was obtained at a 0% moisture content, which indicated that the totally dried pericarps were the most brittle in this condition and more prone to elastic deformation. These changes were similar to those of the dynamic moduli in the oscillation shear tests. It was further shown that pericarps with a high moisture content were more prone to permanent deformation and conducive to peeling.
Fig. 4. Creep curves of the Sapindus pericarp with different moisture contents
Storage Modulus, Loss Modulus, and Loss Tangent
The pericarps of the samples behaved as a viscoelastic solid with a storage modulus higher than the loss modulus, which indicated a dominant solid behavior of the pericarps. This result was consistent with the viscoelastic behavior of carrot and Korla pear tissues (Martínez et al. 2005; Wu and Guo 2010; Xu and Li 2015). However, in some cases, the storage and loss moduli were close, and the differences between the two values were slightly different under different conditions.
Effect of the moisture content on the viscoelasticity
The viscoelasticity curves for different moisture contents obtained from the frequency and temperature scanning are shown in Figs. 5 to 7. Figure 5 shows that the moisture content ranged from 0% to 20%.
The change in the viscoelastic behavior with the moisture content is shown in Fig. 5. The three moduli followed almost the same trend with an increase in the moisture content for both frequency and temperature scanning. The dynamic moduli at different frequencies are shown in Figs. 5a and 5b. The dynamic moduli decreased with an increase in the moisture content, which showed that the tissues became more viscous and elastic after treatment.
These results were different from those for osmotic dehydrated apples and melon tissues (Martínez et al. 2005; Loredo et al. 2013). When the moisture content was low, the storage modulus greatly exceeded the loss modulus. However, at a higher moisture content, the difference between the two moduli was less. The storage modulus had a sharp and then gentle decline, whereas the loss modulus had a relatively gentle decline across the whole range. The loss tangent had a trend different from the trends of the other two moduli. The loss tangent first increased, reached a maximum at a moisture content of approximately 11%, and then decreased. Furthermore, the frequency tended to have little effect compared with the moisture content.