- Email: [email protected]
Improved characterization of nanofibers from bacterial cellulose and its potential application in fresh-cut apples
International Journal of Biological Macromolecules 149 (2020) 178–186
Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Improved characterization of nanofibers from bacterial cellulose and its potential application in fresh-cut apples Xichuan Zhai, Dehui Lin ⁎, Wenwen Li, Xingbin Yang ⁎ Shaanxi Engineering Laboratory for Food Green Processing and Safety Control, and Shaanxi Key Laboratory for Hazard Factors Assessment in Processing and Storage of Agricultural Products, College of Food Engineering and Nutritional Science, Shaanxi Normal University, Xi'an 710062, China
a r t i c l e
i n f o
Article history: Received 18 September 2019 Received in revised form 14 January 2020 Accepted 22 January 2020 Available online 24 January 2020 Keywords: Bacterial cellulose Bacterial cellulose nanofibers Physicochemical properties Fresh-cut apples
a b s t r a c t The present research aimed to study the nanofibers from bacterial cellulose (BC) by HCl hydrolysis and explore its new potential application in fresh-cut apples. Bacterial cellulose nanofibers (BCNs) showed low and more homogeneity particle size, as well as higher zeta potential and transparency in comparison with BC, which was confirmed by morphological analysis. Physical properties analysis showed that BCNs was more excellent semicrystalline polymer with higher thermal stability as compared with BC. Rheological results displayed that BCNs suspensions presented a shear thinning behaviour with higher apparent viscosity, storage (G′) and loss (G′′) moduli at the same concentration in comparison with BC. Furthermore, BCNs suspensions were more stable than BC suspensions under storage condition of 4 °C. Additionally, 2% (wt%) of BCNs suspensions were coated on fresh-cut apples. Results showed that the samples coated with BCNs suspension displayed more excellent properties of keeping fresh-cut apples as compared with that coated with BC suspensions, including delaying weight loss, improving firmness and soluble solids content, reducing browning index and titratable acidity. Therefore, the low cost and high biocompatibility of BCNs can be used as new coatings for fresh-cut apples and have great potential to coat fresh-cut fruits and vegetables in food industry. © 2020 Elsevier B.V. All rights reserved.
1. Introduction Fresh-cut apples are normally found in markets with great demands by consumers due to their year-round availability and convenience. However, the storability, shelf life and microbial safety are still major problems for fresh-cut fruit industry and consumers due to their decomposition processes caused by physical lesions, such as enzymatic browning, texture reduction, water losing, increased susceptibility to microbial spoilage, and production of undesirable odors and flavors [1,2]. Therefore, one of the potential approaches to tackle the above problems are edible coatings, which comprises a thin layer of edible material formed as a coating on a food product [3]. And nowadays, many edibility and biodegradability biopolymers, such as polysaccharides, proteins, and lipids are regarded as excellent coating materials and extensively used in fresh-cut fruits and vegetables [2–5]. Bacterial cellulose (BC), an extremely pure natural microbe exocellular polysaccharide, can form hydrogels with better material properties including high purity, high crystallinity, high degree of porosity, high water-uptake capacity, high tensile strength, excellent biocompatibility and 3D nanofibrillar cellulosic network properties [6–8]. Therefore, BC represents a potential alternative to plant or tunicin⁎ Corresponding authors. E-mail addresses: [email protected] (D. Lin), [email protected] (X. Yang).
https://doi.org/10.1016/j.ijbiomac.2020.01.230 0141-8130/© 2020 Elsevier B.V. All rights reserved.
derived cellulose for specific applications in bio-medicine [9,10], cosmetics and papermaking [11], food processing and packaging and other applications [12]. Recently, it has been showed that natural nanofibers, nanocrystals or nanowhiskers are promising potential in making nanocomposite films and coatings [13]. Therefore, treatment methods, including biological, mechanical or chemical methods, were proposed to prepare CNCs from various kinds of materials [14]. Due to the high cost and pollution of biological method and inefficient of mechanical method [14,15], chemical method (strong mineral acids, e.g. hydrochloric and sulfuric acids) could obtain the exact size, dimensions [16]. However, cellulose nanocrystals from sulfuric acid hydrolysis can be endowed with sulfate groups on their surface, this lead to the decrease of its thermostability and addition process of removing sulfate with hydrogen peroxide [17–19]. In addition, few studies have shown the effective methods to prepare nano-fibers from BC, and there is also no systematic investigation in the literature to explore its new potential application in fresh-cut apples. Hence, bacterial cellulose nanofibers (BCNs) were prepared from BC with the chemical method and characterized by atomic force microscopy (AFM), Fourier-transform infrared (FT-IR), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). And then the stability, rheological properties, water holding capacity (WHC), water swelling ability (WSA) and water release rate (WRR) of BC and BCNs were compared. Finally, suspensions of BC and BCNs were used as coatings of
X. Zhai et al. / International Journal of Biological Macromolecules 149 (2020) 178–186
fresh-cut apples to study their fresh-keeping properties. The parameters of the coated fresh-cut apples, including weight loss, browning index, firmness, soluble solid contents (SSC) and titratable acidity (TA), were determined in the present work. 2. Methods and materials 2.1. Preparation of BC BC was cultured according to our previous method [20]. In brief, the Komagataeibacter hansenii strain CGMCC 3917 was cultured at 30 °C under static conditions in liquid fermentation medium at pH 5.0. After 14 days cultivation, cellulosic membranes were obtained and swilled with tap-water overnight, and then dipped in NaOH solution (0.1 M) at 80 °C for 2 h, and then swilled with distilled water until the pH value of BC membranes was neutral. The purified cellulosic membranes were smashed and stored in sterile water at 4 °C. The purified wet BC membranes were disrupted in a blender (CWFJ, Changzhou, China) at 15,000 rpm for 5 min at room temperature. Then the aqueous cellulose suspensions were centrifuged (4000 rpm) for 15 min to remove the free water, followed by hydrolysis with HCl according to our previous method [20]. 2.2. Preparation of bacterial cellulose nanofibers (BCNs) BCNs were prepared with hydrochloric acid according to our previous study [21]. Briefly, 5.0 g BC was mixed with 75 mL of HCl (3 M), and then the hydrolysis processes were carried out at 70 °C for 4 h under condition of vigorous mechanical stirring at 200 rpm. The suspensions were naturally cooled down to ambient temperature and the hydrolysate was obtained by centrifugation for 10 min at 10000 g and the sediment was hydrolyzed for second time with the same method described above. After hydrolyzation, the sediment was collected and washed with distilled water until its pH was neutral. 2.3. Characterization of BC and BCNs 2.3.1. Size, zeta potential, and transmittance of BC and BCNs The particle size of BC and BCNs was detected with Malvern particle size analyzer (Nano ZS90, Malvern Instruments Ltd., UK) with dynamic light scattering (DLS) according to previous studies [21,22]. The transmittance of BC and BCNs was recorded using a UV–visible spectrophotometer (UV-300, InsMark, Shanghai) in the range of 400 nm to 800 nm. 2.3.2. Morphological analysis by transmission electron microscope (TEM) and atomic force microscopy (AFM) The morphologies of BC and BCNs were observed by TEM and AFM. For TEM, 10 μL of BC or BCNs samples (0.1%, w/v) was dripped on a new carbon-coated electron microscope grid and then dried at ambient temperature, followed by observation with TEM (HT-7700, Hitachi, Japan) at 80 kV [21]. For AFM, the surface topography of BC and BCNs were determined using an atomic force microscopy (Dimension ICON, Bruker, USA). The BC or BCNs suspensions (0.05%, w/v) were deposited on about 1 cm2 piece of freshly cleaved mica for 60 s and the extra water was removed with filter paper. Then, the samples were placed on an AFM specimen disc and observed after drying at ambient temperature [23]. AFM photos (10 × 10 μm2) of the films were obtained in the tapping mode on their airside. 2.3.3. Infrared analysis of BC and BCNs The infrared spectrometer (Tensor27, Brucher, Germany) was used to investigate the primary structure of BC and BCN samples. Dried BC and BCNs were blended with anhydrous KBr powder, ground completely and then milled thoroughly and pressed into a 1 mm slice
179
for infrared detection. Infrared spectra of the samples was performed at the wavelength of 4000–400 cm−1 at ambient temperature [24]. 2.3.4. Crystallinity test of BC and BCNs Automatic X-ray diffractometer (D/Max-3c, Rigalcu, Japan) was used to determine the crystallinity of BC and BCN samples referring to previous study [25]. The testing conditions were set as follows: scanning angle was at range of 5–60°, scanning speed was 4°/min, the ray of Cu/Ka was filtrated by Ni slice and formed X-ray, the wavelength of Xray was 1.54056 × 10−10 m, voltage and electricity of X-ray tube was 40 kV and 80 mA, respectively. Relative crystallinity of the sample was calculated as the following formula: Cr ð%Þ ¼
I200 −Iam 100 I 200
where, Cr is the percentage of relative crystallinity, I200 is the intensity value of the crystallographic zone, Iam is the intensity value of the amorphous zone (2θ = 18°). 2.3.5. Thermogravimetric analysis (TGA) of BC and BCNs Thermoanalyzer Systems (Q1000DSC + LNCS+FACS Q600SDT, TA, USA) was carried out to analyze the thermostability of BC and BCNs. The testing conditions were as follows: 5 mg of sample was tested in a nitrogen atmosphere, N2 flow was 20 mL/min, the testing temperature started at 40 °C and increased to 800 °C for 10 °C/min. 2.3.6. X-ray photoelectron spectroscopy (XPS) analysis of BC and BCNs For X-ray photoelectron spectroscopy (XPS) analysis, the BC and BCNs suspensions were centrifuged through 50 kDa MWCO Amicon filter, and then freeze-dried. Then the experiment was carried out on an AXIS ULTRA XPS with a monochromated Al Kα X-ray source. The elements were recognized based on the specific binding energy (eV). 2.4. Rheological properties of BC and BCNs The rheology characters of BC and BCNs aqueous suspensions were measured using a AR-G2 dynamic rheometer (TA Instruments, U.S.A). The dynamic rheometer was furnished with coarse surface parallelplate geometry (2 cm diameters) to restrain sample slide. Gap and strain were set at 1.0 mm and 1.0%, respectively. Samples were recorded at ambient temperature with three replicates and experimental flow curves were screened by comparison to the Herschele-Bulkley (HB) model (The model was selected by the software until the highest regression value of R2 ≥ 0.982) [26]: τ−τ0 ¼ Κ γ n where, τ is shear stress (Pa); τ0 is the yield stress (Pa); Κ is the consistency index (Pa·sn) and n is the flow index (n b 1 for a shear-thinning fluid and n = 1 for a Newtonian fluid). 2.5. Stability of BC and BCNs The stability of BC and BCNs aqueous suspensions were determined according to the previous report with small modifications [23]. BC and BCNs aqueous suspensions were severally kept at −20 °C, 4 °C and 25 °C for 28 d. And then 10 g of BC or BCNs aqueous suspensions were centrifuged for 10 min at 4000 g. The sediments were gathered and weighed. The specific calculation formula for the stability of suspensions is as follows: Stability ð%Þ ¼
W1 100 W2
where, W1 is the quality of sediments (g) and W2 is the total quality of the suspensions (g).
180
X. Zhai et al. / International Journal of Biological Macromolecules 149 (2020) 178–186
2.6. Water release rate (WRR), water swelling ability (WSA) and water holding capacity (WHC) For WRR, BC and BCNs aqueous suspensions were centrifuged for 15 min at 5000 g. The sediments were collected and weighed, followed by incessantly measuring the weight of the samples stored at room temperature with various intervals. The loss of water at various intervals was graphed against time [27]. For WHC, BC and BCNs samples were kept at 4 °C and 25 °C for one month and the water content in sediments was measured by centrifugation at a different time interval. For WSA, freeze-dried of BC and BCNs were determined by our previous method with minor revision [23]. Specifically, 0.5 g of the dried BC and BCNs samples were soaked in deionized water under stirring (100 rpm) at 25 °C for re-watering. After re-watering, the BC and BCNs suspensions were centrifuged for 15 min at 5000 g. The sediments were collected and weighed. The WSA of BC and BCNs at various intervals were plotted as a function of time. The specific calculation formula for the stability of dried BC and BCNs samples is as follows: WSA ð%Þ ¼
W1−W 100 W
where, W is the quality of dried BC or BCNs sample (g); W1 is the quality of BC or BCNs sample after absorbed water (g). 2.7. Coating effect of BC and BCNs on fresh-cut apple 2.7.1. Preliminary treatment of apples The uniform apple slices (about 5 mm thick) were obtained and used in this study. The control and coated samples were allocated stochastically. Control samples were soaked in chlorinated solution (0.1 mg/mL) for 10 min, but the coated samples were immersed in different concentrations of BC or BCNs suspensions (0.5%, 1% and 2%, wt) for 10 min and the excess solution were drained on absorbent paper. Each treatment was prepared with 8 samples, and all samples were kept for 1, 3, 5 and 7 days at 4 °C for shelf-life studies, respectively [2]. 2.7.2. Measurement of weight loss, firmness, total soluble solids (TSS) content, and titratable acidity (TA) For weight loss, all samples were fetched out at each sampling time point from refrigeration, weighed and calculated through the following formula: Weight loss (%) = (m0-m)/m0 × 100, in which m0 is initial weight and m is present weight [28]. The firmness of all apple pieces in the experiment groups was measured by TA-XT-Plus Texture analyzer (Stable Micro Systems, U.K.). 6 mm diameter cylindrical probe was applied and testing conditions were set based on the previous report with minor modifications [29]: 1 mm/s for testing speed, 5 mm of penetration depth and 5 g trigger force. The firmness of samples was showed as peak force (N), which was the maximum resistance force encountered by samples. The tests were performed 5 times for each group. For the determination of TSS content, the apple slices were crushed to extract juice, then the TSS content was determined with a bench refractometer (Metzer Optical Instruments, Mathura, India) and the values were expressed as Brix. Titratable acid (TA) was determined by titration with 0.1 M NaOH with the indicator of phenolphthalein. TA is expressed as (g/100 g malic acid) [28]. 2.7.3. Detection of browning index (BI) The color values of apple slices were recorded with an automatic colorimeter (WSL-2, Shanghai Precision & Scientific Instrument Co., China) at ambient temperature. The samples were performed at 3 locations and 5 replicates, and the CIELAB scale (L*, a*, and b*) was recorded. The BI was calculated as follows [30]: BI ð%Þ ¼
x−0:31 100 0:172
where; x ð%Þ ¼
a þ 1:75L 100 5:645L þ a −3:012b
2.8. Data analysis All the data were analyzed by SPSS and expressed as mean ± standard deviation (SD) for at least three replicates. Significance was determined at p b 0.05 by analysis of variance (ANOVA) followed by Duncan's multiple comparison tests. 3. Results and discussions 3.1. Particle size, zeta potential and transmittance analysis of BC and BCNs suspensions The uniform properties of BC and BCNs aqueous suspensions were investigated by DLS. As shown in Fig. 1A and Table 1, BC aqueous suspensions exhibited a wide size distribution with low intensity, and its average particle size was 590.9 nm with a polydispersity index (PDI) value of 0.37. However, BCNs obtained by HCl hydrolysis showed difference with BC, characterized with a narrow size distribution with high intensity and displayed an average particle size of 221.4 nm with a PDI value of 0.18 (Fig. 1A and Table 1, p b 0.05). The narrower of size distribution and the smaller of average size indicated the higher uniformity of BCNs suspensions [21], this was possibly due to the elimination of amorphous components caused by hydrogen bonds and the desquamate of the gluycosidic bonds, which was in line with the previous studies [23]. Furthermore, the zeta potential was measured to reflect the stability of colloidal dispersions. As shown in Fig. 1B, BC and BCNs suspensions displayed negative zeta potential, which was in accordance with the reported results that BC fibrilla were negatively charged [31]. In the present study, BC suspensions showed zeta potential at −21.4 mV (Table 1), which was higher than the value of −30 mV reported in the previous studies [25,31]. It was generally considered that BC aqueous solutions with a higher absolute zeta potential value than 30 mV is stable since the strong repulsion forces could prevent the aggregation among the micelles [32], which indicated that BC aqueous suspensions prepared in the present work are not stability. Nevertheless, as compared with BC suspensions, BCNs suspensions exhibited relatively higher zeta potential at about −39.5 mV, indicating that BCNs could be dispersed in aqueous solution due to the strong repulsion forces [32]. And the high zeta potential of BCNs resulting from more exposed carboxyl groups and thus forming more steric hindrance [33], suggesting that more carboxyl groups of BCNs were exposed because of the hydrolysis in the present work. Additionally, the optical transparency of BC and BCNs aqueous suspensions were measured by spectroscopy in the visible light region of 400–800 nm. Results showed that BCNs suspensions displayed higher transmittance than BC suspensions (Fig. 1C), which was in accordance with the result of visible images that BCNs suspensions were more transparent in comparison with the BC suspensions (Fig. 2A). The relatively small size, negatively charged fibrils, and high light transmittance confirmed the excellent colloidal properties of BCNs to be used in the preservation fields of fresh-cut fruits or vegetables. 3.2. Morphological analysis of BC and BCNs The TEM observations of BC and BCNs were presented in Fig. 2B. BC without hydrolysis exhibited ultrafine reticulate fibril arrangements with the entangled micro-fibrils network, which was consistent with the morphology in the previous reports [21,34]. Simultaneously, the ultrafine and extremely pure fibrous network-like structures of BC were also observed in AFM image (Fig. 2C), aggregated and twined fibers
X. Zhai et al. / International Journal of Biological Macromolecules 149 (2020) 178–186
181
Fig. 1. Size distribution (A), zeta potential distribution (B) and UV–vis transmittance (C) of BC and BCNs aqueous suspensions.
were also found in the AFM images, the delaminated and entangled micro-fibrils network and appears irregular reticulate fibril arrangements, in agreement with the AFM images our previously presented in the literature [23]. However, more stochastic interspersion of thin micro-fibrils shucked off from fibril because of the disintegrated hydrogen bonds by strong acid hydrolysis, as well as the formation of short and rodlike microfibrils and the shape of the microfibrils was more irregular, thus resulting to smaller particle size (Table 1, Fig. 2B and C), which was probably due to the reduction of the amorphous parts, in agreement with the previous studies [21,35]. Additionally, the prepared BCNs in the present study were different from the diameters of microfibrils those described in the literature because of the different treatments (e.g., mechanical treatment or H2SO4 hydrolysis) [23,25]. Besides, BCNs also displayed highly gauzy layers in the TEM images and fasciculations or wispy ribbons in the AFM images, suggesting the existence of interfibrous hydrogen bonds among these fibrils, in agreement with the reported results [36]. 3.3. Characterization of BC and BCNs The infrared spectrometer was used to elucidate the primary structural alterations during the hydrolysis treatment. As shown in Fig. 3A,
Table 1 Average size, polydispersity index (PDI) and zeta potential of BC and BCNs. BC Average size (nm) PDI Zeta potential (mV)
BCNs a
590.98 ± 40.12 0.37 ± 0.02a −21.36 ± 3.32b
221.36 ± 20.23b 0.18 ± 0.01b −39.50 ± 4.01a
Data are presented as means ± standard deviation with three replications. Different letters (a and b) show a significant difference in the same row.
BC displayed the absorption peaks at 3420 and 1617 cm−1, which were linked to the stretching vibration and bending vibration of hydroxyl groups (﹣OH), respectively, while the peak shifted to higher wavenumbers for BCNs, which could be attributed to the decrease of hydrogen bonding between BC caused by HCl hydrolysis [37]. The characteristic peaks at around 2965 and 1486 cm−1 were associated with the dissymmetric stretching vibration and unsymmetric distortion vibration of methylene (﹣CH2﹣) [38]. Besides, the absorptions around 1100 cm−1 were attributed to C=O=C stretching vibration of a pyranoid ring in BC and BCNs [24]. Moreover, BCNs revealed the additional feeble absorptions at 1654, 1629 and 1595 cm−1 for the flexural vibration of –OH, which was likely due to the blue shift of BC at 1617 cm−1 for the hydrolysis [25]. In addition, the C_O absorption peak at 1396 cm−1 was enhanced by HCl hydrolysis, suggesting more carboxyl groups in the BCNs, which was different from the BCNs hydrolyzed by H2SO4 due to different treatment in their literature [25]. The above results revealed that HCl treatment could undermine the primal intra-molecular hydrogen bonding of BC [39]. The changes in the crystalline characters of BC and BCNs were investigated via XRD. As displayed in Fig. 3B, acid treatment did not disrupt the whole structure of the cellulosic matrix, both of the BC and BCNs were the typical cellulose I structure with a primary peak at 22.8°, a secondary peak at 16.9° and an amorphous background at 2θ = 18°, which was in accordance with the reported result [40]. However, the diffraction intensity of the BCNs was noticeably enhanced as compared with BC, suggesting the explanation of the amorphous parts after the acid treatment. The relative crystallinity (Cr) of BC and BCNs were 76.6% and 89.2%, respectively, which were coincident with the results of the previous reports [25,41]. Our present result showed that acid treatment could increase the degree of crystallinity of cellulose rather than changing the cellulose morphology, which was different from the alkali treatment and oxidation treatment (bleach) [17,18].
182
X. Zhai et al. / International Journal of Biological Macromolecules 149 (2020) 178–186
Fig. 2. Digital images of BC and BCNs aqueous suspensions (A). TEM (B) and AFM (C) images of BC and BCNs.
The thermogravimetric analysis (TGA) and differential thermograms (DTG) for BC and BCNs were displayed in Fig. 3C. At a low temperature of 50–200 °C, both BC and BCNs displayed small weight reduction stages, which was associated with the volatilization of the physical and chemical bound water or other volatiles [42]. And also, the main degradation of BC and BCNs was not starting at the same point, the thermographs showed different degradation behaviors for BC and BCNs, which was consistent with previous report that acid hydrolysis could increase degree of crystallinity and thus led to the different degradation behaviors [25]. With the increase of temperature, the cellulose showed the degradation at the temperature range of 200–600 °C. Based on the DTG analysis, it could be observed that the BC showed a small
degradation at 200 °C and then showed a relatively small peak with lower degradation, while the BCNs started the main decomposition at above 200 °C, and the degradation increased until the peak appeared, which suggested that BCNs possessed higher thermal stability than that of BC. Furthermore, based on the DTG analysis, peaks at 335 °C and 307 °C were observed in curves of BCNs and BC, respectively. It is well known that the peak of DTG represents the temperature of the maximum weight reduction. The present result showed that the temperature of the maximum weight reduction of BCNs was markedly higher than BC, suggesting that BCNs possessed higher thermal stability in comparison with BC, which was in line with the previous report [25]. The improvement of thermostability of BCNs might be ascribed to the
Fig. 3. FT-IR spectra (A), X-ray diffractograms (B), thermograms (C) and X-ray photoelectron spectroscopy (D) of BC and BCNs.
X. Zhai et al. / International Journal of Biological Macromolecules 149 (2020) 178–186
increased degree of crystallinity and the presence of interfibrillar hydrogen bonds [25,34,36], which were consisted of the results of morphology and XRD analysis (Fig. 3B). XPS was used to analyze the surface elements information of BC and BCNs. Fig. 3D showed the XPS wide-scan spectra of samples, it could be observed that the high signals of carbon (C) and oxygen (O) were detected in BC and BCNs, where BC and BCNs contained 58.5% and 58.1% of C, 40.1% and 40.35% of O, respectively. Moreover, there was no presence of Cl, this indicated that HCl hydrolysis could not bring Cl− into BCNs, which was different from H2SO4 hydrolysis that would bring SO2− 4 into BCNs [43]. 3.4. Rheological properties of BC and BCNs Fig. 4A displayed the dynamical viscosity of the BC and BCNs aqueous suspensions. It could be seen that the viscosity of the BC without acid hydrolysis showed constantly decline with the elevation of shear rate, suggesting a shear-thinning behavior and a characteristic of liquid cellulose whiskers or microcrystalline cellulose hydrogels, in agreement with the results reported in previous literature [44]. However, the viscosity of BCNs suspension (1%, w/v) was relatively higher as compared with that of BC suspension (1%, w/v). Besides, the viscosity of BCNs showed two regions (0.1–10 s−1, 10–100 s−1), which were attributed to the destruction of weak hydrogen bonds and strong hydrogen bond or Van der Waals interaction forces among cellulose whisker regions [23]. The viscosity of BCNs suspension was different from those previously reported results, which was probably due to the cellulose species, cellulose concentration and the diversity of mechanical manipulations [45]. Fig. 4B described the storage modulus (G′) and loss modulus (G′ ′) on the basis of the frequency sweep. Results revealed that G′ was always higher than G′′ for both suspensions of BC and BCNs, indicating that BC and BCNs suspensions could be regarded as dominant elastic properties over the viscosity, thus being classified as a weak gel [46,47]. 3.5. Stabilities of BC and BCNs Fig. 5A showed the stability of BC and BCNs aqueous suspensions after 28 days of storage at −20 °C, 4 °C, and 25 °C. Both BC and BCNs suspensions showed excellent stability during 28 days of storage at﹣20 °C. However, BC showed low stability during 4 weeks of storage at 4 or 25 °C as the sediment increased significantly, which was in consisted with our previous reports [23]. Inversely, BCNs suspensions displayed remarkable stability during 28 days of storage at 4 or 25 °C. Therefore,
183
the stability of BCNs aqueous suspensions was strengthened by the HCl hydrolysis, which might due to the higher zeta potential that maintained the stability of BCNs aqueous system. As can be seen in Fig. 5B, water holding capacity (WHC) of BC was markedly affected by the storage time and temperature, where BC displayed a significant decrease in the WHC during the storage time. Furthermore, the WHC of BC storage at 4 °C was obviously higher than that of BC storage at 25 °C. Nevertheless, WHC of the BCNs suspensions showed more stability during the storage time as compared with that of BC suspensions, and the storage temperature also showed little effect on the WHC of BCNs. The above result implicated that hydrolysis was helpful in keeping WHC of cellulose. It was probably due to that hydrolysis treatment could contribute to the alterations of volume and distance among cellulose chains, thus resulting in a partially steric hindrance to produce new hydrogen bonds, which could enhance the probability of water binding ability for BC [23].. However, the involved mechanisms are very complex and needs to be further studied.
3.6. Effect of BC hydrolysis on WRR and WSA Water release rate (WRR) and water swelling ability (WSA) are vital features to identify the potential applications of BC and BCNs in the food industry [23]. It can be seen in Fig. 6A that the initial water release rates of BC and BCNs suspensions displayed no obvious difference from 0 to 100 min (p N 0.05), which was probably due to that the released water was from the surface of BC and BCNs sheets. However, with the passage of time the WRR of BC and BCNs suspensions decelerated due to the released water from the cellulose network structure. BC showed faster water release than BCNs after 100 min (p b 0.05), indicating that BCNs had better water holding ability than BC. Additionally, the absorbed water was completely evaporated from the BC at 900 min, while the BCNs still contained a little of water at this time period, which coincided with previous result reported in the literature [23,48]. The WSA of BC and BCNs was described in Fig. 6B, it was found that both BC and BCNs had the quick water-absorbing ability at first 20 min, and there was no difference between BC and BCNs (p N 0.05) in the whole experiment. It took about 100 min for the BCNs to completely absorb water, while the BC showed relative low water absorption at the same time as compared with BCNs. The present result showed that BCNs had an obviously faster water absorption than previous studies [23,49], which was possibly attributed to the change in microstructure during hydrolysis processing.
Fig. 4. Rheological properties of BC and BCNs aqueous suspensions. Apparent viscosity as a function of shear rate (A); representative frequency dependence profiles of storage (G′) or loss (G′′) moduli for the corresponding BC and BCNs aqueous suspensions (B). All the frequency sweep experiments were performed in the viscoelastic range (with a strain, γ of 0.5%).
184
X. Zhai et al. / International Journal of Biological Macromolecules 149 (2020) 178–186
Fig. 5. Effects of temperature and time on the stability of BC and BCNs aqueous suspensions (A). Water-holding capacity (WHC) of BC and BCNs at 4 °C and 25 °C during 4 weeks (B). Data are presented as means ± standard deviation with three replications.
3.7. Effect of BC and BCNs suspensions on weight loss, color, firmness, TSS, and TA of fresh-cut apples The potential applications of the BCNs suspensions in the preservation field of fresh-cut fruits and vegetables were investigated. In the present work, the fresh-cut apples were used as samples and coated with different concentrations of BC and BCNs suspensions (0.5%, 1% and 2%, w/v). The parameters of the coated fresh-cut apples, including weight loss, color, firmness, total soluble solids (TSS) content and titratable acidity (TA), were determined during 7d of storage at 4 °C. Fig. 7A described the effects of treatments with different concentrations of BC and BCNs suspensions on the weight loss of fresh-cut apples. In general, weight of fresh-cut apples for all treatments decreased as storage time prolonged. Interestingly, weight in the control group was evidently lower (p b 0.05) than the treated groups. However, treatments with BC and BCNs could effectively reduce weight loss, especially for the concentration of 2%. The weight loss is mainly due to the water evaporation from the surface of fresh-cut fruits and many coatings have been developed to prevent the loss of water [1,2]. Additionally, the fresh-cut apples coated with BCNs suspensions showed obviously smaller weight loss as compared with those samples coated with BC suspensions, which was probably attributed to the results described above that the BCNs showed higher WHC and slower WRR. Our results were consistent with the reports that the treatments might coat the fruit surface and slowed down the water evaporation or reduced respiration rate [1,29].
Color is an important index of the extent of freshness [28]. As displayed in Fig. 7B, the browning index displayed significant differences between the control group and all treated groups. Compared with the control group, the samples coated with BC and BCNs suspensions exhibited the lower browning index with the storage time prolonged (p b 0.05). Notably, the samples coated with 2% of BCNs suspensions displayed the lowest brown index. This may be due to that the coating exhibited higher barrier properties toward O2 and CO2 and controlled the respiration, resulting in the delay of the browning process and prolong the shelf life of the fresh-cut apples [1,28]. Firmness reflecting the quality of food texture is one of the critical factors of fresh-cut fruits [2]. As displayed in Fig. 7C, there were no remarkable diversities among all the samples after 1 day of storage, while the firmness was obviously decreased after 7 days of storage in the control group (p b 0.05), this might be due to the loss of water and resulted in wilting and shrinking of fruits [29]. However, the firmness of samples coated with BC and BCNs suspensions was remarkably higher than that of the untreated control group (p b 0.05), particularly the samples coated with 2% of BCNs suspensions. This result was in line with the previous reports that coating is an effective way of preserving the firmness of fruits [5,29]. For TSS content, there were no remarkable differences among all of the coated samples (p N .05) after 1 day storage (Fig. 7D), while TSS content obviously declined in the control group after 1 day storage. Interestingly, with the increasing of the coating concentration, the values of
Fig. 6. Effects of hydrolysis on water release rate (WRR) and water swelling ability (WSA) of BC. (A) WRR of BC and BCNs, (B) WSA of BC and BCNs at 25 °C. Data are presented as means ± standard deviation with three replications. Different letters show a significant difference at the same time point.
X. Zhai et al. / International Journal of Biological Macromolecules 149 (2020) 178–186
185
Fig. 7. Effects of BC and BCNs suspensions coatings on weight loss, browning index, firmness, soluble solid contents (SSC) and titratable acidity (TA) of fresh-cut apple. (A) weight loss; (B) browning index; (C) firmness; (D) total soluble solids content (SSC); (E) titratable acidity (TA) of fresh-cut apple. Data are presented as means ± standard deviation with three replications. Means within same storage time with different small case letters (a, b, c, d, e and f) are significantly different; means for same treatments on different storage times with different capital letters (A, B, C and D) are significantly different.
TSS decreased slowly. Furthermore, there were no significant TSS content changes in samples coated with 2% of BC or 2% BCNs suspensions during 7 days of storage. It has been demonstrated that higher TA is related with lower pH, thus resulting in the decreased in freshness of the fresh-cut apples. Fig. 7E showed that TA remarkably increased in control samples during the storage time, suggesting that the pH values of the control samples obviously decreased during storage time [29].. While samples coated with BC or BCNs suspensions, especially the concentration of 2%, displayed slow decrease in TA values in comparison with the control samples. This indicated that samples coated with BC or BCNs suspensions could decrease respiratory rate and generation of lowlevel CO2, which showed effects on the glycolytic enzyme systems and the accumulation of acids [29,50].
4. Conclusion This research basically focused on the characterization of nanofibers from BC by HCl hydrolysis and the new potential application of nanofibers in fresh-cut apples. Bacterial cellulose nanofibers (BCNs) was prepared from purified wet bacterial cellulose (BC) by HCl hydrolysis and then the physical characteristics were investigated to assess the effects of hydrolysis on BC aqueous suspensions. The results showed that hydrolysis treatment could result in the lower and more homogeneity particle size, higher zeta potential and transparency, which was confirmed by morphological analysis of TEM and AFM. Physical properties analysis by XRD and TGA analysis showed that BCNs was a kind of excellent semi-crystalline polymer with higher thermal stability as compared
186
X. Zhai et al. / International Journal of Biological Macromolecules 149 (2020) 178–186
with BC. The rheological results revealed that BCNs suspensions possessed a shear thinning behavior with higher apparent viscosity, storage (G′) and loss (G′′) moduli as compared with BC. Furthermore, WHC and WSA of BCNs suspensions showed high stability and the WRR enhanced after hydrolysis process during storage time at 25 °C. The prepared BCNs and BC suspensions were employed as the coatings of fresh-cut apples. The fresh-cut apples coated with BCNs suspensions displayed more excellent parameters including delaying weight loss, improving firmness and soluble solids content, reducing browning index changes and titratable acidity as compared with fresh-cut apples coated with BC suspensions at the same concentration especially for long time preservation (N3 days), while the higher concentration of BC suspensions have also improved the qualities of fresh-cut apples as compared with the control group (without any treatment) and even better than lower concentration of BCNs. Taken together, both BCNs and BC suspensions prepared in the present work can be used as new coatings in fresh-cut apples and may possess great potential to coat fresh-cut fruits or vegetables in food industry. Acknowledgments This study was supported by grants from the National Natural Science Foundation of China (C31701662), Science and Technology Innovation as a Whole Plan Projects of Yulin City (2016182), the Development Program for Innovative Research Team of Shaanxi Normal University, China (GK201801002), the Key Projects of Central Universities, Shaanxi Normal University (grant number GK201902012), the supporting program for youth talent (20190206), and the key research and development plan in Shaanxi province, China (2017NY-102). References [1] Z. Feng, G. Wu, C. Liu, D. Li, B. Jiang, X. Zhang, Food Hydrocolloid 79 (2018) 179–188. [2] M.M. Alves, M.P. Goncalves, C.M.R. Rocha, Lwt-Food Sci. Technol. 80 (2017) 409–415. [3] E. Tavassoli-Kafrani, H. Shekarchizadeh, M. Masoudpour-Behabadi, Carbohydr. Polym. 137 (2016) 360–374. [4] J. Jeevahan, M. Chandrasekaran, J. Mater. Sci. 54 (19) (2019) 12290–12318. [5] V.M. Azevedo, M.V. Dias, H.H.D. Elias, K.L. Fukushima, E.K. Silva, J.D.S. Carneiro, N.D.F. Soares, S.V. Borges, Food Res. Int. 107 (2018) 306–313. [6] D. Lin, P. Lopez-Sanchez, M.J. Gidley, Food Hydrocolloid 52 (2016) 57–68. [7] U. Römling, M.Y. Galperin, Trends Microbiol. 23 (9) (2015) 545–557. [8] D. Lin, P. Lopez-Sanchez, M.J. Gidley, Carbohydr. Polym. 126 (2015) 108–121. [9] M. Ul-Islam, T. Khan, J.K. Park, Carbohydr. Polym. 89 (4) (2012) 1189–1197. [10] N. Mohamad, M.C.I.M. Amin, M. Pandey, N. Ahmad, N.F. Rajab, Carbohydr. Polym. 114 (2014) 312–320. [11] P.R. Chawla, I.B. Bajaj, S.A. Survase, R.S. Singhal, Food Technol. Biotech. 47 (2) (2009) 107–124.
[12] M. Pereda, A. Dufresne, M.I. Aranguren, N.E. Marcovich, Carbohydr. Polym. 101 (2014) 1018–1026. [13] Y. Khaledian, M. Pajohi-Alamoti, B. Bazargani-Gilani, J. Food Process. Pres. 0 (0) (2019), e14114. [14] M.A. Shamsabadi, T. Behzad, R. Bagheri, Fiber Polym 16 (3) (2015) 579–584. [15] H. Takagi, A. Asano, Compos. Part a-Appl. S. 39 (4) (2008) 685–689. [16] J.G. Zhang, T.J. Elder, Y.Q. Pu, A.J. Ragauskas, Carbohydr. Polym. 69 (3) (2007) 607–611. [17] B. Sun, H.-Y. Yu, Y. Zhou, Z. Huang, J.-M. Yao, Ind. Crop. Prod. 89 (2016) 66–77. [18] W.P. Flauzino Neto, H.A. Silvério, N.O. Dantas, D. Pasquini, Ind. Crop. Prod. 42 (2013) 480–488. [19] L. Zhong, S. Fu, X. Peng, H. Zhan, R. Sun, Carbohydr. Polym. 90 (1) (2012) 644–649. [20] X. Zhai, D. Lin, Y. Zhao, W. Li, X. Yang, Food Funct. 9 (10) (2018) 5260–5272. [21] X. Zhai, D. Lin, D. Liu, X. Yang, Food Res. Int. 103 (2017) 8. [22] V. Mikulcova, R. Bordes, V. Kasparkova, Food Hydrocolloid 61 (2016) 780–792. [23] D.H. Lin, R. Li, P. Lopez-Sanchez, Z.X. Li, Food Hydrocolloid 44 (2015) 435–442. [24] X. Zhai, D. Ren, Y. Luo, Y. Hu, X. Yang, Food Funct. 8 (7) (2017) 2536–2547. [25] H.Q. Yan, X.Q. Chen, H.W. Song, J.C. Li, Y.H. Feng, Z.F. Shi, X.H. Wang, Q. Lin, Food Hydrocolloid 72 (2017) 127–135. [26] Z. Long, M.M. Zhao, Q.Z. Zhao, B. Yang, L.Y. Liu, Food Chem. 131 (3) (2012) 748–753. [27] O. Shezad, S. Khan, T. Khan, J.K. Park, Carbohydr. Polym. 82 (1) (2010) 173–180. [28] F. Chen, H. Liu, H. Yang, S. Lai, X. Cheng, Y. Xin, B. Yang, H. Hou, Y. Yao, S. Zhang, G. Bu, Y. Deng, Food Chem. 126 (2) (2011) 450–459. [29] J.X. Chong, S. Lai, H. Yang, Food Control 53 (2015) 195–205. [30] G.I. Olivas, D.S. Mattinson, G.V. Barbosa-Cánovas, Postharvest Biol. Tec. 45 (1) (2007) 89–96. [31] P. Paximada, E.A. Dimitrakopoulou, E. Tsouko, A.A. Koutinas, C. Fasseas, I.G. Mandala, Carbohydr. Polym. 150 (2016) 5–12. [32] H. Yan, X. Chen, J. Li, Y. Feng, Z. Shi, X. Wang, Q. Lin, Carbohydr. Polym. 136 (2016) 757–763. [33] H. Tu, Y. Yu, J. Chen, X. Shi, J. Zhou, H. Deng, Y. Du, Polym. Chem. 8 (19) (2017) 2913–2921. [34] E.E. Kiziltas, A. Kiziltas, D.J. Gardner, Carbohydr. Polym. 124 (2015) 131–138. [35] W.A. Khattak, T. Khan, M. Ul-Islam, M.W. Ullah, S. Khan, F. Wahid, J.K. Park, J. Food Sci. Tech.Mys. 52 (12) (2015) 8343–8349. [36] Y. Nishi, M. Uryu, S. Yamanaka, K. Watanabe, N. Kitamura, M. Iguchi, S. Mitsuhashi, J. Mater. Sci. 25 (6) (1990) 2997–3001. [37] X.J. Wang, N. Ullah, X.C. Sun, Y. Guo, L. Chen, Z.X. Li, X.C. Feng, Int. J. Biol. Macromol. 96 (2017) 353–360. [38] C. Huang, H.J. Guo, L. Xiong, B. Wang, S.L. Shi, X.F. Chen, X.Q. Lin, C. Wang, J. Luo, X.D. Chen, Carbohydr. Polym. 136 (2016) 198–202. [39] M.S. Islam, M.R. Karim, Colloid. Surface A. 366 (1) (2010) 135–140. [40] R. Moriana, F. Vilaplana, M. Ek, Carbohydr. Polym. 139 (2016) 139–149. [41] Z. Li, L. Wang, J. Hua, S. Jia, J. Zhang, H. Liu, Carbohydr. Polym. 120 (2015) 115–119. [42] J.I. Moran, V.A. Alvarez, V.P. Cyras, A. Vazquez, Cellulose 15 (1) (2008) 149–159. [43] Z. Hu, S. Ballinger, R. Pelton, E.D. Cranston, J. Colloid. Interf. Sci. 439 (2015) 139–148. [44] A. Kuijk, R. Koppert, P. Versluis, G. van Dalen, C. Remijn, J. Hazekamp, J. Nijsse, K.P. Veikov, Langmuir 29 (47) (2013) 14356–14360. [45] X.J. Jia, Y.W. Chen, C. Shi, Y.F. Ye, M. Abid, S. Jabbar, P. Wang, X.X. Zeng, T. Wu, Food Hydrocolloid 39 (2014) 27–33. [46] W. Jo, J.H. Bak, B. Yoo, Int. J. Biol. Macromol. 114 (2018) 263–269. [47] P.E.D. Augusto, M. Cristianini, A. Ibarz, J. Food Eng. 108 (2) (2012) 283–289. [48] M. Ul-Islam, T. Khan, J.K. Park, Carbohydr. Polym. 88 (2) (2012) 596–603. [49] D.H. Lin, P. Lopez-Sanchez, R. Li, Z.X. Li, Bioresour. Technol. 151 (2014) 113–119. [50] H. Liu, F.S. Chen, H.S. Yang, Y.Z. Yao, X.Z. Gong, Y. Xin, C.H. Ding, Food Res. Int. 42 (8) (2009) 1131–1140.
Contents lists available at ScienceDirect
International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Improved characterization of nanofibers from bacterial cellulose and its potential application in fresh-cut apples Xichuan Zhai, Dehui Lin ⁎, Wenwen Li, Xingbin Yang ⁎ Shaanxi Engineering Laboratory for Food Green Processing and Safety Control, and Shaanxi Key Laboratory for Hazard Factors Assessment in Processing and Storage of Agricultural Products, College of Food Engineering and Nutritional Science, Shaanxi Normal University, Xi'an 710062, China
a r t i c l e
i n f o
Article history: Received 18 September 2019 Received in revised form 14 January 2020 Accepted 22 January 2020 Available online 24 January 2020 Keywords: Bacterial cellulose Bacterial cellulose nanofibers Physicochemical properties Fresh-cut apples
a b s t r a c t The present research aimed to study the nanofibers from bacterial cellulose (BC) by HCl hydrolysis and explore its new potential application in fresh-cut apples. Bacterial cellulose nanofibers (BCNs) showed low and more homogeneity particle size, as well as higher zeta potential and transparency in comparison with BC, which was confirmed by morphological analysis. Physical properties analysis showed that BCNs was more excellent semicrystalline polymer with higher thermal stability as compared with BC. Rheological results displayed that BCNs suspensions presented a shear thinning behaviour with higher apparent viscosity, storage (G′) and loss (G′′) moduli at the same concentration in comparison with BC. Furthermore, BCNs suspensions were more stable than BC suspensions under storage condition of 4 °C. Additionally, 2% (wt%) of BCNs suspensions were coated on fresh-cut apples. Results showed that the samples coated with BCNs suspension displayed more excellent properties of keeping fresh-cut apples as compared with that coated with BC suspensions, including delaying weight loss, improving firmness and soluble solids content, reducing browning index and titratable acidity. Therefore, the low cost and high biocompatibility of BCNs can be used as new coatings for fresh-cut apples and have great potential to coat fresh-cut fruits and vegetables in food industry. © 2020 Elsevier B.V. All rights reserved.
1. Introduction Fresh-cut apples are normally found in markets with great demands by consumers due to their year-round availability and convenience. However, the storability, shelf life and microbial safety are still major problems for fresh-cut fruit industry and consumers due to their decomposition processes caused by physical lesions, such as enzymatic browning, texture reduction, water losing, increased susceptibility to microbial spoilage, and production of undesirable odors and flavors [1,2]. Therefore, one of the potential approaches to tackle the above problems are edible coatings, which comprises a thin layer of edible material formed as a coating on a food product [3]. And nowadays, many edibility and biodegradability biopolymers, such as polysaccharides, proteins, and lipids are regarded as excellent coating materials and extensively used in fresh-cut fruits and vegetables [2–5]. Bacterial cellulose (BC), an extremely pure natural microbe exocellular polysaccharide, can form hydrogels with better material properties including high purity, high crystallinity, high degree of porosity, high water-uptake capacity, high tensile strength, excellent biocompatibility and 3D nanofibrillar cellulosic network properties [6–8]. Therefore, BC represents a potential alternative to plant or tunicin⁎ Corresponding authors. E-mail addresses: [email protected] (D. Lin), [email protected] (X. Yang).
https://doi.org/10.1016/j.ijbiomac.2020.01.230 0141-8130/© 2020 Elsevier B.V. All rights reserved.
derived cellulose for specific applications in bio-medicine [9,10], cosmetics and papermaking [11], food processing and packaging and other applications [12]. Recently, it has been showed that natural nanofibers, nanocrystals or nanowhiskers are promising potential in making nanocomposite films and coatings [13]. Therefore, treatment methods, including biological, mechanical or chemical methods, were proposed to prepare CNCs from various kinds of materials [14]. Due to the high cost and pollution of biological method and inefficient of mechanical method [14,15], chemical method (strong mineral acids, e.g. hydrochloric and sulfuric acids) could obtain the exact size, dimensions [16]. However, cellulose nanocrystals from sulfuric acid hydrolysis can be endowed with sulfate groups on their surface, this lead to the decrease of its thermostability and addition process of removing sulfate with hydrogen peroxide [17–19]. In addition, few studies have shown the effective methods to prepare nano-fibers from BC, and there is also no systematic investigation in the literature to explore its new potential application in fresh-cut apples. Hence, bacterial cellulose nanofibers (BCNs) were prepared from BC with the chemical method and characterized by atomic force microscopy (AFM), Fourier-transform infrared (FT-IR), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). And then the stability, rheological properties, water holding capacity (WHC), water swelling ability (WSA) and water release rate (WRR) of BC and BCNs were compared. Finally, suspensions of BC and BCNs were used as coatings of
X. Zhai et al. / International Journal of Biological Macromolecules 149 (2020) 178–186
fresh-cut apples to study their fresh-keeping properties. The parameters of the coated fresh-cut apples, including weight loss, browning index, firmness, soluble solid contents (SSC) and titratable acidity (TA), were determined in the present work. 2. Methods and materials 2.1. Preparation of BC BC was cultured according to our previous method [20]. In brief, the Komagataeibacter hansenii strain CGMCC 3917 was cultured at 30 °C under static conditions in liquid fermentation medium at pH 5.0. After 14 days cultivation, cellulosic membranes were obtained and swilled with tap-water overnight, and then dipped in NaOH solution (0.1 M) at 80 °C for 2 h, and then swilled with distilled water until the pH value of BC membranes was neutral. The purified cellulosic membranes were smashed and stored in sterile water at 4 °C. The purified wet BC membranes were disrupted in a blender (CWFJ, Changzhou, China) at 15,000 rpm for 5 min at room temperature. Then the aqueous cellulose suspensions were centrifuged (4000 rpm) for 15 min to remove the free water, followed by hydrolysis with HCl according to our previous method [20]. 2.2. Preparation of bacterial cellulose nanofibers (BCNs) BCNs were prepared with hydrochloric acid according to our previous study [21]. Briefly, 5.0 g BC was mixed with 75 mL of HCl (3 M), and then the hydrolysis processes were carried out at 70 °C for 4 h under condition of vigorous mechanical stirring at 200 rpm. The suspensions were naturally cooled down to ambient temperature and the hydrolysate was obtained by centrifugation for 10 min at 10000 g and the sediment was hydrolyzed for second time with the same method described above. After hydrolyzation, the sediment was collected and washed with distilled water until its pH was neutral. 2.3. Characterization of BC and BCNs 2.3.1. Size, zeta potential, and transmittance of BC and BCNs The particle size of BC and BCNs was detected with Malvern particle size analyzer (Nano ZS90, Malvern Instruments Ltd., UK) with dynamic light scattering (DLS) according to previous studies [21,22]. The transmittance of BC and BCNs was recorded using a UV–visible spectrophotometer (UV-300, InsMark, Shanghai) in the range of 400 nm to 800 nm. 2.3.2. Morphological analysis by transmission electron microscope (TEM) and atomic force microscopy (AFM) The morphologies of BC and BCNs were observed by TEM and AFM. For TEM, 10 μL of BC or BCNs samples (0.1%, w/v) was dripped on a new carbon-coated electron microscope grid and then dried at ambient temperature, followed by observation with TEM (HT-7700, Hitachi, Japan) at 80 kV [21]. For AFM, the surface topography of BC and BCNs were determined using an atomic force microscopy (Dimension ICON, Bruker, USA). The BC or BCNs suspensions (0.05%, w/v) were deposited on about 1 cm2 piece of freshly cleaved mica for 60 s and the extra water was removed with filter paper. Then, the samples were placed on an AFM specimen disc and observed after drying at ambient temperature [23]. AFM photos (10 × 10 μm2) of the films were obtained in the tapping mode on their airside. 2.3.3. Infrared analysis of BC and BCNs The infrared spectrometer (Tensor27, Brucher, Germany) was used to investigate the primary structure of BC and BCN samples. Dried BC and BCNs were blended with anhydrous KBr powder, ground completely and then milled thoroughly and pressed into a 1 mm slice
179
for infrared detection. Infrared spectra of the samples was performed at the wavelength of 4000–400 cm−1 at ambient temperature [24]. 2.3.4. Crystallinity test of BC and BCNs Automatic X-ray diffractometer (D/Max-3c, Rigalcu, Japan) was used to determine the crystallinity of BC and BCN samples referring to previous study [25]. The testing conditions were set as follows: scanning angle was at range of 5–60°, scanning speed was 4°/min, the ray of Cu/Ka was filtrated by Ni slice and formed X-ray, the wavelength of Xray was 1.54056 × 10−10 m, voltage and electricity of X-ray tube was 40 kV and 80 mA, respectively. Relative crystallinity of the sample was calculated as the following formula: Cr ð%Þ ¼
I200 −Iam 100 I 200
where, Cr is the percentage of relative crystallinity, I200 is the intensity value of the crystallographic zone, Iam is the intensity value of the amorphous zone (2θ = 18°). 2.3.5. Thermogravimetric analysis (TGA) of BC and BCNs Thermoanalyzer Systems (Q1000DSC + LNCS+FACS Q600SDT, TA, USA) was carried out to analyze the thermostability of BC and BCNs. The testing conditions were as follows: 5 mg of sample was tested in a nitrogen atmosphere, N2 flow was 20 mL/min, the testing temperature started at 40 °C and increased to 800 °C for 10 °C/min. 2.3.6. X-ray photoelectron spectroscopy (XPS) analysis of BC and BCNs For X-ray photoelectron spectroscopy (XPS) analysis, the BC and BCNs suspensions were centrifuged through 50 kDa MWCO Amicon filter, and then freeze-dried. Then the experiment was carried out on an AXIS ULTRA XPS with a monochromated Al Kα X-ray source. The elements were recognized based on the specific binding energy (eV). 2.4. Rheological properties of BC and BCNs The rheology characters of BC and BCNs aqueous suspensions were measured using a AR-G2 dynamic rheometer (TA Instruments, U.S.A). The dynamic rheometer was furnished with coarse surface parallelplate geometry (2 cm diameters) to restrain sample slide. Gap and strain were set at 1.0 mm and 1.0%, respectively. Samples were recorded at ambient temperature with three replicates and experimental flow curves were screened by comparison to the Herschele-Bulkley (HB) model (The model was selected by the software until the highest regression value of R2 ≥ 0.982) [26]: τ−τ0 ¼ Κ γ n where, τ is shear stress (Pa); τ0 is the yield stress (Pa); Κ is the consistency index (Pa·sn) and n is the flow index (n b 1 for a shear-thinning fluid and n = 1 for a Newtonian fluid). 2.5. Stability of BC and BCNs The stability of BC and BCNs aqueous suspensions were determined according to the previous report with small modifications [23]. BC and BCNs aqueous suspensions were severally kept at −20 °C, 4 °C and 25 °C for 28 d. And then 10 g of BC or BCNs aqueous suspensions were centrifuged for 10 min at 4000 g. The sediments were gathered and weighed. The specific calculation formula for the stability of suspensions is as follows: Stability ð%Þ ¼
W1 100 W2
where, W1 is the quality of sediments (g) and W2 is the total quality of the suspensions (g).
180
X. Zhai et al. / International Journal of Biological Macromolecules 149 (2020) 178–186
2.6. Water release rate (WRR), water swelling ability (WSA) and water holding capacity (WHC) For WRR, BC and BCNs aqueous suspensions were centrifuged for 15 min at 5000 g. The sediments were collected and weighed, followed by incessantly measuring the weight of the samples stored at room temperature with various intervals. The loss of water at various intervals was graphed against time [27]. For WHC, BC and BCNs samples were kept at 4 °C and 25 °C for one month and the water content in sediments was measured by centrifugation at a different time interval. For WSA, freeze-dried of BC and BCNs were determined by our previous method with minor revision [23]. Specifically, 0.5 g of the dried BC and BCNs samples were soaked in deionized water under stirring (100 rpm) at 25 °C for re-watering. After re-watering, the BC and BCNs suspensions were centrifuged for 15 min at 5000 g. The sediments were collected and weighed. The WSA of BC and BCNs at various intervals were plotted as a function of time. The specific calculation formula for the stability of dried BC and BCNs samples is as follows: WSA ð%Þ ¼
W1−W 100 W
where, W is the quality of dried BC or BCNs sample (g); W1 is the quality of BC or BCNs sample after absorbed water (g). 2.7. Coating effect of BC and BCNs on fresh-cut apple 2.7.1. Preliminary treatment of apples The uniform apple slices (about 5 mm thick) were obtained and used in this study. The control and coated samples were allocated stochastically. Control samples were soaked in chlorinated solution (0.1 mg/mL) for 10 min, but the coated samples were immersed in different concentrations of BC or BCNs suspensions (0.5%, 1% and 2%, wt) for 10 min and the excess solution were drained on absorbent paper. Each treatment was prepared with 8 samples, and all samples were kept for 1, 3, 5 and 7 days at 4 °C for shelf-life studies, respectively [2]. 2.7.2. Measurement of weight loss, firmness, total soluble solids (TSS) content, and titratable acidity (TA) For weight loss, all samples were fetched out at each sampling time point from refrigeration, weighed and calculated through the following formula: Weight loss (%) = (m0-m)/m0 × 100, in which m0 is initial weight and m is present weight [28]. The firmness of all apple pieces in the experiment groups was measured by TA-XT-Plus Texture analyzer (Stable Micro Systems, U.K.). 6 mm diameter cylindrical probe was applied and testing conditions were set based on the previous report with minor modifications [29]: 1 mm/s for testing speed, 5 mm of penetration depth and 5 g trigger force. The firmness of samples was showed as peak force (N), which was the maximum resistance force encountered by samples. The tests were performed 5 times for each group. For the determination of TSS content, the apple slices were crushed to extract juice, then the TSS content was determined with a bench refractometer (Metzer Optical Instruments, Mathura, India) and the values were expressed as Brix. Titratable acid (TA) was determined by titration with 0.1 M NaOH with the indicator of phenolphthalein. TA is expressed as (g/100 g malic acid) [28]. 2.7.3. Detection of browning index (BI) The color values of apple slices were recorded with an automatic colorimeter (WSL-2, Shanghai Precision & Scientific Instrument Co., China) at ambient temperature. The samples were performed at 3 locations and 5 replicates, and the CIELAB scale (L*, a*, and b*) was recorded. The BI was calculated as follows [30]: BI ð%Þ ¼
x−0:31 100 0:172
where; x ð%Þ ¼
a þ 1:75L 100 5:645L þ a −3:012b
2.8. Data analysis All the data were analyzed by SPSS and expressed as mean ± standard deviation (SD) for at least three replicates. Significance was determined at p b 0.05 by analysis of variance (ANOVA) followed by Duncan's multiple comparison tests. 3. Results and discussions 3.1. Particle size, zeta potential and transmittance analysis of BC and BCNs suspensions The uniform properties of BC and BCNs aqueous suspensions were investigated by DLS. As shown in Fig. 1A and Table 1, BC aqueous suspensions exhibited a wide size distribution with low intensity, and its average particle size was 590.9 nm with a polydispersity index (PDI) value of 0.37. However, BCNs obtained by HCl hydrolysis showed difference with BC, characterized with a narrow size distribution with high intensity and displayed an average particle size of 221.4 nm with a PDI value of 0.18 (Fig. 1A and Table 1, p b 0.05). The narrower of size distribution and the smaller of average size indicated the higher uniformity of BCNs suspensions [21], this was possibly due to the elimination of amorphous components caused by hydrogen bonds and the desquamate of the gluycosidic bonds, which was in line with the previous studies [23]. Furthermore, the zeta potential was measured to reflect the stability of colloidal dispersions. As shown in Fig. 1B, BC and BCNs suspensions displayed negative zeta potential, which was in accordance with the reported results that BC fibrilla were negatively charged [31]. In the present study, BC suspensions showed zeta potential at −21.4 mV (Table 1), which was higher than the value of −30 mV reported in the previous studies [25,31]. It was generally considered that BC aqueous solutions with a higher absolute zeta potential value than 30 mV is stable since the strong repulsion forces could prevent the aggregation among the micelles [32], which indicated that BC aqueous suspensions prepared in the present work are not stability. Nevertheless, as compared with BC suspensions, BCNs suspensions exhibited relatively higher zeta potential at about −39.5 mV, indicating that BCNs could be dispersed in aqueous solution due to the strong repulsion forces [32]. And the high zeta potential of BCNs resulting from more exposed carboxyl groups and thus forming more steric hindrance [33], suggesting that more carboxyl groups of BCNs were exposed because of the hydrolysis in the present work. Additionally, the optical transparency of BC and BCNs aqueous suspensions were measured by spectroscopy in the visible light region of 400–800 nm. Results showed that BCNs suspensions displayed higher transmittance than BC suspensions (Fig. 1C), which was in accordance with the result of visible images that BCNs suspensions were more transparent in comparison with the BC suspensions (Fig. 2A). The relatively small size, negatively charged fibrils, and high light transmittance confirmed the excellent colloidal properties of BCNs to be used in the preservation fields of fresh-cut fruits or vegetables. 3.2. Morphological analysis of BC and BCNs The TEM observations of BC and BCNs were presented in Fig. 2B. BC without hydrolysis exhibited ultrafine reticulate fibril arrangements with the entangled micro-fibrils network, which was consistent with the morphology in the previous reports [21,34]. Simultaneously, the ultrafine and extremely pure fibrous network-like structures of BC were also observed in AFM image (Fig. 2C), aggregated and twined fibers
X. Zhai et al. / International Journal of Biological Macromolecules 149 (2020) 178–186
181
Fig. 1. Size distribution (A), zeta potential distribution (B) and UV–vis transmittance (C) of BC and BCNs aqueous suspensions.
were also found in the AFM images, the delaminated and entangled micro-fibrils network and appears irregular reticulate fibril arrangements, in agreement with the AFM images our previously presented in the literature [23]. However, more stochastic interspersion of thin micro-fibrils shucked off from fibril because of the disintegrated hydrogen bonds by strong acid hydrolysis, as well as the formation of short and rodlike microfibrils and the shape of the microfibrils was more irregular, thus resulting to smaller particle size (Table 1, Fig. 2B and C), which was probably due to the reduction of the amorphous parts, in agreement with the previous studies [21,35]. Additionally, the prepared BCNs in the present study were different from the diameters of microfibrils those described in the literature because of the different treatments (e.g., mechanical treatment or H2SO4 hydrolysis) [23,25]. Besides, BCNs also displayed highly gauzy layers in the TEM images and fasciculations or wispy ribbons in the AFM images, suggesting the existence of interfibrous hydrogen bonds among these fibrils, in agreement with the reported results [36]. 3.3. Characterization of BC and BCNs The infrared spectrometer was used to elucidate the primary structural alterations during the hydrolysis treatment. As shown in Fig. 3A,
Table 1 Average size, polydispersity index (PDI) and zeta potential of BC and BCNs. BC Average size (nm) PDI Zeta potential (mV)
BCNs a
590.98 ± 40.12 0.37 ± 0.02a −21.36 ± 3.32b
221.36 ± 20.23b 0.18 ± 0.01b −39.50 ± 4.01a
Data are presented as means ± standard deviation with three replications. Different letters (a and b) show a significant difference in the same row.
BC displayed the absorption peaks at 3420 and 1617 cm−1, which were linked to the stretching vibration and bending vibration of hydroxyl groups (﹣OH), respectively, while the peak shifted to higher wavenumbers for BCNs, which could be attributed to the decrease of hydrogen bonding between BC caused by HCl hydrolysis [37]. The characteristic peaks at around 2965 and 1486 cm−1 were associated with the dissymmetric stretching vibration and unsymmetric distortion vibration of methylene (﹣CH2﹣) [38]. Besides, the absorptions around 1100 cm−1 were attributed to C=O=C stretching vibration of a pyranoid ring in BC and BCNs [24]. Moreover, BCNs revealed the additional feeble absorptions at 1654, 1629 and 1595 cm−1 for the flexural vibration of –OH, which was likely due to the blue shift of BC at 1617 cm−1 for the hydrolysis [25]. In addition, the C_O absorption peak at 1396 cm−1 was enhanced by HCl hydrolysis, suggesting more carboxyl groups in the BCNs, which was different from the BCNs hydrolyzed by H2SO4 due to different treatment in their literature [25]. The above results revealed that HCl treatment could undermine the primal intra-molecular hydrogen bonding of BC [39]. The changes in the crystalline characters of BC and BCNs were investigated via XRD. As displayed in Fig. 3B, acid treatment did not disrupt the whole structure of the cellulosic matrix, both of the BC and BCNs were the typical cellulose I structure with a primary peak at 22.8°, a secondary peak at 16.9° and an amorphous background at 2θ = 18°, which was in accordance with the reported result [40]. However, the diffraction intensity of the BCNs was noticeably enhanced as compared with BC, suggesting the explanation of the amorphous parts after the acid treatment. The relative crystallinity (Cr) of BC and BCNs were 76.6% and 89.2%, respectively, which were coincident with the results of the previous reports [25,41]. Our present result showed that acid treatment could increase the degree of crystallinity of cellulose rather than changing the cellulose morphology, which was different from the alkali treatment and oxidation treatment (bleach) [17,18].
182
X. Zhai et al. / International Journal of Biological Macromolecules 149 (2020) 178–186
Fig. 2. Digital images of BC and BCNs aqueous suspensions (A). TEM (B) and AFM (C) images of BC and BCNs.
The thermogravimetric analysis (TGA) and differential thermograms (DTG) for BC and BCNs were displayed in Fig. 3C. At a low temperature of 50–200 °C, both BC and BCNs displayed small weight reduction stages, which was associated with the volatilization of the physical and chemical bound water or other volatiles [42]. And also, the main degradation of BC and BCNs was not starting at the same point, the thermographs showed different degradation behaviors for BC and BCNs, which was consistent with previous report that acid hydrolysis could increase degree of crystallinity and thus led to the different degradation behaviors [25]. With the increase of temperature, the cellulose showed the degradation at the temperature range of 200–600 °C. Based on the DTG analysis, it could be observed that the BC showed a small
degradation at 200 °C and then showed a relatively small peak with lower degradation, while the BCNs started the main decomposition at above 200 °C, and the degradation increased until the peak appeared, which suggested that BCNs possessed higher thermal stability than that of BC. Furthermore, based on the DTG analysis, peaks at 335 °C and 307 °C were observed in curves of BCNs and BC, respectively. It is well known that the peak of DTG represents the temperature of the maximum weight reduction. The present result showed that the temperature of the maximum weight reduction of BCNs was markedly higher than BC, suggesting that BCNs possessed higher thermal stability in comparison with BC, which was in line with the previous report [25]. The improvement of thermostability of BCNs might be ascribed to the
Fig. 3. FT-IR spectra (A), X-ray diffractograms (B), thermograms (C) and X-ray photoelectron spectroscopy (D) of BC and BCNs.
X. Zhai et al. / International Journal of Biological Macromolecules 149 (2020) 178–186
increased degree of crystallinity and the presence of interfibrillar hydrogen bonds [25,34,36], which were consisted of the results of morphology and XRD analysis (Fig. 3B). XPS was used to analyze the surface elements information of BC and BCNs. Fig. 3D showed the XPS wide-scan spectra of samples, it could be observed that the high signals of carbon (C) and oxygen (O) were detected in BC and BCNs, where BC and BCNs contained 58.5% and 58.1% of C, 40.1% and 40.35% of O, respectively. Moreover, there was no presence of Cl, this indicated that HCl hydrolysis could not bring Cl− into BCNs, which was different from H2SO4 hydrolysis that would bring SO2− 4 into BCNs [43]. 3.4. Rheological properties of BC and BCNs Fig. 4A displayed the dynamical viscosity of the BC and BCNs aqueous suspensions. It could be seen that the viscosity of the BC without acid hydrolysis showed constantly decline with the elevation of shear rate, suggesting a shear-thinning behavior and a characteristic of liquid cellulose whiskers or microcrystalline cellulose hydrogels, in agreement with the results reported in previous literature [44]. However, the viscosity of BCNs suspension (1%, w/v) was relatively higher as compared with that of BC suspension (1%, w/v). Besides, the viscosity of BCNs showed two regions (0.1–10 s−1, 10–100 s−1), which were attributed to the destruction of weak hydrogen bonds and strong hydrogen bond or Van der Waals interaction forces among cellulose whisker regions [23]. The viscosity of BCNs suspension was different from those previously reported results, which was probably due to the cellulose species, cellulose concentration and the diversity of mechanical manipulations [45]. Fig. 4B described the storage modulus (G′) and loss modulus (G′ ′) on the basis of the frequency sweep. Results revealed that G′ was always higher than G′′ for both suspensions of BC and BCNs, indicating that BC and BCNs suspensions could be regarded as dominant elastic properties over the viscosity, thus being classified as a weak gel [46,47]. 3.5. Stabilities of BC and BCNs Fig. 5A showed the stability of BC and BCNs aqueous suspensions after 28 days of storage at −20 °C, 4 °C, and 25 °C. Both BC and BCNs suspensions showed excellent stability during 28 days of storage at﹣20 °C. However, BC showed low stability during 4 weeks of storage at 4 or 25 °C as the sediment increased significantly, which was in consisted with our previous reports [23]. Inversely, BCNs suspensions displayed remarkable stability during 28 days of storage at 4 or 25 °C. Therefore,
183
the stability of BCNs aqueous suspensions was strengthened by the HCl hydrolysis, which might due to the higher zeta potential that maintained the stability of BCNs aqueous system. As can be seen in Fig. 5B, water holding capacity (WHC) of BC was markedly affected by the storage time and temperature, where BC displayed a significant decrease in the WHC during the storage time. Furthermore, the WHC of BC storage at 4 °C was obviously higher than that of BC storage at 25 °C. Nevertheless, WHC of the BCNs suspensions showed more stability during the storage time as compared with that of BC suspensions, and the storage temperature also showed little effect on the WHC of BCNs. The above result implicated that hydrolysis was helpful in keeping WHC of cellulose. It was probably due to that hydrolysis treatment could contribute to the alterations of volume and distance among cellulose chains, thus resulting in a partially steric hindrance to produce new hydrogen bonds, which could enhance the probability of water binding ability for BC [23].. However, the involved mechanisms are very complex and needs to be further studied.
3.6. Effect of BC hydrolysis on WRR and WSA Water release rate (WRR) and water swelling ability (WSA) are vital features to identify the potential applications of BC and BCNs in the food industry [23]. It can be seen in Fig. 6A that the initial water release rates of BC and BCNs suspensions displayed no obvious difference from 0 to 100 min (p N 0.05), which was probably due to that the released water was from the surface of BC and BCNs sheets. However, with the passage of time the WRR of BC and BCNs suspensions decelerated due to the released water from the cellulose network structure. BC showed faster water release than BCNs after 100 min (p b 0.05), indicating that BCNs had better water holding ability than BC. Additionally, the absorbed water was completely evaporated from the BC at 900 min, while the BCNs still contained a little of water at this time period, which coincided with previous result reported in the literature [23,48]. The WSA of BC and BCNs was described in Fig. 6B, it was found that both BC and BCNs had the quick water-absorbing ability at first 20 min, and there was no difference between BC and BCNs (p N 0.05) in the whole experiment. It took about 100 min for the BCNs to completely absorb water, while the BC showed relative low water absorption at the same time as compared with BCNs. The present result showed that BCNs had an obviously faster water absorption than previous studies [23,49], which was possibly attributed to the change in microstructure during hydrolysis processing.
Fig. 4. Rheological properties of BC and BCNs aqueous suspensions. Apparent viscosity as a function of shear rate (A); representative frequency dependence profiles of storage (G′) or loss (G′′) moduli for the corresponding BC and BCNs aqueous suspensions (B). All the frequency sweep experiments were performed in the viscoelastic range (with a strain, γ of 0.5%).
184
X. Zhai et al. / International Journal of Biological Macromolecules 149 (2020) 178–186
Fig. 5. Effects of temperature and time on the stability of BC and BCNs aqueous suspensions (A). Water-holding capacity (WHC) of BC and BCNs at 4 °C and 25 °C during 4 weeks (B). Data are presented as means ± standard deviation with three replications.
3.7. Effect of BC and BCNs suspensions on weight loss, color, firmness, TSS, and TA of fresh-cut apples The potential applications of the BCNs suspensions in the preservation field of fresh-cut fruits and vegetables were investigated. In the present work, the fresh-cut apples were used as samples and coated with different concentrations of BC and BCNs suspensions (0.5%, 1% and 2%, w/v). The parameters of the coated fresh-cut apples, including weight loss, color, firmness, total soluble solids (TSS) content and titratable acidity (TA), were determined during 7d of storage at 4 °C. Fig. 7A described the effects of treatments with different concentrations of BC and BCNs suspensions on the weight loss of fresh-cut apples. In general, weight of fresh-cut apples for all treatments decreased as storage time prolonged. Interestingly, weight in the control group was evidently lower (p b 0.05) than the treated groups. However, treatments with BC and BCNs could effectively reduce weight loss, especially for the concentration of 2%. The weight loss is mainly due to the water evaporation from the surface of fresh-cut fruits and many coatings have been developed to prevent the loss of water [1,2]. Additionally, the fresh-cut apples coated with BCNs suspensions showed obviously smaller weight loss as compared with those samples coated with BC suspensions, which was probably attributed to the results described above that the BCNs showed higher WHC and slower WRR. Our results were consistent with the reports that the treatments might coat the fruit surface and slowed down the water evaporation or reduced respiration rate [1,29].
Color is an important index of the extent of freshness [28]. As displayed in Fig. 7B, the browning index displayed significant differences between the control group and all treated groups. Compared with the control group, the samples coated with BC and BCNs suspensions exhibited the lower browning index with the storage time prolonged (p b 0.05). Notably, the samples coated with 2% of BCNs suspensions displayed the lowest brown index. This may be due to that the coating exhibited higher barrier properties toward O2 and CO2 and controlled the respiration, resulting in the delay of the browning process and prolong the shelf life of the fresh-cut apples [1,28]. Firmness reflecting the quality of food texture is one of the critical factors of fresh-cut fruits [2]. As displayed in Fig. 7C, there were no remarkable diversities among all the samples after 1 day of storage, while the firmness was obviously decreased after 7 days of storage in the control group (p b 0.05), this might be due to the loss of water and resulted in wilting and shrinking of fruits [29]. However, the firmness of samples coated with BC and BCNs suspensions was remarkably higher than that of the untreated control group (p b 0.05), particularly the samples coated with 2% of BCNs suspensions. This result was in line with the previous reports that coating is an effective way of preserving the firmness of fruits [5,29]. For TSS content, there were no remarkable differences among all of the coated samples (p N .05) after 1 day storage (Fig. 7D), while TSS content obviously declined in the control group after 1 day storage. Interestingly, with the increasing of the coating concentration, the values of
Fig. 6. Effects of hydrolysis on water release rate (WRR) and water swelling ability (WSA) of BC. (A) WRR of BC and BCNs, (B) WSA of BC and BCNs at 25 °C. Data are presented as means ± standard deviation with three replications. Different letters show a significant difference at the same time point.
X. Zhai et al. / International Journal of Biological Macromolecules 149 (2020) 178–186
185
Fig. 7. Effects of BC and BCNs suspensions coatings on weight loss, browning index, firmness, soluble solid contents (SSC) and titratable acidity (TA) of fresh-cut apple. (A) weight loss; (B) browning index; (C) firmness; (D) total soluble solids content (SSC); (E) titratable acidity (TA) of fresh-cut apple. Data are presented as means ± standard deviation with three replications. Means within same storage time with different small case letters (a, b, c, d, e and f) are significantly different; means for same treatments on different storage times with different capital letters (A, B, C and D) are significantly different.
TSS decreased slowly. Furthermore, there were no significant TSS content changes in samples coated with 2% of BC or 2% BCNs suspensions during 7 days of storage. It has been demonstrated that higher TA is related with lower pH, thus resulting in the decreased in freshness of the fresh-cut apples. Fig. 7E showed that TA remarkably increased in control samples during the storage time, suggesting that the pH values of the control samples obviously decreased during storage time [29].. While samples coated with BC or BCNs suspensions, especially the concentration of 2%, displayed slow decrease in TA values in comparison with the control samples. This indicated that samples coated with BC or BCNs suspensions could decrease respiratory rate and generation of lowlevel CO2, which showed effects on the glycolytic enzyme systems and the accumulation of acids [29,50].
4. Conclusion This research basically focused on the characterization of nanofibers from BC by HCl hydrolysis and the new potential application of nanofibers in fresh-cut apples. Bacterial cellulose nanofibers (BCNs) was prepared from purified wet bacterial cellulose (BC) by HCl hydrolysis and then the physical characteristics were investigated to assess the effects of hydrolysis on BC aqueous suspensions. The results showed that hydrolysis treatment could result in the lower and more homogeneity particle size, higher zeta potential and transparency, which was confirmed by morphological analysis of TEM and AFM. Physical properties analysis by XRD and TGA analysis showed that BCNs was a kind of excellent semi-crystalline polymer with higher thermal stability as compared
186
X. Zhai et al. / International Journal of Biological Macromolecules 149 (2020) 178–186
with BC. The rheological results revealed that BCNs suspensions possessed a shear thinning behavior with higher apparent viscosity, storage (G′) and loss (G′′) moduli as compared with BC. Furthermore, WHC and WSA of BCNs suspensions showed high stability and the WRR enhanced after hydrolysis process during storage time at 25 °C. The prepared BCNs and BC suspensions were employed as the coatings of fresh-cut apples. The fresh-cut apples coated with BCNs suspensions displayed more excellent parameters including delaying weight loss, improving firmness and soluble solids content, reducing browning index changes and titratable acidity as compared with fresh-cut apples coated with BC suspensions at the same concentration especially for long time preservation (N3 days), while the higher concentration of BC suspensions have also improved the qualities of fresh-cut apples as compared with the control group (without any treatment) and even better than lower concentration of BCNs. Taken together, both BCNs and BC suspensions prepared in the present work can be used as new coatings in fresh-cut apples and may possess great potential to coat fresh-cut fruits or vegetables in food industry. Acknowledgments This study was supported by grants from the National Natural Science Foundation of China (C31701662), Science and Technology Innovation as a Whole Plan Projects of Yulin City (2016182), the Development Program for Innovative Research Team of Shaanxi Normal University, China (GK201801002), the Key Projects of Central Universities, Shaanxi Normal University (grant number GK201902012), the supporting program for youth talent (20190206), and the key research and development plan in Shaanxi province, China (2017NY-102). References [1] Z. Feng, G. Wu, C. Liu, D. Li, B. Jiang, X. Zhang, Food Hydrocolloid 79 (2018) 179–188. [2] M.M. Alves, M.P. Goncalves, C.M.R. Rocha, Lwt-Food Sci. Technol. 80 (2017) 409–415. [3] E. Tavassoli-Kafrani, H. Shekarchizadeh, M. Masoudpour-Behabadi, Carbohydr. Polym. 137 (2016) 360–374. [4] J. Jeevahan, M. Chandrasekaran, J. Mater. Sci. 54 (19) (2019) 12290–12318. [5] V.M. Azevedo, M.V. Dias, H.H.D. Elias, K.L. Fukushima, E.K. Silva, J.D.S. Carneiro, N.D.F. Soares, S.V. Borges, Food Res. Int. 107 (2018) 306–313. [6] D. Lin, P. Lopez-Sanchez, M.J. Gidley, Food Hydrocolloid 52 (2016) 57–68. [7] U. Römling, M.Y. Galperin, Trends Microbiol. 23 (9) (2015) 545–557. [8] D. Lin, P. Lopez-Sanchez, M.J. Gidley, Carbohydr. Polym. 126 (2015) 108–121. [9] M. Ul-Islam, T. Khan, J.K. Park, Carbohydr. Polym. 89 (4) (2012) 1189–1197. [10] N. Mohamad, M.C.I.M. Amin, M. Pandey, N. Ahmad, N.F. Rajab, Carbohydr. Polym. 114 (2014) 312–320. [11] P.R. Chawla, I.B. Bajaj, S.A. Survase, R.S. Singhal, Food Technol. Biotech. 47 (2) (2009) 107–124.
[12] M. Pereda, A. Dufresne, M.I. Aranguren, N.E. Marcovich, Carbohydr. Polym. 101 (2014) 1018–1026. [13] Y. Khaledian, M. Pajohi-Alamoti, B. Bazargani-Gilani, J. Food Process. Pres. 0 (0) (2019), e14114. [14] M.A. Shamsabadi, T. Behzad, R. Bagheri, Fiber Polym 16 (3) (2015) 579–584. [15] H. Takagi, A. Asano, Compos. Part a-Appl. S. 39 (4) (2008) 685–689. [16] J.G. Zhang, T.J. Elder, Y.Q. Pu, A.J. Ragauskas, Carbohydr. Polym. 69 (3) (2007) 607–611. [17] B. Sun, H.-Y. Yu, Y. Zhou, Z. Huang, J.-M. Yao, Ind. Crop. Prod. 89 (2016) 66–77. [18] W.P. Flauzino Neto, H.A. Silvério, N.O. Dantas, D. Pasquini, Ind. Crop. Prod. 42 (2013) 480–488. [19] L. Zhong, S. Fu, X. Peng, H. Zhan, R. Sun, Carbohydr. Polym. 90 (1) (2012) 644–649. [20] X. Zhai, D. Lin, Y. Zhao, W. Li, X. Yang, Food Funct. 9 (10) (2018) 5260–5272. [21] X. Zhai, D. Lin, D. Liu, X. Yang, Food Res. Int. 103 (2017) 8. [22] V. Mikulcova, R. Bordes, V. Kasparkova, Food Hydrocolloid 61 (2016) 780–792. [23] D.H. Lin, R. Li, P. Lopez-Sanchez, Z.X. Li, Food Hydrocolloid 44 (2015) 435–442. [24] X. Zhai, D. Ren, Y. Luo, Y. Hu, X. Yang, Food Funct. 8 (7) (2017) 2536–2547. [25] H.Q. Yan, X.Q. Chen, H.W. Song, J.C. Li, Y.H. Feng, Z.F. Shi, X.H. Wang, Q. Lin, Food Hydrocolloid 72 (2017) 127–135. [26] Z. Long, M.M. Zhao, Q.Z. Zhao, B. Yang, L.Y. Liu, Food Chem. 131 (3) (2012) 748–753. [27] O. Shezad, S. Khan, T. Khan, J.K. Park, Carbohydr. Polym. 82 (1) (2010) 173–180. [28] F. Chen, H. Liu, H. Yang, S. Lai, X. Cheng, Y. Xin, B. Yang, H. Hou, Y. Yao, S. Zhang, G. Bu, Y. Deng, Food Chem. 126 (2) (2011) 450–459. [29] J.X. Chong, S. Lai, H. Yang, Food Control 53 (2015) 195–205. [30] G.I. Olivas, D.S. Mattinson, G.V. Barbosa-Cánovas, Postharvest Biol. Tec. 45 (1) (2007) 89–96. [31] P. Paximada, E.A. Dimitrakopoulou, E. Tsouko, A.A. Koutinas, C. Fasseas, I.G. Mandala, Carbohydr. Polym. 150 (2016) 5–12. [32] H. Yan, X. Chen, J. Li, Y. Feng, Z. Shi, X. Wang, Q. Lin, Carbohydr. Polym. 136 (2016) 757–763. [33] H. Tu, Y. Yu, J. Chen, X. Shi, J. Zhou, H. Deng, Y. Du, Polym. Chem. 8 (19) (2017) 2913–2921. [34] E.E. Kiziltas, A. Kiziltas, D.J. Gardner, Carbohydr. Polym. 124 (2015) 131–138. [35] W.A. Khattak, T. Khan, M. Ul-Islam, M.W. Ullah, S. Khan, F. Wahid, J.K. Park, J. Food Sci. Tech.Mys. 52 (12) (2015) 8343–8349. [36] Y. Nishi, M. Uryu, S. Yamanaka, K. Watanabe, N. Kitamura, M. Iguchi, S. Mitsuhashi, J. Mater. Sci. 25 (6) (1990) 2997–3001. [37] X.J. Wang, N. Ullah, X.C. Sun, Y. Guo, L. Chen, Z.X. Li, X.C. Feng, Int. J. Biol. Macromol. 96 (2017) 353–360. [38] C. Huang, H.J. Guo, L. Xiong, B. Wang, S.L. Shi, X.F. Chen, X.Q. Lin, C. Wang, J. Luo, X.D. Chen, Carbohydr. Polym. 136 (2016) 198–202. [39] M.S. Islam, M.R. Karim, Colloid. Surface A. 366 (1) (2010) 135–140. [40] R. Moriana, F. Vilaplana, M. Ek, Carbohydr. Polym. 139 (2016) 139–149. [41] Z. Li, L. Wang, J. Hua, S. Jia, J. Zhang, H. Liu, Carbohydr. Polym. 120 (2015) 115–119. [42] J.I. Moran, V.A. Alvarez, V.P. Cyras, A. Vazquez, Cellulose 15 (1) (2008) 149–159. [43] Z. Hu, S. Ballinger, R. Pelton, E.D. Cranston, J. Colloid. Interf. Sci. 439 (2015) 139–148. [44] A. Kuijk, R. Koppert, P. Versluis, G. van Dalen, C. Remijn, J. Hazekamp, J. Nijsse, K.P. Veikov, Langmuir 29 (47) (2013) 14356–14360. [45] X.J. Jia, Y.W. Chen, C. Shi, Y.F. Ye, M. Abid, S. Jabbar, P. Wang, X.X. Zeng, T. Wu, Food Hydrocolloid 39 (2014) 27–33. [46] W. Jo, J.H. Bak, B. Yoo, Int. J. Biol. Macromol. 114 (2018) 263–269. [47] P.E.D. Augusto, M. Cristianini, A. Ibarz, J. Food Eng. 108 (2) (2012) 283–289. [48] M. Ul-Islam, T. Khan, J.K. Park, Carbohydr. Polym. 88 (2) (2012) 596–603. [49] D.H. Lin, P. Lopez-Sanchez, R. Li, Z.X. Li, Bioresour. Technol. 151 (2014) 113–119. [50] H. Liu, F.S. Chen, H.S. Yang, Y.Z. Yao, X.Z. Gong, Y. Xin, C.H. Ding, Food Res. Int. 42 (8) (2009) 1131–1140.