Miscibility and Antimicrobial Properties of m-Aramid

Aug 19, 2013 - ABSTRACT: m-Aramid/chitosan hybrid films and nanosized webs were produced. The m-aramid and chitosan salt were dissolved in dimethyl ...
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Miscibility and Antimicrobial Properties of m‑Aramid/Chitosan Hybrid Composite Sam Soo Kim† and Jaewoong Lee*,‡ †

Department of Textile Engineering and Technology, Yeungnam University, Gyeongsan 712-749, South Korea Korean Intellectual Property Office, Daejeon, 302-701, South Korea

‡

ABSTRACT: m-Aramid/chitosan hybrid films and nanosized webs were produced. The m-aramid and chitosan salt were dissolved in dimethyl sulfoxide followed by coagulation in a sodium hydroxide solution to produce the m-aramid/chitosan film. An electro-spinning process was used to prepare the nanosized web to increase the surface area of the m-aramid/chitosan hybrid composite for antimicrobial properties. The morphology of the m-aramid/chitosan hybrid composite was analyzed by scanning electron microscopy. The miscibility of the hybrid composite was determined by Fourier transform infrared spectroscopy and Xray diffraction. The antimicrobial efficacy was measured by a swatch test with bacterial suspensions. The nanosized webs, which had enhanced surface area, produced a 7-log reduction against Escherichia coli and Staphylococcus aureus.

1. INTRODUCTION Industrial polymeric fibers require a range of characteristics, such as heat stability, durability, and high strength, depending on their applications. In particular, because aromatic amides (aramid) possess high-performance characteristics, such as relatively high modulus, glass transition temperature, or melting point, they have been used as one of the major materials for a range of industrial fields. Aramid is defined as a material containing more than 85% of repeating units that consist of phenyl rings and amide groups.poly(p-phenyleneterephthalamide), p-aramid, has been applied to areas requiring high tenacity, dimensional stability, and heat resistance. Unlike paramid, poly(m-phenyleneisophthalamide), m-aramid, does not possess high tenacity and dimensional stability, but the heat resistance and flame retardant properties of m-aramid are the same as p-aramid.1−4 In addition, the flexible and elongated characteristics of m-aramid result in a better affinity for fabric applications than p-aramid. In other words, between p- and maramid in the same denier, m-aramid would be preferred for a heat protective cloth.5−7 In the case of fire fighting applications, m-aramid fibers are easily exposed to wet conditions due to a fire fighter’s job. Wet textiles are a good substrate for the propagation for microorganisms. Therefore, under wet conditions, protective clothing for fire fighters might provide an opportunity for microorganisms to proliferate. Chitosan is a natural basic polysaccharide and the second most abundant material next to cellulose. Chitin in the castoff skin of crustaceans, such as crabs, shrimp, and prawns, can be changed to chitosan by deacetylation using a strong alkali at high temperatures. Because chitosan contains one amine group in its repeating unit, it can be changed easily to have cationic properties (−NH3+), which can have antimicrobial properties.8−12 Furthermore, the hydroxyl and amine groups in the molecular structure of chitosan support hydrogen bonds and have relatively high compatibility with other polymers. Therefore, a range of polymers, e.g., starch,13−15 silk fibroin,16,17 poly(vinyl alcohol),18−20 and poly(ε-caprolactone),21,22 were blended with chitosan. Chitin is soluble in © 2013 American Chemical Society

many solvents, such as formic acid/dichloroacetic acid, trichloroacetic acid/dichloroethane, dimethylacetamide (DMAc)/LiCl, N-methylpyrrolidinone/LiCl, and dimethyl sulfoxide (DMSO).23,24 On the other hand, most organic solvents other than acids cannot dissolve chitosan directly. Therefore, there are some barriers for blending between chitosan and conventional fiber forming polymers including aramid. Interestingly, Sashiwa et al.25 reported that chitosan can be made soluble in DMSO by salt formation. In this study, the heat resistant polymer, m-aramid, which is soluble in DMSO, was blended with chitosan. To the best of the authors’ knowledge, there are no reports of m-aramid/chitosan hybrid composites. The aim of this study was to prepare m-aramid/ chitosan hybrid composites and measure the antimicrobial activity of the samples. The morphology of the m-aramid/ chitosan hybrid composites was analyzed by scanning electron microscopy (SEM). The m-aramid/chitosan hybrid samples were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The mechanical properties of the hybrid films are also discussed. The antimicrobial efficacy of the m-aramid/ chitosan hybrid composites was examined by placing the composites in contact with bacterial solutions.

2. EXPERIMENTAL SECTION 2.1. Materials. DMSO, lithium chloride anhydrous (LiCl), DMAc, and methane sulfonic acid (CH3SO3H) were purchased from Daejung Reagent Chemicals (Shiheung, Korea) and used as received. Chitosan (viscosity 13−14 cps, degree of deacetylation was 95.4%) was obtained from YB Biochemical (Yeongdeok, Korea). The m-aramid was a Yantai Spandex fiber product (Yantai, China). Received: Revised: Accepted: Published: 12703

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Figure 1. Surface images of the m-aramid/chitosan hybrid films: (a) 100/0, (b) 50/50, and (c) 35/65 wt %.

2.2. Preparation of m-Aramid/Chitosan Blended Films. m-Aramid was scoured with distilled water (∼60 °C), ethanol, and acetone followed by drying overnight at ambient temperatures. Each scouring process other than drying was repeated two times. A homogeneous 10 wt % m-aramid solution was prepared by adding 10 g of m-aramid fiber into 100 mL of DMSO containing 8 g of LiCl with constant stirring at 130 °C for 4 h. Chitosan (1.5 g) was dissolved in 100 mL of distilled water with methane sulfonic acid (1 g) for 1 h at ambient temperature followed by freeze-drying (Freeze-dryer, FDA5518, Ilshin Engineering, Siheung, Korea) for 48 h.25 A freeze-dried chitosan−methane sulfonic acid salt (10 g) was added to 100 mL of DMSO and stirred at ambient temperature for 1 h. The homogeneous m-aramid and chitosan−methane sulfonic acid salt were blended at the following ratios: 100/0, 85/15, 75/25, 65/35, 50/50, 35/65, 0/100 (m-aramid/chitosan salt = wt %/wt %). The blended solutions were stirred constantly at 60 °C for 2 h followed by film manufacturing on a glass plate using a film maker (Baker Applicator, YBA-4, Yoshimitsu Seiki, Tokyo, Japan). The films formed on the glass plates were dipped into a 1% sodium hydroxide (NaOH) solution at ambient temperature for 6 h followed by rinsing with abundant distilled water. The films were then soaked in distilled water at ambient temperature for 6 h and at 80 °C for 20 min. The films were dried at ambient temperature. The thickness of the films was measured by a dial thickness gauge (Peacock Dial Thickness Gauge H, Ozaki Mfg, Ozaki, Japan) after drying. The thickness of films was approximately 150−200 μm. 2.3. Preparation of m-Aramid/Chitosan Nanosized Webs. When the m-aramid/chitosan solutions in DMSO were used for electro-spinning, the formation of an electro-spun web was difficult to control. Therefore, the m-aramid/chitosan solutions for electro-spinning were similar to those used for the preparation of films except that m-aramid was dissolved in DMAc. In other words, a homogeneous 10 wt % m-aramid solution was prepared by adding 10 g of m-aramid fiber into 100 mL of DMAc containing 8 g of LiCl with constant stirring at 130 °C for 4 h. An electro-spinner equipped with a high voltage DC generator (Chungpa EMT Co., Ltd., Seoul, Korea) and a syringe pump (Legato 200, KD Scientific, Holliston, MA, USA) was used. The voltage, flow rate, and distance between the needle of the syringe and collector were 20 kV, 1.5 mL/h, and 20 cm, respectively. Aluminum foil was used as the collector wrapped on a metal drum, and the metal drum collector was rotating during collection. The electro-spun webs were soaked in a 1% NaOH solution for 6 h followed by rinsing

with abundant distilled water and then soaking in distilled water at ambient temperature for 6 h and at 80 °C for 20 min. The electro-spun webs were dried at ambient temperature. The thickness of the webs was approximately 200 μm. 2.4. Scanning Electron Microscopy (SEM). The samples were coated with platinum under an argon purge before the observations. The resulting samples were observed by SEM (Hitachi S-4100, Tokyo, Japan) at a 15 kV accelerating voltage. To examine the cross sections, the samples were placed in liquid nitrogen to freeze them over 30 min and samples were broken for the observations. 2.5. Fourier Transform Infrared Spectroscopy (FTIR). The surface of the pure and blended samples was examined by FTIR (Spectrum 100, PerkinElmer Co., Waltham, MA, USA) spectroscopy in attenuated total reflectance (ATR) mode. Germanium (Ge) ATR crystals were used as the internal reflection elements. The sample was held against one side of the Ge crystal. The background spectra were obtained with a Ge crystal and nitrogen gas in the absence of the sample. The FTIR spectra of the samples were scanned after drying at 105 °C for 2 h from 700 to 4000 cm−1 for 64 scans at a 2 cm−1 resolution. 2.6. X-ray Diffractometer (XRD). XRD (D/Max, Rigaku Co., Tokyo, Japan) was performed to examine crystal formation under the molecular interaction between m-aramid and chitosan. Cu Kα radiation (wavelength = 1.5418 Å) was used as an incident X-ray source (40 kV, 200 mA), and the samples were scanned from 10 to 80° 2θ at a rate of 2°/min. The samples were prepared as films and powder (m-aramid/ chitosan, 0/100 wt %). 2.7. Thermogravimetric Analysis (TGA) and Differential Thermal Analysis (DTA). The thermal character of the m-aramid/chitosan hybrid composites was examined by TGA and DTA (Q500, TA Instruments, New Castle, DE, USA) at a scan rate of 30 °C/min from room temperature to 600 °C under a nitrogen purge and from 600 to 800 °C under an oxygen purge. 2.8. Universal Testing Machine. The tensile strength and elongation at break were carried out in a universal testing machine (Instron, model 3345, Instron Co., Canton, MA, USA) at a cross head speed of 300 mm/min according to the KS K 0521: 2006 method. The gauge length (distance between jaws) was 200 mm. Ten samples were tested in each case, and the average was taken. 2.9. Antimicrobial Test. The films and webs of the maramid and m-aramid/chitosan hybrid composites were challenged with E. coli (KCTC 2441) and Staphylococcus aureus 12704

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Figure 2. SEM images of opposite sides of the m-aramid/chitosan hybrid films: (a) 100/0, (b) 50/50, and (c) 35/65 wt %.

Figure 3. Cross-sectional SEM images of the m-aramid/chitosan hybrid films: (a) 100/0, (b) 50/50, and (c) 35/65 wt %.

relatively slowly. As shown in Figure 1, m-aramid and maramid/chitosan films showed a relatively even surface. The maramid and m-aramid/chitosan solutions were assumed to have coagulated properly under direct contact with 1% NaOH solution as a coagulation system. On the other hand, the opposite sides of m-aramid and m-aramid/chitosan films in contact with the glass plate showed many voids (Figure 2). This could be evidence of different coagulation forms of blended solutions, even in the same coagulation system.17,28 To prevent voids on the film surface, the opposite side in contact with the glass plate might be exposed immediately to a coagulation system, similar to the surface in contact with air. Lee et al. suggested that m-aramid dissolved in DMAc coagulates with a range of surface structures under different coagulation solutions including water, methanol, ethanol, and propanol.26 The crosssectional images of the films, considered as the medium coagulation speed between the surface and opposite side, were prepared after freezing with liquid nitrogen followed by breaking (Figure 3). As shown in Figure 3, the cross-section of pure m-aramid film showed a relatively smooth surface, unlike the 50/50 wt % and 35/65 wt % (m-aramid/chitosan) films, which contained voids. In particular, the number of voids with a rough cross-section increased with increasing chitosan ratio in the hybrid films. m-Aramid dissolved in DMSO/LiCl was assumed to be more suitable for coagulation than chitosan dissolved in the same solvent during coagulation in a 1% NaOH solution. In addition, it was suggested that, for membrane applications with abundant voids, the rate of coagulation of the m-aramid/chitosan solution should be as slow as possible and the amount of chitosan can be adjusted to be as high as

(KCTC 1621) using the modified AATCC Test Method 1002004. Both bacteria were incubated under optimal medium conditions and temperature. A 1 mL aliquot of the culture fluid was then diluted with 9 mL of sterile distilled water followed by incubation using the streak-plate method at 37 °C for 24 h. The total bacterial concentrations of E. coli and Staphylococcus aureus were 8.60 × 107 and 6.20 × 107 cfu/mL, respectively. Bacterial suspensions (25 μL) prepared with a pH 7 phosphate buffer were added to 1 in.2 sample swatches. A second swatch was sandwiched over the first to ensure contact between the suspension and the films or webs. At contact time of 60 min, the samples were quenched with 5.0 mL of sterile 0.85% saline solution. The quenched samples were then diluted with pH 7 phosphate buffer and plated on Typticase soy agar. The plates were incubated at 37 °C for 24 h, and the number of bacteria was counted to determine the presence or absence of viable bacteria. The antimicrobial test was replicated thrice.

3. RESULTS AND DISCUSSION 3.1. Morphology of m-Aramid/Chitosan Hybrid Composites. Figures 1−3 show SEM images of the surface, opposite side, and cross-section of m-aramid/chitosan hybrid films, respectively. Because the freeze-dried chitosan−methane sulfonic acid salt dissolved in DMSO coagulated as particles in a 1% NaOH solution, this sample was excluded from the comparison of the SEM images. During preparation, m-aramid/ chitosan solutions on glass plates were dipped into a coagulation bath, which meant that the surface of the solution in contact with air would coagulate first. In other words, the opposite side in contact with the glass plate might solidify 12705

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Figure 4. SEM images of the m-aramid/chitosan electro-spun webs: (a) 100/0 and (b) 85/15 wt %.

possible. Although voids were observed in the opposite sides and cross-section of the films, there was no evidence of phase separation between m-aramid and chitosan. m-Aramid/chitosan hybrid composites with an enhanced surface area were prepared by electro-spinning, as illustrated in Figure 4. The diameters of the pure m-aramid and m-aramid/ chitosan electro-spun webs ranged from tens to thousands of nanometers. Regarding suitability for electro-spinning, maramid/chitosan dissolved in DMSO could not form a fiber structure, even though the electro-spinning factors had been adjusted. Therefore, a cosolvent system (DMAc/DMSO) was applied to spinning, which allowed fiber formation during electro-spinning. The relatively higher boiling point of DMSO (∼189 °C) might result in a slow rate of evaporation, which would allow fiber formation during electro-spinning. 3.2. Miscibility between m-Aramid and Chitosan. In the structure, m-aramid contains pi electrons on the phenyl rings and amide groups in each repeating unit. On the other hand, chitosan contains two hydroxyl groups (−OH) and one amine group (−NH2) in each repeating unit. Therefore, hydrogen bonds can exist between both polymers. FTIR spectroscopy was conducted to examine the functional groups and molecular interaction in m-aramid/chitosan hybrid films (Figure 5). Because the presence of humidity on the samples can interrupt the band of hydroxyl groups at approximately 3200−3500 cm−1,15 the films in this study were dried at 105 °C for 2 h before measuring the FTIR spectra. As shown in Figure 5, pure m-aramid revealed distinguished absorption features at approximately 3253, 1651, 1580, and 1532 cm−1, which were assigned to the N−H stretching vibration, amide CO stretching, CC stretching vibration of the aromatic ring, and N−H in-plane bending modes of m-aramid, respectively.27 On the other hand, pure chitosan exhibited a number of absorption bands at approximately 3460, 2857, and 1591 cm−1, which were assigned to O−H stretching, C−H stretching, and amine band, respectively.18 A comparison of the FTIR spectra between the m-aramid film and m-aramid/chitosan hybrid film showed that no new peaks appeared other than those around 3000−3500 cm−1. Interestingly, the FTIR band at 3253 cm−1 for pure m-aramid shifted to 3296 cm−1 for the m-aramid/chitosan (75/25 wt %) film. The N−H stretching vibration band of m-aramid had shifted to higher wavenumbers after blending with chitosan. Many papers on polymer blending reported that a band shift in the FTIR spectra after blending was caused by molecular

Figure 5. FTIR spectra of the m-aramid/chitosan hybrid films: (a) 100/0, (b) 85/15, (c) 75/25, (d) 65/35, (e) 50/50, (f) 35/65, and (g) 0/100 (powder) wt %.

interactions and such a band shift might indicate H-bonding and the miscibility between the blended polymers.13,18,19 Regarding the conditions of samples for the FTIR measurement in this study, the FTIR band at approximately 3200 cm−1 might not be disrupted by humidity. Therefore, the band shift of m-aramid after blending with chitosan up to 35/65 wt % (maramid/chitosan) might be due to a molecular interaction, and m-aramid and chitosan might be miscible. Because m-aramid and chitosan both contain hydrophilic functional groups, the m-aramid/chitosan hybrid composite might retain its relatively high compatibility. Therefore, during solidification, the crystal structure of the m-aramid/chitosan hybrid composite may be affected by each component. XRD was performed to observe crystal formation of the m-aramid/ chitosan hybrid composites, as shown in Figure 6. Pure chitosan showed distinguishable peaks at 10° 2θ and 20° 2θ.21,28 On the other hand, the pure m-aramid film showed a peak at 23° 2θ.26 The broad peak of the pure m-aramid film at 23° 2θ showed that the drawing process of m-aramid might assist in crystallization. Therefore, the m-aramid fiber after the drawing process could show a relatively sharp and strong peak, unlike the m-aramid film without drawing (Figure 6). 12706

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Figure 6. XRD patterns of the m-aramid/chitosan hybrid films: (a) 100/0, (b) 85/15, (c) 75/25, (d) 65/35, (e) 50/50, (f) 35/65, and (g) 0/100 (powder) wt %. Figure 7. TGA curves of the m-aramid/chitosan hybrid films: (a) 100/ 0, (b) 85/15, (c) 75/25, (d) 65/35, (e) 50/50, (f) 35/65, and (g) 0/ 100 (powder) wt %.

XRD of the m-aramid/chitosan hybrid film showed a shift in the peak at 23° 2θ to 25° 2θ. The crystal of m-aramid/chitosan hybrid film was assumed to be different from that of the pure m-aramid film, and the molecular interaction between both polymers might affect the crystal structure of m-aramid. On the other hand, when the ratio of chitosan was 35 wt % (m-aramid/ chitosan, 65/35 wt %), a peak for chitosan at 20° 2θ was observed in the m-aramid/chitosan hybrid film but its intensity was quite weak. The crystal of chitosan in the m-aramid/ chitosan hybrid film retained its own structure, unlike the crystal of m-aramid after blending. The FTIR band for pure m-aramid shifted from 3259 cm−1 to higher wavenumbers after blending with chitosan, and the XRD peak shift before and after blending confirmed that molecular interactions and miscibility existed between m-aramid and chitosan. In addition, it could be suggested that the samples were predominantly phase mixed at modest chitosan loading. m-Aramid, however, might be affected relatively more by the effect of molecular interactions than chitosan in m-aramid/ chitosan hybrid films. 3.3. Thermal and Mechanical Characteristics. Figures 7 and 8, respectively, show the results of thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of maramid and m-aramid/chitosan hybrid films. As shown in Figure 7, the weight reduction of the samples at approximately 100 °C was assigned to the evaporation of water. The weight loss for chitosan at approximately 245 °C was assigned to the decomposition of chitosan. Regarding the decomposition temperature (Td), both the pure m-aramid and m-aramid/ chitosan hybrid films decomposed at approximately 440 °C (Figure 8). Although chitosan has a lower decomposition temperature than m-aramid, the m-aramid/chitosan hybrid film retained the decomposition temperature of pure m-aramid. The m-aramid/chitosan hybrid films revealed a flash point temperature (Tf) at approximately 605 °C. On the other hand, the pure m-aramid films showed a Tf at 629 °C. Interestingly, pure chitosan showed small weight reduction at approximately 700 °C, which might be the flash point temperature of chitosan. The mechanical properties including strength and elongation at break of m-aramid/chitosan hybrid films were measured (Figures 9 and 10). In general, the tenacity and elongation at break of m-aramid fibers are approximately 5 g/d and 15%, respectively.4 On the other hand, chitosan is a difficult material

Figure 8. DTA curves of the m-aramid/chitosan hybrid films: (a) 100/ 0, (b) 85/15, (c) 75/25, (d) 65/35, (e) 50/50, (f) 35/65, and (g) 0/ 100 (powder) wt %.

Figure 9. Tensile strength of the m-aramid/chitosan hybrid films: (a) 100/0, (b) 85/15, (c) 75/25, (d) 65/35, and (e) 50/50 wt %. 12707

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the cell wall consisting of sialic acid or phospholipid. Deformation of the bacterial cell wall might result in the leaking of protoplasm and inactivation of the bacteria.28,29 The dramatic increase in the surface area of the nanosized web (Figure 4b) is believed to have enhanced the contact areas between the chitosan and bacteria. Therefore, more cationic amine groups in the nanosized web than in the films might inactivate the bacteria.

4. CONCLUSIONS A unique antimicrobial hybrid composite for thermal applications was presented. FTIR and XRD revealed a molecular interaction between m-aramid and chitosan. The thermal stability of the m-aramid/chitosan hybrid composite was similar to that of pure m-aramid. The m-aramid/chitosan (85/15 wt %) nanosized webs exhibited sufficient antimicrobial efficacy against Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive), but the m-aramid/chitosan films did not reveal satisfactory antimicrobial efficacy. Although different coagulation systems should be developed to enhance the mechanical properties of m-aramid/chitosan films, this study revealed a new approach to producing hybrid composites for innovative applications in the m-aramid industry.

Figure 10. Elongation at break of the m-aramid/chitosan hybrid films: (a) 100/0, (b) 85/15, (c) 75/25, (d) 65/35, and (e) 50/50 wt %.

for making fibers due to its brittleness. The tensile strength of the 85/15 and 50/50 wt % (m-aramid/chitosan) samples retained 25.6% and 5.7% of the tensile strength of pure maramid film, respectively (Figure 9). Regarding the elongation at break, the 85/15 and 50/50 wt % (m-aramid/chitosan) samples possessed 27.0% and 9.0% of the value for pure maramid films, respectively. Therefore, the strength and elongation property of m-aramid were lost after blending with chitosan. This was attributed to the increasing number and size of the voids with increasing amounts of chitosan in the maramid/chitosan hybrid films. 3.4. Antimicrobial Efficacy. As outlined in the Introduction, the purpose of this study was to produce antimicrobial maramid/chitosan hybrid composites. The pure m-aramid and maramid/chitosan hybrid films in this research possessed both a relatively even surface and another surface that contained many voids (Figure 1 and 2). To eliminate the possibility of contacting different surface areas of the samples against bacteria, only the even surfaces of the films were used in the antimicrobial test. The antimicrobial efficacies against Gramnegative bacteria (E. coli) and Gram-positive bacteria (Staphylococcus aureus) were measured, and the results are listed in Table 1. The pure m-aramid film and m-aramid/chitosan film both showed small log reductions of bacteria after a 60 min contact time. On the other hand, the nanosized web containing 15 wt % chitosan (m-aramid/chitosan, 85/15 wt %) destroyed all bacteria of both species after a 60 min contact time. Although the components between m-aramid/chitosan films and nanosized web were identical, the antimicrobial efficacy varied. The antimicrobial mechanism of chitosan involved contact of the cationic amine group (−NH3+) of chitosan with

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82-42-481-8716. Fax:+82-42-472-3558. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank Dr. Byungjoo Seo, Department of Applied Microbiology and Biotechnology, Yeungnam University, for the preparation of the antimicrobial test. The authors are also grateful to Kyu Yul Ryoo and Minji Kim for their helpful discussion and for preparing the samples.

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REFERENCES

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Table 1. Antimicrobial Efficacy of the Films and Nanosized Webs Escherichia colia sample (m-aramid/chitosan, wt %) film (100/0) film (85/15) nanoweb (100/0) nanoweb (85/15)

Staphylococcus aureusb

contact time (min)

bacterial no. (cfu/mL)c

log reduction

bacterial no. (cfu/mL)c

log reduction

60

7.36 ± 0.65 × 10 2.20 ± 0.27 × 107 6.90 ± 0.39 × 107 0

0.07 0.59 0.09 7.93

4.30 ± 0.83 × 107 2.82 ± 0.36 × 107 4.63 ± 0.33 × 107 0

0.16 0.34 0.13 7.79

7

Total bacteria: 8.60 × 107cfu/mL. bTotal bacteria: 6.20 × 107cfu/mL. cBacterial no. data are expressed as mean ± standard deviation of a triplicate analysis.

a

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