Abstract
The chemical constituents of essential oil extracted from the young inflorescence of torch ginger plant by hydrodistillation method using gas chromatography-mass spectrometry was analyzed. Additionally, the extracted essential oil and the crude solvent extracts (distilled water, absolute ethanol, and 50% ethanol) of the inflorescence were screened for their potential antimicrobial activities. Both the essential oil and solvent extracts exhibited antibacterial activities against Gram-positive bacteria (Bacillus cereus, B. subtilis, Staphylococcus aureus, Listeria monocytogenes). Bacillus subtilis was found to be more susceptible to torch ginger inflorescence essential oil (MIC, 0.78 mg/ml) compared to others under study. Of the Gram-negative bacteria screened, only Klebsiella pneumoniae was susceptible to the essential oil (MIC, 1.56) and solvent extracts (MIC, 3.1 and 1.6 for water and 50% ethanol, respectively). However, neither the essential oil nor the extracts exhibited any activities against the screened fungi or yeasts. The results obtained clearly indicated that the oil and extracts derived from the inflorescence of torch ginger has rich antibacterial activity and possess great potential to be used as a preservative in the food and pharmaceutical industries.
INTRODUCTION
The past few decades have witnessed the use of several aromatic plant species for improving the healthy life style of humans the world over. The health benefits provided by these plants are attributed to the presence of various bioactive compounds. Among the various bioactive compounds (e.g., phenolic compounds like flavonoids, flavonols, etc.), plant essential oils have gained considerable attention mainly due to their pharmaceutical and therapeutic potentials.[ Citation1 ] Of late, oil extracted from plant parts (leaves, flowers, barks, rhizomes, and fruits) of aromatic plants have been identified for their applications in food, drugs, and cosmetic products.[ Citation2,Citation3 ]
Preservation of food against the deleterious action of spoilage and pathogenic microorganisms is essential in food industries, considering the safety and quality issues. In food industries, chemicals or the synthetic antimicrobial preservatives are routinely used for preservation purposes. The presence of chemical residues and development of resistance in microorganisms leading to adverse health problems have rendered synthetic chemical preservatives to be unsuitable for food applications.[ Citation4 ] Additionally, there is an increase in demand from consumers for minimally processed food products that are free from traditionally used chemical preservatives. This has forced the manufacturers to explore for newer strategies for stabilization of food products. Recent years have witnessed a renewed interest in screening for bioactive compounds from plants and also to look for its potential to be used as a natural preservative of food.[ Citation5 − Citation7 ] Additionally, essential oils from aromatic plants are also screened for their potential antimicrobial and antioxidant activities.[ Citation7 ]
The Zingiberaceae family is recognized for aromatic plant species and comprises over 1400 species with 47 genera.[ Citation8 ] The torch ginger plant (Etlingera elatior Jack.) belonging to the Zingiberaceae family, is one of the most popular aromatic plants found in the South-Asian regions of the world. This plant bears a bright pink colored inflorescence with a mild and pleasant aroma. The young inflorescence is used as a spice for food flavoring and in preparation of some traditional foods of Malaysia, such as “asam laksa,” “nasi kerabu,” and “nasi ulam.”[ Citation9,Citation10 ] The essential oil extracted from torch ginger plant parts (leaves, stems, flowers, and rhizomes) has been reported previously.[ Citation11 ] Additionally, pharmacological and toxicological activities of the methanolic extracts of the ‘torch ginger flower’ has been reported.[ Citation12 ] However, to our knowledge no detailed reports are available on the antimicrobial activities of the essential oil or solvent extracts (results on water and ethanol are useful for food applications) of torch ginger inflorescence. Providing scientific details on torch ginger inflorescence assumes importance, as inflorescence is the part that is widely used for culinary purposes rather than the mature flowers. Hence, the present study was designed with an objective to (i) analyze the chemical composition of essential oil using gas chromatography-mass spectrometry (GC-MS) and (ii) to evaluate the antimicrobial (antibacterial and antifungal) activities of essential oil as well as the solvent extracts of the inflorescence. It is expected that the results obtained in the present study might provide more details on the chemical constituents of the aromatic oil, which could be further explored on a commercial scale in food and pharmaceutical industries, both as a preservative as well as a flavoring component.
MATERIALS AND METHODS
Plant Material Preparation
Inflorescences of the torch ginger plant were collected from the local wet market in Penang, Malaysia. The inflorescence used were fresh, unopened, and of equal maturity with no apparent physical defects. The inflorescences were surface cleaned cautiously to remove all the adhering debris. Further, the inflorescence samples were freeze dried for 48 h in a freeze dryer (Model 7754511, Labconco Corporation, Kansas City, MO, USA). After freeze drying, the samples were ground to a fine powder using a commercial kitchen blender (Model BL 335, Kenwood, Selangor, Malaysia) and stored at 4°C in amber-colored glass bottles, covered with aluminium foil (to prevent direct exposure to light) until further analysis.
Hydrodistillation of Essential Oil
A known quantity of the dried inflorescence powder (15 g) was subjected to continuous hydrodistillation using a Likens-Nickerson distillation unit for 2–3 h.[ Citation13 ] The distillate obtained was trapped in diethyl ether. Extracted oil was dried over anhydrous sodium sulphate and flushed with nitrogen gas to obtain pure oil extract.
Solvent Extraction
A known amount of the dried sample (10 g) was extracted twice with 250 ml of the solvent (50 and 100% ethanol and distilled water) for 90 min on a hotplate with continuous magnetic stirring (Model FSMQ 143551, Fisher Scientific, Kuala Lumpur, Malaysia) (at 1100 rpm at room temperature 25 ± 1°C). The resultant extract was filtered through Whatman No. 1 filter paper under vacuum. Further, the filtrates were pooled and the solvent was evaporated in a rotary vacuum evaporator (ELITA, Tokyo, Japan) at 35°C to obtain a final volume of 15 ml of concentrate. This concentrate of the extract was vacuum dried (at 35°C) to yield a crude extract of 21.3, 22.60, and 20.1% in water and in 50 and 100% ethanol, respectively.
Analysis of Volatile Compounds by GC-MS
GC-MS analysis was performed for analyzing volatile compounds by using a GC system coupled to a mass selective detector (Clarus 6007, Perkin Elmer, MA, USA). The column equipped was HP-5 (30 m × 0.25 mm id × film thickness 0.25 μm). The temperature programming for the operating condition was as follows: initial oven temperature, 60°C for 1 min increased up to 250°C at a rate of 5°C/min and held for 10 min; injector temperature, 250°C; split ratio, 25:1; carrier gas, Helium, solvent delay for 1 min; transfer temperature, 250°C; ion source temperature, 250°C; and mass range 22 to 600 Da.
Antimicrobial Activity
Microorganisms and growth conditions
A total of eight foodborne pathogenic bacteria obtained from our culture collection center (Food Microbiology Laboratory, School of Industrial Technology, Universiti Sains Malaysia) were used for evaluation of antimicrobial activities of the essential oil and solvent extracts. Four Gram-positive (Bacillus cereus, B. subtilis, Staphylococcus aureus, Listeria monocytogenes) and four Gram-negative (Escherichia coli, Salmonella typhimurium, S. enteritidis, Klebsiella pneumoniae) bacteria were used for performing the antibacterial assay. Additionally, two yeast (Candida tropicalis, C. utilis) and two fungal (Aspergillus niger, Mucor sp.) species were also evaluated. As there are high chances of inevitable risk of cross-contamination in a laboratory culture, from time to time the cultures were reconfirmed with regard to their pathogenicity by employing various standard molecular techniques/methods available. Bacterial cultures were maintained on nutrient agar (Merck, Darmstadt, Germany) slants and the fungal cultures were maintained on Sabouraud Dextrose Agar (SDA; Merck) plates at 4°C. All the cultures were re-cultured every 2–3 weeks. Working cultures of bacteria were activated in nutrient broth, while fungi were activated in SDA broth at 30°C for 18 h.
Disc diffusion assay
Agar disc diffusion assay[ Citation14 ] was employed for the assessment of antimicrobial activities or the inhibitory potential of the essential oil and inflorescence extracts. Except for the essential oil, the three dried crude solvent extracts were dissolved in deionized water at a concentration of 100 mg/ml and further filtered using sterilized 0.2-μm syringe filters (Sartorius minisart cellulose acetate syringe filter, Sartorius Stedim, Göttingen, Germany). Correspondingly, the essential oil was dissolved in absolute ethanol at a concentration of 50 mg/ml and filtered in the same manner as mentioned above and was used for the disc diffusion assay. By employing total plate count and yeast and molds count method, the concentration of the inoculums were standardized to a given concentrations of 108 colony forming units/ml (CFU) for bacteria, 106 CFU/ml for yeast, and 104 CFU/ml for molds, and this activated inoculums of the microorganisms was used for assays. The cell counts were adjusted by using a hemocytometer. Bacteria were inoculated on Mueller-Hinton agar, while yeast and molds were inoculated on SDA. Sterile blank paper discs (Oxoid, Hampshire, England) with a diameter of 6 mm were impregnated with 20 μl of the prepared extracts, left for drying, and placed on the inoculated agar plates. Water and ethanol served as the common negative control, while ampicillin (20 μg/disc, Biobasic Inc., Markham, ON, Canada) served as control for the bacteria. Nystatin (20 μg/disc, Biobasic Inc.) served as positive controls for fungi. Plates were inverted and incubated at 37°C for 24 h for bacteria and at 25 ± 1°C for 48 h for fungi. Antimicrobial activity was determined by measuring the diameter of the clear zones of inhibition and three replicates were done for each case.
Determination of minimum inhibitory concentration (MIC)
We used the method described by Sahin et al.[ Citation15 ] with some modifications for determining the MIC of the bacterial strains. Only those microorganisms that were found to be sensitive to the extracts and essential oil in disc diffusion assay were further analyzed for the MIC. In brief, the broth cultures activated for 18 h were adjusted to 108 CFU/ml. The essential oil obtained was dissolved in ethanol to a maximum concentration of 50 mg/ml and the rest of the extracts were dissolved in distilled water at 100 mg/ml. All the extracts and essential oil were two-fold serially diluted (eight times) to have a concentration series. From each dilution, 100 μl of the extract was transferred into concecutive wells in a sterile flat-bottomed 96-well plates. Each well was made to 200 μl by adding 95 μl of nurient broth and 5 μl of standardized inoculum (the activated inoculums that were standardized to the given concentration). The last well containing 95 μl of nuritent broth, 100 μl of distilled water, and 5 μl of standardized inoculum served as the negative control. Ampicillin and nystatin at a concentration ranging from 1000 μg/ml to 7.8 μg/ml served as positive controls for bacteria and fungi, respectively. Plates were covered with sterile lids and incubated at 37°C for 24 h for bacteria and 25 ± 1°C for 48 h for fungi. All the studies were performed in triplicate and at the end of the incubation period, 40 μl of ethanolic (0.2 mg/ml) p-iodonitrotetrazolium violet (INT) (Sigma-Aldrich, Seelze, Germany) was dispensed into each well. This was further incubated under the same temperatue range for another 30 min. MIC was recorded as the lowest concentration where pink color of INT changed or disappears (in the presence of INT medium with metabolically active microorgnisms appears as pink color).
Statistical Analysis
The results on the disc diffusion assay are presented as mean values ± S.D. Analysis of variance was performed and the significant difference recorded between mean values were determined by Tukey's pair wise comparison test (level of significance of p < 0.05). Statistical analyses were conducted using SPSS 12.01 (SPSS Inc., Chicago, IL, USA).
RESULTS AND DISCUSSION
Chemical Composition of Essential Oil
Essential oils are secondary metabolites that are formed in aromatic plants. These oils are characterized by the presence of an intense and unique odor and vary among different aromatic plant species. These plant essential oils are volatile in nature and are complex compounds.[ Citation2 ] In the present study, hydrodistillation of dried powder of torch ginger inflorescence yielded 0.67 ± 0.02% essential oil (white color/colorless). The percentage composition and the retention time of the identified compounds are presented in . Results of the GC-MS analysis of the oil revealed the presence of a total of 55 compounds. The identified compounds were sub-divided as terpene hydrocarbons (monoterpenes and sesquiterpenes) and oxygenated compounds (phenols, aldehydes, ketones, esters, lactones, coumarins, ethers, and oxides). The predominant chemical classes of the essential oil were alcohol (44.25%) followed by acids (24.42%), aldehydes (19.54%), esters (10.51%), and sesquiterpenes (0.99%).
Table 1 Chemical composition of essential oils of E. elatior inflorescence.z
The chemical compound 1-dodecanol (25.2%) was the most abundant alcohol in the inflorescence followed by hexadecen-1-ol, trans-9 (12.7%). These two alcohols have been reported to exhibit rich antimicrobial activities against microbial pathogens.[ Citation16,Citation17 ] Dodecanoic/lauric acid contributed to 20.4% of the acid's essential oil, while dodecanal (17.5%) was the major aldehyde identified. Dodecanal has wide applications in perfumery industries and the presence of this in the inflorescence might prove to be advantageous for the dependent industry too. Earlier, dodecanol (42.5%), dodecanal (14.5%), α-pinene (22.2%), cyclododecane (40.3%), 1,1-dodecanediol, diacetate (24.4%), and α-pinene (6.3%) have been isolated as the major compounds of the essential oil of the torch ginger inflorescence.[ Citation11 ] On comparison to this report, results of our study showed slight variations in the composition, which can be attributed to the regional variations of the samples, differences in the stage of maturity of the inflorescence, as well as disparities in the extraction conditions. Earlier, variations in the chemical composition of plant essential oil have been attributed to the genesis of the plant, the stage of plants maturity, plant part used, prevailing environmental conditions during the growing stages, and to the storage conditions employed.[ Citation11,Citation18,Citation19 ]
Antimicrobial Activity of Essential Oil and Solvent Extracts of Torch Ginger
In the present study, we used deionized water to dissolve the dried crude sample extracts of torch ginger inflorescence. Alcohols were not used, as they are known to exhibit inhibitory activities against bacterial spores.[ Citation20 ] However, the essential oil of the torch ginger inflorescence was dissolved in 100% ethanol. This solvent was used because plant essential oil cannot be dissolved in water, which is mainly attributed to the hydrophobic nature of aliphatic long chain hydrocarbons. Negative controls were maintained for ethanol and water, which showed no inhibitory effect on the tested microorganisms.
Results on disc diffusion assay of the oil and solvent extracts are shown in . Overall, Gram-positive bacteria showed high susceptibility to the essential oil of torch ginger inflorescence and solvent extracts compared to Gram-negative bacteria. However, Klebsiella pneumoniae was the only Gram-negative bacteria that was susceptible to the essential oil (40 mm, diameter of inhibition zone) and to the extracts. The results on the MIC levels of the oil and the solvent extracts of torch ginger inflorescence against Gram-positive and Gram-negative bacteria are depicted in . Results showed Gram-positive bacteria, B. subtilis, to be more susceptible to torch ginger inflorescence essential oil (MIC, 0.78 mg/ml) compared to others under study. The results obtained clearly indicated essential oil to be more efficient in growth inhibition of bacteria compared to all the solvent extracts. With regard to the tested fungi (Aspergillus niger, Mucor sp.) and yeasts (Candida tropicalis, Candida utilis), high resistance were recorded for both the essential oil and extracts, and they did not exhibit any activities (results not shown in the tables).
In the present study, the terpene fraction of the essential oil was around 1% and the rest of the identified compounds were oxygenated hydrocarbons. Previously, Adams[ Citation21 ] has reported that antifungal activity of essential oil is governed to a greater extent by oxygenated terpenes. Due to limited hydrogen capacity (less hydrogen binding to hydrocarbons) and low water solubility of hydrocarbons, they tend to be a relatively inactive compound irrespective of their functional groups.[ Citation22 ] Hence, this might be the main reason for the torch ginger inflorescence not to exhibit any antifungal activity. The rate of penetration of essential oil through the cell wall and cell wall membrane structures of any microbes determines its susceptibility to the compounds present in the oil. The germicidal action of the essential oil is mainly determined by their capacity to destruct the cellular barriers of a microbe, which leads to a loss of chemiosmotic control.[ Citation23 ]
Table 2 Antimicrobial activity of torch ginger essential oil, ethanolic, and water extracts against some selected foodborne pathogens.Footnote z
Table 3 Minimum inhibitory concentration (MIC) levels of the essential oil and the extracts of torch ginger inflorescence against Gram-positive bacteria and Gram-negative bacteria
In this study, one of the major constituents of the essential oil was dodecanoic acid (lauric acid) (20.35%) and its derivative compounds. Lauric acid is already a well established antimicrobial compound in foods.[ Citation24 ] Studies conducted by Kabara et al.[ Citation25 ] to assess the antimicrobial action of fatty acids on Gram-negative and Gram-positive bacterial strains have shown lauric acid to exhibit higher inhibition of Gram-positive bacteria. Further, the same authors have reported that when the free carbonyl group of the fatty acid is reduced to corresponding aldehyde or alcohol, these compounds can be more effective than its acid. An earlier study,[ Citation15 ] has also reported on the positive effects of fatty acid alcohols on the growth inhibition of Streptococcus mutans (Gram-positive carcinogenic bacteria), which was attributed to the presence of 1-dodecanol.
Apart from the major (dominant) chemical compounds present in an essential oil, some of the compounds present in low levels (such as myristic acid) can also exhibit rich antimicrobial activities against foodborne pathogens. Reports are available wherein compounds present in low levels are also known to exhibit potential synergetic effects or antimicrobial activities.[ Citation26,Citation27 ]
Additionally, the main factor influencing the antimicrobial activity of essential oil includes the lipophilic and hydrophilic characters of their hydrocabon skeleton and their functional groups. It has been reported that the antimicrobial activity of different chemical compounds decline in the order: phenols > aldehydes > ketones > alcohols > ethers > hydrocarbons.[ Citation4,Citation28 ] The antimicrobial activity observed for ethanol and water extracts in the present study could also be attributed to the presence of bioactive compounds in the inflorescence of torch ginger. In our previous study on the torch ginger inflorescence extracts,[ Citation10 ] we had reported the presence of bioactive phenolic compounds (polyphenols, flavonoids, anthocyanins, and tannins). These phenolic compounds are known to exhibit antimicrobial activities too.[ Citation29,Citation30 ] However, in the present study, the low antimicrobial activity observed for 100% ethanol extracts compared to its aqueous mixture (50% ethanol) and distilled water might be attributed to the variations in the extractability of bioactive compounds.
CONCLUSIONS
In conclusion, results obtained in the present study clearly showed that the essential oil and solvent extracts of torch ginger inflorescence have potential antibacterial activities against tested foodborne pathogenic bacteria. However, no inhibitory effect was found against the fungi or yeasts tested. Since the extracts exhibited potential antibacterial activity against some of the most common spectra, the results obtained provide a strong base for utilization of torch ginger inflorescence in salads, ready-to-eat meals, or fresh-cut foods (meat, fish, and other seafoods) to reduce the detrimental effects associated with the foodborne pathogens, thus assuring better safety of the food. Further detailed investigations are required to evaluate the interactions between the essential oil with other food ingredients or bioactive compounds. In vivo clinical evaluations are needed for further successful commercialization of torch ginger to benefit both food and pharmaceutical industries.
ACKNOWLEDGMENT
M. M. Jeevani Osadee Wijekoon acknowledges Universiti Sains Malaysia for the postgraduate fellowship awarded.
Related Research Data
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