First published in May 09
Published: 18 November, 2009
Even with the current slump in hide and skin prices, the cost of raw materials for leather production is significant. So, the preservation of stock is still fundamental to good quality control and making a living for tanners. Yet biocides by their very nature are environmentally unfriendly. However, help may be at hand, with research into new ecofriendly brands of bacterial control.
Dr Graham Lampard reports
There is a wonderful chapter in Procter’s ‘Principles of Leather Manufacture’ that elucidates the pros and cons of antiseptics and disinfectants.1 He suggests a myriad of chemicals from lime to common salt, mercuric chloride to camphor and essential oils. OK, so the book was written at the turn of the last century, and chemicals such as mercuric chloride and benzoic acid are inappropriate today, but the ideas are still valid. Indeed there has been much recent work in finding alternatives to common salt for preservation, and replacing ‘dangerous’ biocides with apparently safer alternatives. Dr Graham Lampard reports
The idea of ‘essential oils’ Procter mentions is used by Bayramoglu et al, from Ege University, in Turkey. She used the Origanum species to extract oregano essential oils, which are produced from the oregano plant through the process of steam distillation. It had already been found that Origanum minutiflorum essential oil has antifungal activity during pickling and wet-blue processes and its effect improves with increasing concentration rates.2 1%, based on float, of the essential oils from three different oregano species and the oil of fennel were tested for their antimicrobial activity in parallel with 7-25% phenol and 4-chloro-3-methylphenol as commercial bactericides commonly used in leather industry. The results showed that three essential oils of oregano had a much stronger bactericidal activity than the commercial ones, and may find use as bactericidal agents in the leather industry.
The use of heavy metals in tanning is seen by many
environmentalists as cause for concern and the articles published recently in Leather International from BLC Leather Technology Centre, highlighting the various methods for determining metal species concentrations seem to confirm this. So the paper from Gu Haibin et al3 using copper (II) complexes may seem out-of-step with current thinking, but the authors assure the reader that the complex is: ‘an effective, harmless and broad spectrum antimicrobial compound.’ The paper discusses a new antimicrobial complex of copper (II) with benzothiazole derivative. The antimicrobial activity of the complex against representative bacteria and fungi found in shoe leathers showed MICs (minimal inhibitory concentrations) of 100-200 mg/l to bacteria, 1-50 mg/l to moulds and 1 mg/l to yeasts. Results indicated that in trials, >5 ppm of the complex inhibited 99.9% micro-organism growth for at least 7 days. The MICs of the copper complex and the inhibition ratios on shoe linings are shown in tables 1 and 2.
Waterglass
Dr K H Munz has been promoting waterglass, in all its various forms, for a myriad of uses in the leather production ever since I started working for this publication more than 15 years ago. The latest reincarnation is as an agent for curing raw hides.4 He said a European funded R&D programme had developed new curing methods in collaboration with tanners and supply houses. The objective was to replace common salt by neutralised alkali silicates, applied as a powder or by drum penetration. This led to a reduction in total dissolved salts, and the almost total elimination of common salts in soaking liquors. The drum penetration method involves soaking the hides and skins in aqueous (5-30%) waterglass: after 2-5 hours the liquor is drained and, after refloating, neutralised, preferably with citric acid to pH 5.0-5.5. They are then horsed for dewatering. The dry hides can be stored for many months, and although they look like parchment, after resoaking leather quality is unaffected. The bacterial counts are similar to that for salt cured hides, see table 3.
For the powder method, commercial sodium silicate was neutralised, washed to remove neutral salts, dried and ground. The resultant fine powder was applied as for common salt. Trials showed that about half the amount of silicate, based on the weight of the skin, was required when compared with common salt. Acceptable storage was obtained for up to six months. A final advantage is that rehydration of dried hides was found to be extremely good; hides with a moisture content of 12% up to a standard of 60% could be rehydrated in less than one hour. Bacterial counts, TDS levels and common salt concentrations are given in table 4.
One of the latest pieces of research into alternatives to regular biocides comes from Turkey again. Mutlu et al looked at ozone as a biocide in soaking. In a series of trials they compared ozone application with a sodium dimethyl dithiocarbamide based bactericide.
Ozone is a powerful oxidising agent which, when dissolved in water, produces a broad spectrum biocide that destroys all types of bacteria. It is effective over a wide pH range from pH 6.5 - >9.5. Its biocidal effectiveness is due to its ability to oxidise organic materials in bacterial membranes, which weakens the cell wall leading to rupture and immediate death of the cell. In contrast, all other oxidising and non-oxidising biocides must be transported across the cell membrane in order to interfere with either the nuclear reproductive mechanism or various enzymatic life giving reactions in the cell, in either event resulting in substantially less biocidal efficiency. It is used in many industries, including pharmaceutical applications in drug safety.
In the Turkish work, samples of sheepskin were stirred with either 0.15% of the dithiocarbamide, 20g/hour ozone, applied every 5, 10 or 15 minutes per hour, or with no added biocide. The CFU results from plating showed that the untreated soak bath had 7 x 106 cfu/mL at the end of the trials, after 7 hours; enough to damage the skin and lead to grain damage. This compares with 8 x 105 for the biocide, and <1 x 105 – 3.5 x 105 for the ozonated samples, depending on the concentration applied. These figures are considered high – the authors suggesting the reason being they carried out the trials in the height of summer, which in Turkey means temperatures of 38-40°C! – but they do show the effectiveness of ozone as a biocide, both in terms of stopping grain damage, and the fact there are no byproducts provided the ozone is handled safely in the first place.
Unique biocide for the leather industry
Published: 04 June, 2008
Eser Eke Bayramoglu, Faculty of Engineering, Department of Leather Engineering, Ege University, Türkiye, highlights the use of essential oils as potential biocides for the leather industry
AbstractIn this investigation, 1% of essential oils from three different oregano species and the oil of Foeniculum vulgare (fennel) were tested for their antimicrobial activity against 4-chloro-3-methylphenol and commercial bactericides commonly used in the leather industry.
The results showed that three essential oils of oregano had much stronger bactericidal activity than the commercial bactericides or the fennel. The findings suggest that these oregano essential oils could be used as bactericidal agents in the leather industry.
Introduction
Microbial activity is responsible for the degradation of organic substances leading to some undesirable consequences. To avoid such activity in processing, which can have severe economic consequences for the tanner, specific care must be exercised2.Soaking is the first process which rehydrates and cleans hides and for a short soaking period bacteria aren't a huge problem. But, when the hides are too dry and longer soaking periods are needed, this can lead to exponential microbial growth which may also cause permanent damage.
Various researchers have reported that a wide variety of bacteria can be isolated from the soaking water, including species of Bacillus, Chromobacter, Pseudomonas, Clostridium, Lactobacillus and Serratia marcescens3. Since the raw material is not sterile4 during the soaking process, bacterial attack can lead to putrefactive odours and hair slip and, depending on the extent of the damage, tiny abrasions known as ‘pin prick' may develop. The resultant leather may have grain loss3 and with further damage to the epidermal layer, a matt and lustreless grain, loss of ubstance etc, can also occur.
The bactericidal agents currently used in the industry are generally harmful to human health and nature, and their use has been either restricted or banned in certain countries, for example, pentachlorophenol (PCP)5, while the use of polyhalogenated phenolic compounds (TCP/TBP) has also been restricted6.
As can be seen from these particular examples, it is essential to discover new chemicals that can be used safely in the leather industry to counter undesirable effects of microorganisms. It has been known for many years that essential oils are 100% natural chemicals displaying varying degrees of antimicrobial activity. These oils are volatile compounds of a plant's secondary metabolism, and may be used as phytoproctective agents7,8.
The objective of this study was to determine whether the essential oils could be used as alternative bactericides in the leather industry. Essential oils of three different oregano species plus an organic fennel oil were tested for their bactericidal activity against a commonly used antimicrobial agent. This preference was based on the earlier reports pointing to their use in the food industry and health related issues as antimicrobial agents 1,9,10,11,12,13,14.
Materials and Methods
Raw skinFive native sheepskins (salted-dry) were selected with six pieces (weighing about 100 grams each) used in soaking tests, which contained a known bactericidal agent (phenol, 4-chloro-3 methylphenol, sodium salt), three different essential oils and the fennel oil.
Essential oils and bactericidal agents
The essential oils: Origanum onites (Turkish oregano, potmarjoram), Origanum sp (1) and Origanum sp (2) and fennel oil were purchased from a commercial company. The oils were prepared in ethanol in equal volumes and the final concentration was adjusted to 1% in the soak water. 1% of 4-chloro-3 methylphenol was chosen as the control.
The percentage constituents of the essential oils within the three oregano species and F. vulgare (fennel oil) are presented in Table 115,16,17,18.
Soaking process
All washing procedures were performed in sterile distilled water (DW). Pre-washing steps (for 1 hour) of the samples and the consecutive soaking steps (8-24 hours) took place in 1 litre aliquots of sterile DW. The aim of the pre-washing process was to let the water diffuse through the skin, removing the dirt, blood, conservation salt, dust etc. The bactericides were added prior to secondary soaking steps and the samples were mechanically agitated for ten minutes per hour.
Growth media
PCA (Plate Count Agar-Oxoid) medium was used for total
bacterial counts (aerobic mesophyllics). 1 ml aliquots were taken from each of the soak waters after eight hours and 24 hours respectively. The aliquots were further diluted (101 to 106) in sterile saline and 0.1ml samples in duplicates were poured into PCA medium (55°C in a water bath). The Petri plates were incubated for 48 hours at 37ºC and the bacterial counts were averaged and expressed as cfu/ml.
Results and dıscussıon
Bacterial counts obtained from six different soaking regimes of the five sets with eight and 24 hour intervals were averaged. As can be seen, Origanum onites (O0), Origanum sp (O1) and Origanum sp (O2) displayed varying degrees of antibacterial activity at concentrations around 1% compared with the control (C).
The data, when examined statistically on the logarithmic scale (Figure 5), indicated that the dimension or direction of bacterial growth was not the same for all groups between eight and 24 hour soaking periods. As time proceeded, the number of bacteria increased for the bactericide, control and the fennel sample, and it decreased for Origanum onites, Origanum sp (1) and Origanum sp (2) samples respectively. This can be attributed to a meaningful group-time interaction (p<0.05).
The changes observed from eight to 24 hours in each group were subjected to the Paired Samples t-test. The increase in bacterial numbers for the control, the bactericide and fennel oil and, a decrease for Origanum sp (1) and Origanum sp (2) groups were statistically significant, whereas the Origanum onites group were not (p=0.196 and p>0.05).
The differences between the groups, by the end of the eighth hour, were evaluated with Oneway Anova and the differences between the groups by the end of 24th hour were organized according to the values of the eighth hour, and evaluated with Ancova (Analysis of Covariance) analysis respectively. In conclusion, the difference among the groups from the 8th to 24th hour has been found significant.
Examining the variations between the groups at the eighth hour, a significant difference was observed reciprocally between the following: the control and Origanum onites, the bactericide and fennel oil, Origanum onites and the control, Origanum sp (2)-fennel oil, Origanum sp (1)-fennel oil, Origanum sp (2)-Origanum onites and fennel oil, and finally between fennel oil and all other groups (p<0.05).
The differences among the groups at the 24th hour were as follows: a significant difference was found among all the groups except that of the control and fennel oil (p<0.05).
The differences scored as significant among those groups were between the bactericide and all other groups, Origanum onites and the control, the bactericide and fennel oil, Origanum sp (1) and the control, the bactericide and fennel oil, Origanum sp (2) and the control, the bactericide and fennel oil.
The use of fennel oil as an antimicrobial agent has been reported19. Our results were also supported by this report. Although fennel oil seemed to increase the bacterial growth during an eight hour soaking period, at the end of the 24 hour period the bacterial counts were less than the control groups.
The study indicated that all oregano oils used during the experiments had a greater effect than that of the bactericide and the fennel oil. The most effective oil was Origanum onites, of which carvacrol content was the highest (84.48%).
Conclusion
As a result of the study, it can be concluded that Oregano essential oils displayed both antifungal and antibacterial activities. Further investigation as a bactericide in leather production is recommended.
Acknowledgements
The authoress would like to express her gratitude to Prof Dr N Gürdal Alaeddinoglu for the information he provided and for the interest he has shown in the study. The author's heartfelt thanks are also due to Türer Tarım for providing essential oils and to Ege University Research Fund Department for the financial support.
References
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