Alternative biocides – an optimistic future

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.
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.