Enzymes – the sustainable choice26 November 2009
Dr Victoria Addy, technical director, BLC Leather Technology Centre, reviews the latest developments in enzyme technology for the leather industry
The process of converting hides and skins into leather involves a sequence of complex chemical reactions and mechanical processes. Although the leather industry is environmentally important as a user of a by-product, it is perceived as a consumer of resources and a producer of pollutants. In order to reach a position of sustainability, the industry must aim to reduce the consumption of chemical, water and energy resources. The impact of this processing on the environment and the safety of the resulting leather to be used throughout the supply chain to the end consumer is related to the chemical applied, the raw materials used and the effluents, waste and emissions generated through-out processing. Provisions to control these outputs and ensure eventual consumer
safety are, therefore, paramount and at the forefront of the
consumer products industry.
With the ever increasing emphasis on cleaner production, sustainable processing and greener materials, the application of enzymes to both replace and enhance chemical systems has never been more relevant. Enzyme systems that can minimise the effluent burden and replace chemicals which have an undesirable environmental profile will become of increasing interest. In an environment where brands and retailers are keen to demonstrate good environmental credentials, material selection based on green criteria will become even more important.
Green chemistry, also called sustainable chemistry, is a chemical philosophy encouraging the design of products and processes that reduce or eliminate the use and generation of hazardous substances. Green chemistry embraces many of the chemical disciplines including inorganic chemistry, organic chemistry, analytical chemistry and biochemistry and its major application is in industrial processing.
The application of enzymes fits many of the principles of green chemistry. The world market for industrial enzymes was estimated as $2.3 billion in 2008 and is estimated to rise at a growth rate of 4% per annum to $2.7 billion in 20121.
Enzymes are vital for sustaining life; without them chemical reactions in biological systems would rarely occur. Enzymes, therefore, act as powerful catalysts and are capable of increasing the rate of reactions by at least a factor of 1 million. Enzymes are generally small globular proteins, although some structural proteins such as the protein filaments of muscle can also have a catalytic role. Enzymes have a number of advantages over their chemical counterparts in that they are highly specific, operate at low temperatures and at atmospheric pressure and act rapidly. Furthermore, being diverse, they can be used over a wide range of temperature and pH conditions and can, therefore, be selected or even engineered for a specific purpose. There are thousands of known enzymes which, coupled with the capabilities of genetic manipulation and protein engineering, make the possibilities for enzyme technology almost limitless.
Enzymes in leathermaking
Enzymes have an important role in leather manufacture and the use of biocatalysts is now considered to be state-of-the-art for the processing of animals skins and hides.
The most important enzyme types for leather processing are proteases and lipases, which are active against protein and fat components of the skin respectively. Other enzyme activities against other biological components present in the skin may also present future opportunities for green processing, especially in the beamhouse. Currently proteases are used extensively for the soaking, liming and bating operations and lipase formulations are commercially available to assist with the dispersion of natural fat within the skin substrate.
Research into the use of enzymes for other applications include the use of transglutaminase to facilitate tanning and dyeing reactions and phospholipase to target specific complex lipids such as sebaceous grease has also shown promise. In addition there has been a renewed emphasis on the use of enzymes to add value to waste products from the leather making process; examples include the use of lipases to produce biodiesel from waste animal fats and the use of a thermophilic protease to convert waste protein to bioethanol. The use of proteases to recover chromium from chrome containing protein waste has been the subject of considerable research.
Whilst commercial enzymes are a staple component of many commercial leather processing systems, their use is often feared due to the fact that good process control is required to both optimise their activity and prevent side activities which can cause complications such as grain damage. Furthermore, in view of the fact that enzymes are biological molecules, they are sensitive to their environment in terms of both physical conditions and chemical content and concentration that may act as both activators and inhibitors of the enzyme. An important consideration for enzyme activity is their local environment which is governed by the chemicals added to the processing vessel. Typically, this could include biocides, surfactants, metal cations and specific anions such as sulphide and chloride ions, all of which may have either a positive (activation) or negative (inhibition) effect on activity (see Figure 1).
Whilst there are a number of factors to take into consideration when handling and using enzymes, if used appropriately their benefits far outweigh any sensitivities associated with their use and often the results achieved are not possible with simple chemical systems. This is particularly relevant with properties such as softness, area yield and uniformity of the resulting leather.
One area of particular value in terms of environmental improvement is in the unhairing process. Chemical unhairing of cattle hides relies on significant concentrations of alkali, most usually calcium hydroxide which has a very low solubility, and sulphide based depilatory chemicals, both of which contribute a significant environmental burden.
Enzymes have been used successfully to aid unhairing either as a pre-treatment in the soaking process or as an addition or pre-cursor to the liming process. When used effectively, enzymes can both speed up the process and reduce the amount of chemicals used with the resulting hides often being cleaner.
There remains much interest in the future direction of enzyme research for the leather industry. Based on the understanding that enzymes are available to carry out almost all chemical reactions, the future options for leather applications are considerable. Enzymology and green chemistry will continue to play a major part in the evolution of the leather making process, ensuring cleaner processing and the production of greener materials.
1. BCC Research Report B10030E, 2007.