Today, there is a significant interest in alternatives to fossil fuels. This has arisen given the increasing public concern over the environment as well as the general scientific consensus on the greenhouse effect caused by CO2 emissions. Consequently, any realization of a renewable energy source that is an effective alternative to gasoline and diesel would prove significant.
Plant material is a renewable energy source. Trees, shrubs and herbs grow all over the world, and in climates such as the tropics, even all year round. Via photosynthesis they convert CO2 and sunlight into lignocellulose – lignocellulose is the bulk material of a plant cell.
Lignocellulose is constituted of cellulose (long chains of glucose, up to 30.000 units – figure 1), hemicellulose (the single units resemble glucose but are slightly different) and lignin (figure 2). Lignin is derived from the latin word lignum, meaning “wood”, and is constituted of a structurally complex mixture of polyphenols, called monolignols (Figure 3). Lignin is what makes plants “stand” by effectively providing structural support. Cotton, for example, is pure cellulose without any lignin. Therefore cotton is soft, and nothing like wood.
The cellulose in plants is what makes lignocellulosic material so interesting in bioethanol production: The cellulose can be degraded back into glucose with enzymes and in turn, organisms such as yeast are able to (under anaerobic conditions) ferment the glucose into ethanol. Once the ethanol has been distilled, it can then utilized to run vehicles or machinery that function via a modified combustion engine.
Overall, this is tantamount to a process that can create an alternative renewable energy source for the transport sector, literally in a manner similar to the production of alcoholic beverages.
The origins of the use of ethanol as a combustion fuel began in 1878, when the first spark-ignition engine was developed. Soon after, Ford developed an ethanol utilizing car, the Ford T. Currently USA and Brazil have large scale production of bioethanol, which is actually used as fuel in the transport sector. Unfortunately, this production is based only on the starch and sugar obtained from maize and sugar cane, which constitutes a small part of the total plant material – about 1-2 % in most cases. As well, this production is not very energetically favourable, with approximately 50 -70% of the energy yield lost to the production process. As a result, this use of maize and sugar cane is currently not a very attractive alternative to fossil fuels.
However, if it was possible to utilize the rest of the plant’s lignocellulosic material, then conceivably almost all of the plant could be used to produce the ethanol, representing a 50 to 100 fold increase in efficiency. In effect, the world’s most abundant form of biomass could become available for bioethanol production. Cereal straw, maize cops, saw dust, logging residues, municipal solid waste or energy crops, could all contribute.
Unfortunately the process of fermenting lignocellulosic material is not as simple as the process of fermenting starch and simple sugars. Indeed, the lignocellulosic material has to first be degraded into much smaller molecules, i.e. single sugar monomers, in order to be fermented effectively. Whilst degradation of lignocellulosic material by high pressure, high temperature and usually sulphuric acid hydrolysis can be effective, it is a very costly way of releasing these sugar residues. The method itself requires a lot of energy, and use of sulphuric acid is problematic being extremely corrosive to equipment and contributing to large amounts of waste product in the form of calcium sulphate.
Consequently, alternatives are currently being investigated. Here, a strategy is to mimic nature, whereby it is known that many organisms are capable of degrading plant material using enzymatic processes. Such organisms include fungi and bacteria and the enzymes capable degrading lignocellulosic material are generally called cellulases. In effect, they “tear” up the fibres containing cellulose and degrade it to smaller sugar residues.
As well, small pieces of lignin, e.g. monolignols from the degradation of lignocellulose also pose a problem during fermentation. These have an inhibitory effect on the fermenting organism, baker’s yeast (Saccharomyces cereviseae) – the aromatic rings of the monolignols (i.e. the six membered ring) are actually toxic to yeast.
Again, nature also has a possible answer for this obstacle. Enzymes such as laccases can modify the monolignols. In particular, fungi, such as certain white rot species, have forms of laccases capable of degrading lignin so that metabolism can occur. As well, laccases have been reported to facilitate processes that allow the monolignols to aggregate to a degree such that they are then too large to enter any yeast cell. Consequently, the use of this enzyme can possibly be applied in the industry to neutralize the inhibitory effect on yeast by lignin.
In conclusion, at this point in time, bioethanol production using enzymatic tools still needs more refinement. Ways of optimizing the process have to be further investigated as it still contains large inefficiencies at almost all the steps entertained in this essay. Therefore, the challenge still exists for scientists to make the industrial degradation of lignocellulosic material more efficient and less costly, so that it can be a profitable, realistic, and needed alternative to the use of fossil fuels.