Ethanol Fermentation

Most of us would think of ethanol fermentation very simply as the essence of brewing in cider and beer production or in winemaking.

Ethanol is a product of the aerobic fermentation of sugars by that classic microorganism Saccharomyces cerevisiae. Ethanol is a commodity product – at least 1 x 10E6 tons of industrial ethanol is produced annually in the USA alone. The majority is manufactured by catalytic conversion of ethylene. The only time  fermentation becomes attractive is when there is a petrol shortage or issues in supply chains. Fermentation is attractive because at least all the materials used are renewable and dare I say it ‘sustainable’.

Ethanol from Molasses

Typical fermentation media include molasses from cane sugar which is an extensive industrial production method in those countries where the sugar industry is important. 

Molasses is a viscous liquid that results from the refining of sugar cane and sugar beet which is turned into sugar. It is cheap because it is this waste by-product from sugar processing. In India for example, it is roughly 10% of the total pure sugar cost. That makes it a potent source of carbon. The quality of this sugary material is highly variable in its sugar content, type of extraction and the age of the sugar manufacturing plant. It has many uses in cooking and baking.

Molasses is a very complex mixture of nitrogenous material and sugars. It is 50% w/w fermentable sugar and between 0.5 and 1 % w/w nitrogen. For many fermentation purposes, the carbon to nitrogen ratio is extremely important for fermentation. For example it is a ratio totally unsuited to yeast production (Johnson et al., 1995).  

Beer Brewing

In brewing, beer which contains between 3 and 7.5% w/w ethanol is produced in a batch fermentation.  More sophisticated fermentation systems use fed-batch where sugar is fed intermittently when this substrate is nearly used up or continuously. Other systems use immobilised cell reactors as a way of developing the fermentation process further. The technology is especially suited to the continuous production of ethanol rather than relying on batch fermentation methods.

A series of fermentation techniques was explored by Cysewski & Wilke (1978).

Continuous Ethanol Production

From the 70s onwards, it was reasoned that moving away from a batch fermentation system to a continuous system would be a natural progression. Ethanol production lends itself very well to this technology. It is of course extremely well understood and a great deal of knowledge has been accrued over the years in continuous beer production well before it was being explored in terms of process optimisation.

The Brewing Research Foundation at Redhill in Surry, UK explored the possibility of a plug flow reactor for continuous production by feeding a yeast system with wort (Baker & Kirsop, 1973). The yeast was retained by a filter using kieselguhr as the support material. Kieselguhr is commonly used in filtration systems to maintain flow before blocking of the sinter or membrane occurs. The main limitation in this case was kieselguhr as expected, blocking the retaining filter.  

In the very late 70s and early 80s, researchers started to examine continuous ethanol production using immobilized Saccharomyces cerevisiae (Linko & Linko, 1981). In one classic example, they immobilised this yeast in a typical carbohydrate polymer – calcium alginate sourced from seaweed. These were in the form of small beads which could be placed into packed-bed column reactors. The cells were fed on substrates such as glucose and cane molasses which are both relatively cheap and plentiful for ethanol. A beer was also produced continuously using barley malt wort.

The continuous process was claimed to be operated for a number months by supplying both a well-balanced nutrient composition and the ‘periodical’ regeneration of cells through judicious addition of nutrients and aeration.

Success can be measured by the fact that a 7 per cent (w/v) ethanol solution was generated using cane molasses as the substrate and a typical beer of 4.5 per cent (w/v) from barley wort. It’s not clear what the palatability of the beer was like but it does illustrate a particular principle of ethanol production.

In the case of producing the 7% ethanol solution, the cane molasses was fed into the column reactor at 17.5% (w/v) total reducing sugars with a residence time of between 4 and 5 hours. To generate the beer the barley wort passed through the same type of reactor in a residence time of 2 hours.

References

Baker, D. A., & Kirsop, B. H. (1973). Rapid beer production and conditioning using a plug fermentor. Journal of the Institute of Brewing, 79(6), 487-494 (Article).

Von W Johnson and others (1995) Utilization of molasses for the production of fat by an oleaginous yeast, Rhodotorula glutinis IIP-30, Journal of Industrial Microbiology, 14 (1), pp. 1–4, (Article) 

Linko, Y.Y., Linko, P. (1981) Continuous ethanol production by immobilized yeast reactor. Biotechnol Lett 3, pp. 21–26 (1981) (Article).