During the fermentation process, several by-products are produced together with ethanol. In co-culture fermentation which involves different strains, different side-products besides ethanol are produced. The list of by-products and their applications in industry are listed in Table 5.

The production of by-products somehow reduces the ethanol yield due to the competition from other metabolic conversions. The inhibition of lactic and acetic acids on yeast for ethanol production in corn mash was examined when both the acids synergistically reduced the rates of ethanol synthesis and the final quantities of ethanol produced by the yeast (Graves et al., 2007). The inhibitory effects of the acids were more apparent at elevated temperatures. So, a reduction in the formation of by-products is needed to achieve higher ethanol yield.

Alternatively, a fermentation process should not be only aimed for higher conversions of raw materials and ethanol productivity, but should rather take the advantage of the byproducts released during the transformation of feed stocks and convert them into valuable co-products. To reduce the inhibition effect, in-situ separation can be applied to separate the valuable co-product from the process. In this way, economical and environmental criteria can be met. However, depending on the objective and the economic analysis of the particular ethanol plant, the by-products may either generate extra revenue for the plant or just an inhibition the conversion process.

Among the ethanol byproducts, glycerol and lactic acid are used extensively by industries and can increase the production profit. These fermentative products have attracted interest owing to their prospect environmental friendliness and of using renewable resources instead of petrochemical. These byproducts have broad applications which can generate lucrative profit for the processes i. e. lactic acid. The global market for lactic acid is predicted to reach 328.9 thousand tonnes by 2015 (Plastics Today, 2011). The world consumption of lactic acid is stimulated by its applications in key industries such as cosmetics, biodegradable plastics and food additives. Lactic acid is used as a pH balancer in shampoos and soaps, and other alpha hydroxy acid applications, was expected to elevate the consumption in the market. Polymer lactic acid (PLA) for biodegradable plastics has properties similar to petroleum derived plastic and was expected to increase the demand for environmental friendly packaging. Food additives will continue to be the largest application area for lactic acid globally, but biodegradable plastics represent the fastest growing end-use application.

Glycerol or glycerin is a simple alcohol produced by S. cerevisiae during glucose fermentation to ethanol to maintain the redox balance. The global market for glycerin is forecasted to reach 4.4 billion pounds by 2015 (PRWeb, 2010). The increased demand for glycerin was reported to originate from various end-use area such as oral care, personal care, pharmaceutical and food and beverage. In fact, there are over 1,500 end-uses for the chemical. In most products, glycerin is used in very small portions with exception in a few end-uses which require a significant amount of glycerin in their formulation. Glycerin is also used in several novel applications such as propylene glycol, syngas and epichlorohydrin and it is expected to improve the glycerin demand.

Glycerol also can be potentially used as fuel additives for diesel and biodiesel formulation that assist to a decreasing in particles, hydrocarbons, carbon monoxide and unregulated aldehydes emissions. It can also act as cold flow improvers and viscosity reducer for use in biodiesel and antiknock additives for gasoline (Rahmat et al., 2010). Since glycerol is also produced in the fermentation broth, it is attractive as an entrainer to reduce the use of fresh entrainer in extractive distillation of azeotrope mixture of ethanol-water system.

2. Conclusion

Cassava is an attractive alternative as the carbon substrate for ethanol production especially where water availability is limited as it can tolerate drought and yields on relatively low fertility soil. The conventional method for the ethanol production involves liquefaction, saccharification and fermentation steps which are time consuming and cost ineffective, in view of the use of enzymes. Therefore, direct fermentation with integrated steps that incorporating recombinant or co-culture strains in a single reactor offers a more convenient method for the production of ethanol and its high value by products. By co-culture fermentation, high starch concentration can be used to reduce water usage in fermentation and subsequently in ethanol-water separation system. Furthermore, the fermentation medium can be prepared at lower temperature or raw starch can be used for direct fermentation to reduce the energy consumption. From the safety, economic and production process aspects, single-step bioconversion using co-culture microorganisms is a better alternative as far as production of ethanol and its by products from starch is concerned. The ethanol by-products such as lactic acid and glycerol can be value added co-products to generate extra revenue.