**1. Introduction**

The raw materials for ethanol production can be classified based on the type of carbohydrates they contain, i.e., sugar, starch, or cellulose by fermentation. Sucrose, glucose, or fructose for ethanol production for simultaneous saccharification and fermentation process are derived from any of the two classes of raw materials namely, starchy and cellulosic materials [1].

Ethanol production from simple sugars derived from sugarcane molasses, beet sugar is commercially well established. The yeast or bacterial cells can metabolize the simple sugars directly without the necessity of pretreatment step. The starch and cellulose polymers must be hydrolyzed to simple sugars before they can be fermented by yeast or bacteria [2–4]. Although cellulosic materials are available in plenty than starchy and sugar-containing raw materials, the process of conversion of it to fermentable sugars is often a very expensive pretreatment step using

enzymes [5, 6]. Starch-containing substrates must be hydrolyzed by enzymes or acid to simple sugars and can be used for the production of ethanol. The carbon, hydrogen, and oxygen are normally provided by a complex carbohydrate source such as cane or beet molasses in industries. Vitamins and minerals may be added as additional nutrients. The sources of nitrogen are generally ammonium sulfate and urea, but they require biotin for effective utilization [7]. Other cheaper raw materials such as spent sulfite liquors, and whey also are sources of fermentable sugars. The sugar concentration in the above-mentioned industrial effluents is very much lower than in usual starchy and cellulosic substrates. Spent sulfite liquors contain 20–30 gL<sup>−</sup><sup>1</sup> of hexose while whey contains 40–50 gL<sup>−</sup><sup>1</sup> of lactose. Cellulosic raw materials on acid or enzyme hydrolysis give a maximum sugar concentration of around 40–60 gL<sup>−</sup><sup>1</sup> [8]. Ammonium or potassium phosphate provides the potassium and phosphorous required for growth of yeast. The magnesium sulfate, chloride and biotin can be provided as additional supplements [8, 9]. In a study by Qureshi and Manderson [10] four renewable agricultural resources were considered, namely wood, molasses, whey permeate, and starch. He reported that molasses sugars were cheaper than sugars derived from the other raw materials.

The simultaneous saccharification and fermentation (SSF) process was conceptualized in the late 1970s by Wright et al., Takagi et al., and Blotkamp et al. [11, 12]. This process employs fermentative microorganisms in combination with amylolytic enzymes in a single fermenter. Sugar accumulation in the fermenter is minimized in this process that favors increased hydrolysis and ethanol yield when compared to separate hydrolysis and fermentation. The main advantage process over separate hydrolysis and fermentation is that high substrate concentration, long residence time and high enzyme concentration can be used in same reactor. Optimization of process variables namely substrate concentration, enzyme concentration, pH and temperature are important to maximize the ethanol yield.

Starches that can be used for ethanol production by fermentation, includes grains, cassava (manioc, tapioca), sweet potato, sweet sorghum, and Jerusalem artichoke, corn, wheat, rice, potatoes, and sugar beets are the mostly used feedstocks in Europe and North America, sugarcane, molasses, cassava, babassu nuts, and sweet potatoes appear to provide the most promising feed for ethanol for countries such as Brazil.
