Explain how fermentation helps to meet the demand for fuel in Brazil
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Explanation:
Brazil today is home to the world's largest fleet of cars that use ethanol derived from sugarcane as an alternative fuel to fossil fuel based petroleum. Twenty-seven million cars, 73% of the total, can use a mix of ethanol and gasoline....
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Ethanol Production in Brazil: The Industrial Process and Its Impact on Yeast Fermentation 89 3.3 The fermentation The process operates in fed-batch (75 % of the distilleries), also mentioned as Melle-Boinot process, or in continuous mode, both utilizing yeast cell recycling. In both process, after the end of fermentation, yeast cells are collected by centrifuging and re-used in a next fermentation cycle. Up to 90-95 % of the yeast cells are recycled, resulting in high cell densities inside the fermentor [10 to 14 % (wet weight basis/v)]. Cell reuse reduces the need for intensive yeast propagation, and less sugar is deviated for biomass formation. It is estimated that yeast biomass increases 5 to 10 % (in relation to initial biomass) during a fermentation cycle, which is enough to replace the yeast cells lost during the centrifugation step. This high yeast biomass inside the fermentor is responsible for a very short fermentation time of 6 to 10 h, when compared to 40 to 50 h in corn fermentation process. Normally, temperature is kept around 32 to 35 °C, yet due to the short fermentation time, cooling is not always efficient enough in removing heat, and temperature may reach up to 40 °C, especially in the summer season (Lima et al., 2001; Laluce, 1991; Wheals et al., 1999; Lima et al., 2001; Amorim et al, 2004; Andrietta et al., 2002). Generally, fermentation starts by adding cane must (prepared by mixing cane juice and molasses at any proportion), which contains 18 to 22 % (w/w) total reducing sugars (TRS), to a yeast cell suspension. This yeast suspension (with ca 30 % of yeast cell, on wet basis) represents 25 to 30 % of the total volume of fermentation, and is performed in tanks of 300 to 3,000 m3. This large inoculum is normally prepared by mixing 2 to 12 ton of baker’s yeast with 10 to 300 kg of selected strains in active dry yeast form. The feeding time normally lasts for 4 to 6 h and fermentation is finished within 6 to 10 h, resulting in ethanol titres of 8 to 12 % (v/v). When fermentation ceases, yeast cells are separated by centrifugation, resulting in a concentrated yeast cell suspension (the yeast “cream”) with 60 to 70 % (wet weight basis/v) of cells. The yeast cream is diluted with water (1:1), and treated with sulphuric acid (pH 1.8-2.5) for 2 h, in order to reduce bacterial contamination and to be re-used as starter for a next fermentation cycle. This recycling trait makes the Brazilian process quite peculiar in that yeast cells are reused at least twice a day over the production season of 200-250 days. Tank stirring is also desirable at a low power, only to avoid cells compacting at the bottom of the tank and to keep a higher contact surface with substrate. After centrifuging, the “beer” or “wine” is driven to distillation for ethanol recovery, normally using distillation tray technology. After distilling a liquid stream called stillage or “vinasse” is produced at the ratio of 10-15 litters per litter of produced ethanol and is delivered into the cane fields for use as irrigation water and fertiliser (adding to soil potassium, calcium, magnesium, others micronutrients and some organic matter). When compared to the continuous mode, Melle-Boinot process (Figure 1) presented higher yields and productivity, lower levels of contamination and it was easier to clean (Zhang, 2009), but according to Andrietta et al. (2007) beneficial traits of a continuous version are masked by the improper engineering conception of low-cost adaptations of batch to continuous plants.