Preparation of Substrate by Microbial Fermentation,Ethanol Production,Cellulose,Nitrogen,Volatile Acidity,Acidic,Moisture,Glucoamylase,Alpha Amylase,Tapica

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Home >> Industrial and Microbial Biotechnology >> Microbes and Microbial Genomics for Industry >> Preparation of Substrate by Microbial Fermentation

Preparation of substrate
For commercial ethanol production, following three substrates are used as renewable raw materials: (i) roots, tubers or grains that are rich in starch, (ii) molasses or juice derived from sugarcane, palm or sugarbeet, and (iii) wood or waste products (rich in cellulose) from processed wood. In India ethanol is produced mainly by fermentation of molasses, which are produced as a by-product in sugar factories and are quite rich in fermentable sugars.

Chemical composition of molasses (approximate quantities).

Component	Percentage
Total reducing sugar -fermentable -non-fermentable	45.00 6.00
Total nitrogen	0.36
Total sulphur	0.05
Volatile acidity (as acidic)	0.18
Ash content	11.00
Moisture	28.00

However for the current licensed production capacity of about 1500 million litres of alcohol per annum (by 193 distilleries) adequate quantity of molasses is not available. Therefore, other substrates (rich in starch) have often been used. These alternate substrates include (i) grains of rice, maize, millets and barley containing usable starch and (ii) roots of sweet potato, sugarbeet and cassava. Potato and tapioca are also used as raw materials. Approximate cost of the production of one litre of alcohol from these starch rich raw materials is given in the table.

Production cost of one litre of alcohol from different starch rich raw materials

Raw material	Cost (Rs)
1. Potato	15.30
2. Maize	10.00
3. Tapioca	3.00
4. Rice	6.50
5. Sweet potato	12.30
6. Sugar beet	5.15

Starting with those parts of plants which are rich in starch such as seed, the material is first ground, filtered and dried. The starch is liquefied by boiling under pressure, cooled and then hydrolysed enzymatically. For glucose production as an intermediate, glucoamylase and alpha-amylase are added after the boiling and cooling process. Approximately, one litre commercial alpha amylase and 3.5 litres glucoamylase are added for one tonne of starch. Glucose produced thus can be used for ethanol production.

Fermentation.
Various types of continuous fermentation plants are available for producing ethanol, where sometimes several fermenters are connected in a series. In a typical fermenter using molasses as a substrate, diammonium phosphate (as a source of nitrogen for the yeast) is mixed up with the substrate. The pH is maintained at 5.0 and fermentation is carried out at 30-35°C. After fermentation, yeast cells are separated by centrifugation or sedimentation and used again.

Distillation.
Ethanol from the fermented mixture is recovered by distillation process. The first distillation yields about 80% pure ethanol but by repeated distillation, ethanol concentration of 99.4% or more can be achieved. Final dessication can be achieved by chemical dessicants and by absorption.

In recent years, cellulose also seems to have become a distinct possibility as a renewable source of alcohol. Bagasse, sulphite waste, liquor pf paper industry and agriculture biomass (hay) are the sources of cel1ulose. The biomass is first pretreated through physical or chemical processes leading to reduction in size. This helps in hydrolysis of cellulose and hemicellulose by enzymatic or chemical treatment. The soluble sugar obtained after hydrolysis are subjected to fermentation in the usual manner. Cellulolytic microorganisms can also be used for degradation of biomass, so that direct fermentative production of alcohols or acids will be possible. Biochemical Engineering Research Centre (BERC) at L.I.T., Delhi is conducting research for developing improved methods for cellulose pretreatment.

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