SSF Using a Mixture of Saccharification and Fermentation Microorganisms
In this type of simultaneous saccharification and fermentation (SSF) technique pretreated biomass is exposed to a cocktail of enzymes that can convert cellulosic material to ethanol in one reactor. Cellulases and xylanases in the cocktail first convert the carbohydrate polymers to fermentable sugars, and these enzymes are notoriously susceptible to feedback inhibition by the products—glucose, xylose, cellobiose and other oligosaccharides [67]. Therefore, this process has an enhanced rate of hydrolysis because sugars are concurrently removed by the fermentation step by yeast or bacteria in the solution. This SSF technique requires lower enzyme loading, results in higher bioethanol yields, and reduces the risk of contamination. Compatibility of enzymes and fermentation conditions is the major issue in this SSF technique, and it is essential to match enzymes and microorganisms that can operate under similar pH, temperature and substrate concentrations [68]. In many cases, the low pH, e. g., lower than 5, and high temperature, e. g., > 40°C, may be favorable for enzymatic hydrolysis, whereas the low pH can surely inhabit the lactic acid production and the high temperature may adversely affect the fungal cell growth [69]. For example, Trichoderma reesei cellulases, which constitute the most active preparations, have optimal activity at pH 4.5 and 55°C. On the other hand, Saccharomyces cultures are typically operated at pH 4.5 and 37°C [70].
Various forms of biomass such as corn stover [71-73], wheat straw [58], rice straw [74-77], barley straw [78], oat straw [79], switchgrass [80,81], sugarcane bagasse [82,83], sorghum bagasse [84-86], cogon grass [87], napier grass [88], guinea grass [89], Paja Brava straw [90], lespedeza stalks [91], eucalyptus wood [92-94], pine wood [95,96], aspen wood [97], algae [98], seaweed [99], paper sludge [100,101], and waste paper [102] have been tested for bioethanol production using this type of simultaneous saccharification and fermentation (SSF) technique. A selected sample of SSF on various biomass forms, saccharification enzymes used, operating conditions and ethanol yields are shown in Table 8.6.
In conclusion, there are advantages and disadvantages in the simultaneous saccharification and fermentation (SSF) using a mixture of biomass saccharification microorganisms which are outlined below [105].
The main advantages of SSF are:
1. Significantly reduces the enzyme inhibition by conversion of sugars that inhibit the cellulase activity compared to SHF, because immediate consumption of sugars by the microorganism produces low sugar
concentrations in the fermenter, which results in increased saccharification rates.
2. Lower enzyme requirement.
3. Higher product yields.
4. Lower requirements for sterile conditions since glucose is removed immediately and bioethanol is produced.
5. Shorter process time compared to SHF.
The main disadvantages of SSF are:
1. Different temperature optima for saccharification and fermentation can make it difficult to optimize the process.
2. A typical fermentation will take 5-7 days; the long residence time may make contamination control difficult in a continuous process, but may be manageable in a batch process.