In batch processes, all nutrients required for fermentation are present in the medium prior to cultivation. Batch technology had been preferred in the past due to the ease of operation, low cost of controlling and moni­toring system, low requirements for complete sterilization, use of unskilled labor, low risk of financial loss, and easy management of feed­stocks. However, overall productivity of the process is very low, because of long turnaround times and an initial lag phase [9].

In order to improve traditional batch processes, cell recycling and application of several fermentors have been used. Reuse of produced cells can increase productivity of the process. Application of several fermentors operated at staggered intervals can provide a continuous feed to the distillation system. One of the successful batch methods applied for industrial production of ethanol is Melle-Boinot fermen­tation. This process achieves a reduced fermentation time and increased yield by recycling yeast and applying several fermentors operated at staggered intervals. In this method, yeast cells from pre­vious fermentation are separated from the media by centrifugation and up to 80% are recycled [9, 68]. Instead of centrifugation, the cells can be filtered, followed by the separation of yeast from the filter aid using hydrocyclones and then recycled [69].

In well-detoxified or completely noninhibiting acid hydrolyzates of lignocellulosic materials, exponential growth will be obtained after inoc­ulation of the bioreactor. If the hydrolyzate is slightly inhibiting, there will be a relatively long lag phase during which part of the inhibitors are converted. However, if the hydrolyzate is severely inhibiting, no conversion of the inhibitors will occur, and neither cell growth nor fer­mentation will occur. A slightly inhibiting hydrolyzate can thus be detoxified during batch fermentation. However, very high concentration of the inhibitors will cause complete inactivation of the metabolism [18].

Several strategies may be considered for fermentation of hydrolyzate to improve the in situ detoxification in batch fermentation and obtain higher yield and productivity of ethanol. Having high initial cell den­sity, increasing the tolerance of microorganisms against the inhibitors by either adaptation of cells to the medium or genetic modification of the microorganism, and choosing optimal reactor conditions to minimize the effects of inhibitors are among these strategies.

Volumetric ethanol productivity is low in lignocellulosic hydrolyzates when low cell-mass inocula are used due to poor cell growth. Usually, high cell concentration, e. g., 10 g/L dry cells, have been used in order to find a high yield and productivity of ethanol in different studies. In addition, a high initial cell density helps the process for in situ detox­ification by the microorganisms, and therefore, the demand for a detox­ification unit decreases. In situ detoxification of the inhibitors may even lead to increased ethanol yield and productivity, due to uncoupling by the presence of weak acids, or due to decreased glycerol production in the presence of furfural [21]. Adaptation of the cells to hydrolyzate or genetic modification of the microorganism can significantly improve the yield and productivity of ethanol. Optimization of reactor condi­tions can be used to minimize the effects of inhibitors. Among the dif­ferent parameters, cell growth is found to be strongly dependent on pH [18, 21].