One of the proposed technologies for the development of high-performance processes using starchy materials consists of the fermentation of high and very high gravity mashes. During high gravity fermentation, the solids concentration in the medium exceeds 200 g/L, which implies a high substrate load and, con­sequently, high ethanol concentrations at the end of fermentation. Furthermore, a lower amount of process water is required as well as lower energy demands. However, this process implies more prolonged cultivation times and, sometimes, incomplete fermentation due to end-product inhibition, high osmotic pressure, and inadequate nutrition (Barber et al., 2002). To accelerate high gravity fermen­tations, the controlled addition of small amounts of acetaldehyde during the fer­mentation allows the reduction in cultivation time from 790 h to 585 h for initial glucose concentration of 300 g/L without effect on ethanol yield. It is believed that this positive effect may be caused by the ability of acetaldehyde to replenish the intracellular acetaldehyde pool and restore the cellular redox balance (Barber et al., 2002). Continuous operation can improve the performance of high grav­ity fermentations. In particular, continuous fermentation can reach almost the same ethanol production as batch fermentation, although it is likely that there is a threshold concentration of initial glucose above which an ethanol yield no longer increases (Zhao and Lin, 2003).

For industrial ethanol production, fermentation of wheat mashes of very high gravity (VHG) has been proposed. These mashes consist of wheat starch hydro — lyzates containing 300 g or more of dissolved solids per liter of mash. VHG fer­mentation technology enables high ethanol concentrations to be obtained from very concentrated sugar solutions. To this aim, very low levels of dissolved oxygen are required as well as nitrogen sources that do not limit the cell growth, such as urea or ammonium salts (Jones and Ingledew, 1994a). In this way, 21.1% (by volume) ethanol concentrations are obtained in only four days of fermentation from VHG wheat mash. One of the strategies employed is the addition of com­mercial proteases during the VHG fermentation in order to release amino acids from soluble proteins contained in the wheat mash compensating, in this way, the addition of exogenous nitrogen sources (Jones and Ingledew, 1994b). Thomas et al. (1996) emphasize that considerable amounts of water can be saved by applying this technology to fuel ethanol production. Moreover, the implementation of VHG fermentation increases the throughput rate of an ethanol plant without the need of increasing the plant capacity. These authors provide a theoretical method for pre­dicting the maximum concentration of ethanol in fermented mash that takes into account changes in the weight and volume of mash during fermentation. Bayrock and Ingledew (2001) designed and tested a system that combines the multistage continuous culture fermentation and the VHG cultivation for feed containing 150 to 320 g/L of glucose using S. cerevisiae. The maximum ethanol concentration obtained in the process was 132.1 g/L indicating the feasibility of implementing this technology in the industry, particularly in the continuous production of etha­nol from wheat starch (Sanchez and Cardona, 2008).

VHG technology has been tested with successful results for oats, barley, rye, and triticale, as cited by Wang et al. (1999). The pretreatment of feedstock can play an important role when process integration is analyzed during this type of fer­mentation processes. Wang et al. (1999) propose the integration of a pretreatment process, pearling by abrasion of cereal grains such as rye or triticale, with VHG fermentation technology. The pearling of cereals removes approximately 12% of grain dry matter, which increases its starch content to 7 to 8%. This increase in starch content combined with the employment of high concentrations of sugars, which is the main feature of VHG fermentation, allows the increase in the final ethanol concentration of 64% in comparison to the use of nonpearled grains in conventional fermentations.