Fermented broth or “mash” typically contains 2-12% ethanol. Further­more, it contains a number of other materials that can be classified into microbial biomass, fusel oil, volatile components, and stillage. Fusel oil is a mixture of primary methylbutanols and methylpropanols formed from a-ketoacids and derived from or leading to amino acids. Depending on the resources used, important components of fusel oil can be isoamylal- cohol, n-propylalcohol, sec-butylalcohol, isobutylalcohol, n-butlyalcohol, active amylalcohol, and n-amylalcohol. The amount of fusel oil in mash depends on the pH of the fermentor. Fusel oil is used in solvents for paints, polymers, varnishes, and essential oils. Acetaldehyde and trace amounts of other aldehydes and volatile esters are usually produced from grains and molasses. Typically, 1 L of acetaldehyde and 1-5 L of fusel oil are produced per 1000 L of ethanol [9, 47].

Stillage consists of the nonvolatile fraction of materials remaining after alcohol distillation. Its composition depends greatly on the type of feedstock used for fermentation. Stillage generally contains solids, resid­ual sugars, residual ethanol, waxes, fats, fibers, and mineral salts. The solids may be originated from feedstock proteins and spent microbial cells [9].