Butanol is a 4-carbon alcohol, which is considered a better biofuel than its 2-carbon counterpart, ethanol. It is usually produced by ABE (acetone-butanol-ethanol) fermentation using agricultural feedstocks, by bacteria called, Clostridia. Among butanol's better biofuel characteristics (relative to ethanol) are: (1) higher energy density, (2) higher hydrophobicity (doesn't attract water, therefore, can be shipped in existing fuel piepelines; also less corrosive and blends better with gasoline at higher levels). However, the barriers which hinder commercialization of butanol production are its low yield, low productivity and low titer, due to the product toxicity of the fermenting microorganisms. Low titers are said to increase the cost of separating butanol from the fermentation broth. Scientists from the Ohio State University attempted to improve butanol productivity and titer using a single-path continuous fermentation operation in a fibrous bed bioreactor (FBB). They immobilized a non-spore-forming mutant strain of Clostridium beijerinckii in the fibrous bed bioreactor (FBB), and optimized the process (in terms of culture medium and operating conditions) for increased butanol production. The scientists also mentioned that the "mutator strain in combination with the FBB-based adaptation method, will help more rapid evolution of the [butanol]-producing Clostridium beijerinckii towards higher butanol tolerance..