Alkanes, sugar substrates

One of the more recent innovative approaches was to look for new micro-organisms and novel carbohydrate substrates. The early fermentations used sugar beet or cane molasses, various syrups, sweet potato starch or glucose itself and the micro-organism was always an Aspergillus spp. In the early 1930 s it was found that yeasts would produce dtric add from acetate. Since then a variety of yeasts, prindpally Candida spp., has been shown to convert glucose, w-alkanes or ethanol to dtric add with great effidency. 

Citric acid can be produced in high productivity and high yield by fermentation of simple sugars mainly by the mycelial fungus, Aspergillus niger, although some processes use the yeast Yarrowia lipolytica (Candida lipolytica or Saccharomycopsis lipolytica) with n-alkane as the substrate. 

Yeast and bacteria can produce biosurfactants, biological surfactants from various substrates including sugars, oils, alkanes and wastes [5]. Some types of biosurfactants are glycolipids, lipopeptides, phospholipids, fatty acids, neutral lipids, polymeric and particulate compounds [6]. Most are either anionic or neutral, while only a few with amine groups are cationic. The hydrophobic part of the molecule is based on long-chain fatty acids, hydroxy fatty acids or a-alkyl-jS-hydroxy fatty acids. The hydrophilic portion can be a carbohydrate, amino acid, cyclic peptide, phosphate, carboxylic acid or alcohol. 

Copolymers containing hydroxyalkanoic acids with a chain length ranging from 3 to 14 carbon atoms have been produced from various carbon substrates (such as sugars, alkanoic acids, alcohols, and alkanes) by a variety of bacteria over 70 strains (Steinbiichel 1991). The PHA compositions produced by a bacterimn are dependent of the substrate specificities of enzymes in the PHA biosynthetic pathway. 

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