Production of Organic Acids by Fungi

Abstract

Fungi, in particular aspergilli are well known for their potential to overproduce a variety of organic acids. These micro-organisms have an intrinsic ability to accumulate organic acids and it is generally assumed that this ability provides the fungi with an ecological advantage since they grow rather well at pH 3–5, while some species even tolerate pH values as low as 1.5. Organic acid production can be stimulated and in a number of cases conditions have been found that result in almost quantitative conversion of carbon substrate into acid. This is exploited in large-scale production of a number of organic acids, e.g. citric, gluconic and itaconic acid. Table 1 lists the most important organic acids for which a production process employing fungi has been described. In this chapter we will discuss different aspects of organic acid production, including biochemistry, production and applications. Since citric acid is by far the most important organic acid, in production volume as well as in knowledge available, emphasis will be on production of citric acid by Aspergillus niger. Production of itaconic acid, gluconic acid, and other acids for which fungal production processes have been described will be discussed in less detail. Finally, the reader is referred to a number of excellent reviews that have been written on organic acid production by fungi (Kubicek and Rohr 1986; Mattey 1992; Rohr et al. 1992, 1996a-c; Zidwick 1992; Kristiansen et al. 1999).