Effect of water activity on invertase production in solid state fermentation by improved diploid strains of Aspergillus niger

Invertase production under solid state fermentation (SSF) was determined using two overproducing mutants (Aw96-3 and Aw96-4) isolated previously from the wild type strain Aspergillus niger C28B25, as well as one diploid (DAR1) and two autodiploid strains (AD96-3 and AD96-4) constructed by parasexual crossings among these mutants. Using polyurethane foam (PUF) as an inert carrier, two initial water activity (Aw) values were evaluated (0.99 and 0.96). At Aw=0.99, maximal activity was reached by diploid AD96-4 (48.91 IU/ml) representing 30- and 13-fold increases with respect to maximal values achieved by the wild type and the haploid parental mutant (Aw96-4), respectively. Similar levels were achieved by this strain at Aw=0.96. However, diploid DAR1 only produced high levels of invertase at Aw=0.96 (43.90 IU/ml), whereas strain AD96-3 reached its highest production (31.10 IU/ml) at Aw=0.99. Both productivity and yields were also analysed for every strain at each Aw value.