I would like to know, is there are rule to how long the fermentation period lasts ?
I'm working with a strain of Aspergillus niger to produce an enzyme and the highest production I obtain is on day 9(Very high), But the problem is that the culture then no longer contains pellets, All the pellets become destroyed; only debris remain, I think the organism is then in its lag phase. I work in 250 ml flasks, rpm 200, temp 28
Is it wrong to wait 9 days on an enzyme that is produced from day 1?

Here a couple of Aspergillus fermentation papers that may help. The fermentation period will vary depending on many factors...temp, strain (mutant vs wild type), growth medium, enzyme of interest etc.

Citric acid fermentation by mutant strain of Aspergillus niger GCMC-7using molasses based medium
EJB Electronic Journal of Biotechnology ISSN: 0717-3458 Vol.5 No.2, Issue of August 15, 2002
http://www.scielo.cl/pdf/ejb/v5n2/a05.pdf

Studies on the Use of Agricultural Wastes for Cellulase Enzyme Production by Aspegillus niger
Research Journal of Agriculture and Biological Sciences 1(4): 325-328, 2005
http://www.insinet.net/rjabs/325-328.pdf

Morphological development of Aspergillus niger in submerged citric acid fermentation as a function of the spore inoculum level. Application of neural network and cluster analysis for characterization of mycelial morphologyMicrob Cell Fact. 2006; 5: 3.
http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1386700&blobtype=pdf

Thanks for your reply
I wish if you know any links to informative sites on fermentation or downloadable books , You let me know.

In my work, I don't try to follow the changes that happens to the strain, I don't try to see if mutations occur or not. Is that wrong?