Geosmin depuration from fish

Off-flavour described as ‘earthy-musty’ is commonly reported in fish raised in land-based aquaculture systems. Bioconcentration in fish of lipophilic geosmin produced by microbiota is regarded as the most important cause for this off-flavour. Despite that off-flavour in fish is an extensively studied and well documented problem, satisfactory solutions are lacking. The most promising solution on a short notice is the optimization of geosmin removal from fish prior to harvest, i.e., the off-flavour depuration process. This depuration process however is not always effective nor reliable. Hence, further optimization is needed to prevent market entrance of off-flavoured fish. The general objective of this thesis was therefore to improve off-flavour depuration processes, with a focus on geosmin excretion. The experimental work conducted in this thesis leads to the following conclusions regarding the bioconcentration of geosmin in fish:

• Rainbow trout bioconcentrates waterborne geosmin, but in vivo bioconcentration is less than the general fish bioconcentration model for moderately lipophilic compounds predicts based on theoretical rate constants.
• Geosmin distribution within the body of rainbow trout and Nile tilapia is not exclusively governed by the lipid content of tissues/organs.
• Geosmin depuration from European eel is not affected by the water renewal rate of depuration tanks.
• Geosmin depuration from Atlantic salmon is enhanced by increased water renewal rate of depuration tanks.
• Geosmin elimination from European eel does not follow the generally accepted passive diffusion mechanism for excretion of lipophilic chemicals. We assume that geosmin biotransformation by the eel is probable.
• Exercise (swimming) enhances geosmin excretion by European eel and reduces the time required to depurate off-flavours from fish.
• Fed Nile tilapia eliminate geosmin faster from their ovaries compared to starved fish.
• The rate of geosmin elimination from muscle tissue and ovary is similar in Nile tilapia.

This thesis shows that there is more to geosmin bioconcentration in fish than passive partitioning over water and lipid compartments. The physiology underlying the bioconcentration process remains to be fully elucidated. Yet this thesis presents various measures for the optimization of off-flavour depuration processes that can be readily adopted by the aquaculture industry.