Technological problem: off flavour in aquaculture products
Off flavour is the presence of undesired sensory properties in food items. Most common in aquaculture products are earthy-musty off flavours caused by the presence of geosmin and 2-methyl-iso-borneol (MIB) in fish tissues. Geosmin and MIB are secondary metabolites produced and excreted by a wide range of micro-organisms common to land-based aquaculture systems. Following excretion to the water, these compounds are rapidly taken up by the fish via their gills and bio-accumulated in body fat until a dynamic equilibrium between the water and lipid fractions in the system is reached. Presently aquaculture producers utilize the reversibility of this process to depurate off flavours from fish by placing them in water free of geosmin and MIB. This so-called off-flavour depuration process is however unreliable, partly ineffective, costly and not sustainable. New solutions that prevent the presence of off flavour causing compounds in fish crops are therefore clearly needed.
Competitive threats and economic impact of off flavour
Off flavour in farmed fish is one of the most significant economic problems for the aquaculture industry (Vallod et al., 2007, Robin et al., 2006). For the US channel catfish industry it is estimated that more than 30% of the potential revenue is lost due to off flavour (Smith et al., 2008). Various studies estimated that off flavour adds 0.04 to 0.26 US$/kg fish to the total production costs (in Tucker, 2000).
For the European aquaculture industry the economic impact of off flavour has not been systematically studied. We estimate that for the European aquaculture industry the annual loss of fish biomass due to off flavour depuration has a value of more than 8 million euro. (Table 3.2, section 3.1). In addition it is clear that off flavour seriously impinges upon profit margin and economic feasibility of individual fish producers in RAS (see section 3.1). Economic damage results from consumer rejection of off flavoured fish. Producers suffer direct losses when they are forced to withdraw fish crops from the market or are forced to sell fish crops at lower prices. Indirect losses are caused by a reduction of market volumes and prices due to low consumer appreciation of (certain) aquaculture products. In addition, removing off flavours from fish crops by depuration procedures prior to market entrance results in loss of biomass and the operational costs of depuration significantly contribute to the costs price of the farmed fish.
Rationale for this project – soundness of concept
Off flavour in farmed fish is an extensively studied and well documented problem. Despite this, satisfactory solutions for the problem are still lacking today. The well documented chain of events leading to the presence of geosmin and MIB in fish tissues (see 1.1.1) allows for different approaches to reduce off flavour incidence in farmed fish. We distinguish three main strategies:
- Preventive strategy: reducing the microbial production of geosmin and MIB in aquaculture production systems;
- Curative strategy: removing geosmin and MIB from the fish culture water;
- Alternative strategy: optimizing the depuration process.
Each of these three strategies is aimed at different parts of the route from microbial production of geosmin and MIB to their bio-accumulation in fish tissues, resulting in off flavour. Each individual strategy can ultimately result in the absence of off flavour in aquaculture crops (Fig. 1.1). We choose however not to focus on one single strategy for the following reasons:
- Targeting the entire chain of events leading to off flavour increases the chances of success in the project;
- Future commercial application of measures to prevent off flavour should rely on more than one single strategy for security reasons;
- In case single strategies are partially successful, a combination of strategies may be required to prevent off flavour;
- The distance to commercial application is different for the three strategies. Addressing all three strategies provides short term improvement and contributes to solutions on the long term.
The Preventive strategy targets the off flavour problem at its source: elimination of microbial production of geosmin and MIB. As proper functioning of aquaculture production systems relies on microbiological communities and given the wide and diverse range of microbiota capable of geosmin and MIB production, total elimination of these producers from the system is not realistic. Hence, prevention of geosmin and MIB production should be addressed via the functional physiology of microbiota. It is highly likely that microbiota synthesize relatively complex organic molecules such as geosmin and MIB for a specific reason related to specific (environmental) conditions. At present the functions of geosmin and MIB as well as the conditions leading to their synthesis are not known. Recent advances in the measurement of gene-expressions related to microbial geosmin synthesis provide an excellent tool for systematic investigations on the conditions leading to geosmin production. Once these conditions have been established, we can proceed to prevent conditions that induce geosmin and MIB production in aquaculture systems.
The Curative strategy, removal of geosmin and MIB from the water, targets these compounds after microbial production but prior to bio-accumulation in fish. Past attempts to remove geosmin and MIB from the water by chemical oxidation using ozone were unsuccessful (Schrader et al., 2010). Selective microbial degradation of geosmin and MIB however provides a new curative strategy. Cumulative advances in this field (Izaguirre et al.,1988; ; Lauderdale et al., 2004; Elhadi et al., 2006; Hoefel et al, 2006; Persson et al., 2007;; Ho et al., 2007, McDowell et al., 2007; Eaton and Sandusky, 2009) open the way to the design of biological reactors that remove geosmin and MIB from the culture water.
The Alternative strategy targets the off flavour at the end of chain of events leading to off flavoured aquaculture products: depurating geosmin and MIB from fish prior to market entrance. This procedure is based on the reversibility of the bio-accumulation process; the equilibrium between fish and water re-establishes once the fish are placed in water free of these compounds, resulting in a net flux from the fish to the water. This eliminates off flavour entirely or partly depending on, among others, depuration time and initial fish tissue concentrations. This procedure is currently the main remedy against off flavour. Results are however variable and partly unpredictable. Depuration can be improved by taking into account physiological mechanisms underlying geosmin and MIB absorption and excretion by fish. Short term improvement is possible based on existing studies on absorption and excretion of xenobiotics in fish in general. Specific studies on geosmin and MIB lead to further optimization. Optimization includes shortening of the depuration procedure as well as a higher predictability and lower variation among individual fish.