Ensiling in organic acids (e.g. phosphoric, formic, lactic or acetic acids) is an effective way of treating the carcasses of diseased fish. For example, the European Union requires fish showing clinical signs of infectious salmon anaemia to be ensiled in formic acid before rendering (Stagg 1999); ensiling is considered best practice for reducing the risk of spread of this disease from this material (SEPA 2007). Another method, sometimes used in tropical climates, involves the addition of simple sugars, such as molasses, and a lactic acid bacterial culture, which generates lactic acid through the natural breakdown of the sugar (Gill 2000).
The fish must be thoroughly macerated before loading into the ensiling plant, to ensure rapid contact with the acid. Formic acid (15 L of 85% formic acid per 500 kg of fish material) is added to the macerated fish to produce a pH of <3.9. The pH needs to be maintained below 3.9 for a minimum of 24 hours (FRS Marine Laboratory 2000). The suspension must be stored in appropriate leakproof, acid-proof containers and must be agitated regularly to aid liquefaction (Lo et al. 1993). The use of concentrated acids requires the greatest care and should only be done by experienced staff.
Ensiled material will require further heat treatment at a minimum of 60 °C for 2 hours to kill residual pathogens. Heating of Aeromonas salmonicida, Mycobacterium chelonei and Renibacterium salmoninarum in the presence of acid affected each bacterial species differently (Gill 2000):
- A. salmonicida was less heat resistant at pH 4.0 (acid) than at a neutral pH.
- M. chelonei was slightly more heat resistant (survived for slightly longer) at acid pH than at a neutral pH.
- R. salmoninarum was significantly less heat resistant at acid pH; it survived for more than 3 hours at 55 °C in buffer at pH 7, but was destroyed within 1 minute in fish silage heated to 55 °C.
Smail et al. (1993a) detected infectious pancreatic necrosis (IPN) virus in the faeces of cattle fed a diet containing fish silage made from fish infected with the virus. Neither the silage process (at pH 3.8–4.0) nor the acidic conditions within the bovine digestive system (pH 1.1–1.3) was capable of destroying the IPN virus. Salmonella Typhimurium is also able to survive the ensiling process (Salte & Hellemann 1982).
Heat-treated silage can be used commercially (e.g. as fertiliser) or, if appropriate, can be mixed with molasses and crop residues to be used as feed for terrestrial animals (Ayangbile et al. 1997; Samuels et al. 1991). The fish oils can be separated and used in manufacture of other products such as biodiesel. Alternatively, whole silage can be used as biogas feedstock or disposed of in accordance with local regulations and by-laws (Scottish Government 2005).
Ensiling has been successfully used with prawn heads and hulls (Fagbenro & Bello-Olusoji 1997) and would also be applicable to whole prawns. The prawn carcasses must be finely ground to ensure complete solubilisation. Ensiling may be less effective for crustaceans with hard shells, such as crabs and lobsters, although the low pH will eventually dissolve shells. Crabs have been used to produce silage for terrestrial animal feed, although the feed had low digestibility and marginal economic benefits (Samuels et al. 1991). Ensiling is not considered an appropriate method of disposal for molluscs because their thick shells tend to resist chemical breakdown.
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