Valorisation of Fish Bones: Effects of Preservation by Formic Acid on AL-extractable calcium, Phosphorus, Magnesium, and Potassium

Fishbones contain significant amounts of plant nutrients. Fish residues may be preserved by acidification to pH < 4, which may affect the extractability and plant availability of nutrients when applied as fertilisers. Grinded bone material from cod ( Gadus morhua ) heads was mixed with formic acid to investigate if acidification would increase the concentrations of ammonium acetate-lactate (AL)-extractable nutrients. Two degrees of fineness of fishbones (coarse 2-4 mm; fine < 0.71 mm) were compared at pH 3.0 and 4.0 plus a water control in a laboratory study over 55 days. Samples for extraction of AL-soluble P, Ca, Mg and K were taken on day 2, 15, 34 and 55. Whereas acidification clearly increased the concentrations of AL-extractable calcium (Ca-AL) and magnesium (Mg-AL), extractable phosphorus (P-AL) was only significantly increased in finely grinded bones at pH 3. After 34 days, 6% of the total content of P was extracted by AL in fine fishbones at pH 3. Without acidification about 1% of the P was extracted, possibly from phospholipids. This P-AL concentration was well above P-AL extracted from acidified coarse fishbones and from fine fishbones acidified to pH 4. With acidification, about 30% of total Ca and 10% of total Mg were extracted by AL, and the Ca-AL and Mg-AL concentrations were closely correlated. The reason for lower P-AL in coarse fishbones at pH 3 and 4, and fine fishbones at pH 4 than in water controls may be a precipitation of apatite from phospholipids and dissolved calcium.