Enzymatic processing of mussel shells to produce biorenewable calcium carbonate in seawater

Abstract

Mussels are a high protein food grown using aquacultural methods around the world. By-product streams from this industry include raw discards (small or damaged product) and cooked shells that retain their posterior adductor muscle. Mollusc shells are primarily calcium carbonate and for blue mussels, Mytilus edulis, the shell accounts for up to 70% of their mass. The shell by-product streams could be used in a range of applications but shell storage without protein removal is problematic. In this study, we studied the potential of two enzymes (Multifect PR 6L and PR 7L) to remove protein from mussel shells using an initial screening and Design of Experiments approach. The enzymatic cleaning experiments were monitored using either thermogravimetric analysis or via manual recovery and weighing of protein residue still on shells. 20 g mussel shells with raw protein attached were efficiently cleaned using 1.0–2.0 μL g⁻¹ Multifect PR 6L in seawater or tap water for 4 h at 55.0 °C. 20 g mussel shells with cooked protein attached were efficiently cleaned using 6.0 μL g⁻¹ Multifect PR 7L in seawater or synthetic seawater for 10 h at 25.0 °C. As the developed methods do not require any additives (i.e. no need for pH adjustment), scale up to integrate into existing seafood processing facilities will be relatively straightforward and would provide two product streams: a biorenewable calcium carbonate and protein hydrolysate. Initial trials using freeze-dried mussel protein hydrolysate from this work to feed zebrafish (Danio rerio) over the course of 18 months have shown success.