The application of seafood processing by-products in the food industry
Posted: 18 August 2016 | | No comments yet
It has been reported by numerous popular media that fish stocks are declining and several commercial fisheries are currently over-exploited and will collapse by the mid-century. On the other hand, as with other foods, seafood processing generates large quantities of by-products. A typical example is fish filleting to recover boneless and skinless marketable fillets. The frames, heads, and viscera would be typical by-products and the fillets would be considered the main product. When fish are processed for fillets, the by-products account for 60–70% and fish meat and oil left on the by-products account for 20–30% and 5–15% of the whole fish weight, respectively.
These by-products are nutrient rich but are often ground and discarded without any effort for nutrient recovery. Normally these materials are used as plant fertilisers or livestock feeds, which are considered as a low value-addition. However, recovery and conversion of the by-products to human food, or specialty food, would result in a higher value-addition and this is estimated to increase fivefold within the next decade. Therefore, an appropriate recovery method of seafood processing by-products can result in added revenue for a processor, as well as reduce environmental pollution due to disposal of the processing by-products.
Fish muscle proteins can be recovered from the by-products through a relatively novel processing technology called isoelectric solubilisation/precipitation (ISP)1 . This technology relies on the isoelectric point (pI) of muscle proteins. The pI of a protein is the pH at which the net (i.e., overall) electrostatic charge of the protein is zero. This means that the pI value can affect the solubility of protein at a given pH.
There are five steps to the recovery of proteins and lipids from seafood processing by-products using ISP. In the first step, by-products are homogenised in water at a ratio of 1:6 (by-products: water, wt/wt). The aim is to provide a reaction medium and increase the available surface area for the subsequent protein solubilisation reaction.
In the second step the muscle protein is dissolved by the addition of an acid or base to the solution. This Incorporation of acid or base, moves the pH away from the pI – therefore, fish muscle proteins assume a more negative or positive surface charge for alkaline or acidic conditions, respectively. This leads to protein– protein electrostatic repulsion, which weakens the protein-protein hydrophobic interactions with a simultaneous increase of protein–water electrostatic interactions. Ultimately, these changes lead to protein solubility in water.