Protein isolation from herring (Clupea harengus) using the pH-shift process - Protein yield, protein isolate quality and removal of food contaminants

Doctoral thesis, 2012

Herring (Clupea harengus) contain valuable proteins but is difficult to process into high-quality foods due to its small size and high content of bones, heme-proteins and lipids. Herring is among the most abundant fish species in the world, but is currently utilized largely for fish meal and oil production. The work presented in this thesis has aimed at evaluating pH-shift processing as a method to isolate proteins from herring and thereby increase its potential as a food raw material. The pH-shift process solubilizes muscle proteins at low or high pH (pH ≤3 or ≥10.8) whereafter impurities can be removed and the solubilized purified proteins are precipitated near the isoelectric point (~pH 5.5). The focus has been the yield and quality of the proteins. Specific aims have been to investigate: i) possible differences between the acid and alkaline version of the pH-shift process, ii) the possibility to remove dioxins and PCBs, and iii) the effect of alkaline pH-shift processing on the microstructure, salt solubility and in vitro digestibility of the proteins. The acid and alkaline versions of the pH-shift process performed similarly when applied to gutted herring, with protein yields of 57-59%. The protein isolates had significantly higher protein concentration and less ash and lipids than the gutted herring, and also a significantly improved color and a well-balanced amino acid profile. The two process versions isolated proteins with similar ability to form a gel, but the acid process version induced proteolysis. Furthermore, the pH-shift process was highly efficient at removing dioxins and PCBs from contaminated Baltic herring, which was correlated to the removal of lipids. The microstructure analyses of the alkali-processed herring proteins revealed a loose protein network, with no remaining myofibrillar structure. The salt solubility of the proteins was significantly decreased after processing, and this was mainly due to exposure to low pH (5.5) during precipitation of the proteins. Precipitation at pH 6.5 was therefore evaluated and resulted in higher protein salt solubility, less lipid oxidation and higher gelation ability of the proteins compared to precipitation at pH 5.5. Despite the changes in protein salt solubility and microstructure, the in vitro digestibility of the alkali-processed proteins precipitated at pH 5.5 remained the same as that of the herring raw material. To conclude, pH-shift processing is a promising tool to isolate proteins from herring and other small pelagic fish species, resulting in high protein yield and an isolate with good gelation capacity, nutritional characteristics, and low content of lipophilic contaminants. Protein isolation using the pH-shift process therefore has the potential to enhance the value of small pelagic fish species and increase their use for human consumption.