Process characteristics and quality evaluation of fish protein hydrolysates

Abstract

Fish protein hydrolysates (FPH) were prepared from raw silver hake (Merluccius billinearis), raw herring (Clupea harengus), ethanol extracted herring mince and from herring presscake using Alcalase and papain enzyroes. The degree of hydrolysis of the hydrolysates was determined as influenced by species, method of fat extraction before hydrolysis and the type of enzyroe used for hydrolysis. The yields, physico-chemical, microbial and sensory properties of hydrolysates were determined as well as the molecular weight distribution by gel filtration chromatography. Samples of spray-dried hydrolysates were stored for 3 months at room temperature and their storage stability evaluated in terms of colour, non-enzyroatic browning, Thiobarbituric acid (TBA) value and Free fatty acid content (FFA). The degree of hydrolysis of fish protein hydrolysate was not influenced by species. Samples that had no fat extraction before hydrolysis were hydrolysed to higher degrees than samples that had been solvent-extracted or cooked and pressed in that order. Alcalase enzyme hydrolysed samples to a higher degree than papain. Yields of hydrolysates were higher in hydrolysates prepared from raw fish than in samples with fat removed before hydrolysis. Proximate composition, TBA, FFA and microbial counts were within limits observed in Iiterature for fish protein hydrolysates. Higher ash contents were observed in Alcalase hydrolysed samples than papain hydrolysed samples. Microbial counts were significantly higher (p>0.05) in papain hydrolysed samples than in Alcalase hydrolysed samples. The solubility of fish protein hydrolysates as determined by the nitrogen solubility index (NSI%) was higher in samples with lower fat contents. An evaluation of the intensity of bitterness and fishy odour in hydrolysates showed that Alcalase hydrolysed samples were less bitter than papain hydrolysed samples and fishy odours were significantly influenced by the levels of residual lipids in the hydrolysates. Ethanol extracted herring hydrolysates were evaluated as having no fishy odour as well as having only slight levels of bitterness. Fish protein hydrolysates showed only slight increases in lipid oxidation over a three-month storage period (less than 35% by Free fatty acid determination and less than 20% by Thiobarbituric acid determination). The properties of colour and non-enzymatic browning indicated increases resulting in darkening and may have been influenced by the slight degradation of lipid occurring during storage