Studies of enzymatic processes important for ripening of herring
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This thesis presents a study of lipolytic and proteolytic enzymatic processes during rapid-ripening of herring filets. The aim of this study is to investigate the enzymatic activity in such a ripening process, and how the enzymes affect the biochemical parameters through such a process. In particular, proteolytic enzyme activity, its effect on the changes in amino acid profiles depending on different experimental parameters and the lipolytic enzyme activity will be analyzed. This is done by setting up a herring ripening experiment with samples taken in regular intervals through a time series, and investigating how the enzymatic activity profiles, both specific and general, change over time. In addition, to investigate the effect of proteolytic enzyme activity the free amino acid composition is analyzed throughout the time series. Herring caught in the Trondheim Fjord were gutted and cut into filets before being added to a sugar-salt brine with low salt concentration. To enhance the ripening process, ground pyloric caeca isolated from the herring intestines were added to the brine. Samples of this ripening recipe, as well as samples prepared without pyloric caeca, prefrozen filets, and filets with the skin still intact respectively, were taken every day for 5 days and analyzed. Proteolytic and lipolytic activity is studied in a series of ripening samples, and changes in enzymatic activity, free amino acid content and biochemical parameters such as water content and ash based upon different ripening recipe parameters are presented. General proteolytic activity is analyzed and related to changes in free amino acid content. Activity of the proteolytic enzymes chymotrypsin, trypsin, elastase and cathepsin B in the muscle extract samples are presented. The thesis show how the enzymatic processes in herring filets change with different biochemical parameters. It studies the changes in proteolytic and lipolytic activity over the ripening time period, and how changes in proteolytic activity affects the free amino acid content based upon differences in ripening recipes. All muscle extract samples showed an increase in water content and ash, with the prefrozen samples increasing the most and the filets with skin the least in water content. General proteolytic and lipolytic activity profiles in the muscle extract were similar in all samples, while the brine samples showed a significantly lowered proteolytic activity when pyloric caeca was not added to the ripening brine. For specific enzyme activity measurements, adding pyloric caeca to the brine generally increased activity in the muscle extract samples, but in a varying degree. Trypsin showed almost no activity without added pyloric caeca. Chymotrypsin and cathepsin B activity remained at a stable low level without added pyloric caeca, while the other samples showed varying and generally higher enzymatic activity. Activity comparisons showed chymotrypsin activity to be 10 times higher than trypsin, the second highest. HPLC measurements of 17 amino acids showed very high contents of free histidine, around a third of the total free amino acid content, and high contents of glutamic acid, glycine/arginine, lysine and alanine, making up another third of the total. Total free amino acid content remained fairly stable in muscle extract samples, while brine samples showed a generally increasing total content from start to end of the ripening time series.