Role of protein degradation in fermentation of fish sauce

Raksakulthai, Nongnuch (1986) Role of protein degradation in fermentation of fish sauce. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (34MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

Male inshore capelin were used to prepare fish sauce, a fermented liquid product used as a condiment in South East Asia. Fermentation of mince capelin with salt (4:1 w/w) alone did not give satisfactory results in terms of extractable soluble nitrogen, free amino acid formation or sensory evaluation score. Supplementation of the salted mince with 2.5% (w/w) squid hepatopancreas (SHP) significantly increased the degree of protein hydrolysis (P<0.01), the free amino acid content of the finished product (2.1-fold) and the sensory evaluation score (P<0.05). The free amino acid content in control and SHP-supplemented sauce were 242 and 520 mM, respectively. The failure of the heat-treated SHP (100°C, 30 min) to aid the fermentation process indicated that SHP aids the fermentation of fish sauce by virtue of its enzymes. Acidification of the salted mince containing 2.5% SHP with HC1 to pH 4.5 gave a product having a lower sensory score, although the degree of protein hydrolysis significantly increased (P<0.05) and the free amino acid content increased to 1.5-fold the control (370 mM). -- The results of fermentation of salted mince at initial pH values ranging from 3-8, at ambient temperature and at 37°C and at concentration of NaCI ranging from 15-30% indicated that the conditions recommended for fermentation of capelin fish sauce were at 25% salt (w/w), at ambient temperature (20-25°C) and at natural pH of fermentation (pH 5-7). -- Enzymes associated with the viscera were found to contribute to the hydrolysis of protein during fish sauce fermentation but the sensory evaluation score of the finished product prepared from round and gutted capelin did not show any significant difference. Enzymes from the viable bacteria in salted mince were of little importance in the fermentation process as indicated by insignificant difference between protein hydrolysis of the antibiotic-treated sample and the control. When minced capelin was held at ambient temperature for 24 h prior to addition of salt, the number of total viable bacteria increased significantly as did the amount of protein hydrolysis in salted mince (P<0.05). However, delayed salting for 24 h did not significantly improve the sensory quality of the fish sauce. -- The importance of aging or ripening on quality of fish sauce was investigated by comparison of fish sauce kept at -20°C and at ambient temperature for 4-6 months after fermentation. It was found that ripening process had no significant effect on free amino acid content, color or sensory evaluation score of fish sauce although a trend toward darker color of the aged samples was resulted. -- The contribution of free amino acids and peptides to flavor of fish sauce was examined. Fish sauce was filtered through an ultrafiltration unit, M.W. cut off 10,000. The results indicated that removal of larger molecules from fish sauce lowered the acceptability (P<0.001). Gel filtration chromatography indicated that the apparent molecular size of peptides and amino acids in fish sauce ranged between 100-300 daltons. Regression analysis of preference score and free amino acid content indicated a significant correlation of (0.724 (P<0.05). -- To characterize, partially, the enzymes retained in fish sauce after 6 months fermentation, fish sauce was concentrated 3.5-fold using an ultrafiltration unit, M.W. cut off 10,000. Protease activity of the enzymes retained in fish sauce prepared with SHP was greater than that of control (3-fold on azocasein and 5-fold on hide powder azure substrate). The pH optimum of the residual enzymes in low concentration (0.2 M) of NaCl was at pH 4. At salt concentration of 1.5 M, the optimal pH shifted to pH 5.0, and at 4.0 M NaCl the maximal activity was at pH 6. Salt partially inhibited the proteolytic activity, however, the residual enzymes in SHP-supplemented sauce appeared to be more tolerant to salt than those in the control sauce. Protease activity at low salt concentration on azocasein was partially inhibited by ethylenediaminetetraacetate, iodoacetate, p-chloromercuribenzoic acid, mercuric chloride and soybean trypsin inhibitor. In the presence of 4 M NaCl, inhibitors for sulfhydryl proteases appeared to be the only effective inhibitors against hydrolysis of azocasein. Hydrolase activity of cathepsin C was less inhibited by salt than was the azocasein hydrolysis. It was apparent that cathepsin C and other sulfhydryl proteases were of importance to the fish sauce fermentation.

Actions (login required)

View Item