Abstract
The collection of eggs produced by fish is referred to as fish roe and is a low-value by-product generated from fish processing (3-30%) depending on fish species and countries of origins. Fish roe by-products are good source of highly beneficial components such as protein, vitamins, minerals, and contains substantial amounts of omega-3 and omega-6 long chain polyunsaturated fatty acids (LC-PUFAs), such as eicosapentaenoic acid (EPA, C20:5n3), docosahexaenoic acid (DHA, C22:6n3), and arachidonic acid (ARA, C20:4n6). The use of fresh fish roe is limited in Western countries, but processed fish roe is a prized commodity in many countries such as in Asia (e.g., Japan, Korea, and Taiwan) and other Mediterranean and European countries (e.g., Egypt , Greece, Italy, Spain, and Norway).
Chinook salmon (Oncorhynchus tshawytscha), also known as king salmon, is one of the most important fish species farmed commercially in New Zealand, but salmon roe is an unattractive by-product. This present study aimed to evaluate different processing methods for salmon roe. Salting, drying, fermentation, or combinations of these processes, are potentially useful methods for processing salmon roe to add value and enhance consumer interest, while retaining nutritional value. However, little information is available in relation to nutritional changes occurring in salmon roes during processing.
Salted-drying and salted-fermentation of the salmon roe was conducted at 4oC and 20oC over 20 and 30 days, respectively. Samples for analysis of the fresh roe were retained at day 0, and for the salt-dried and salt-fermented processing, samples were collected at 4 day- and 6 day intervals, respectively. Physicochemical and biochemical changes (colour parameters, proximate composition, salt content, pH, acidity, water activity, fatty acid profiles, vitamin E and carotenoid contents), and changes in microbial numbers (total bacterial count and lactic acid bacteria), that occurred in the salmon roe over the salted-drying and salted-fermentation period were determined. Separation of lipid classes was carried out by thin layer chromatography (TLC) analysis. The determination of cholesterol content and fatty acids of the roe samples were performed using gas chromatography with flame ionization detector (GC-FID), with prior total lipid extraction carried out following the Bligh and Dyer method. 13C and 1H nuclear magnetic resonance (NMR) spectroscopy were used to investigate positional distribution of polyunsaturated fatty acids. Carotenoids and tocopherols were determined by reverse phase (RP)-HPLC. A metagenomics approach using 16S rRNA gene sequencing method was also conducted for the detection, differentiation, and identification of microbes that might be involved during salted-drying and fermentation.
The physicochemical, biochemical and microbiological properties were evaluated in Chinook salmon roe over 20 days of salted-drying processing. It was found that moisture, colour parameters (L*, a*, and b*), water activity (aw) and the pH value were decreased at the end of the salted-drying processing. Total bacterial and lactic acid bacteria (LAB) counts (log CFU/g) varied over the 20 days of processing. Saturated fatty acids, such as palmitic (C16:0) and stearic (C18:0), and unsaturated fatty acids such as C16:1n9, C18:1n7, and C20: 4n6, were significantly decreased (p < 0.05), whereas C18: 3n6, and C22:5n3 contents were increased (p < 0.05) by salted-drying time. No significant (p > 0.05) differences in the EPA and DHA contents were observed due to the salted-drying process. Monoacylglycerol (MAG), diacylglycerol (DAG), triacylglycerol (TAG), cholesterol (CHOL), and steryl ester (SE) lipid classes were separated using TLC technique and hexane-diethyl ether-acetic acid (80:20:0.5, v/v/v) as a development solvent. There were no free fatty acid (FFA) present in either fresh or processed samples. Cholesterol and tocopherol contents were reduced after salted-drying. In terms of carotenoids, the lutein contents increased (p < 0.05) after salted-drying, while other carotenoids such as fucoxanthin, violaxanthin, astaxanthin, zeaxanthin, and canthaxanthin contents did not change (p > 0.05) after salted-drying process.
Fermentation affected the physicochemical, biochemical, and microbiological compositions of Chinook salmon roe. While the water activity (aw) and pH value were decreased at the end of the fermentation process, while the total bacterial and LAB counts (log CFU/g) fluctuated over the processing period. There were FFA present in the fermented roe lipid samples. The SFA content decreased (p < 0.05), MUFA content did not change (p > 0.05), whereas the PUFA content was significantly increased (p < 0.05) by the increase in fermentation time. Cholesterol and tocopherol contents were decreased (p < 0.05) as a result of fermentation. Carotenoids such as fucoxanthin, violaxanthin, zeaxanthin, canthaxanthin, α- and β-cryptoxanthin, echinenone, and α-carotene were increased after salted-fermentation process, but astaxanthin and lutein were decreased.
Lecithin, phospholipids (PLs), and neutral lipid (NL) yield were increased with fermentation time. The phospholipid classes that increased with fermentation time were; phosphatidylcholine (PC), lysophosphatidylcholines (LPC), phosphatidylethanolamine (PE), phosphatidylinositol (PI), phosphatidylserine (PS), and lysophosphatidylethanolamine (LPE). Among the dominant phospholipid classes, only PC, PE, and LPC had higher EPA and DHA contents due to fermentation. Further, fermentation resulted in a PUFA/SFA ratio > 0.45, which is recommended value to assess the nutritional quality of the lipid fraction of foods.
DHA content did not appear to change because of salted-drying and fermentation as demonstrated by proton NMR analysis. In addition, carbon NMR analysis revealed that DHA was mainly esterified at the sn-2 position of triacylglycerol (TAG), while EPA and stearidonic acid (SDA) were found mainly at the sn-1,3 position.
Microflora taxonomy demonstrated by 16S rRNA gene sequencing identified nine phyla both in fresh and processed salmon roe. The dominant relative abundance of phyla was the nanoarchaeaeota (20 – 81%), followed by the archaea (15 – 76%), and the woesearchaeia (0.2 – 3.2%). In salt-dried roe product, the highest abundance of nanoarchaeaeota was recorded in sample SD4_S3 (80.90%), while in salt-fermented product was found in sample SF24_S1 (70.12%). The phylum nanoarchaeaeota is recognized present in hyper environments such as at a low temperature, in an acidic condition or in a high salt concentration.
Overall, this study has highlighted the potential for converting salmon roe into products using salted-drying and salted-fermentation processing that have potential for achieving beneficial effects on the nutritional and functional characteristics of the processed product. Salmon roe and its products have attributes that position it well as an alternative to delicacy food for consumers.