Due to the global issues related to climate change and population growth, more sustainable use of existing food resources is required. Fish and other aquatic organisms are rich in health beneficial polyunsaturated fatty acids, protein, and several vitamins and minerals. Fish and other aquatic resources are estimated to play an even more vital role in the future. While many popular fish stocks have already been overexploited, several species are currently not utilized at all or are directed to low-value, non-human uses, such as feed.
One example of such species is Baltic herring (Clupea harengus membras). Baltic herring is the most important commercial capture species in Finland, based on both value and quantity. However, only a small fraction of the total catch is used as food. Some factors limiting the food use of Baltic herring include the decreasing size of the caught fish, its high susceptibility to oxidation and other degradation leading to quality loss, as well as its distinct odor and flavor that are not preferred by some consumers. Roach (Rutilus rutilus) along with other cyprinid fish, are not commercially important fish species, and are utilized for human consumption even less than Baltic herring. In addition to providing a valuable source of proteins, lipids and other nutriens, increasing the capture of roach and other cyprinid fish could result in beneficial effects in reducing eutrophication.
This thesis focused on alternative ways of increasing utilization of underutilized fish, with a focus on Baltic herring and roach. The two processing methods studied were the pH shift and enzymatic hydrolysis, both of which have been widely studied for other fish species but not Baltic herring or roach. Both processes have the advantage that whole fish or fish side streams without any pre-processing can be used as raw materials. Further, the addition of natural antioxidants was investigated as a means of inhibiting lipid oxidation and degradation of sensory quality during frozen and refrigerated storage of minced Baltic herring. Lingonberry juice press residue, sea buckthorn juice press residue, sea buckthorn juice press residue after supercritical CO2 extraction of lipids, and a commercial extract mixture as natural additions were compared to conventional antioxidants ethylenediaminetetraacetic acid (EDTA) and combination of α-tocopherol and ascorbic acid.
The composition of proteins and lipids in Baltic herring and roach protein isolates and hydrolysates was significantly affected by the processing type (pH shift or enzymatic hydrolysis). Compared to enzymatic hydrolysis, the pH shift led to enrichment of essential amino acids, phospholipids, and in case of acid extraction, also polyunsaturated fatty acids such as docosahexaenoic acid (DHA). The pH shift, especially alkaline pH shift, however, induced considerable lipid and protein oxidation. The fish protein hydrolysates (prepared using enzymatic hydrolysis) showed more moderate formation of hydroperoxides, secondary volatile oxidation products, and protein carbonyls. The incomplete elimination of fishy odors and flavors during preparation of fish protein isolates and hydrolysates is a major factor limiting their use. Based on this research, while not completely eliminating these odors, the alkaline pH shift reduced the total intensity of odor and intensity of the fishy odor in Baltic herring, which was reflected in the quantity and quality of odor-active compounds.
The natural antioxidants, lingonberry juice press residue (3%), sea buckthorn juice press residue (3% w/w), and commercial extract mixture (0.1%) retarded lipid oxidation in Baltic herring mince stored at –20 °C more effectively than EDTA or combination of α-tocopherol and ascorbic acid, as indicated by lower loss of polyunsaturated fatty acids, lower PV, and/or lower formation of secondary oxidation related volatiles, such as 1-penten-3-ol and hexanal. During refrigerated storage (1 °C), sea buckthorn juice press residue after supercritical CO2 oil extraction and lingonberry juice press residue were effective antioxidants also at the lower concentrations (1.5%, 1%). Based on the odor and flavor profile of Baltic herring minces, particularly sea buckthorn juice press residue after supercritical oil extraction (1.5%), also prevented changes in sensory quality during a 3-day storage. This was further reflected in the odoractive compounds of raw minces. Berry press residues are side streams of berry juice and oil production, and their use as antioxidantive materials would provide added value for these currently under-utilized materials as well.
This thesis provided valuable insights related to increasing utilization of Baltic herring and other under-utilized fish. The results showed that pH-shift processing and enzymatic hydrolysis of Baltic herring and roach produced fish protein isolates and hydrolysates with high nutritional quality. However, lipid and protein oxidation during these processes and storage of Baltic herring poses a challenge for preservation of quality. Use of antioxidants may improve preservation of the nutritional and sensory quality of Baltic herring during processing and storage
