Turid Rustad, Professor at NTNU sheds light on innovative methods to preserve pelagic fish species
Pelagic fish species, such as anchovies, sardines, mackerel, tuna, shad and menhaden are all valuable as food sources. They contain both valuable and easily digestible proteins, have a high content of essential fatty acids, such as DHA and EPA, and contain fat-soluble vitamins (E and D). However, this also makes the pelagic raw material highly susceptible to degradation (quality loss) during storage. Since the fisheries for many of the pelagic species are seasonal, this leads to a need for efficient preservation methods. The composition and stability of pelagic fish are also varying with both the seasons and fishing ground. This is also challenging with regard to finding efficient preservation methods.
In the project ProHealth (Innovative processing to preserve positive health effects in pelagic fish products), a project under the Joint programming initiative, a Healthy diet for a healthy life (JPIHDHL) has focused on developing new and improved methods for extending the shelf life of pelagic fish. The aim of the project is to develop a comprehensive toolbox of optimised existing and novel technologies for developing healthy, high quality, safe and sustainable fish products from pelagic fish species. The project has partners from Norway (NTNU and SINTEF), Ireland (Teagasc), Poland (NMFRI) and Italy (University of Perugia).
One of the most efficient methods for long-term preservation of pelagic fish is freezing and frozen storage. Freezing is one of the best and most used methods for storing and reserving muscle food for long periods of time. Freezing and frozen storage of mackerel have been shown to preserve the lipid quality while proteins are oxidised after long time storage. This could lead to reduced raw material quality, such as reduced water holding capacity.
Fresh mackerel has a short shelf life as a chilled product. Fresh fish is usually transported and sold on flaked ice, resulting in a temperature of close to 0°C. Cooling and refrigeration will increase shelf life by slowing down both microbial growth and enzymatic reactions, but it will not kill microorganisms or stop enzymatic reactions. It is important to start cooling as soon as possible after catch/harvest. In addition to chilling on ice flakes, chilled seawater (a mixture of ice and seawater) or mechanically refrigerated seawater is used to chill fish onboard fishing vessels. This has been shown to be more efficient in preserving fish than the use of ice.
Studies on sardine have shown reduced oxidation and lower development of trimethylamine nitrogen (microbial development) (Sampels, 2015). Slurry ice (or chilling on ice) can also be combined with the addition of natural antioxidants, ozone or organic acids.
Superchilling or deep chilling is defined as chilling the products to a temperature close to or below the initial freezing point. For fish, this is usually between -0.5 to -2.5 °C. Superchilling can inhibit most autolytic and microbial changes in fish compared to normal chilling.
Partial freezing of the water in the fish will lead to increased concentrations of solutes in the unfrozen phase. This may again result in increased enzymatic activity and protein denaturation. In the project Prohealth, it has been shown that superchilling will prolong the microbial shelf life of mackerel – however, some challenges related to protein denaturation needs to be solved by optimising the process and the storage conditions.
Today, consumers want healthy food products that are safe to eat, have a high nutritional value and also a very high sensory quality, are fresh-like and are free of or have a reduced content of chemical additives. This has led to an increased interest in new non-thermal food processing and preservation methods. These include high- pressure processing (HPP), ultrasound (US), pulsed electric field (PEF) pulsed light (PL), cold plasma (CP) and ozone.
One of the most promising of these methods is high-pressure processing (Zhao et al., 2018). This is a process where the food or the raw material is treated with pressures from 200 to 800 MPa at temperatures between 5°C and 35°C. HPP leads to inactivation of microbes because of changes to the cell membrane, denaturation of proteins. This includes the inactivation of enzymes and changes to the structure of the cell. HPP will lead to inactivation of living bacteria while spores are highly resistant. HPP is usually done on packaged products and this reduces the probability of recontamination of the products.
The use of HPP has been studied on several fish species, including herring and mackerel. Results as reviewed by Zhao et al. show that HPP can significantly extend the microbial shelf life of herring and mackerel using pressures from 200 to 450 MPa. Higher pressures lead to a longer extension of shelf life, but it also to larger changes in the sensory quality of the fish. HPP leads to some lipid oxidation and colour changes. Work in ProHealth has shown that HPP can be used for pretreatment of smoked mackerel to extend its shelf life. The use of HPP can also be used for stabilisation of fish mince, but pressures that are too high lead to a reduction of the gelling properties of mince.
Cold plasma is another method that has been investigated as a preservation method for fish. Plasma is defined as the fourth state of matter in addition to solid, liquid and gas (Zhao et al.; 2018). Cold plasma was found to reduce the number of spoilage bacteria on mackerel while the pH and colour remained stable. However, the treatment leads to increased lipid oxidation. The treatment should, therefore, be combined with the use of antioxidants.
Several of the novel preservation methods are promising – but a better understanding of the mechanisms behind these methods is needed. It is also very important to remember that a low temperature in the value chain is essential.
Zhao, Y-M., de Alba, M. ; Sun, D-W and Tiwari, B. 2018. Principles and recent applications of novel non-thermal processing methods for the fish industry – a review. Crit. Rev. Food Sci. and Technol. https://doi.org/10.1080/10408398.2018.1495613
Sampels, S. 2015. The effects of storage and preservation technologies on the quality of fish products: A review. J. Food Proc. Pres. 39:1206-1215.
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