Freeze-dried salmon could be the latest thing among health-conscious astronauts and military types and mountain climbers, thanks to work at the University of Alaska Fairbanks (UAF).
Researchers have devised a process that removes around 97 per cent of the moisture from salmon while preserving qualities such as taste and colour. The product is also quick to hydrate and can be stored at room temperature.
“Conventional drying takes temperature and time. During the process the water in the muscle changes from liquid to vapour and that causes some of the cellular structure to collapse,” Alexandra Oliveira told FoodProductionDaily. “In freeze-drying the water passes from solid to gas without ever passing through liquid and that preserves the structure.”
Scientists at UAF have been working on the project together with the US Agricultural Research Service’s (ARS) Subarctic Agricultural Research Unit (SARU). So far the work has focused on high-quality salmon for high-value applications, in which people are looking for light-weight products packing a big nutritional punch.
“We’ve been working with high quality fish to produce a product with very high levels of omega-3 fatty acids – mainly DHA and EPA,” explained Oliveira. “It’s a high-quality product that can be stored at room temperature in modified atmosphere packaging to prevent oxidation of the lipids.”
However, one reason the ARS is so interested in the work is as a potential outlet for so-called “watermarked” salmon, which yield less desirable meat as they approach the spawning season.
“The fish approach their home river to spawn and as they come into fresh water they undergo a number of morphological changes,” explains Oliveira. “They also reduce their feeding and start consuming their energy reservoirs, which in salmon are mainly lipids stored in the muscle.”
The lipid-soluble pigments in the flesh migrate to the skin – hence the watermarking - and the water content of the muscle rises, leading to undesirable changes in the colour and texture of the meat. ARS hopes that a freeze-dried product might be high-value way of using this meat, which is otherwise treated as a low-value by-product.
Oliveira stressed that more work is needed to see whether their approach would be useful with watermarked fish: “None of the work so far has been with watermarked fish. The pale meat would have to be closely investigated for texture and variability.”
Oliveira and her colleagues managed to create a drying regime that took only nine hours, compared to 20 hours or more. To reduce the time and remove moisture from the salmon cubes at a faster rate, the temperature during the freeze-drying process was raised from –40˚C to 0˚C.
The team worked with three species of salmon - pink, sockeye, and chum. The novel two-stage freeze drying process forms cubes with less than 10 per cent moisture that shrank by only 12 per cent. It’s also quick to rehydrate, making it ideal for soup mixes.