The researchers said the aim of the study, which was published in the journal Food Control, was to evaluate the effect the use of natural additives, superchilling, and modified atmosphere packaging (MAP) would have on shelf life of salmon, as they note that there has not been previous research focused on such a preservation method.
“There are reports on the use of MAP on fish preservation; although unification of criteria is difficult due to differences in fish species, gas mixtures and storage temperatures,” continued the authors.
Norway and Chile are the main producers of Atlantic salmon, and export to countries such as the US, Japan and Germany. The authors report that the shelf life of fresh fish is normally between seven to eleven days using traditional packaging, and the burden lies with the producer to ensure original product quality right to the final destination.
MAP works by replacing the normal atmosphere in the pack by a specified gas mix, the main components of which are oxygen, carbon dioxide and nitrogen. The modified atmosphere in the package helps prevent the growth of microbiological organisms and bio-chemical reactions and the consequent spoilage of goods.
It is reported to be a reliable and cost effective alternative to refrigeration or preservatives.
The researchers claim the success of MAP in extending seafood shelf life depends on many factors, including initial product quality, hygiene during slaughter, packing material selection, packing equipment, appropriate gas mixture and gas-to-product volume (g/p) ratio for the product:
“Maintenance of the process temperatures where the amount of CO2 dissolved into the product and the storage temperature are the most critical factors,” they said.
The authors said they undertook three different sets of experiments: the first focused on evaluating the effect of including natural additives together with superchilling and MAP. The other two were to determine the influence of the gas concentration (CO2:N2) and the gas-to-product volume (g/p) ratio on fillet preservation.
A control (exposed to air) sample was included in the three sets of experiments, they added.
The researchers said that defatted Atlantic salmon fillets (170–185 g) without skin and bones were obtained from a fish farm in Chile and were processed under strict hygienic conditions to obtain a raw material with low initial bacterial counts.
For the initial experiment, they said the fillets were submerged in a solution made from rosemary extract and one from a natural bio-active protein and then placed in plastic trays.
The superchilling process involved placing the fish into the centre of a freezing tunnel cooled by air convection, and keeping them in the tunnel for 30 minutes, in order to reach −1.5 °C in the centre of fillets: “This temperature was maintained constant until the packaging process,” added the authors.
And they explained that the fillets were placed in bio-oriented polyamide pouches with a base of three layers of polyolefins and then gas packed in a Multivac A-316 vacuum-packing machine.
The fish pouches, they added, were stored in a chilled room (2 ± 2 °C) until their analyses, with samples in triplicate taken for sensory, chemical and microbiologic analysis every six or seven days throughout the 28 day storage period.
The results showed that the combined effect of the three technologies of natural additives, MAP and superchilling did not improve the salmon shelf life, and also impacted negatively on appearance and odour of the salmon.
However, the authors found that the use of the combination of superchilling and MAP incorporating a CO2 concentration of 90 per cent and a g/p ratio of 2.5 resulted in the longest shelf-life of 22 days as compared to 11 days in a control sample.
“The lowest salmon shelf life was 16 days, and the highest 22 days, values that are representative for the samples with the lowest and highest CO2 concentration and g/p ratio, respectively,” they said.
They said that, based on the sensory analysis, the variable with the greatest influence on the shelf-life extension seems to be the g/p ratio. “The samples with lower g/p ratio reached the sensory rejection limit first; this effect could be caused by a higher reduction of the CO2 level in the pouch and the correspondent microbial growth rate increase.”
The researchers added that during the storage time, the physicochemical properties (drip loss, pH, TVN and K) of the fillets were under the acceptable limits.
Source: Food Control
Published online ahead of print: DOI: 10.1016/j.foodcont.2008.12.010
Title: Shelf-life extension on fillets of Atlantic Salmon (Salmo salar) using natural additives, superchilling and modified atmosphere packaging
Authors: K. Fernández, E. Aspe and M. Roeckel