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Combined preservation methods for shelflife extension of fresh fish. Grímur Ólafsson Icelandic fisheries laboratories. Introduction. Fresh fish is exported by air, whole or filleted, iced in styrofoam boxes. The maximum expected shelf life for whole fish is 14-17 days at 0-4°C.
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Combined preservation methods for shelflife extension of fresh fish Grímur Ólafsson Icelandic fisheries laboratories
Introduction • Fresh fish is exported by air, whole or filleted, iced in styrofoam boxes. • The maximum expected shelf life for whole fish is 14-17 days at 0-4°C. • The maximum expected shelf life for fresh fillets is 10-12 days at 0-4°C.
Active packaging • Packaging is called active if it performs some desired role other than being an inert barrier to external conditions. • Example: modified atmosphere packaging, scavengers (O2, CO2, ethylene, amides, sulphur compounds. Packages that emit C02, ethanol and other preservation compounds.
Oxygen scavengers • Compounds that absorb O2 have been in use in Japan since 1980. • Mostly metal compounds are used i.e. iron but also organic acids and enzymes • Scavengers based on iron have become marked leading and are used commercially in Japan and USA.
Vapour generators • Ethanol vapour for food packaging is produced by dissolving it in silica gel in a porous bag which allows controlled vaporisation. • Ethanol vapour generators have been used for shelf life extension of backed goods and dry and semi-dry fish products
Fish spoilage • Fish-spoilage is primarily caused by bacterial growth • Shelf life estimation is based on bacterial and sensorial evaluation • Fish exceeding 500.000 cfu/g is considered unfit for human consumption • Total count and sensory evaluation do not always agree
Specific Spoilage Bacteria (SSO) • Metods used: TVC and SSO • Pseudomonas species and Shewanella putrefaciens are examples of spoilage bact. in chilled fish • Spoilage compounds: e.g. sulphuric compounds and TMA
Modified atmosphere • Photobacterium phosphoreum • P. phosphorerum is a powerful TMA-producer and extremely tolerant to CO2 • Lactic acid bacteria becomes dominant in this type of package (Lactobacillus)
Strict anaerobes • Little is known on the growth of strict anaerobic bacteria in fish packed in MAP or vacuum packs • C. botulinum and C. perfringens
Aim of the project Two main objectives: • To study if quality and shelf life could be increased by using MAP, O2-scav. and ethanol vapour separately or combined to produce a hurdle effect • To examine the importance of strict anaerobes as fish spoilage bacteria.
Experimental procedure The experiment was divided into: • Screening experiment • Main experiment
Screening experiment Fillets of 2-3 days old cod was packed with: 1. MAP (70%CO2/30%N2) 2. O2 scavenger 3. Ethanol vapour generator 4. Oxygen scavenger + Ethanol vapour 5. MAP + O2 scavenger 6. MAP + Ethanol vapour 7. MAP +O2 scavenger + Ethanol vapour 8. K1 - airtight plastic bags 9. K2 - Chilling (ice)
Results from screening exp. • Microbial growth was slowest in MAP with or without other preservation methods. • MAP +EtOH and/or O2 scav. was not significantly better than MAP alone. • EtOH and/or O2 scav. did not give significantly longer shelf life than storage on ice.
Main experiment • Based on results from the screening tests, MAP and O2-scav. were selected for further experiments. • Fresh cod fillets (2-3 days old) were packed and stored at 0-2°C.
Main experiment Packaging: 1. MAP (70%CO2/30%N2) 2. O2 scavenger 3. MAP +O2 scavenger 4. Chilling (ice) Samples were taken at days: 0, 4, 7, 11 and 14.
Analytical procedures • Microbiological measurements: TVC and SSO (H2S producing) • Chemical: TMA-N and TVB-N • Sensory evaluation: freshness, taste and smell vas evaluated on raw and cooked fillets and graded according to the Torry scale
Results and discussion Raw material when packed • TVC: 70.000/g . • SSO: 100/g. • TMA-N: 0 mg N/100 mg
Raw material Bacterial composition: Shewanella 4 Alteromonas 5 Coryneforms 2 Pseudomonas: 9 LAB1 Cytophaga-Flvavobacterium 6
Cod fillets on ice • Storage on ice proved to be good with respect to sensory evaluation, in fact as good as in MAP • 12-14 days shelf life • bacterial count was high • Main spoilage bacteria: • Alteromonas • Aeromonas • Shewanella • TMA values were low in these samples
Oxygen scavenger • The scavengers used, create a totally anaerobic environment in less than 20 hours • This method gave very poor results. Microbial growth was high, sensory evaluation was poor and TMA-N values were high • Main spoilage bacteria: • Shewanella • Cytophaga - Flavobacterium • Pseudomonas • Aeromonas
MAP • Shelf life similar as for fillets on ice in sensory evaluation • TMA-N values were much higher. • TVC much lower than in fish on ice. • Main spoilage bacteria: • LAB
MAP + Oxygen scavenger • Results were almost the same as when MAP was used separately. • Bateriological genus evaluation showed almost entirely lactic acid bacteria as in the MAP.
Discussion • MAP packaging of fish has been done previously and is a commercially used method. • The use of oxygen scavengers and EtOH generators have mostly been used on dry and semi-dry foods.
Discussion • Oxygen scavengers gave worse results than expected and worse than reported results from vacuum packaging (approx. 10 days). This was somewhat surprising and has not been explained. • Combination of MAP and Ox.scav and/or EtOH did not seem to make a significant difference.
