Studies on cold-smoked fish production and their relation to controlling spoilage and safety

  • Maria Manuela Ramos Vieira da Silva (Student)

Student thesis: Doctoral Thesis


Smoked fish products are currently part of the Portuguese diet, being accessible to a large group of people. They are presented on Portuguese commercial point of sale packed in a vacuum, in small sliced pieces or in the form of a fillet. Fishes such as salmon, salmon-trout and swordfish, with European provenance, are used as raw material for the cold smoking production. The shelf-life depends on the type of cold smoking applied, ranging from 2 to 6 weeks at refrigeration temperatures ≤5ºC. Microbiological and physicochemical characteristics have demonstrated the dominance of Lactic Acid Bacteria, Enterobacteriaceae, and vibrios, among others, with the possibility of other groups of microorganisms being present, such as the pathogen Listeria monocytogenes. Attemptsto create a quality index ofcold-smoked fish products havebeen the subject of several studies in order to establish a correlation between the useful shelf-life of the product and the physicochemical, sensorial and microbiological characteristics. The main objective of this research was to characterize the microbial ecology of vacuum packed cold-smoked fish available in a Portuguese commercial point of sale and at a pilot-scale production by the identification of determinant variables, which influence the microbiological and physicochemical quality of the cold smoked fish. The investigation was conducted to characterize commercial cold-smoked fish products available in the Portuguese market, mainly salmon (Salmo salar) and salmon trout (Oncorhynchus mykiss), considering microbiological and physicochemical studies. From microbiological characterization, the ability of different bacteria isolated from cold-smoked fish to produce biogenic amines was evaluated, using different descarboxylation agar growth medium. A pilot-scale cold smoking controlled experiments using salmon trout were conducted to study: a) the effect of the application of ozone as a disinfected agent on whole fresh fish and fillets on reduction of microorganisms, including L. innocua; b) the effect of a previous freezing step (-20°C) of individual samples of vacuum packed cold-smoked salmon trout before product commercialized at chilled storage on microbial ecology c) the effect of combined treatments of salting/drying/smoking (wet or dry salting, addition of sugar in the salting mixture and long and short smoking) on microbiological and physicochemical properties followed by the use of vacuum and modified atmospheres packaging chilled storage conditions at 5ºC.Results from samples obtained at the Portuguese point of saleevidenced differences on shelf-life products, as well as on microbiological numbers of the cold-smoked samples. Some of the cold-smoked samples would be at the limit of the allowed microbial load for RTE products, even before reaching shelf-life ́s limit. The results also suggested the decrease in coefficient of variation of samples for aerobic plate counts and for numbers of Enterobateriaceaein controlled time and temperature laboratory conditions. The results demonstrated the suitability of growth culture medium on selection of bacteria producing biogenic amines. Some bacterial strains belonging to the group of LAB and Enterobacteriaceaewere positive for tyramine production and less for histamine production. Complementary results obtained from HPLC determinations, showed higher concentration of tyramine production by Carnobacterium divergensand by Lactoccocus lactis lactis.From gaseous ozone treatments applied to whole and fresh fillets salmon trout, a decrease of less than 1Log10in L. innocua(as a surrogate for the pathogen L. monocytogenes) numbers occurred on ozone treated samples in all sampling occasions. Aerobic Plate Count was slightly lower on fresh fillets after treatment and during three weeks of storage. From ozone treatments applied to whole fresh fish, a reduction greater than 1 Log10/g of L. innocuaoccurred on smoked samples at the end of the storage period. These results were more pronounced when the slime present on the whole fish surface was removed. From a previous freezing step (-20ºC) the results showed an general effect on microbial load throughout storage of previously frozen samples at initial stage of chill storage (1stweek) for total aerobic plate count, LAB and H2S producing bacteria, as compared to non-frozen cold-smoked samples only chilled. Asignificant increase in numbers of H2S-producing bacteria was observed in previously frozen samples, independently of the type of salting applied. Enterobacteriaceaegroup wasless affected by the previously freezingstep. DSC thermograms showed changes in muscle structure after salting/cold-smoking process. The stability of myofibrillar proteins were affected by salting/smoking treatment and the additional freezing step can result in decrease in product quality. Results from combined treatments of salting/drying/smoking on production of cold-smoked salmon trout revealed that wet salting treatment (especially for shorter salting times) did not produce the same results on microbiological and physicochemical characteristics on the end product, compared with dry salting. Overall, dry salting is preferable to brining for reducing microbial growth in cold-smoked salmon trout stored in VP.Higher sugar content in the salting mixture (salt:sugar|3:1) induced an increase in microbiological numbers. The smoking process characterized by long drying and short smoking times (Group II –Dry 6h and Smoke 2h) encouraged a general increase in microbiological numbers of cold-smoked samples, with asignificant increase in LAB counts, but a negative effect on the samples regarding microbiological quality, with significant increase in Enterobacteriaceae and H2S-producing bacteria. After 3 week storage the average of samples presented levels of trimethylamine(TMA) (up to 30 mg in 100 g of fish). A positive effect of short dry and long smoke exposure on microbial ecology was observed (Group I –Dry 2h and Smoke 6h) in cold-smoked fish, dry salted and packaged either in VP or MAP. MAP represents an alternative of packaging to VP, reducing the microbiological activity of some spoilage bacteria.The present research highlights how essential is the improvement of careful control in three areas of work that compose the production of cold smoked fish: (1) the control of quality of the raw material; (2) the design of technological procedures applied during the curing/smoking preservation process and (3) type of packaging and chill storage conditions. Advances in the integrated production of cold smoked fish using the preservative combined effects as the 'hurdle concept' should be applied defining technical procedures to product stability and safety commercialization.
Date of Award30 Jul 2019
Original languageEnglish
Awarding Institution
  • Universidade Católica Portuguesa
SupervisorPaul Gibbs (Supervisor)


  • Cold-smoked fish
  • Shelf life
  • Cold-smoking process
  • Microbial ecology
  • Salmon
  • Salmon-trout
  • Spoilage bacteria
  • Vacuum package
  • Modified atmospheres package
  • Biogenic amines
  • Trimethylamine
  • Quality safety


  • Doutoramento em Biotecnologia

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