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Model H2S-producing Shewanella*: Microbial spoilage model for aerobically stored fresh fish
Reference Dalgaard P. (1993) Evaluation and Prediction of Microbial Fish Spoilage, Ph.D-thesis, Technological Laboratory, Danish Ministry of Fisheries, Lyngby, Denmark, 169 p.
Primary growth model Log-transformed 3-parameter Logistic model 
Secondary growth model Square-root model
Factor in model Temperature
Product validation studies Cod, Haddock, Hoki, Orange roughy, Smooth oreo dory, Sea bream and Snapper (Dalgaard, 1999).
Range of applicability Fresh seafood from temperate/cold waters stored aerobically at chill temperatures. The range of applicability in SSSP is 0-10°C
SSSP includes a simple square-root model for the effect of temperature on the maximum specific growth rate (µmax, h-1) of H2S-producing Shewanella spoilage bacteria when growing in fresh fish under aerobic storage. The range of applicability has been set to 0 - 10°C in SSSP although the more exact spoilage domain remains to be determined in storage trials. The maximum population density has been set to 109 cfu/g.

* H2S-producing Shewanella bacteria from chilled seafood were previously often identified as Shewanella putrefaciens. However, based on more recent taxonomy  most of the H2S-producing Shewanella bacteria from chilled cold-water fish is more correctly identified as Shewanella baltica (Ziemke et al. 1998; Fonnesbech et al. 2005).

 

Primary growth model:

                                                           Eqn. 1

 

In eqn. 1, Nt (cfu/g) is the concentration at time t, N0 (cfu/g) the initial concentration, Nmax the maximum concentration (cfu/g) and mmax the maximum specific growth rate (h -1).

 

Secondary growth model:

                                                           Eqn. 2

 

This model for growth of H2S-producing Shewanella bacteria has been validated by comparison of observed and predicted growth rates in different fresh seafoods. The model predicted growth rates accurately in fresh fish stored aerobically at low temperatures. Data for higher temperatures remain to be evaluated in validation studies (See Table below). The model substantially overestimated growth rates in tropical seafood (bias factor = 1.44) and it can not be used for accurate shelf-life prediction for fresh fish from tropical/warm waters.

Evaluation of microbial spoilage model for growth of H2S-producing Shewanella bacteria in aerobically stored fresh fish

Origin of seafoods

Atmosphere

Temp. (oC)

na

Bias factor

Accuracy Factor

Temperate/cold waters

Air

0 - 3

19

1.13

1.21

Tropical/warm waters

Air

0 – 30

15

1.44

1.60

a) Number of growth curves evaluated in product validation studies (Dalgaard, 1999)

To predict shelf-life, the initial concentration of H2S-producing Shewanella bacteria is required and it can be determined using Iron agar Lyngby (Gram et al., 1987) or Peptone Iron agar (Levin, 1968). For H2S-producing Shewanella bacteria, minimal spoilage levels from 106 cfu/g to 108 cfu/g has been reported in the literature and SSSP uses a minimal spoilage levels of 107 cfu/g. Eqn. 3 is used to calculate product shelf-life.

 

                                                     Eqn. 3