Module |
CO_{2
}equilibrium concentrations in MAP seafood |

Reference | Ross, T. and Dalgaard, P. (2004). Secondary models.
In: McKeller, R.C.and Lu, X. (Eds.), Modeling Microbial Responses in
Foods. CRC Press, Boca Raton, USA, pp. 63-150. |

Factors in model | Temperature, initial
gas/product ratio, initial percentage of CO_{2} |

Range of applicability | FSSP uses this module for seafood with 0-8% NaCl in the water phase, pH between 5.6 and 7.7 and storage temperatures from 0°C to 25°C. |

When seafood is packed in modified atmospheres with CO_{2}
this gas dissolves into the water and lipid phases of the products.
CO_{2} dissolves gradually in chilled fresh fish with 50% of
the equilibrium concentration being dissolved in less than 5 hours and >90% being dissolved
after 24 hours (Sivertsvik et al. 2004; Rotabakk et al. 2008). At equilibrium the relation
between the concentrations of CO_{2} in the headspace gas phase
and the concentration dissolved in the water/lipid phase of fresh fish is determined by Henry's law (Eqn.
1).
Eqn. 1 In Eqn. 1 _{
} is Henry’s constant (mg l^{-1} atm^{-1})
and pCO the partial pressure of CO_{2}_{2} (atm.).
Compared to oxygen and nitrogen large amounts of CO_{2} can be
dissolved in fresh fish. Therefore, the gas composition in the headspace of MAP fish may change significantly
after packaging
with a given initial gas mixture and gas/product ratio. The equilibrium gas composition is influenced
by several factors e.g. the percentage of CO_{2}
in the initial headspace gas (%CO_{2}^{Initial}),
the initial gas/product volume ratio (G/P), temperature,
pH and of corse permeability of the packing film. For flexible packaging
films with low gas permeability the equilibrium concentration of CO_{2}
in the headspace gas of MAP fresh fish can be predicted at different
temperatures using eqn. 1-3 together (Ross and Dalgaard 2004). These
equations take into account the effect of (i) the percentage of CO_{2}
in the initial headspace gas (%CO_{2}^{Initial}),
(ii) the initial gas/product volume ratio (G/P) and (iii) temperature. The
equations does not take into account the effect of pH and results should
not be considered highly accurate.
Eqn. 2
Eqn.
2 describes the temperature dependance of Henry's constant. In this
equation, _{2}/liter
of water or lipid/atm.
Eqn. 3
In
Eqn. 3 _{2} (1.976 g/l). |

To allow users of FSSP to conveniently calculate
equilibrium concentrations of CO_{2} in MAP seafood the
equations presented above have been included in a specific FSSP module
'CO_{2
}equilibrium concentrations in MAP seafood'. As shown by the FSSP
dialog box below, fresh cod fillets (~80 % water, 0% lipid) packed with an
initial CO_{2} concentration of 60% and a gas/fish ratio of 2:1 (i.e. 200 ml gas and 100 ml water in
the fish)
at 2°C have
an equilibrium concentration of close to 40% CO_{2} due to
solubility of CO_{2} in the fish. With the same initial CO_{2}
concentration but a gas/fish-water ratio of 4:1 the equilibrium
concentration of CO_{2} will be close to 50% CO_{2}. As
also shown below the 'CO_{2
}equilibrium concentrations in MAP seafood' module in FSSP allows users to calculate the relation between the equilibrium
concentrations of dissolved and headspace CO_{2}
at different temperatures. See Rotabakk et al. (2008) for more recent
developments in the calculation of CO_{2
}equilibrium concentrations in MAP seafood' |