CO2

Module	CO₂equilibrium 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 CO₂
Range of applicability	SSSP 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 CO₂ this gas dissolves into the water and lipid phases of the products. CO₂ 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 CO₂ 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 K_H is Henry’s constant (mg l^-1 atm^-1) and pCO₂ the partial pressure of CO₂ (atm.). Compared to oxygen and nitrogen large amounts of CO₂ can be dissolved in fresh fish. Therefore, the gas composition in the headspace of MAP fish may change significantly after packaging with an given initial gas mixture and gas/product ratio. The equilibrium gas composition is influenced by several factors e.g. the percentage of CO₂ in the initial headspace gas (%CO₂^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 CO₂ 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 CO₂ in the initial headspace gas (%CO₂^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, K is the absolute temperature (Carroll et al. 1991). These authors expressed K_Has MPa/mole fraction and in eqn. 2, 101325 Pa/atm. and 2.4429 was used to convert this unit into mg CO₂/liter of water or lipid/atm.

Eqn. 3

In Eqn. 3 dCO₂ is the density of CO₂ (1.976 g/l).

To allow users of SSSP to conveniently calculate equilibrium concentrations of CO₂ in MAP seafood the equations presented above have been included in a specific SSSP module 'CO₂equilibrium concentrations in MAP seafood'. As shown by the SSSP dialog box below, fresh cod fillets (~80 % water, 0% lipid) packed with an initial CO₂ 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% CO₂ due to solubility of CO₂ in the fish. With the same initial CO₂ concentration but a gas/fish-water ratio of 4:1 the equilibrium concentration of CO₂ will be close to 50% CO₂. As also shown below the 'CO₂equilibrium concentrations in MAP seafood' module in SSSP allows users to calculate the relation between the equilibrium concentrations of dissolved and headspace CO₂ at different temperatures. See Rotabakk et al. (2008) for more recent developments in the calculation of CO₂equilibrium concentrations in MAP seafood'