Article pp.146-149 du Vol.23 n°1 (2003)

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146-149 Sci. Aliments 23(1), 2003 L. Bengaida et al.

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Estimation of equilibrium properties in meat preservation processes by a group-contribution method

L. Bengaida, C.G. Dussap, J.B. Gros¹

INTRODUCTION

Equilibrium properties of water are considered as reference properties in food processing. Water activity is the best known property. This work concer- ned the use of a method, which in addition to the equilibrium properties of water, affords equilibrium properties of all components in an aqueous solution.

It is based on the UNIFAC method, a group-contribution method, which estima- tes equilibrium properties from the excess Gibbs energy of all compounds within the solution. Completed by material balances, this gives vapour-liquid properties, liquid-liquid or solid-liquid properties, osmotic properties, and knowing dissociation constants values, this affords pH. Two examples are pre- sented, osmotic dehydration and modified atmosphere packaging of meat.

MODEL

The solution model developed by Achard et al. (1994) is the sum of two terms:

1) the UNIFAC model (Universal Functional Group Activity Coefficient, Fre- denslund et al., 1975) as modified by Larsen et al. (1987) for estimating activity coefficients of non-ionic molecules in the mixtures. It relies on the idea of group-contribution in which each molecule is considered as a collection of basic building blocks, the functional groups. Activity coefficients are the sum of two terms: the first accounts for surface and volume of groups, the second for energetic interactions between

1. Laboratoire de génie chimique et biochimique, Université B. Pascal, CUST, 24, avenue des Landais, BP 206, 63174 Aubière cedex, France.

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Estimation of equilibrium properties in meat preservation processes by a group-contribution method 147

groups. The UNIFAC model represents short-range forces between all the species.

2) a model for activity coefficients of electrolytes. It represents long-range ion-ion interactions and is based on the Debye and Hückel theory as modified by Pitzer (1973). Thus,

It was decided to take into account the solvation of charged species giving clusters by means of an hydration number. This number characteri- ses a specific ion, not a given salt, which facilitates the treatment of mixtures containing many dissociated or partly dissociated compounds. The hydration number for anions was set to zero.

The model gives the detailed concentrations of ionic or non-ionic species in the solution. Particularly it gives pH defined as:

where a_H+ stands for activity of H⁺. For systems of weak electrolytes (amino acids, phosphates, carbonates…) a new automatic procedure was used to generate the whole species in the mixture; for example, when Na₂HPO₄ is dissolved in water, the species Na⁺, H⁺, OH^–, HPO₄^2–, PO₄^3–, H₂PO₄^–, H₃PO₄may be present in solution and should be considered.

RESULTS

This model was used to estimate equilibrium properties needed when designing processes such as osmotic dehydration (a_w, activity coefficients of sugars, etc.) or production of aromas by biotransformation by micro-organisms (pH, Henry's coefficient). It is tested in predictive microbiology to estimate the time course of a_w and pH.

Osmotic dehydration

In osmotic dehydration processes, foods are immersed in aqueous concen- trated solutions containing salts, sugars, or other water activity depression agents. Dehydration of meat can be achieved in sugars and salts solutions; the processing time depends, among others, on water activity differences between the solution and the meat, and on the relative values of diffusion coefficients of water, salts and sugars in meat. There are many ways to lower the activity of water; some solutions defined to obtain an activity of water of 0.85 were tested in a meat dehydration process and a solution containing 1 kg water, 200 g NaCl, 420 g sucrose and 1 g xanthan gum was finally selected (Eman Djomeh et al., 2000).

Lnγi = LnγiUL + LnγiPDH

pH = –log₁₀(a_H+)

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148 Sci. Aliments 23(1), 2003 L. Bengaida et al.

Figure 1 shows that the model gave further informations, namely it eviden- ces the very low influence of sucrose concentration on the mean activity coefficient of NaCl in the mixture.

Figure 1

Experimental values of the mean activity coefficient of NaCl in a water-sucrose-NaCl solution at different sucrose molalities (● 0. ■ 0.31 ◆ 0.68 and ▲ 1.11 mol/kg)

compared to calculated values(^_____).

Figure 2

Calculated influence of the partial pressure of carbon dioxide in a CO₂–N₂ mixture at 1 atm. total pressure, on the pH of fresh meat.

Controlled atmosphere preservation

The used of increased concentration of CO₂ to extend the shelf live of fresh meat is hardly a new technique. High oxygen modified atmosphere packaging

γ± = (γ+.γ–)^1/2

Activity coefficient

0,5 0,6 0,7 0,8 0,9 1

0 0,5 1 1,5 2 2,5

NaCl molality (mol/kg)

γ

Modified atmosphere

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9

5,42 5,44 5,46 5,48 5,5 5,52 5,54

pH Pc02(atm)

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Estimation of equilibrium properties in meat preservation processes by a group-contribution method 149

(MAP) which have atmospheres of 20% CO₂ and 80% O_2,or 25% CO₂, 70% O₂ and 5% N₂, are used for both to extend the colour stability and to delay microbial spoilage.

For cooked meats, MAP systems with 50% CO₂ and 50% N₂ are used, for French saucissons or sliced hard salamis 10% CO₂ and 90% N₂ are used.

Figure 2 shows the influence of CO₂ percentage on pH of a fresh meat initially at pH=5.63. The model meat considered for calculation was limited to its solu- ble fraction, the composition of which is: (g/1000g solution) water 755; K⁺ 3.9;

Na⁺ 1.0; Mg⁺⁺ 0.2; Ca⁺⁺ 0.012; P 2.0; lactic acid 9.0; (mol/1000g) the dipeptides carnosine and anserine, and histidilimidazole residues of myofibrillar proteins 0.035 (Puolanne and Kivikari, 2000). It can be observed that in this particular case, carbon dioxide can be used at very different concentrations without influence on the pH of post rigor meat; nevertheless CO₂ dissolved as bicarbo- nate is not negligible.

These results computed using our model are in agreement with those of Pua- lanne and Kivikari (2000) who determined the buffering capacity of dark and light beef pork and poultry muscles and found: for pork m. longissimus dorsi: pH=5.44

± 0.06; minimum buffering capacity 38.9 ± 2.2 mmol H⁺pH^–1kg^–1 at pH=5.56 ± 0.04; mean buffering capacity in pH range 5.5-7.0, 52 mmol H⁺pH^–1kg^–1, for pork m. triceps brachii: pH=5.90 ± 0.14; minimum buffering capacity 32.2 ± 1.9 mmol H⁺ pH^–1kg^–1 at pH=5.64 ± 0.04; mean buffering capacity in pH range 5.5-7.0, 45 mmol H⁺pH^–1kg^–1.

CONCLUSION

Group-contribution models help estimating equilibrium properties between food products and their environment. They can help designing new processes or defining improved process conditions.

REFERENCES

ACHARD C., DUSSAP C.G., GROS J.-B.,1994.

Prediction of pH in complex aqueous mixtures using a group-contribution method.

AIChE J., 40, 1210-1222.

FREDENSLUND A., JONES R.L., PRAUSNITZ J.M., 1975. Group-contribution estimation of activity coefficients in nonideal mixtures. AIChE J. 21, 1086.

LARSEN B.L., RASMUSSEN P., FREDENS- LUND A., 1987. A modified UNIFAC group-

contribution model for prediction of phase equilibria and heats of mixing. Ind. Eng.

Chem. Res., 26, 2274-2286.

PITZER K S., 1973. Thermodynamics of electrolytes.1. Theoretical basis and general equation. J. Phys. Chem., 77, 268-277.

PUOLANNE E., KIVIKARI R., 2000. Determina- tion of buffering capacity of post rigor meat. Meat Science, 56, 7-13.