Kinetic modelling of colour changes during beef roasting

S.M. GOÑI; V. O. SALVADORI

Atenas

Congreso; 11 International Congress on Engineering and Food; 2011

During beef roasting, the muscle suffers several changes associated with its internal temperature evolution. At the end of cooking, beef’s colour, which can vary from "very rare" to "well done", is the principal indicator of the degree of doneness as it is perceived by consumers. On the other hand, mathematical modelling of beef roasting has been proved to be a useful tool in the calculation of cooking time in order to achieve safe products. The objective of this work was to develop a simple kinetic model to describe the colour changes produced during beef roasting. Furthermore we test coupling it to a previous developed and validated model of beef roasting, which describe simultaneous heat and mass transfer during the process. Thin slices (4 mm thickness) of beef semitendinosus muscle were used to obtain colour change information. The samples were subjected to different time-temperature treatments (2.5-30 minutes, 40-100ºC) using a thermostatic water bath. After each heat treatment, instrumental measures of surface colour in the CIEL*a*b* colour space, was obtained, using a MINOLTA colorimeter. In all the treatments the a*-value was lower than the raw ones, the variation was more pronounced at high temperatures. At long process time the values remain in a plateau, these were considered as equilibrium values at the given temperature. The L*-value of cooked samples was higher than the value of raw ones, and the variation was less pronounced at high temperatures and long process time. Variations in b*-value did not exhibit a clear trend, being in a narrow region. From this analysis, variations of the a*-value appears to be the most important, since it determines the change from a pink-red colour of raw beef to a gray-brown colour of cooked beef; such changes are actually the ones perceived by consumer and can determine product acceptability. Then, a simple kinetic model describing the variation of a*-value was proposed. A first order kinetic model was used, being reaction rates correlated with temperature according to an Arrhenius relationship. The model fits well to the experimental data, with an absolute relative error of 6.45%; the pre-exponential factor was found as k0=1.44821010 s–1, and activation energy was found as Ea=80.7397 kJ/mol. Later, the developed kinetic model was successfully coupled to a beef roasting model, predicting a non uniform internal distribution of a*-values, which is actually observed in large cooked samples. From a model-based framework, the usefulness of beef cooking models describing the relevant heat and mass transfer mechanisms can be improved incorporating other important issues, related to safety restrictions, i.e. to reach a given temperature or lethality at certain point, and also consider quality features, as colour or texture, among others. In this sense, the developed model, which describes the redness variation during beef roasting, can be used to establish operating strategies to reach desirables colour values or colour uniformity at end of cooking. Furthermore, other beef muscles (or muscles of another kind of animal) can be considered provided that appropriate information of colour variation is known.