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This paper presents the results of thermophysical characterization of the powder resulting from the solar drying of Moringa oleifera leaves. The desorption isotherms of the powder, are determined by the gravimetric static method. The models of B.E.T, Smith, Henderson, Iglesias and GAB are used for the smoothing of the experimental points. The Clausius-Clayperon and GibbsHelmholtz equation are used for the determination of thermodynamic properties. The isotherms obtained are with type II. The theoretical curves of GAB present a better similarity with the experimental curve at 55°C. The monolayer moisture content (Xm) is 6.32 and 7. 24 % at 30° C; 5.13 and 6.3% at 35 °C; 5.1 and 5.9 % at 45 °C; 4.22 and 4.66 % at 55 °C respectively for BET and GAB models. Isosteric and entropic heats are more important for low values of moisture content. The activation energy of desorption of the powder in the range of temperatures tested is -10.1009539 kJ/mol. Isokinetic temperature (Tβ) and the mean harmonic temperature (Thm) are respectively 301.2435K and 300.12342K and the free energy (ΔGβ) is 302.18 J/mol. The desorption process of the Powder is non-spontaneous and is done by controlled mechanisms enthalpic.

RESUME La qualité du beurre de karité dépend des processus qui ont conduit à son obtention. Dans la présente étude, sont exposés les résultats de l'influence des opérations thermiques impliquées dans les procédés traditionnels d'extraction du beurre de karité (Vitellaria paradoxa) au Bénin. L'effet de l'interaction des opérations thermiques (de cuisson, de séchage, de torréfaction, de friture, de fumage et de chauffage) appliquées au cours de différents procédés d'extraction (Lokpa et Peulh au Bénin, Tchabal au Cameroun) a été évalué. Différents échantillons de beurre ont été obtenus par la méthode Soxhlet à partir des fruits, des noix cuites, des noix cuites séchées, des amandes séchées, des amandes séchées et torréfiées, des amandes séchées et frites, des amandes fumées. Les indices d'acide, de peroxyde et de réfraction des différents échantillons de beurre ont été déterminés; de même que la teneur en eau et la composition en acide gras. Les résultats obtenus ont montré que les opérations thermiques permettent la réduction de l'acidité des beurres. Quant au taux de peroxyde, il diminue avec des températures supérieures à 145 °C. Par ailleurs, les multiples résultats obtenus montrent que la chaleur n'a pas d'effet sur la composition en acides gras du beurre.

The aim of this paper is to show the interest of the covariance analysis applied to measurement error in the particular case of the identification of a drying characteristic curve from experimental drying data. The modelisation of drying by use of the Drying Characteristic Curve (DCC) method is first presented with usual specifications (power function, critical moisture content. . .). The experimental procedure used to obtain drying curves and the data processing are detailled and analysed. Measurements errors are identified at the first step of the DRYING TECHNOLOGY Vol. 20, No. 10, pp. 1919-1939, 2002 procedure and their effects on the estimation error of the exponent of the power function are estimated. Three different methods for estimating are presented under their matrix form: the least square method and two methods based on the hhGauss-Markovii or hhMaximum likelihoodii theorem, firstly under a simplified form suited if the estimation errors are uncorrelated and secondly under a complete form suited even if the estimation errors are correlated. These three methods are applied to experimental results obtained with ginger roots drying. The value of the exponent of the power function and then the distances between the three corresponding theoretical drying curves (representing product water content vs. time) and the experimental points are studied. It is shown that in this particular application, the complete Gauss-Markov method leads to the better fitting and that the simplified Gauss-Markov method, since it is a priori non appliable in this case where errors are correlated, gives quite better results than the oridnary least squares method. The covariance matrices of the estimation errors of reduced water content, reduced drying rate and exponent are also presented in order to show the correlations existing between the measurement errors of each variable during a drying cycle.