Natural Convection Drying at Low Temperatures of Previously Frozen Salted Meat G. Clemente, J. Bon, J. V. Garcı ´a-Pe ´rez, and A. Mulet Grupo de An alisis y Simulaci on de Procesos Agroalimentarios, Departamento de Tecnologı ´a de Alimentos, Universidad Polite ´cnica de Valencia, Spain The drying of raw cured ham proceeds under natural convection. The aim of this work was to determine the effect of external resistance on the drying of frozen and salted Biceps femoris and Semimembranosus. Natural convection drying kinetics were obtained at 5, 10, 15, and 20  C. The D e values obtained neglecting external resistance were underestimated. Activation energy, E a , agreed with literature results. The use of a D e value from literature and a calculated E a allowed us to calculate D e for the temperatures used in this study. The mass transfer coefficient (k) was estimated from a model taking external resistance and the calculated D e into account. The k values agreed with the ones in literature; thus, under natural convection conditions, external mass transfer resistance must be considered. Keywords Drying; External resistance; Meat; Modeling; Natural convection INTRODUCTION Raw cured meat products are very important in the Spanish food industry and dry-cured ham is one of the most important. The main raw material used for its elabor- ation is the hind leg of pork, with Biceps femoris and Semimembranosus as the main muscles. Nowadays, the industrial use of frozen ham as raw material is becoming more common. [1] The production of dry-cured ham con- sists of several steps. The raw hams are first salted and then cured=dried, the final salt content lies between 3 and 9%. [2] Drying is the most time- and energy-consuming step. The mechanisms involved and the modeling of drying have to be established in order to carry out optimization and scale-up. [3] The drying of meat is generally modeled as a diffusion process [4] and in diffusion-based models some process simplifications are common. For this reason, the diffusion coefficient is considered to be an effective one. [5] Effective diffusivity can be estimated by fitting a diffusion model to experimental drying kinetics. The model considered is useful for estimating the drying rate and for the purposes of drier design [6] and, as a consequence, it is important to know the effective diffusivity in order to ana- lyze, optimize, and design drying systems. [7] The effective diffusivity is influenced, among other factors, by tempera- ture, and this influence is generally described by the Arrhenius equation. Although when the D e of food is determined, the product is usually assumed to be isotropic; some research can be found in literature into the effect of the unisotropy of meat on D e . However, these works are scarce and con- tradictory as some authors have found larger D e values parallel to the fibers than perpendicular to them, [8–10] whereas others have not found this difference. [11,12] The salt, NaCl, which is the most important ingredient in dry-cured ham, could decrease the D e value, [13] but this depends not only on how much is present but also on the dehydration temperature. [11,14] As long as the method of drying is taken into account, external resistance to mass transfer can be neglected when a diffusion model is formulated. Effective diffusivity is a property that is linked to the material and should not be influenced by external conditions. As such, when being identified the model must take experimental conditions into account. It is necessary to know the mass transfer coefficient, which can be obtained from semi-empirical equations from the literature or calculated from the system model [15] when external resistance to mass transfer is considered. The freezing of meat produces many changes, both physical and physicochemical. These changes also take place during the conservation and thawing of frozen meat. [16] For this reason, the structure of the thawed meat is different to that of the fresh meat and some of its physical properties may change. The literature provides effective diffusivity figures for frozen and thawed meat that has been dehydrated in a convective dryer, both when external Correspondence: G. Clemente, Grupo de An alisis y Simulaci on de Procesos Agroalimentarios, Departamento de Tecnologı ´a de Alimentos, Universidad Polite ´cnica de Valencia, Camino de Vera s=n; 46022, Valencia, Spain; E-mail: gclemen@tal.upv.es Drying Technology, 25: 1885–1891, 2007 Copyright # 2007 Taylor & Francis Group, LLC ISSN: 0737-3937 print/1532-2300 online DOI: 10.1080/07373930701677967 1885