Comparison of rheological properties in fatty liver from duck and goose by oscillatory plate-plate rheometer measurements

Texture of fatty liver, as a multiparameter attribute, has been examined more and more during the last decade. The instrumental texture analysers used are developed on the analogy of penetration tests to imitate the action of human jaws. In this study a Physica UDS200 rheometer was used, fitted with a MP31 top plate and a Peltier bottom plate. Rotational rheometer measurements using plate-plate systems seem to represent feasible off-line methods for objective texture characteristics of fatty liver. INTRODUCTION Rheological characterisation of food products like fatty liver from force fed goose and ducks, have to be expressed as objective measures. There are many reasons for such measurements. Some reasons may be in order to decide, or achieve a defined quality of the product, to optimize manufacturing processes and as a method of grading. Special challenges are associated with rheological methods examining temperature sensitive biological fatty products like “foie gras”. Both, the fat content and the fatty acid composition in such products, will influence on rheological properties which will vary even at small temperature changes. Existing laboratory instruments measure accurately and objectively rheological parameters. Since rheological behaviour of food with high fat content is very dependent on temperature variations, this will complicate the feasibility of these methods. Grading of fatty liver from goose and duck all over the world is mainly based on manual examination. This can be considered as a sensory test where the most critical step is the estimation of texture characteristics. The texture is influenced by the tissue structure. It is a complex rheological attribute, which can be detected by several ways; the most important ones being touch/ pressure. Traditionally, in Hungary graders are trained by practical presentations. Studies have shown that the reproducibility of this is relatively poor. The texture of fatty liver, as a multiparameter attribute, has been examined more and more during the last decade. The instrumental texture analysers used are developed on the analogy of penetration tests to imitate the action of human jaws. In accordance to this, K. Horváth-Almássy and O.Bara-Herczegh demonstrated that a Stevens QTS 25 Texture Analyser could be used to characterise the quality of goose liver. In this study it was concluded that this instrumentation could be an important supplement of traditional grading method. It could help training graders in supervising and being up-to-date. Comparison of rheological properties in fatty liver from duck and goose by oscillatory plate-plate rheometer measurements. E.O.Rukke, X. Fernandez and R.B. Schüller 1 Department of Chemistry, Biotechnology and Food Science, Norwegian University of Life Sciences, P.O.Box 5003, N-1432 Ås, Norway. 2 Tissus Animaux, Nutrition, Digestion, Ecosystèmes et Métabolisme, INRA-ENSAT, Avenue de l`Agrobiopole – BP 32607, F-31326 Auzeville Tolosane P. Chartrin et al, 2006, used another objective method for texture analysis to evaluate sensory characteristics in duck breast meat. They used a single-blade (Warner-Bratzler) shear test, performed with a universal testing machine (MTS Synergie 200, MTS Systems, Ivry sur Seine, France). In this study another off-line method for objective texture characteristics using rotational rheometer equipment with plateplate systems was tested on fatty liver. One important advantage using this equipment is the temperature control system. Accurate temperature control is essential when performing rheological measurements since viscosity generally depends on temperature. The objectives of this work were as follows; To investigate the feasibility of an off-line objective rotational rheometer measurement method using plate-plate systems studying texture characteristics of fatty liver. Using an objective rheological measurement method comparing texture differences between fatty liver from goose and duck, which are analysed regarding fatty acid composition. Using a method with strict temperature control during performance of rheological measurements; both at constant temperature and during temperature scans from +4+40C and back down to +4C. MATERIALS AND METHODS Liver samples French fatty liver from force fed goose (Maxipalm) and duck (Mule ducks) were used in this study. The animals were slaughtered in a conventional way after 10 hours feed withdrawal. After electric stunning, the animals were bled within 1215 seconds after cutting the carotids and the jugulars. The samples used were horizontally slices about 25 mm in diameter taken from the superior lobs of the liver. The samples were collected immediately after plucking, frozen in liquid Nitrogen, and stored vacuum-packed in plastic pads at – 20 oC before analysis. Rheological measurements were carried out after thawing the samples at +4 oC. The liver sample was then placed as a thin formed “disc” at the bottom plate (the Peltier plate) of the rheometer, Fig. 1. Instrumental analysis The MP31 top plate was placed in a bucket containing melting ice, Fig. 2. The Peltier temperature was set to +4 °C. The liver sample was placed on the Peltier plate and gently formed to cover the plate. The MP31, taken from the ice bucket at 0 oC, was dried with a towel and connected to the rheometer. The MP31 was then lowered very slowly squeezing the liver sample gently to obtain the desired plate – plate clearance of 2 mm. The excess liver was then removed from the rim of the MP31, Fig. 1. Figure 1: Liver sample prior to removal of excess