FOOD HYDROCOLLOIDS Food Hydrocolloids 21 (2007) 287–296 Transfers of small analytes in a multiphasic stirred fruit yoghurt model Alice B. Nongonierma a , Philippe Cayot a , Mark Springett b , Jean-Luc Le Que´re´ c , Re´my Cachon d , Andre´e Voilley a,Ã a Ecole Nationale Supe´rieure de Biologie Applique´e a` la Nutrition et a` l’Alimentation, Universite´de Bourgogne, 1, Esplanade Erasme, 21000 Dijon, France b Danone Vitapole, Centre de Recherche Daniel Carasso, R.D. 128, 91767 Palaiseau, France c UMR FlAVIC (Flaveur, Vision et Comportement du Consommateur) INRA-ENESAD, 17, rue Sully, 21034 Dijon Cedex, France d Laboratoire de Microbiologie, UMR UB/INRA 1232, ENSBANA, 1 esplanade Erasme, 21000 Dijon, France Received 15 December 2004; accepted 12 April 2006 Abstract The transfer of small analytes in a multiphasic stirred fruit yoghurt model, made of a pectin gel aimed to mimic fruit pieces and of a dairy gel done with milk acidified by glucono-d-lactone hydrolysis, have been studied. The concentration gradients between the pectin gel and the dairy gel were the driving force for the migration of small analytes (i.e. water, protons and colorants). Water migrated from the dairy to the pectin gel, causing modifications in the water content of both gels and an equilibration of their water activity at 0.93870.003. Inversely, protons migrated from the pectin to the dairy gel. These changes in composition of both gels being likely to have induced structure modifications. Migration of relatively small molar mass colorants did not depend on steric considerations, but rather on their charge and structure. In both gels, most of the colorants studied were charged at the equilibrium pH 4.1. This charge was linked to their diffusivity in the pectin gel. Negatively charged colorants were thought to be affected by the attractive electrostatic forces between the pectin carboxylic groups and Ca 2+ , causing a decrease in their diffusivity. Transfer of colorants in the dairy gel were affected by electrostatic interactions with milk proteins and ions or by hydrophobic interactions with milk proteins and fat. r 2006 Elsevier Ltd. All rights reserved. Keywords: Diffusion; Colorants; Protons; Water; Coupled transfers; Dairy gel; Pectin gel 1. Introduction Sensory evaluations have indicated a marked preference for the fruity and sweet characteristics of yoghurt (Barnes, Harper, Bodyfelt, & McDaniel, 1991). The addition of fruits in yoghurts is made using a preparation made from fruit pieces and a syrup which contains sucrose, thickeners, colorants, flavours and preservatives. The pH of these preparations varies between 3.4 and 4.0 according to the fruits used and the desired acidity. Their dry matter content is generally higher than 60% (Fellows, Chang, & Shazer, 1991). When the fruit preparation is added to the yoghurt, the sugars and pectin from the syrup are mixed with the yoghurt, causing an increase in its apparent viscosity (Afonso & Maia, 1999; Ramaswamy & Basak, 1992). Pectins are used to avoid syneresis in acidified dairy products (Tromp, de Kruif, van Eijk, & Rolin, 2004). They adsorb on casein micelles reversibly (Maroziene & De Kruif, 2000), inducing an increase in the steric repulsion and thus avoiding their massive aggregation. Due to the heterogeneous composition and structure of fruit yoghurts, there are concentration gradients between the fruit pieces, the dairy gel, the aqueous bulk phase, the fat globules, etc. The corresponding gradients between these phases are the driving force for transfers of small analytes from one phase to the other. During the storage of ARTICLE IN PRESS www.elsevier.com/locate/foodhyd 0268-005X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2006.04.007 Abbreviations: % water, water content of the pectin gel (% w/w); Ag, Silver; C, concentration of colorant in the sample (mol L 1 ); C j i , concentration of the colorant at the interface in the gel j (mol L 1 ); C j , concentration of the colorant in the gel j (mol L 1 ); Cl, chlorine; COO  , carboxylate group; dg, dairy gel; D j , diffusion coefficient of the colorant in the gel j (m 2 s 1 ); dM, mass variation of the pectin gel (g); erf, error function; pg, pectin gel; t, time (s); x, distance of the colorant from the interface (m) Ã Corresponding author. Tel.: +33 3 80 39 66 59; fax: +33 3 80 39 66 11. E-mail address: voilley@u-bourgogne.fr (A. Voilley).