-In the first part of this study we used mathematical modelling for prediction of hydrothermal pathways (time evolution of temperature and water content) of a whey protein-rich product during its spray drying in a co-current configuration. The model was validated through drying assays, where the inlet/outlet air temperatures were fixed at 170/85• C, 217/107• C, 247/125• C or 260/138• C, the other spray-drying operation variables being unchanged. Then, the spray-dried powders were characterised before and after re-hydration for evaluation of a threshold air operating temperature value leading to changes in protein structure-functionality, in comparison with the nonspray-dried protein solution (WP-L). The predicted hydro-thermal pathways of the whey proteinrich product during its spray drying indicated that all of the operation variables used were accompanied by a product wet bulb temperature value lower than 60• C. However, the resulting powders presented different internal porosity and wall thickness, and the protein solutions obtained after the powders' re-hydration behaved differently from the non-spray-dried protein solution (WP-L). Particularly, it seemed that powders obtained at outlet air temperature and water content higher than 100• C and lower than 4%, respectively, presented a higher internal porosity and lower free lactose content. Furthermore, they were accompanied by lower protein solubility and conformation stability and by a slight development of hydroxymethylfurfural and covalently-bound protein aggregates. In addition, in situ evaluation of foam formation and stability after air injection into the re-hydrated powders showed that increasing spray-drying air temperatures led to powders with more and more enhanced foaming properties, relative to the whey protein-rich solution before spray drying. The powder characteristics obtained in the present study, added to those recently published [Relkin et al., Lait 87 (2007) 337-348] are presented and discussed in regards to effects of spray-drying operation variables on protein structural changes and enhancement of foaming properties without additional processing equipment. • C (les autres paramètres d'opération de séchage étant maintenus constants), nous avons caractérisé les poudres obtenues avant et après ré-hydratation. Nous avons observé que pour toutes les conditions opératoires testées, la température de thermomètre humide reste inférieure à 60• C, pendant une durée diminuant de 10 à 5 s pour une température de sortie d'air (produit) variant de 85• C (59. Cependant, les poudres sortant à une température supérieure à 100• C et un taux d'humidité inférieur à 4 %, semblent subir une montée en température beaucoup plus brutale après un palier à 60• C (température de thermomètre humide) d'une durée inférieure à 7 s. Ces poudres présentent une porosité interne qui augmente ( 40 %) et une épaisseur de paroi (visualisée par microscopie électronique à balayage) qui semble diminuer avec l'intensité du traitement thermique. Les solutions protéiques...