An experimental study of laminar-turbulent transition in complex diluted fruit purees flows in circular ducts, is presented in this work. Data measured using a rectilinear pipe viscosimeter are analyzed to single out useful critical values of both the wall shear stress and the flow rate at which transition-to-turbulence occurs for a given dilution degree. A comparison between the Dodge-Metzner-Reed methods and the classical Mishra & Tripathi and Hanks correlations to estimate the critical generalized Reynolds number, is also discussed. The emerging discrepancies can reasonably be attributed to viscoelastic effects, which probably become important near the point where transition-to-turbulence occurs. This analysis therefore has important practical implications for the prevention of the breakdown of the structure due to turbulent mechanical stresses [J. Food Engng. 52 (2002) 397]. (C) 2003 Elsevier Ltd. All rights reserved.