Citrus Rheology

Abstract

“Rheo” comes from the Greek word meaning “to flow,” thus rheology is the study of flow or, in the citrus industry, the study of the flow of citrus juices and concentrates. Fluids are divided into two basic types in rheology. Newtonian fluids are chemically pure homogenous fluids whose viscosity, or resistance to deformation, does not change with shear rate. (Shear rate can be described as the flow rate, the stirring rate, or the rate at which one surface passes over another.) Non-Newtonian fluids have different apparent viscosities at different shear rates. The latter description generally characterizes heterogeneous solutions such as citrus juices and concentrates. The non-Newtonian nature of citrus juices is primarily due to pulp and cloud material within the juice. Removal of these components renders the resulting serum Newtonian in nature. The term “consistency” or “apparent viscosity” is generally used with non-Newtonian fluids, while “viscosity” is generally used with Newtonian fluids.