Shear Rate Effect on the Viscosity of Homemade and Commercial Tomato Sauce

Abstract

Rheological characterization of tomato products is important not only for design of unit operations, but also to optimize processes and guarantee high quality of food products. Time dependence is related with structural changes due to shear rate. Thus, rheological characterization as a function of time is extremely important to understand the changes that occurred in food products during commercial processes. However, these characterizations are rare in the literature for tomato products. Those rheological properties depend of several parameters, such as agronomic, structural and process parameters. In this study, the effect of shear rate on the viscosity at room temperature (22 °C) of two kinds of tomato sauce (homemade and commercial) is analyzed; using a Brookfield viscometer model 4535, Lab-Line Instruments. Homemade sauce was prepared with fresh saladette tomatoes from a local market. Rheological analysis was performed at shear rates from 0.05 to 10.47 s -1 ; using all the spindles available. Statistical analysis was made using LSD and Duncan tests with a confidence interval of 95% (p-value of 0.05). It was demonstrated that homemade and commercial tomato sauce behaved as a thixotropic fluid, due to the typical decrease on viscosity samples observed when testing time was increased. When tomato sauce is at rest, it has a gelled tridimensional structure; and, as soon as a shear stress is applied, the movement generates an alignment of tomato constituent chains in flow direction, breaking physic entanglements and thus causing a decrease in viscosity as a function of time. Furthermore, it was discovered that the viscosity of homemade sauce was higher than the viscosity of commercial sauce. The latter may be due to the pre-thermal treatment. During this unit operation, polymeric chains of lycopene most likely break into smaller chains of isoprene, causing the observed decrease in viscosity. Besides, commercial sauce contains additives, seasonings, preservatives and thickeners that are not in a homemade sauce.