44C-29 |
Effect of shear on kinetics of serum separation and some rheological properties of ketchup |
H. SIGFUSSON1, G. R. Ziegler2, and J. N. Coupland2. (1) Food and Agricultural Products Research & Technology Center, Oklahoma State University, FAPC RM 106, Stillwater, OK 74078-6055, (2) Department of Food Science, The Pennsylvania State University, 102 Borland Lab, University Park, PA 16802 Syneresis is an important defect in ketchup products that can be affected by several factors such as paste preparation, tomato cultivars, total solid content, water-insoluble solids, and pH. Stress applied to the product during shipping/storage/handling may exacerbate the problem. The objectives were to examine the effect of a pre-shearing treatment on the syneresis kinetics and rheological properties of ketchup. Ketchup samples were pre-sheared in a temperature controlled (20°C) shaker bath at full speed for 5 and 30 min for comparison to unsheared samples. Samples were loaded into the sample cell and allowed to equilibrate for 20 min prior to measurement. The yield stress was determined using vane geometry in a Rheometrics SR-2000 stress-controlled rheometer. Viscosity of samples as a function of shear-rate (0-1000 s-1), and G' and G'' (at 5 Hz) as a function of percent strain (0-1000%), were determined using a concentric cylinder in a Rheometrics RFS-II shear-controlled rheometer. Sample temperatures were maintained at 20°C during the measurements. Serum separation from similar samples, as measured by mass drainage through a screen, was monitored over a period of 4 weeks. Pre-shear resulted in a lower ketchup viscosity over the range of shear-rate tested. It also significantly (p < 0.05) lowered the initial yield stress (t0) as compared to unsheared samples. Furthermore, shear resulted in a shift in the crossover point of G' and G'' to a lower strain (%) value. The rate of initial serum separation increased significantly (p < 0.05) with pre-shear treatment, although the final amount of serum was not affected. The weak gel structure of ketchup and possibly other food dispersions such as purees and mustard is important for entrapping serum. When the gel is disrupted by shear the resulting structure is weaker and more prone to syneresis. Control of shear history is therefore vital in maintaining product quality.
Session 44C, Fruit & Vegetable Products: Chemistry
|