29C-26 |
Rheological behaviour of apple tissue at thermally and/or osmotic dehydration treatment |
V. Y. Martínez1, A. B. Nieto2, P. E. Viollaz2, and S. M. ALZAMORA3. (1) Departamento de Industrias, Facultad de Ciencias Exactas y Naturales - Universidad de Buenos Aires, Ciudad Universitaria. Int. Güiraldes S/N, Ciudad de Buenos Aires, 1428, Argentina, (2) Departamento de Industrias, Argentina, (3) Departamento de Industrias, Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires, Santa Maria 300. Tortuguitas, Buenos Aires, 1667, Argentina Viscoelastic properties of fruits change vastly during processing because of the alteration of the structural components at different levels and their chemical and physical interactions. Blanching as a pretreatment is usually carried out to inactivate enzymes, reduce the initial microbial load and sensitize survival microorganisms to further processing. Osmotic dehydration is a potential pretreatment for improving the quality of fruit products and has been extensively applied to the partial dehydration of vegetable tissues. Our objective was to analyze the rheological behaviour of fresh, thermally treated and/or osmotic dehydrated apple tissue. Different treatments were performed on fruit samples and then apple cylinders (0.6 cm thick) were loaded between the parallel plates (3 cm in diameter) of a Paar Physica CR 300 rheometer (USA). The treatments were glucose, glucose-calcium and sucrose impregnation at atmospheric pressure, glucose impregnation under vacuum, steam blanching and combination of these treatments. Linear viscoelastic limiting strain was determined at an angular frequency of 10 1/s. Storage (G´) and loss (G´´) moduli of apple tissue were obtained through the frequency sweep test (0.1 – 100 1/s), using an amplitude of 0.05%. For creep testing, the samples were subjected to a constant shear stress of 35 Pa during 60 seconds. G´ for fresh and treated tissues slightly increased with increasing angular frequency. Blanched samples showed a significant decrease in G´ as compared with the raw tissue, but all impregnated samples had a lower G´ value than the blanched ones, without significant differences between them. Calcium presence did not change the observed behaviour. A generalized Kelvin model with six elements was found to satisfactorily describe the creep compliance function with correlation coefficients > 0.999. Initial compliance and decay compliances significantly increased with the treatments while retardation times were approximately constant. These results would make possible improvements in quality assessment of commercial apples.
Session 29C, Food Engineering: Rheology and texture
|