Effect of high levels of calcium on the viscoelastic behaviour of apple tissue.

DOCTOROVICH, R.; MARTÍNEZ, V.; SALVATORI, D; ALZAMORA, S. M.

Nueva Orleans, USA

Congreso; 2005 Institute of Food Technologists Annual Meeting (IFT); 2005

Institute of Food Technolgists

Currently there is a growing interest in the design of functional vegetable matrices with added physiologically-active components in order to increase the consumption of a specific vitamin, mineral or other compound. Viscoelastic properties of fruits change after calcium incorporation because of the alteration of the structure at different levels and their chemical and physical interactions. The objective of this work was to determine the effect of calcium impregnation at atmospheric pressure (AI) or under vacuum (VI) on the viscoelastic properties of apple slices using oscillatory shear and creep tests. AI experiments were conducted at room temperature by immersion of cylindrical apple samples in agitated isotonic glucose aqueous solutions with calcium salts (5.88 % calcium gluconate and lactate). Samples were taken out of the osmotic medium at selected time intervals (2, 6, 10, 22 h). For VI, a pulse of 10 min was applied at 50 mmHg and then atmospheric pressure was restored. After treatments apple cylinders (0.6 cm thick) were loaded between parallel plates (3 cm in diameter) of a Paar Physica CR 300 rheometer (USA). Storage (G´) and loss (G´´) moduli were obtained through the frequency seep test (0.1 and 100 1/s), using an amplitude of 0.05% (within linear viscoelastic range). For creep test, samples were subjected to a constant shear of 30 Pa during 60 s. All samples had a viscoelastic solid behavior with G´ dominating the viscoelastic response. Treated samples with or without calcium showed a pronounced decrease in G´ as compared with the fresh tissue. A generalized Kelvin model with six elements was found to satisfactorily describe the creep compliance function with correlation coefficients > 0.99. Initial compliance and decay compliances significantly increased with the treatments while retardation times were approximately constant. These results would allow improvements in quality of calcium impregnated apples.