Viscoelastic characteristics and water mobility of osmotic dehydrated apple

CASIM, SILVINA M.; SALVATORI, DANIELA; NIETO, ANDREA B.; MATIACEVICH, SILVIA B.; BUERA, PILAR; ALZAMORA, STELLA M.

Viña del Mar, Chile

Congreso; International Congress on Engineering and Food; 2008

Instituto Chileno de Ingeniería para Alimentos (IChIA), International Association of Engineering and Food (IAEF)

Osmotic dehydration of fruit tissues takes place simultaneously with complex physical, microstructural and macrostructural modifications that influence the mechanical behaviour. The objectives of this study were 1) to analyze the linear viscoelastic characteristics of blanched and non blanched apple parenchymatous tissue (as determined by dynamic oscillatory shear and creep/recovery tests) after glucose osmotic dehydration at atmospheric pressure (ODA) or under vacuum (ODV), with or without calcium, and 2) to evaluate  the  water mobility changes by measuring spin-spin relaxation times (T2). For ODA, fresh cut apples (@ 60 mm x 60 mm x 6mm) (Granny Smith cv.) were immersed at 25ºC into a 20% w/w aqueous glucose solution to reach aw 0.97. In some experiments, calcium lactate (1000 ppm) was included in the osmotic medium. For ODV, the samples were immersed into a 59% w/w aqueous glucose solution and a vacuum pressure (21 Pa) was applied for 10 min. Previous blanching was performed in saturated vapor (1.5min). Dynamic oscillatory tests were performed in the controlled strain mode (20°C, Paar Physica CR 300 rheometer; 30mm diameter parallel plate geometry; 0.1 – 100 s-1, strain amplitude 0.05%).). Creep–recovery tests were conducted by applying a constant shear stress of 35 Pa till 60 s (recovery: 120 s). CPMG sequence was applied to acquire T2 (Bruker Minispec 20 Hz pulsed NMR; 25ºC). The storage modulus G’ greatly exceeded the viscous modulus G” for raw and treated tissues but there was a decrease in G’ due to treatments. A mechanical model consisting of a spring in series with two Voigt elements and a dashpot element properly predicted the creep compliance response. Overall compliance significantly increased by the treatments. There were not significant differences between creep compliance parameters or G’ values of apples subjected to the different treatments. The rheological response was correlated with previous microscopic observations (loss of turgor pressure, cell wall alteration). T2 values decreased for all treated samples, indicating a reduction of water mobility.  The significant decrease in G’ and overall compliance would mean that this lower water mobility was not enough to counteract the effect of structural tissue damage on rheological properties.