Effect of relative humidity on shrinkage and color changes in dehydrated strawberry

MARCELA AGUDELO; CAROLINA SCHEBOR; MARÍA DEL PILAR BUERA

Querétaro

Simposio; 11TH INTERNACIONAL SYMPOSIUM ON THE PROPERTIES OF WATER (ISOPOW11); 2010

Discoloration and shrinkage of dehydrated fruits lowers the quality and economical value of products. The objective of this work was to assess the effect of relative humidity (RH) on color alterations and shrinkage in freeze-dried strawberry slices. A method was set up to evaluate visually detectable changes, such as radial shrinkage and non-uniform color changes occurring after equilibration at 20ºC (at various RHs) and during storage at 45ºC. The changes in reflectance and surface area of the slices were analyzed using a computer vision system. The chromatic coordinate a* (CIELAB color space) was employed to measure color changes. Shrinkage was determined as the % of radial area reduction relative to the control sample. An exponential decay of the area occurred in the samples during 14 days of humidification at 20ºC, reaching a 50 % area reduction at 93 %RH. The observed shrinkage was caused by storage temperatures well above the glass transition temperature. No further shrinkage was observed upon 22 hours of storage at 45ºC after humidification. The variable a* did not change significantly during humidification, but it was modified upon storage at 45ºC: it decreased as increasing time and/or RH. The final color is the result of loss of redness due to pigment destruction and of browning development. As strawberry slices are heterogeneous materials, the image segmentation through the L* and a* coordinates. The pixels proportion of light areas in the pictures was greater at low RH and short storage times. As increasing RH and storage time the pixels proportion of dark areas and the homogeneity color distribution increased. The segmented image analysis was appropriate to evaluate some relevant characteristics of the visual appearance changes related to the humidification level and storage time.