Non Destructive Measurements of Moisture, Soluble Solids Content and Water Self-Diffusion Coefficient of Ripe and Unripe Osmo-dehydrated Kiwifruit Slices by NIR Spectroscopy

TYLEWICZ URSZULA; SANTAGAPITA, PATRICIO R.; PANARESE, VALENTINA; ROCCULI, PIETRO; DALLA ROSA, MARCO

Perugia

Congreso; Frontiers in Water Biophysics; 2012

Dept Physics, Dept Chemistry Università di Perugia

Kiwifruit is one of the most suitable fruit for osmotic dehydration process (OD), because offers the possibility to treat unripe fruits to obtain final products with acceptable firmness and organoleptic characteristics. The effect of OD on the quality and the water state of kiwifruit has been widely studied [1], [2], [3]. The aim of present work was to evaluate the feasibility of a non-destructive and rapid method based on NIR spectroscopy to determine the changes on moisture, soluble solids content and water self-diffusion coefficient promoted by OD on kiwifruit at different ripening stages. Two kiwifruit (A. deliciosa cv Hayward) groups with refractometric index values of 9.5 ± 1.1 (LB) and 14.1 ± 0.9 (HB) °Bx were selected. The OD process was carried out by dipping the samples (10 mm thick slices) in 61.5% (w/v) sucrose solution at 25°C, with continuous stirring for 0, 30, 60, 180 and 300 min. The moisture content was determined gravimetrically and soluble solids content (SSC) by measuring the refractive index with a digital refractometer. The proton transverse relaxation time (T2) and its peak intensity were determined at 24°C through the CPMG sequence using a 20 MHz spectrometer. Moreover, water self diffusion coefficient (Dw) measurements through pulsed field gradient spin-echo (PGSE) sequence were performed [3]. Non-destructive moisture, SSC and Dw of kiwifruit slices were determined by means of FT-NIR spectrometer using a fibre optic probe. Diffuse reflectance spectra were acquired in the spectral range between 4,000 and 12,000 cm-1, by averaging 32 scans at a resolution of 4 cm-1. OD leads to a great water loss (WL) from kiwifruit and the simultaneous counter-diffusion of solutes from the hypertonic solution into the kiwifruit tissues. The highest WL rates occurred during the first treatment hour, because the dehydration driving force was the greatest. The samples with LB presented the higher WL. As expected, the kiwifruit Dw coefficient values measured in the raw kiwifruit were lower than the free water one, as the structures and solutes of raw kiwifruit reduce water mobility, and decreased even more during OD, due to the water loss and sugar gain. For both the maturity groups, a principal component analysis (PCA) was carried out in order to set up a model to analyze the spectral data. Four PCs were obtained, which explained more than the 95% of the variance. Partial least square (PLS) regressions were performed between the NIR spectra and the analyzed physico-chemical parameters. The best predictive models showed coefficient of determination (R2) values of 0.897, 0.882 and 0.763 (cross validations) for moisture content, SSC and water self-diffusion coefficient, respectively. A multy-analytical approach is often needed to deeply investigate the behaviour of vegetable tissues during minimal processing. Then, by knowing the extent of changes on mass transfer parameters and water behaviour during OD, which in turn modified the structural and textural characteristics of the final product, it is possible to exploit both raw unripe and ripe kiwifruits from a technological point of view. In this scenario, NIR spectroscopy appears as an interesting tool to take on-line decisions about the optimum OD time, according to a specific target of final product.