The aim of this work was to investigate the feasibility of near infrared (NIR) spectroscopy both in quantifying the residual water content of osmo–air dehydrated apple rings and in differentiating them on the basis of the sugar solution used in the osmotic pretreatment so identifying the osmotic syrup used. In order to achieve this, a set of products obtained by different processing conditions was created. Apple rings were dipped for different lengths of time (30, 60 and 90 min) in three different sugar solutions at the same water activity (aw = 0.90): sucrose (59% wt / wt), maltose (62% w / wt) and a mixture (53% wt / wt) of fructose, glucose and sucrose having the same composition as apple. Air drying was then performed at different temperatures (70°, 80°, 90°) and the obtained osmo–air dehydrated apple rings were packed in polypropylene film under vacuum in order to avoid water absorption phenomena. NIR spectra, in the range 10000–4000 cm^–1, were recorded directly on packed apple rings using a FT-NIR spectrometer equipped with fiber optic probe. In calibrating the residual water content the coefficient of determination (R²) was 0.93, the RPD value was 3.33 and RER value was 13.79, which could be considered, for our purposes, suitable for controlling the dehydration process. Moreover, applying a classification statistic method on NIR data, such as PLS discrimination analysis, it was also possible to identify correctly which sugar solution was used in the osmotic pre-treatment independently of the conditions used. Two-dimensional correlation analysis was also carried out, in order to investigate how the ratios between the main constituents, sugars and water, involved in the osmo–air dehydration varied as a function of the process variable used.

Keywords: air drying, osmotic dehydration, apple, water content