Optical properties–microstructure–texture relationships of dried apple slices: Spatially resolved diffuse reflectance spectroscopy as a novel technique for analysis and process control

Abstract The potential of spatially resolved diffuse reflectance spectroscopy in the 500–1000 nm range by means of a fiber-optics probe was investigated for acquiring scattering and absorption properties of air dried apple rings subjected to different pre-treatment conditions: without osmo-dehydration (TQ) and with osmo-dehydration for 1 (OSMO1) and 3 h (OSMO2). The fresh apple rings were produced from ‘Golden Delicious’ apples at harvest (H) and 5 month storage at 2 conditions: controlled atmosphere (CA) and normal atmosphere (NA). Microstructure properties of the dried apple rings were also obtained from X-ray micro-CT measurements. The TQ samples were found to have significantly higher scattering properties, thicker tissue, smaller pore sizes, were less crispy, and required higher snapping work or rupture energy than the OSMO1 and OSMO2 samples. On the other hand, no significant differences were observed between the scattering properties, microstructure, and textural quality of the OSMO1 and OSMO2 apple rings. From these results, it was concluded that there is a clear process–microstructure–quality relation in osmo-air-dried apples which can be measured non-destructively with spatially resolved diffuse reflectance spectroscopy. Therefore, this study confirmed the potential of spatially resolved diffuse reflectance spectroscopy for non-destructive quality assessment of air-dried apple slices, which provides perspectives for drying process optimization. Industrial relevance Dried fruit is an important category of processed foods on the market with a worldwide annual production of 9.5 × 10 9  kg in 2012, which is 13% higher than the production in 2011 (International Nut and Dried Fruit Council, 2013). Besides the nutritional value the textural properties also have a strong impact on the consumption quality as they determine the taste sensation and digestibility. Therefore, the food industry is demanding fast and non-destructive measurement techniques which could be used on-line/in-line for the evaluation of the texture and microstructure of individual products and for realtime process control and optimization. The spatially resolved spectroscopy technique investigated in this research allows to detect textural quality differences in dried food products (apples in this study) and has high potential for use in the food industry thanks to the non-invasive, sensitive and fast interaction of the propagating light with food matrices.

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