Opacity Studies in Dehydrated Fruits in Relation to Proton Mobility and Supramolecular Aspects

Abstract

Water content is one of the variables affecting light scattering in foods, which on time determine gloss, translucence, and transparency. The objective of this study was to evaluate the effect of freeze-drying, humidification, and storage time on the changes of light distribution inside fruit tissues and their relationship with glass transition and proton mobility. Sliced and powdered freeze-dried apple, pear, and melon humidified at different relative humidities were employed. All the studied materials were translucent when fresh and became opaque after the freeze-drying process. In freeze-dried sliced fruits, the cellular structure contributed to maintain air inside the matrices, and then the opacity of the materials was almost constant, independently of the relative humidity. In the powdered materials, the compacted structure retained less air than sliced samples, and the powders were translucent due to the lower number of light-matter interfaces than in the porous-dried fruits, leading to a lower internal diffusion. The opacity decrease when increasing the water mass fraction followed a variation parallel to the Tg curve. Opacity decrease occurred at T-Tg values above 38 °C and was coincident with the observation of a proton population of higher mobility than that observed below the water content hydration limit value. Proton mobility at a molecular scale could thus be related to supramolecular events affecting food appearance, and this information may help to develop food products with the desired appearance.