Physical and mechanical properties of raspberries subjected to osmotic dehydration and further dehydration by air- and freeze-drying

Abstract

The aim of this study was to analyze the effect of the application of dry and wet sucrose infusions, as pretreatments previous to air-and freeze-drying, on mechanical and physical properties of raspberries: water sorption, glass transition temperature (T g), molecular mobility, texture and rehydration properties. Different dry and wet sugar infusions were prepared using combinations of additives: sodium bisulphite, citric acid, sodium bisulphite and citric acid, and no additives. These specific pretreatments are often used to obtain better sensorial characteristics of fruits upon further drying. After the dehydration step (air-or freeze-drying), all the samples were in the supercooled state. Pretreated samples presented lower T g values and lower spin-spin relaxation times than control samples. Regarding texture, pretreated samples showed lower firmness than control samples. Also, freeze-dried pretreated samples showed higher firmness and lower deformability than air-dried pretreated ones. When considering the hygroscopicity, freeze-dried samples were more hygroscopic than air-dried ones. The fresh-like dried raspberries obtained could be directly consumed as snacks or incorporated in a composite food, such as a cereal mix. In this latter case, pretreated fruits would be more suitable, since their rehydration capacity at short times was relatively low.

Valorization of postharvest sweet cherry discard for the development of dehydrated fruit ingredients: compositional, physical, and mechanical properties

“…Shrinkage (S) was evaluated as volume reduction percentage, and hygroscopicity (H) was analyzed by exposing the ingredients to a 75% RH. Both determinations were done in triplicate according to procedures described by Sette et al …”

“…Water transport from the solid matrix causes changes in mechanical and structural properties, resulting in material with reduced molecular mobility due to low water content and a high concentration of solids. The phenomenon of volumetric shrinkage that occurs during drying also causes a change in food size and shape . On the other hand, freeze drying usually offers higher quality food due to minimal shrinkage, high rehydration capacity, and nutrient retention in the dried product.…”

“…This phenomenon affects the rehydration capacity of dried products, the rate and extent of the rehydration being strictly related to the duration and severity of the previous dehydration process . If the volumetric shrinkage is minimal, the presence of well defined intercellular holes may increase the rate of rehydration . The application of osmotic dehydration as pretreatment before drying causes partial removal of water from the tissues together with the entry of solutes, with a high impact on quality parameters, mainly in terms of color, volume, texture, and nutrients.…”

Cost-effectiveness assessment of dried raspberry production

“…The products made with this method have relatively high production costs compared to the convective drying method where straw is used as the main source of energy. Sette et al, (2016) indicate that the freezedried raspberry provides product with a higher quality in terms of strength and the reduction of the deformation of the dried fruit as compared to a drying process based on the thermal energy. In addition to freeze drying, raspberries can be treated by a conventional method (convection drying) shown in this paper and whose potentials have not been sufficiently explored in domestic literature and practice.…”

Comparison of energy consumption in the convective and freeze drying of raspberries

“…Moreover, the drying time is longer than in other drying processes. Conversely, the convective drying process, as one of the most commonly used methods of drying, results in products of somewhat lower quality (Sette, P. et al, 2016;Kowalski, S.J., et al, 2016). However, the drying process itself features lower energy requirements and a shorter drying time.…”