Abstract
The aim of study was to determine the effects of sonication (S), convective freezing (F), convective freezing preceded by sonication (SF) as well as cryogenic freezing (N) on the osmo-microwave-vacuum drying kinetics, energy usage and properties of dried cranberries such as moisture content, moisture diffusion, water activity, density, porosity, thermal conductivity, thermal diffusivity, volumetric heat capacity, lightness, redness, yellowness, total differences in color, saturation and hue, hardness, cohesiveness, springiness, and chewiness. Osmo-microwave-vacuum drying of cranberries took from 13.5 to 16.0 min. All initial treatments increased the moisture diffusivity and thus reduced the drying time. The most energy effective method was osmo-microwave-vacuum drying preceded by sonication (S) of fruits. Osmo-microwave-drying of cranberries subjected to convective freezing preceded by sonication (SF) resulted in the highest lightness (32.5 ± 0.5), redness (33.9 ± 0.7), and yellowness (11.3 ± 0.5) of fruits, as well as the lowest cohesion (the lowest resistant to stress associated with manufacturing, packaging, storage, and delivery). The lowest hardness, i.e. 12.3 ± 0.4 N and the highest cohesiveness and springiness, i.e. 0.38 ± 0.02 and 0.74 ± 0.03 of dried fruits, were noted for berries subjected to initial cryogenic freezing (N). Cryogenic freezing (N) combined with osmo-microwave-vacuum drying resulted in the largest color changes of fruits and the highest thermal conductivity. Sonicated and convectively frozen (SF) fruits were characterized by the highest thermal diffusivity. Sonication (S), convective freezing (F) and their combination (SF) significantly reduced the volumetric heat capacity of cranberry fruits.
Highlights
Cranberry fruit is a rich source of bioactive compounds with antioxidant properties (Zielinska et al 2017)
Osmo-microwave-vacuum drying of raw cranberries was longer by 33% than microwave-vacuum drying of whole fruits, while osmo-microwave-vacuum drying of fruits subjected to S, F and Sonicated and convectively frozen (SF) treatment was shorter by 7%, 4% and 3% than microwave-vacuum drying of whole fruits (Staniszewska et al 2019)
The results indicated the lowest changes in the local temperature of the material during osmo-microwave-vacuum abcd – the same letters in columns mean no statistical differences between samples (p ≤ 0.05); Symbols: MC – moisture content [kg H2O/kg DM], t – time [min], specific moisture evaporation rate of drying (SMER) – specific moisture evaporation rate during microwave-vacuum drying at a microwave power of 300 W [kg H2O/kWh], R – raw, S – sonication, F – convective freezing, SF – sonication combined with convective freezing, N – cryogenic freezing, OM – osmo-microwave-vacuum drying frozen (SF) as well as cryogenically frozen (N) cranberries was more effective than microwave-vacuum drying of fruits (Staniszewska et al 2019)
Summary
Cranberry fruit is a rich source of bioactive compounds with antioxidant properties (Zielinska et al 2017). Seasonality, production scale, as well as consumer preferences cause that they are often preserved by drying (Zielinska, Markowski 2018). They have become of interest to the food producers as they may constitute valuable ingredients to cookies, cakes, cereals, sauces and salads due to the presence of anthocyanins and their antioxidant activity (Beaudry et al 2003). Microwave dielectric heating results from the ability of a given reagent to absorb the energy assigned to microwave radiation and its conversion into thermal energy This heating, caused by the electrical component of electromagnetic radiation, occurs through the rotation of dipole molecules and ionic conductivity (Nowak, Kowalska 2007). The advantages of microwave-vacuum drying include short drying time, explosion puffing of fruits and vegetables and high quality of the final product (Zielinska et al 2019, Zielinska, Zielinska 2019)
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