The effect of freezing on the hot air and microwave vacuum drying kinetics and texture of whole cranberries

Abstract

The aim of this study was to evaluate the effect of convective and cryogenic freezing, hot air convective drying (HACD) at 60, 70, 80, and 90 °C and microwave vacuum drying (MWVD) at 100, 150, 200, 300, 450, and 500 W on the drying kinetics and texture of whole cranberries. Effective moisture diffusivities and drying rates were higher, whereas drying times were shorter for the samples dried by MWVD compared with the samples processed by HACD. The drying kinetics of cranberries during MWVD was discussed based on the hypothesis postulating that changes in the drying rate of cranberries during MWVD can be explained by and correlated with changes in the pressure gradient on material surface. Cranberries processed by MWVD were characterized by significantly greater hardness, gumminess, and chewiness in comparison with HACD samples. MWVD was found to be an effective method for producing dried snacks characterized by hard and crispy texture and considerable resistance to stress associated with manufacturing, packaging, storage, and delivery. HACD produced brittle fruit that were difficult to store and transport and were not fully suitable for direct consumption. Convective freezing before MWVD improved the overall appearance of cranberries, whereas cryogenic freezing combined with high temperature HACD adversely influenced the drying rate and produced dried cranberries with suboptimal overall appearance.

Keywords:

• Vaccinium macrocarpon
• pretreatment
• drying
• moisture diffusivity
• texture

Acknowledgments

The author would like to thank Waldemar Brandt and Piotr Sadowski of the University of Warmia and Mazury in Olsztyn for technical support.

Additional information

Funding

This study was supported by research grant no. 2015/17/B/NZ9/03601 from the National Science Center in Poland, entitled "The effect of unconventional pretreatment methods on the kinetics of hybrid dehydration and the physical and chemical properties of cranberries (Vaccinium macrocarpon)". This study was also partially supported by the University of Warmia and Mazury in Olsztyn.