Novel contact ultrasound system for the accelerated freeze-drying of vegetables

A contact ultrasound system for the integration into freeze-drying processes for vegetables was developed and process parameters were estimated. Red bell pepper cubes were freeze-dried on a stainless steel screen used as the sound transmitting surface. Ultrasound induced heating effects due to attenuation and absorption in the product and relevant process parameters were investigated. Continuous ultrasound application at an excitation amplitude of 6.7 μm resulted in immediate sample heating at reduced ambient pressure and a loss of freeze-drying conditions. A reduction of the excitation amplitude to 4.9 μm and of the net sonication time to 10% by applying an intermittent treatment with an interval of 10 s ultrasound and 90 s recovery phase allowed freeze-drying at increased sublimation rates without causing sample heating for 7 h of freeze-drying. As drying proceeds less sublimation energy is required in the process and sound energy was partially transformed into heat, which could be used for accelerated moisture removal during secondary drying in combination with further ultrasound effects improving heat and mass transfer (moisture migration, effect on boundary layers, improved evaporation at low pressure cycles). The ultrasound treatment reduced the drying time required to reach a final moisture content of 10% d.b. by 11.5%. The ultrasound treatment did not affect product quality in terms of bulk density, color, ascorbic acid content, and rehydration characteristics.Industrial relevance: Freeze-drying is a unique drying process for the production of high quality food products. However, high energy requirements for sublimation and vacuum generation turn it into a very cost intensive treatment. Current technologies used for the improvement of drying rates (heated plates, infrared radiation, microwave application) are limited to heating effects and can easily impair product quality. In this study a contact ultrasound treatment has been developed close to industrial freeze-drying, where the product is dried on trays. Ultrasound combines the positive effects of heating due to attenuation and adsorption with mechanical effects of pressure waves to improve drying rates and can thus be used to shorten drying times and has the potential to reduce related processing costs.

► A novel contact ultrasound system for freeze-drying of plant based food products is presented.
► Temperature effects in dependence of excitation amplitude and ultrasound interval are discussed.
► Intermittent ultrasound (10% net sonication time) at 4.9 μm excitation amplitude improved drying rate of bell pepper.
► The intermittent ultrasound treatment maintained bell pepper quality.
► Suitability of ultrasound treatments for increased primary and secondary drying is discussed.