Drying food more efficiently

Thin Film Drying

The industrial sector is one of the largest energy consumers now and will remain to be so in the future. Achieving circularity in this sector and moving to more sustainable energy resources will stimulate a more sustainable way of producing materials and food. It is absolutely essential for creating a sustainable future industry that we also focus on making processes more energy efficient. Drying is one of the most energy intensive processes in the industry and especially so in our food industry. The ISPT drying cluster’s Thin Film Drying project focusses on understanding conductive drying processes in order to make more use of this energy efficient technology.

Conductive drying as an efficient way of drying food products

Conductive drying is a completely different drying principle compared to mainstream hot air drying. In conductive drying a liquid feed is indirectly heated via steam condensation when spread as a thin film on a hot wall. Under atmospheric conditions the product is dried at its boiling temperature, i.e. 100 ^oC. A major advantage is that such a process can be operated more energy efficiently than hot air drying. Although the heat load to the product during conductive drying can be large, costing a lot of energy to achieve. That can be improved by operating the process at reduced pressure thus lowering the boiling temperature. In this way food products can be efficiently dried and product quality is retained, especially in terms of nutritional quality and/or colour.

Comparing different conductive technologies

For this project laboratory scale drying experiments with novel custom built drying devices were carried out. The purpose of these experiments was to build an understanding of the conductive drying kinetics and subsequently the degradation and formation dynamics of product quality parameters. In order to compare and study the different conductive drying technologies an experimental pilot scale study and techno-economic assessment were carried out. The comparison included product quality, energy consumption and operational and capital costs. The pilot scale experiments were carried out with tomato puree, skim milk and whole milk. The conductive drying techniques compared were (vacuum) drum drying, refractance window drying and agitated thin film drying, which were available at four different locations.

Formulating food-drying guidelines

The project and all the comparative studies conclude that conductive drying technologies overall show a lower energy consumption, lower operating expenditures and capital expenditure than conventional spray drying, where a liquid feed is atomised and dried by exposure to hot air. The energy consumption was compared on the basis of energy use per ton water removed, where it should be noted that conclusions may ultimately vary per product. By combining the techno-economic assessment and the experimental lab- and pilot-scale results the researchers were able to formulate guidelines for selecting an appropriate conductive drying method for a (tomato-based) product. Moreover, the custom-built lab-scale set-ups and predictive models developed in this study facilitate fast scaling up of these attractive thin film drying technologies.

Thin Film Drying research

PhD Jun Qui presented his results as a part of the Thin Film Drying project. We asked him about his  research, his experience within the cluster and the ISPT.

My name is Jun Qiu. I was born on 26th April, 1989, in Nantong, Jiangsu Province, China. After finishing a 6-years study in the middle and high school affiliated to Nantong No.1 Middle School, I started my bachelor study of Food Science and Technology at South China University of Technology in 2008. In 2012, I started my master programme of Food Technology at Wageningen University and Research. I finished my MSc thesis at the Laboratory of Food Process Engineering under the supervision of Dr. Jacob Bouman and Dr. Maarten Schutyser studying morphology development of whey protein droplets during single droplet drying. I carried out my MSc internship at R&D Unilever Vlaardingen under the supervision of Dr. Seddik Khalloufi on modelling volume reduction of food products during drying. After obtaining my MSc degree in 2014, I started my PhD research at the Laboratory of Food Process Engineering under the supervision of Dr. Maarten Schutyser and Prof. dr. Remko Boom. When I was doing my internship in Unilever, my supervisor Dr. Seddik Khalloufi told me the information about this PhD project. It matched my personal and professional interest very well. Therefore, I applied and got this PhD position.

During my PhD I developed two custom-built experimental systems, which represent (vacuum) drum dryer and agitated thin film dryer (ATFD), respectively. With these devices, I investigated the drying kinetics of conductive drying and ATFD drying behaviour of different food formulations. In addition, I carried out pilot-scale drying experiments of tomato and milk powders, with a vacuum drum dryer (ANDRITZ Gouda, NL), a regular drum dryer (Unilever, NL), an agitated thin film dryer (Bodec, NL) and a refractance window dryer (ILVO, BE). I studied the influence of different conductive drying processes on the final powder quality, i.e. volatile and non-volatile profiles, colour, etc..

1. During conductive drying, three distinct periods could be identified: heating (period 1), boiling (period 2) and conductive drying (period 3).
2. ATFD is suitable for materials that are not too sticky during drying, and have fast crystallisation kinetics or transform into a brittle solid matrix.
3. RWD (before VDD) leads to high retention of volatiles in powders that may be related to a different driving force for evaporation during RWD. In contrast, products dried with drum drying had less volatiles, but retained higher levels of specific non-volatile compounds (i.e. amino acids) that can contribute to taste perception.

My work provided better insights on the principle of conductive drying of foods and to translate insights generated to operation of different mild conductive drying technologies. Working with ISPT is an impressive experience. I really like to work with it. It provided a good platform, with which I can work closely with different companies and learn from them.