Modelling and dynamic optimisation of quality indicator profiles during drying

E.J. Quirijns

Research output: Thesisinternal PhD, WU


Food properties change during processing, causing an altered quality experience by the consumer. Apart from quality considerations, also economic and environmental aspects are important in industry. They are conflicting, leading to the wish to find the optimal operation. Since quality is a subjective quantity, quality indicators are used that can be objectively measured. Those quality indicators are linked to the relevant quality attributes. In many processes a spatial distribution of the quality indicator will arise, the so-called quality indicator profile (QIP). In this thesis, it is investigated whether the consideration of QIPs is necessary in quality optimisation. Drying of the enzyme catalase, immobilised in starch cylinders is used as case study, where the residual catalase activity formed the quality indicator.

Simulation and optimisation studies demonstrate that average values for the quality indicator do not uniquely represent QIPs inside the material. Safety of the product can only be guaranteed by using QIPs in optimisation procedures. By optimisation, the spatial distribution of the quality indicator can be influenced, creating advanced possibilities to control product quality.

Based on these findings a new microscopic drying model is developed, to obtain a proper description of QIPs. In this model a distinction is made between different classes of water present in the material. The conversion between them is described by a sorption process. Since the model consists of both a diffusion and a sorption part, it is called the diffusion-sorption model. In this model all parameters have a physical meaning. The assumptions made in the model are supported by independent sorption experiments, that also provide the essential parameters. The diffusion-sorption model is compared with four other diffusion models known from literature. It performs best in both calibration and validation, confirming the confidence in this model.

Experimental verification establishes the central role of QIPs in quality description and optimisation. The experiments show that QIPs exist during and after drying and that they are significantly influenced by varying the air conditions. Based on independent enzyme inactivation experiments a fractional conversion model is developed that can predict the residual enzyme activity as function of moisture content and temperature.

Both the diffusion-sorption model and the quality model lead to reliable predictions. The two models are combined to predict QIPs during drying. The drying curve agrees with the moisture content measurements. The predicted QIPs are qualitatively comparable with the experimental ones, though higher in enzyme activity.

Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
  • van Straten, Gerrit, Promotor
  • van Loon, Wilko, Co-promotor
Award date24 Apr 2006
Place of Publication[S.l.]
Print ISBNs9789085044284
Publication statusPublished - 24 Apr 2006


  • drying
  • drying quality
  • quality standards
  • quality controls
  • indicators
  • optimization
  • optimization methods
  • dynamic models


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