Elsevier

Advanced Powder Technology

Volume 30, Issue 11, November 2019, Pages 2823-2831
Advanced Powder Technology

Original Research Paper
Optimization of aqueous microgrinding processes for fibrous plant materials

https://doi.org/10.1016/j.apt.2019.08.029Get rights and content

Highlights

  • Surface functionality of organic plant materials evolves with milling process.

  • Oil migration to the surface was found to be the main cause for re-agglomeration.

  • Milling step drives stability of suspensions of micronized food particles.

  • Particle functionality is maintained throughout spray drying and reconstitution.

Abstract

Fibrous plant-based materials are characterized by inhomogeneous structure and composition, which further evolve during wet grinding processes and affect the surface functionality of micronized particles. Therefore, the performance of aqueous microgrinding operations in stirred media mills can be optimized by investigating the interaction between process conditions and material properties of heterogeneous fibrous plant materials.

In this experimental study it is shown how particle size reduction, tendency of re-agglomeration and stability of the suspension of micronized particles are driven by the specific energy input, residence time, temperature and presence of surfactants during the milling process. A structured experimental approach is described to optimize the achievable particle size reduction, expressed by the top cut diameter d90,3. It was found that the applied wet milling process determines the stability of particle suspensions throughout further downstream processing, making the grinding process the core unit operation with respect to the performance and formulation of food products containing micronized particles.

Section snippets

Motivation for fine grinding of plant materials

Milling is a fundamental mechanical process in various industries such as mining (ores, salts), construction (sand, cement), chemistry (pigments, catalysts), paper, pharmaceuticals and food to reduce the size of particles with the aim to increase reactivity, to access inner structures, to improve the homogeneity of mixing and to enhance mass transfer. Common applications in food technology are milling of grains [1], seeds and nuts [2], [3], salt and sugar [4], dried herbs and spices [5], coffee

Principle and physical description of stirred media milling

The general operating principle of stirred media mills is based on grinding media collisions. Mechanical power is supplied via a stirrer into the grinding chamber in which the media is moved intensively. Finally, the energy has to be transferred to the product particles in order to induce particle fragmentation. The efficiency of particle size reduction is strongly dependent on process variables of stirred media milling. Key operating parameters are the stirrer tip speed and grinding media

Materials and methods

In this chapter, the process chain from the educts consisting of fibrous organic plant material to a final reconstituted beverage is presented. Each process step and its parameters are explained. The term “fibrous” is used in this work referring to materials primarily consisting of dietary fibers, such as cellulose. It does not necessarily imply an elongated particle shape.

Premilled roasted coffee particles (RC) were used as feed material for micronization, their initial particle size

Impact of process parameters on the grinding result

The reduction of the top cut particle size x90,3 was defined as key quality criterion to evaluate the micronization of RC particles. First micronization experiments were performed with suspensions containing a solids mass concentration of 0.1 using different grinding media types under variation of the stirrer tip speed.

Fig. 3 shows the particle size x90,3 as function of the specific energy input for micronization with different grinding media materials under variation of the tip speed after one

Conclusions

This study investigated the optimization of wet milling processes of aqueous suspensions of fibrous plant-based food particles, taking the example of roast and ground coffee particles. The objectives were to minimize the top-cut particle size x90,3 and to maintain a stable suspension of micronized particles after milling and after further downstream processing. Specific energy input, residence time, particle concentration and temperature were varied in a systematic experimental approach. Unlike

Acknowledgements

This is a joint publication between Nestlé Research, which funded the study, and the universities TUHH and TUBS. As authors from Nestlé Research, we cannot acknoledge ourselves. It is Nestlé's intention to actively contribute to the scientific progress in Food Science and Technology through publications. Rather than acknowleding for funding, we express the active and collaborative spirit of this scientific project as joint authors.

References (30)

  • B. Belderok

    Developments in bread-making processes

    Plant Foods Hum. Nutr.

    (2000)
  • N.N. Mohsenin

    Physical Properties of Plant and Animal Materials

    (1978)
  • A. Borges et al.

    Effect of water activity on the mechanical properties of selected legumes and nuts

    J. Sci. Food Agric.

    (1997)
  • T. Varzakas et al.

    Food Engineering Handbook – Food Process Engineering

    (2014)
  • W.H. Ukers

    All About Tea

    (1935)
  • View full text