Original Research PaperOptimization of aqueous microgrinding processes for fibrous plant materials
Graphical abstract
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.
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