In our last column, we attempted to show how the sizing of coffee particles is not a trivial exercise. Three-dimensional objects with diverse morphologies, geometries, and topographies are not easily quantified in a simple metric.
Size alone, without qualification, can be rather meaningless. In an effort to better communicate particle characteristics, particle technologists have developed at least five main geometrical size descriptions, as well as two dynamic statistical diameters.
However, how best to quantify the variety of shapes and sizes often encountered in materials is still an active discipline of scientific inquiry, as the answer typically depends on the application. Sizing particles is simply a means to an end, with the end being some correlation of the material’s properties with some process of its usage or preparation.
In coffee, shape and size are the most important characteristics that affect pore geometry, porosity (total pore volume), pore size distribution, and solid surface area – factors which impact extraction.
In this article, we will briefly discuss three different sizing techniques that are often used for coffee, looking at some advantages and disadvantages of each.
“I often refer to sieving as the Cinderella of particle size analysis methods; it does most of the hard work and gets little consideration,” said Harold Heywood, a former Professor at the Imperial College of London and a recognised leader of modern particle sizing science.
It’s been recorded that the early Egyptians once used woven reeds and grasses to size grains. Though the level of sophistication and standardisation has increased since, the basic principles of sieving remain the same: fine materials are separated from coarser material by means of a mesh or perforated vessel. Sieves with smaller and smaller apertures can be stacked together, allowing a sample to filter through the various screens (see Figure 1). After movement of particles into the various sieves has ceased, the material contained within each sieve is weighed.
Note that the largest dimension may pass through a smaller pore size if the particle orients itself a certain way, so that it is classified by the second largest dimension. The particle in Figure 1 has one dimension of 430 micrometres, but is able to pass through the sieve with 400-micrometre pores. This is a known disadvantage of particle sizing via sieving. There is however, a potential to underestimate size characteristics of some particles.
Besides the “second largest dimension” sizing issue shown in Figure 1, some other concerns with sieving involve agglomeration (the tendency for materials to clump together due to their high moisture, high oil content, and/or irregular topography), blinding (plugging of screen openings), and electrostatic charges causing clinging of particles or increased agglomeration and blinding. Despite these concerns, the biggest advantages of sieving are the cost of equipment and the physical separation of particles that results.
n Using the concept of light scattering, laser diffraction is a popular method for rapidly determining particle distribution. The basic principle of light diffraction is that small particles in the path of a light beam scatter the light in predictable patterns.
Based on the intensity pattern of the scattered light, as a function of the sample’s angle to the light source, it is possible to calculate the distribution of particles as they pass through the light source-detector array. Unlike the other two methods presented here, laser diffraction doesn’t look at each particle at once (that is, it’s not a counting method). Rather, it quickly measures the whole sample in what is called an ensemble method.
Commercial laser diffraction equipment utilises algorithms based on the Mie theory of light scattering. Underlying this theory are two important assumptions to consider: particles are smooth and particles are homogenous. Deviations from these assumptions may lead to various biases in the data, such as overestimating particles on the smaller and larger ends of the scale.
Let us return to the 3-D microscopic image of an actual coffee particle. See Figure 2. Note the potential deviations from spherical, smooth, and non-porous.
Further, Schenker and colleagues in the Journal of Food Science (2000) paper Pore structure of coffee beans affected by roasting conditions, use a variety of measurements (volumetry, mercury porosimetry, and electron microscopy) to demonstrate that the degree of roast has a significant impact on pore-structure development in coffee.