Café en poudre l'analyse de la taille et de la forme des particules pour un meilleur goût

Coffee beans are one of the most important commodities, with an estimated 1.4 billion cups of coffee consumed worldwide every day. The taste of coffee is determined by the roasting of the beans, the size distribution of the powder, and the type and quality of preparation. Different preparation methods in the brewing and filtering process (e.g. espresso machine, filter coffee or AeroPress) require different grinds of coffee powder for an aromatic result. When roasted coffee beans are ground into powder, the determination of the particle size distribution plays a decisive role, as it has a significant influence on the brewing and filtering properties and thus on the taste and salubriousness of the beverage.

Lors de la préparation du café, il est important d'obtenir une extraction optimale des ingrédients qui sont dissous du café moulu par l'eau chaude ou la vapeur. Plus la poudre de café est fine, plus il est possible d'extraire d'ingrédients en peu de temps. Si la mouture n'est pas adaptée de manière optimale à la durée et à la température du processus d'infusion, le café risque d'être surextrait ou sous-extrait. Un café sous-extrait (= mouture trop grossière) a peu d'arôme et un goût aqueux. Un café sur-extrait (= mouture trop fine) a un goût amer en raison d'une trop grande quantité de composants dissous (par exemple les acides tanniques). En déterminant de manière fiable la taille des particules, il est possible d'obtenir une mouture reproductible pour le processus de préparation concerné, ce qui permet d'obtenir un café savoureux aux arômes équilibrés. En raison de sa forte teneur en huile, de sa large distribution granulométrique et de sa forme très irrégulière, la poudre de café présente un comportement difficile en tant que matériau en vrac, c'est-à-dire que les particules ont une forte tendance à s'agglomérer et que la poudre est difficile à verser ou à transporter. Les méthodes de mesure mécaniques et optiques doivent tenir compte de ce problème.

Traditionally, the particle size distribution of coffee powder was determined by analytical sieving. However, the laser diffraction method has become increasingly established as the standard method. Both methods have certain limitations in terms of information content, accuracy and sensitivity. Coffee powder, especially when used in capsules or pods, is highly optimized for the respective preparation process and must comply with very tight quality specifications. These can often only be checked with imaging methods, which provide very high-resolution size distributions with simultaneously high sample throughput.

Example Measurements of Coffee Powder with Dynamic Image Analysis, Laser Diffraction and Sieving

Coffee powder typically has a broad particle size distribution with a pronounced fine and coarse part. The fine fraction is less than 200 μm, the coarse fraction can reach up to 2 mm.

Thanks to the patented two-camera system and the resulting very wide dynamic measuring range, the CAMSIZER X2 can determine both with high resolution and good statistical reliability in the case of coffee powder. This is shown in Figure 4 using the example of coffee powder with different grinds.

During the measurement, the particles were dispersed with compressed air in the X-Jet sample feed module (Fig. 2) at 80 kPa. For the vibratory feeding of the coffee powder on a dosing chute, Microtrac developed devices that allow problem-free feeding for measurement even with very poorly flowing coffee powders.

The roasting of the coffee beans influences their brittleness. Ground coffee from brittle beans often consists of angular, or pointed, grains, which lead to lower packing density in compacted coffee. Both, the particle size distribution, and the particle shape affect the bulk density, the filtering and extraction properties of the powder, and thus also the quality of the prepared coffee.

Both figures show that the CAMSIZER X2 can use image analysis to determine both width, length, and circle-equivalent diameter simultaneously and output each as its own distribution curves. Thus, one measurement produces multiple results based on the three different definitions of particle size. The results with respect to these parameters differ significantly, which at the same time describes the irregular grain shape: for spherical particles, the distributions for all three size definitions would be identical.

In comparison, the result of laser diffraction, which assumes the particles to be spherical, provides only an average over the width and length of the particles. Thus, the distribution obtained by laser diffraction most closely matches the "circle equivalent diameter" definition of image analysis, but with a bigger span. In the two examples, the median values (d50) are well comparable, and the same is true for the percentage of fines < 200 µm. Laser diffraction is thus quite capable of characterizing coffee powder reasonably reliably.

However, the correct detection of coarse fraction is difficult. As the diffraction angles become smaller with increasing particle size, this is more difficult to measure. Thus, the resolution of the instruments inevitably becomes poorer for large particles. In addition, small amounts of oversize particles may not generate enough signal to be represented in the results.

This is particularly clear in the example in the second figure: the image analysis finds particles up to 2 mm long, the laser result does not show any particles > 1200 µm!

The problem with sieve analysis of coffee powder is the cohesiveness of the product. The oil content inevitably leads to clumping, adhesion to the sieve frame and clogging of the sieve meshes. Below 200 µm, air jet sieving should be used in any case.

All this represents a high time expenditure to determine a size distribution that ultimately comprises no more than eight data points (= number of sieves used). Comparison with dynamic image analysis shows that the sieve data compare very well with the size definition "particle width".

This is due to the fact that during the sieving process, the grains align in such a way that they pass through a sieve with their smallest possible projection area. Sieve analysis therefore tends to determine particle width. It further follows from these observations that a comparison between sieve analysis and laser diffraction can hardly be reasonably and reliably achieved for irregularly shaped particles such as coffee powder.