Monitoring the Particle Size Distribution of Ground Coffee

Measuring the particle size distribution of samples of ground coffee is a common way of monitoring the grinding operation for consistency. Sieve analysis is one of the more traditional methods of carrying out size measurements. While this method does facilitate grind control, the measurements are complex and time intensive. In addition, the resolution is insufficient to detect the beginning of drift within the grinding loop. As an alternative, laser diffraction is a well-established and convenient method for measuring particle size. It has a measurement range of 0.01 to 3500 mm, and possesses rapid measurement cycles that ensure reproducible manufacture of the coffee grind sizes linked to different coffee grades.

This article demonstrates the process of performing bulk measurements of coffee grounds as part of production and quality control, using the Funnel Sample Feeder (FSF) for the Mastersizer 3000's Aero S dry powder disperser (Figure 1). The experimental setup allows dispersion and measurement of coffee samples of up to 130 ml in volume accurately and rapidly. The FSF includes a sample feed hopper and a funnel capable of holding large sample volumes. Further, the coatings applied on the FSF surfaces are designed to reduce adhesion of oily and sticky samples, which minimizes the need for manual cleaning of the unit between measurements. This is one of the major benefits over sieving method.

The Funnel Sample Feeder attached to an Aero S Dry Dispersion Unit.

Figure 1. The Funnel Sample Feeder attached to an Aero S Dry Dispersion Unit.

In this experiment, FSF is used to measure aliquots of 5 to 6 g of both coarse and fine coffee so as to determine if the achieved measurement reproducibility satisfies the requirements of the guidance provided in ISO13320 – the international standard for laser diffraction measurements. The coarser coffee was selected as the free-flowing sample, while the fine grade coffee was selected for its cohesive behavior.

Measurement of Two Coffee Grades

The process involved tumbling and subsequent measurement of the fine grade coffee in aliquots of 5 g. Because of the greater polydispersity, the coarser grade coffee was tumbled and separated with a spinning riffler so as to ensure reproducibly sampled aliquots of sizes 5 to 6 g.

Based on the procedure defined in ISO13320, the conditions needed for sample dispersion were determined. Within the Aero S dry powder disperser, each sample was found to be completely dispersed at a dispersion pressure of 2 bar. The control of sample flow rate through the Aero S disperser ensures reproducible dispersion and a realistic measurement concentration within the Mastersizer 3000 system.

The versatility of the FSF design enables regulation of sample flow with the help of a specially designed mesh that aids in capturing and dispersing very large or loose agglomerates and with its simple magnetic gates to fine tune the flow gap within the hopper. The optimum feed rates for the coarse and fine grade coffee were estimated to be 80% and 60%, respectively.

Sample Measurement

With the funnel on the FSF, addition of sample to the Aero S dispersion unit has become easier and more straightforward, reducing the time required to obtain measurement results. For both coffee grades, fifteen aliquots of the fine measurements were produced with a time interval of approximately 2 minutes between each measurement (including system initialization, sample measurement and cleaning). Figure 2 shows the average particle size distributions, and Tables 1 and 2 summarize the reproducibility of the analysis.

Result overlay of the 15 measurements made for both coffee grades.

Figure 2. Result overlay of the 15 measurements made for both coffee grades.

Table 1. Variation, as measured by RSD, of 15 aliquots of fine grade coffee.

Record Number Dv10 (µm) ISO 5% RSD Dv50 (µm) ISO 3% RSD Dv90 (µm) ISO 5% RSD
1 33.6 236 485
2 33.3 237 495
3 33.2 239 493
4 33.3 238 495
5 33.2 237 494
6 33.5 236 495
7 33.5 236 488
8 33.8 236 496
9 33.4 234 498
10 33.5 236 495
11 34.3 237 486
12 34.6 239 499
13 34.3 239 495
14 34 236 490
15 34.5 238 500
1RSD (%) 1.42 0.642 0.917

Table 2. Variation of 15 aliquots of coarse grade coffee.

Record Number Dv10 (µm) ISO 5% RSD Dv50 (µm) ISO 3% RSD Dv90 (µm) ISO 5% RSD
1 776 1490 2570
2 766 1480 2550
3 765 1470 2550
4 774 1480 2550
5 783 1500 2570
6 778 1490 2560
7 775 1490 2560
8 782 1500 2570
9 787 1510 2570
10 778 1490 2570
11 780 1490 2560
12 787 1500 2570
13 786 1500 2570
14 792 1500 2570
15 792 1500 2570
1RSD (%) 1.05 0.665 0.305

The results reveal that the differences between each coffee grade can be easily resolved using the Mastersizer 3000. From the data in Table 1, it is evident that FSF can offer excellent reproducibility of analysis within the limits suggested by ISO13320. Moreover, the absence of adhesion of the finer coffee to the sample flow path ensures a fast sample turnaround.

Conclusion

The consistency of the final product during grinding operations can be ensured by controlling the particle size distribution of coffee grounds. The use of FSF for the Mastersizer 3000's Aero S dry powder disperser favors this requirement, thereby ensuring rapid, reproducible bulk coffee measurements. Due to its ease of use and sample throughput, this system serves as a suitable choice over conventional sieving methods.

This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.

For more information on this source, please visit Malvern Panalytical.

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