X-ray Microscopic Examination of Espresso Powder Porosity

Figure 1: Rendered volumetric models of uncompressed (left) and 30 lb compressed espresso powders

As winter sets in across the northern hemisphere, a recent study found that approximately 75 percent of Americans enjoy a coffee beverage each day with the most popular choice being a flavored latte.1 To create a latte, a barista combines steamed and foamed milk with shots of espresso. For long time readers, you may recall we also explored physical changes in coffee beans resulting from the roasting process using our SkyScan 1272 (Coffee Roasting Article – 2021).

Porosity, which measures void spaces, is a fundamental property that affects the thermal, physical, and mechanical properties of a material. The volume of void spaces and pore size distribution in our espresso powder under compression directly affect the mobility of water passing through the espresso puck and consequently the taste of the beverage. Porosity itself is straightforward – the ratio of the volume of voids over the total volume – but the accurate measurement and quantification of porosity is more complex. Micro-CT is a unique tool providing high-resolution 3D characterizations of our compressed espresso samples for analysis of the structure, textures, porosity, permeability, and pore distribution.

For our imaging series this month, we utilized the SkyScan 1272 desktop micro-CT to examine espresso powder at three different compression levels to look for changes in macro-porosity arising from compression of the espresso grains into a puck.

X-Ray Microscopic Imaging of Compressed Espresso Powders

To avoid any concerns of sampling, we selected a pre-ground bulk espresso powder from a local grocer as the base material for this study. A pre-defined volume of espresso powder was added to three plastic tubes. One tube was imaged as the grains were packed just from loading into the tube, to serve as a zero-compression control sample. The next two samples were compressed using a force gauge to either 15 lbs of pressure or 30 lbs of pressure. Classically, 30 lbs of pressure is the standard recommendation when tamping espresso powder before extracting a shot.2 Since espresso pucks are only compressed briefly before extracting a shot, we did not image these samples under static compression but rather allowed them to relax prior to imaging. We examined the grounds using our high-resolution SkyScan 1272 micro-CT at an isotropic voxel size of 5 µm. The SkyScan 1272 is a fine match for this project due to its high resolution and quick sample collection for organic materials.

Figure 2: Planar 2D slices through each dataset showing no compression (top), 15 lb compression (bottom left), and 30lb compression (bottom right)

As shown in Figure 2, after reconstruction with NRecon we can examine a 2D plane through each sample. While coffee beans themselves are quite porous after roasting, for this project we are more interested in the larger macro-pores which exist among individual grains. From these images, it’s clear that compressing the finely ground espresso powder reduces the distance between individual grains as the amount of black air space visible in the slice decreases from the control sample with both the 15 and 30 lb force samples.

Figure 3: Volumetric 3D views through each dataset with pores highlighted in red showing no compression (top), 15 lb compression (bottom left), and 30 lb compression (bottom right)

Using a combination of transfer functions and clipping planes within CTVox, we can highlight the macro-pores within each volume of interest as shown in Figure 3. In this view, we overlaid the pore size data on top of the imaging data and then digitally removed the top half of the espresso powder volume to provide clearer context for the size distribution of pores within each dataset.

Figure 4: Clipped 3D view through the pore data highlighting differences in average macro-pore size between the control sample (left) and the 30 lb compression sample (right)

CTVox allows us to visualize the quantitative pore data as shown in Figure 4. Keeping the color scale the same among the datasets in the study allows for a direct comparison of the results. For this study, the smallest pores are shown in red with increasing size moving to green and blue.

Figure 5: Comparison of calculated macro-pore diameter among the three samples

As shown in Figure 5, the control dataset has the largest range of pore sizes while the 15 lb and 30 lb datasets show a distinct shift to the left in the chart, indicating the prevalence of smaller pores. With careful comparison, the 30 lb dataset is still shifted further left on average than the 15 lb dataset showing that additional compression did yield smaller macro-pores on average. However, additional testing at higher compressions would likely show diminishing returns. In fact, over-compression for espresso pucks would result in limited pathways for water to travel through the puck and would reduce the quality of the resulting espresso shot.

Sample Control 15 lb 30 lb
Average Pore Diameter 88.11 ± 44.37 µm 57.50 ± 27.48 µm 54.16 ± 26.52 µm
Total Porosity (%) 54.07 39.96 36.71

Table 1: Extracted quantitative macro-porosity data for the espresso powder samples

Table 1 documents the extracted qualitative data for the three samples, highlighting the decrease in porosity with increasing compression force. The nominal macro-pore diameter decreases as compression is applied to the sample though no statistical difference is evident between the two compressed samples.

 

 

Figure 6: Maverick 3D renderings of macro-pores present within the control sample (left) and the 30 lb compressed sample (right)

After segmenting the compressed coffee grains, we extracted volumetric data for the pore networks and were able to import these pore models into Maverick Indie for photorealistic rendering as shown in Figure 6.

Conclusion

Among the SkyScan product line, the SkyScan 1272 is a workhorse high resolution instrument which excels with imaging and analysis of organic samples. The small footprint and full self-shielding make the SkyScan 1272 a great fit for most laboratories.

We hope you found this Image of the Month informative and encourage you to subscribe to our newsletter and social media channels in preparation for the continuation of our Image of the Month series next month.

Scan Specifications

Sample Espresso Powders
Voltage (kV) 60
Current (µA) 100
Filter 0.25 aluminum
Voxel Size (nm) 5
Rotation Step 0.2
Exposure Time (ms) 1415
Rotation Extent (deg.) 180
Scan Time (HH:MM:SS) 02:40:24

These scans were completed on our SkyScan 1272 micro-CT system at the Micro Photonics Imaging Laboratory in Allentown, PA. Reconstructions were completed using NRecon 2.0 while visualization and volumetric inspection of the 2D and 3D results were completed using DataViewer and CTVox. The coffee powders and macro-pores were converted to STL volumetric models using Synopsys’ Simpleware ScanIP software with the CAD add-on module (Synopsys, Inc., Mountain View, USA) before 3D rendering using Maverick Render Indie (Random Control, Madrid, Spain).

Would you like your work to be featured in our monthly newsletter? If so, please contact us by calling Seth Hogg at 610-366-7103 or emailing seth.hogg@microphotonics.com.

References

1 https://www.driveresearch.com/market-research-company-blog/coffee-survey/

2 https://www.espresso-services.com/espresso-blog/how-to-tamp-espresso-like-a-pro

*Simpleware software (Synopsys, Inc., Mountain View, USA) enables you to comprehensively process 3D image data (MRI, CT, micro-CT, FIB-SEM…) and export models suitable for CAD, CAE and 3D printing. Use Simpleware software’s capabilities to visualize, analyze, and quantify your data, and to export models for design and simulation workflows. Simpleware™ is a trademark of Synopsys, Inc. in the U.S. and/or other countries.

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