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Effective Application of Particle Size Analysis from Development to Manufacture

September 16, 2013
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The performance-defining nature of particle size makes it a critical parameter for many products, from cement to coffee, powder coatings to pharmaceuticals. In the majority of cases the subsequent need for particle size measurement is met by laser diffraction, a technique that is routinely applied both in the laboratory and for real-time monitoring, at both the pilot and commercial scale. The availability of reliable and robust technology for every stage of the manufacturing lifecycle provides a wealth of opportunities for product and process optimization.
    Here we examine these opportunities, through an exploration of the application of laser diffraction particle size analysis in the coffee industry. The taste of a cup of coffee is directly influenced by the particle size distribution of the grounds used to brew it, making particle size of critical importance to those supplying ground coffee or milling in situ, as well as to manufacturers of coffee grinders or more sophisticated ‘bean to cup’ machines.

Optimizing taste
The taste of a brewed cup of coffee is influenced by:
• the type and roast of the coffee beans used, and their freshness
• the particle size of the coffee grounds used for brewing
• the amount of coffee used to brew each cup
• the temperature at which the brewing or extraction takes place
• the brewing time/pressure

Some of these variables are down to the choice and technique of the maker, but particle size is not something that is routinely measured at the point of use. Rather particle size specifications tend to be targeted as part of the manufacturing process: for ground coffee, stand-alone grinders, or for integrated bean-to-cup machines.
    Over the years, the particle sizing techniques applied to coffee have become more sophisticated, from a simple, but knowledgeable rubbing between finger and thumb, through sieving, to today’s laser diffraction particle size analysis. Underlying knowledge has developed accordingly and it is now clear that the entire particle size distribution rather than just the averaged particle size is critical to achieving desirable flavor in an acceptable brewing time.
    Coarsely ground coffee presents a relatively low contact area for flavor extraction, necessitating long brewing times. A finer grind, in contrast, releases its flavor more rapidly but makes the extraction of bitter components more likely. These can create an unpleasant, burnt taste. Furthermore, very fine grinds pack together closely creating significant resistance to the passage of water during brewing. Fine blends therefore tend to demand high brewing pressures, which can again affect flavor.
    An ongoing task for coffee and manufacturers of coffee makers is to refine their understanding of these correlations in order to perfect a grind, not just for one market, but increasingly to meet specific geographical requirements. Laser diffraction efficiently meets the associated need for particle size analysis within the lab while at the same time providing options for on-line monitoring on the factory floor, where required.

Introducing Laser Diffraction
Laser diffraction particle size measurement is based on the detection and analysis of the light scattering pattern produced as a laser beam passes through a particulate sample. Small particles scatter light relatively weakly at wide angles while larger particles scatter strongly at angles close to the incident beam. The MIE theory of light is applied to calculate the entire particle size distribution of the sample from the detected scattering pattern, negating any requirement for instrument calibration.
    Efficient and non-destructive, laser diffraction is inherently fast, primarily because it is an ensemble particle sizing technique, i.e. it directly measures the particle size distribution of the sample, rather than building up a distribution from measurements of individual particles. Sample preparation is straightforward with appropriate dispersion of the sample the key requirement. For coffee, the primary particle size, the size of the individual grounds rather than the size of any agglomerates, is crucial to flavor release behavior, making efficient dispersion during the measurement a pre-requisite.
    Modern laser diffraction instrumentation for the laboratory is versatile, highly automated, and extremely productive. The latest systems measure across a 0.01 to 3500 micron size range and are driven by intuitive software that brings accurate, reliable measurement within the reach of even novice users. Equally importantly, for those measuring coffee, these systems incorporate extremely powerful and effective dry dispersion engines that enable the swift and efficient measurement of even sticky, oily materials such as those present in coffee grinds.
    Going into a production environment, the speed of laser diffraction measurement comes very much to the fore with on-line systems delivering measurement at a frequency of up to four complete particle size distributions per second. The real-time monitoring that this provides enables substantial process optimization and automated control in milling applications, and can also be used on the production line to enhance, for example, grinder qc (see case study below). The ability to fashion the interface between the analyzer and production line is key here, to ensure relevant and reliable analysis and present data in a way that can be easily digested and used by operators.
    The following case studies include data illustrating how laser diffraction supports each step of the manufacturing cycle, from research and development in the laboratory to routine manufacture on the production floor.

Case Study 1: Comparing the Particle Size Distribution of Different Coffees

Figure 1 shows particle size distributions for three commercially available ground coffees: filter, smooth roast, and espresso coffees. The characterization of each sample was carried out using the Mastersizer 3000 (Malvern instruments, Malvern, UK), equipped with the aero s dry powder dispersion unit. This instrument has a wide dynamic range and successfully detects large particles in the samples as well as precisely quantifying the very fine particles present, all in a single measurement.
    The results show that the espresso product has a significant fine fraction, which is responsible for its characteristic bitter taste. In contrast the smooth, and most especially the filter product, have a much lower fines fraction to give a much smoother flavor. Lab-based analyses of this type can be used in quality control, but also support the extension of our understanding as to how to optimize individual elements of the particle size distribution to deliver great taste. This is an important focus for those working at the forefront of the industry.

Case Study 2: Optimizing Grinder Set-Up
The data in figure 2 show how once a target particle size distribution has been established it can be used to support the set-up of grinders on a production line to ensure that, straight from the box, they deliver grounds that make a desirable cup of coffee for the target customer base.
    In this experiment particle size was measured as a function of grinder gap size, where gap size is the distance between the two elements of the grinder that rotate relative to one another to reduce the size of the coffee. Lower setting numbers equate to a narrower gap setting and can be seen to produce a finer grind. The laser diffraction data effectively tracks the impact of steadily increasing grinder gap providing a secure basis for its manipulation to meet specific requirements. For a customer base that favors bitter, espresso-type blends, setting 1 is preferable, while higher settings will produce a smoother blend.

Case Study 3: Real-Time Grinder QC
At its site in Mignagola di Carbonera, Italy, De'longhi, one of the world’s leading producers of coffee making machines has taken the step of installing on-line laser diffraction to provide real-time qc of the grinders integrated within its complex ‘bean to cup’ machines. Here the brief was to achieve 100%, numerically quantifiable qc testing of the grinders in a timeframe consistent with the dynamics of the production line. On-line measurement proved essential to achieve the real-time acceptance/failure required.
    Figure 3 shows the on-line particle size analyzer in its installed position, beneath the test workbench, and the screen viewed by the operator. To carry out the test the assembly line worker pours fresh coffee beans into the grinder and initiates the test sequence. The grinder starts to work and the resulting ground coffee flows out of the grinding chamber, down towards the analyzer. A venturi disperses the sample into the measurement zone of the instrument. The arrival of ground coffee in the measurement zone reduces the amount of light seen by the detector, automatically triggering measurement, without any manual intervention. The end of the analysis is marked in an identical manner.
    Although particle size distribution data are measured here one of the most interesting features of the application is the way in which data presentation has been precisely tailored to test requirements. The focus of attention for the operator is the green areas of the screen which indicate whether the grinder is giving acceptable performance, and if not, how many turns of the gear wheel are required to correct it. This guidance is automatically generated by comparing the measured particle size against developed standards, within the software. Following any corrective action a second test is always carried out.
    Refining the analytical solution to this level of simplicity and effectiveness required some initial effort but the developed solution is extremely successful. Testing is complete in just a few seconds and the amount of coffee used is relatively small, even though the results produced are both robust and representative. Because the grinder is tested with real coffee, small sample size is a distinct advantage that reduces the ongoing cost of testing. Every grinder is tested and each one leaves the testing station with numerical test data verifying performance.
    In practical terms, testing is now streamlined and straightforward with minimal scope for manual error. The Insitec systems are extremely reliable, with no significant problems encountered during all the years of operation. Each week the operator cleans the analyzer, a simple task that takes just a few minutes, but beyond that maintenance requirements are minimal. As production has expanded new assembly lines have been added, each grinder area having a dedicated on-line particle size analyzer.

Conclusion
Over recent decades laser diffraction has matured into a versatile and powerful technique for industrial particle size analysis. Users that exploit this potential in an imaginative way gain access to timely and robust particle size data that support every aspect of the production cycle, from fundamental R&D to every day manufacture. This exploration of the application of laser diffraction for coffee measurement highlights the potential benefits of such a strategy and its value for those targeting the very highest levels of product and process performance.

Dr. Paul Kippax is divisional product group manager for micrometrics at Malvern Instruments, and has responsibility for the company's Mastersizer, Spraytec, and analytical imaging product ranges. He joined the company in 1997, starting as an application scientist. Prior to that he obtained a degree in chemistry and a PhD in physical chemistry, both at the university of Nottingham in the UK. For more information on Malvern Instruments Ltd, visit www.malvern.com.

Figures

Figure 1: particle size data for espresso, smooth, and filter coffee highlights the impact of fines on flavor, the high fine content of the espresso imparting its characteristic bitter taste.

Figure 2: increasing setting numbers equate to wider grinder gap sizes and can be seen to result in a coarser grind in this investigation of grinder set-up

Figure 3: an on-line particle size analyzer (insitec, malvern instruments) provides real- time qc testing for grinders on a production line (left). The simply presented test results tell the operator the extent to which the gear wheel need to be changed to bring the grinder into specification – in this case a single turn (right).

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