The size and shape of particles have a direct impact on product properties such as appearance, taste, flowability, dissolution, and extraction behavior. Apart from the conventional particle analysis methods such as laser diffraction and sieving, dynamic image analysis (DIA) is largely established as a standard technique in many laboratories.
Higher information content, reduced measurement times, and reduction of workload are the advantages offered by DIA. However, quality control requires any new measurement method to provide results that are comparable to established methods, to ensure that product specifications remain unchanged.
For over two decades, Microtrac MRB's CAMSIZER instruments have been effectively applied in various industries for high-resolution particle analysis of granulates, powders, and suspensions. These measuring systems are utilized for routine quality and process control of different types of foodstuff including flowable bulk materials like coffee beans, sugar, granulated chocolate, salt, almonds, tea, hazelnuts, rice, and breakfast cereals. By using compressed air or wet dispersion the range of sample materials is extended to finer and agglomerated products such as flavors, flour, cocoa powder, granulates (fluidized bed granulation or spray dried, for example, milk powder), ground coffee, or soup powder.
DIA can also be used for particle characterization of pharmaceutical products and dietary supplements.
Unique Measurement Principle
The CAMSIZER instruments are the most reliable and efficient devices for measuring particle size and shape of bulk solids through DIA. The unique dual camera technology of the CAMSIZER P4 (Fig. 1) allows a wide measuring range from 20 µm to 30 mm and enables highly precise particle shape recognition, higher resolution, and more rapid analysis compared to any other optical analyzer. This ensures optimum measurement conditions over the complete size range, without compromising sensitivity or accuracy.
The CAMSIZER P4’s hardware settings are the same for all measurement tasks and do not need to be altered for different sample materials.
The sample is conveyed into the instrument through a vibrating chute, and two CCD cameras individually detect the particles in free fall as shadow projections. The large particles are captured and evaluated by the basic camera, and at the same time, the small particles are accurately detected by the zoom camera. The combination of the two distinctive image scales offers not only excellent measurement statistics through the analysis of a vast number of particles but also highest precision even for fine particles.
Figure 1. CAMSIZER P4. The zoom camera (blue) and basic camera (red) analyze the sample simultaneously.
The CAMSIZER X2, which is optimized for analyzing agglomerating particles and fine powders, uses the same measurement principle. The measurement range of the X2 model is 0.8 µm to 8 mm in combination with highly flexible dispersion options: a free-fall unit and an air-jet dispersion module that isolates particles before detection in an air flow and is most commonly used. It is possible to adjust the dispersion pressure according to the sample properties. As the particles cross the detection zone at high velocities while using air-jet dispersion, the CAMSIZER X2’s cameras have very short exposure time and a high image acquisition rate of over 300 per second. The CAMSIZER X2 features the additional option of analyzing wet samples.
Given below are three application examples from the food industry where DIA has been found to be the best-suited method for particle analysis:
- Coffee powder: using the CAMSIZER X2 as an alternative to laser diffraction to perform particle size analysis
- Crystal sugar: using the CAMSIZER P4 as an alternative to sieve analysis to perform particle size measurement
- Rice: using the CAMSIZER P4 to measure the length of rice grains and to detect broken grains
Example 1: Crystal Sugar
When sugar is produced, crystals with a wide particle size range are generated. Large production sieves are used for fractionating the sugar crystals based on their size, and the resulting fractions are stored in different silos. Depending on the application, sugar producers and sugar processing companies have varying demands in relation to the grain size distributions of the sugar. The different fractions are taken from the silos and customized to the final products based on the specifications of the final processor. For instance, in table sugar, dust content is not allowed, whereas in other industrial applications, rapid solubility of fine particles is desired.
Comprehensive and continuous quality control is required to monitor the screening process during the production and specification of the end product. Traditionally, labor- and time-intensive sieve analysis has been the chosen technique for this purpose. Recently, an increasing number of manufacturers have substituted their sieve shakers with CAMSIZER instruments, hence DIA has been largely established and accepted as a standard procedure.
The CAMSIZER P4 offers a fail-safe and considerably rapid alternative to manual sieve analysis of granulated sugar and crystal sugar, thereby enabling more frequent and accurate production monitoring. A single sieve analysis usually takes over 15 minutes, including weighing and cleaning of the sieves. In contrast, a CAMSIZER P4 analysis runs automatically and is normally finished within a time frame of 2–3 minutes. Yet, a necessary condition for using CAMSIZER in quality control is that it should offer the same results as the sieve analysis.
Figure 2. Two different sugar samples with different size distribution, one coarse (blue), one bimodal (green). CAMSIZER P4 and sieve analysis (black dots).
The graph in Figure 2 illustrates the particle size distributions of two different sugar samples, measured using a CAMSIZER P4. The green curve represents a mixture of two different sugar types from different silos, and the blue curve represents a coarse sugar. The black dots represent the results acquired through sieve analysis. It is evident that the CAMSIZER P4’s measurement results are in absolute agreement with the sieve data.
CAMSIZER P4 Advantages Over Sieve Analysis
|Time-intensive: > 15 minutes incl. weighing, sieving, and cleaning
||Less than 3 minutes
|Result depend on user, sieving motion, sample amount, sieve tolerance
||Always reproducible, independent from user, location etc.
|Sieves are liable to wear
||Maintenance-free, calibration within seconds
|No shape analysis
||Shape analysis possible
|Particle breakage possible
||Contact-free, nondestructive, no cross-contamination
||High level of automation
Example 2: Coffee Powder
Particle analysis of coffee powder is more difficult since it tends to be highly agglomerated due to the the oily nature of the particles, wide size distribution, , and irregular grain shape. Hence the material is difficult to convey, and it is essential to take the poor flowability into account when evaluating optical or mechanical analysis techniques.
Apart from sieve analysis, laser diffraction is also accepted as a standard procedure for characterizing ground coffee. Both sieve analysis and laser diffraction are limited with regard to informative value and significance of the results. When sieve analysis is used, the distribution is restricted to a small number of data points. Moreover, coffee powder exhibits a tendency to agglomerate and is not easily “sievable” using standard vibrational sieving. As a result, a majority of small particles will not pass the appropriate sieve and hence the result is not reliable. One way to resolve this is to use air-jet sieving.
However, this procedure is even more time-intensive since only a single sieve can be used at a time. Laser diffraction provides better dispersion by compressed air or wet measurement and, more importantly, analysis times are very short. Yet, the major disadvantages of this procedure are restricted dynamic measurement range and the poor sensitivity and low resolution specifically for oversize particles. Furthermore, neither of the two techniques offers information related to the particle shape.
Based on the principle of DIA, the CAMSIZER X2 optical particle analyzer is an excellent alternative to laser diffraction, not just for coffee powder. Typically, the particle size distribution of ground coffee is very wide, ranging from 10 µm to the millimeter range, with marked fine and coarse fractions. For samples such as this, the excellence of the dual camera technology becomes evident: the wide dynamic measuring range enables the entire distribution to be reliably detected in a single measurement run without any loss of accuracy for the fine or coarse particles. The size distributions of five coffee samples ground to different fineness are illustrated in Figure 3. Hence, the CAMSIZER X2 is an ideal tool for monitoring grinding processes and for finding out the optimal settings for the mills. Compressed-air dispersion was adopted to carry out the measurements.
Figure 3. CAMSIZER X2 measurement results for five coffee samples ground to different fineness.
Particle Shape of Coffee Powder
The brittleness of coffee beans is influenced by the roasting process. In general, ground coffee from brittle beans often consists of angular grains. Particle shape affects not only the packing density of the powder but also the permeability and extraction properties. As a result, the taste of the beverage is also affected by the shape of the particles.
As shown in Figures 4 and 5, the CAMSIZER X2 digital image analyzer has the ability to simultaneously determine the size distribution based on the length, width, or area-equivalent diameter of the particles. There is a considerable difference in the results regarding these parameters, which is also caused by the irregular grain shape (in the case of perfectly spherical particles, the three curves will overlie on each other). Although the results of laser diffraction, a method in which all particles are presumed to be spherical, only exhibit an average distribution of the actual width and length of the particles, they are largely in agreement with the equal area diameter measured by image analysis. It is evident from Figures 4 and 5 that laser diffraction lacks the ability to accurately detect the coarse part of the sample in the >1 mm range. The particle shape (aspect ratio) can be calculated from the difference between the length and the width.
Figure 4. Size distributions of a coffee sample. CAMSIZER X2 width measurement (green), CMASIZER X2 length measurement (purple), CAMSIZER X2 equal area diameter (red), and laser diffraction (blue).
Figure 5. Size distributions of a coffee sample. CAMSIZER X2 width measurement (green), CMASIZER X2 length measurement (purple), CAMSIZER X2 equal area diameter (red), and laser diffraction (blue).
Example 3: Rice and Broken Rice
ISO 7301 (EN) specifies the quality characteristics for rice. Apart from determining the odor, moisture content, or impurities (such as insects), the percentage of broken grains is important. Compared to whole grains, broken grains release more starch when cooked, thereby making the cooked rice more sticky. The cost and the amount of customs duties on import and export of rice are determined by the amount of broken rice grains. Moreover, the grain length is a significant measure for the optimization of production parameters such as the peeling in the rice mill.
Customarily, the length of the rice grains is manually measured using a ruler or a caliper, or with long-hole sieves. These techniques are inaccurate and time-intensive, not least because only very few grains are measured, rendering the results unrepresentative. The CAMSIZER P4 can be used to measure the length of a representative amount of rice grains within just a few minutes. Since a guidance sheet precisely aligns the individual grains before the measurement, all the particles are brought in perfect orientation for the length measurement. The ISO standard defines a grain as broken if it is shorter than three-fourths of the average grain length of the entire sample (Figure 6).
Figure 6. Result of the length measurement of rice grains with the CAMSIZER P4 (red curve, measurement time 1 minute). The blue curve represents the measurement of 200 rice grains with a caliper (measurement time >1 hour). The CAMSIZER P4 provides identical, reliable, and user-independent results.
The application examples described in this article demonstrate the excellence of DIA over traditional measurement methods with regard to efficient and reliable particle characterization of bulk solids. The combination of simultaneous particle shape determination and very detailed particle size analysis makes the CAMSIZER systems an attractive alternative for quality control laboratories in the food industry. They can be used to perform measurements with considerably less workload and in a shorter period of time, and provide abundant additional information that is not offered by other techniques.
This information has been sourced, reviewed and adapted from materials provided by Microtrac MRB.
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