Particle size distribution of agglomerated products should be measured using an adequately representative sample and with minimum disruption. Care should be taken to prevent swelling, dissolution, or disintegration. For sufficiently stable particles, sieving is still widely in use (dry for particles larger than 30 to 40 ¡um and wet for smaller particles), but over the past few years, laser diffraction has become a standard method due to the availability of a large number of different devices (Coulter, Malvern, Sympatec, etc). The last method is more versatile because it allows the measurement of larger particles with a dry feeder in a relatively nonviolent way and is also applicable for fines in a dispersing liquid. Automatic image analysis systems are a recent development employing charge-coupled device (CCD) camera imaging of particles falling out of a dry feeder.
Agglomerated particles may partially disintegrate when subjected to a more or less violent impact (vibration, shaking) during storage, transportation, and handling. In the case of agglomerated coffee, attrition is the primary cause in separation of fines from the outer surface of agglomerates (12). Formation of so-called secondary particles (shattering) from the disintegrated pieces is also taking place. The strength or resistance of agglomerated products to attrition may be measured by comparing its size distribution before and after rotation for a limited time inside a cylinder (friabilator). Other properties of agglomerates (bulk or free flow density, flowability, cohesion, angles of spatula and repose) are measured, for example, with the Hosokawa (MicroPul) tester.
For powdered foods, information about wetting and dispersing behavior is of special interest to the manufacturer. Wetting of a powder is easily tested by preparing a sample of defined height (5-8 mm) in a cylindrical testing vessel with a slide covering the liquid reservoir. When the slide is pulled out sideways, the powder sits on the liquid surface and is wetted. The wetting time measured in this way is useful for quality control purposes and for product property comparison.
Powder dispersion measurement is more difficult and depends on the ability of measuring the amount of material actually dispersed after the mixture/dispersion has been prepared in a defined way. For many products, this can be achieved by, for example, photometry (milk powder, cocoa beverages), conductometry (powdered extracts of coffee, tea) or refraction index measurement (sugars). The dispersed powder mass divided by the total powder mass in the sample is called degree of dispersibility (which varies between 0 and 1). Milk powder can reach a value of up to 0.8 (depending on fat content and age), instant cocoa beverages up to 0.98 and instant coffee up to 1.
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