The Effect of Measurement Angle and Particle Size on Scattered Light in the Beer Line
Model DSB for measurement of proteins, bacteria, yeast and kieselguhr.
By John Byrnes & Andrea Valentine
Beer appearance is important to brewers because it reflects product quality, and is the single characteristic most apparent to the buyer at the time of purchase. The light transmission and scattering characteristics of beer are used to determine beer haze and, therefore, appearance.
The "shadow box" laboratory instrument is used in most breweries for visual estimates of clarity in the glass. The large volumes of beer now processed demand continuous measurement, and scattered-light hazemeters are currently used in major breweries, installed in the beer pipe after the kieselguhr filter.
Scattered light measurement is characterized by a light source - which sends radiated light through the beer - and a detector which measures the light scattered by the particles. The particles that cause the light to scatter include beer proteins, yeast, kieselguhr (diatomaceous earth), bacteria, fining agents, PVPP and others.
The common practice is to place the measuring light detector at a 90º angle to the axis of the radiated light. The popular Radiometer Hazemeter bench instrument, for example, employed this side scatter principle.
Other measuring instruments locate the detector at a smaller angle off the transmitted light axis, typically 13º. Here, the sensor is located on the side of the beer sample opposite the light source. This is commonly known as a forward scatter hazemeter.
The objective of this work is to present data using both "forward scatter" and "90º scatter" methods. This data is collected holding all other variables constant, so a direct comparison can be made of the difference in light measured at the two angles.
This simultaneous measurement is then used to compare the response of each technique to changing particle size.
Particle sizes were selected to represent the complete range of haze-causing particles present in beer during filtration. Reed, et. al. have shown the high concentrations of small particles (<500 nm) present in rough beer (Fig. 1). Wenn et. al. have shown evidence that these fine particles contribute to formation of large "bits" after filtration. Based on the above evidence, one specific objective was to be able to measure these small protein size particles.
The test instrument uses a flow-cell sensor (Fig.2a) and separate electronic analyzer (Fig.2b). The sensor is designed to be mounted directly in the beer pipe, and the analyzer is wall-mounted nearby, providing digital indicators and electronic output signals. The instrument is the Model DSB, developed by McNab, Incorporated.
A three-inch diameter flow cell was used. It illuminates the beer sample with collimated light. The light source is a single polychromatic and near-infrared lamp. Infrared was not used due to its insensitivity to the broad range of particle sizes measured, especially the smaller particles.
The measuring detector unit measures the light scattered at a 90º angle from the axis of the collimated light (Fig.3). The combination detector contains non-imaging optics to collect the light scattered at a small angle (13º) from the light source axis and the forward scatter detector to measure this light. In general, as this forward scatter angle decreases, sensitivity to particles increases.
The combination detector also contains a direct beam detector located on the axis of the transmitting light. This measures changes of transmitted light. This transmitted light signal is used in a ratio with each scattered light detector. This eliminates the effect of color changes and other common mode interferences. Beer color, for example, is not particulate, but absorbs light and can change the reading of a non-compensated hazemeter. The fourth unit acts as a baffle or "light trap", reducing stray light for the 90º sensor.
This device was prepared as an integrated single unit, instead of two separate instruments, to eliminate inherent variations caused by different light-source wavelength (color), detector wavelength sensitivity, and variations due to zero calibration and span calibration methods.
The electronic analyzer gives separate, simultaneous haze measurements for both forward scatter and 90º scatter signals. Separate 4-20 mA analog outputs were also used for both output channels.
Four sample solutions were prepared, each containing particles of a single characteristic size. The four particle sizes represented were:
Actual solutions of kieselguhr and yeast were prepared. Yeast particles have typical sizes of 20,000 nm, and a typical size for kieselguhr is 2,500 nm. Bacteria were approximated with polymer beads of approximately 1,500 nm size, and protein particles were approximated with polymer beads with typical sizes under 200 nm.
The scattered light measurements were made simultaneously as particle concentration was increased. This was to eliminate errors from measurements made at different times, or if different illumination, sensors or analyzers were used for the two measurements.
The instrument was calibrated per the manufacturer's recommendation. Data from the 90º sensor is shown in EBC turbidity units. Data from the forward scatter sensor are usually presented in units of ppm silica. In this case, we have imposed the readings onto the EBC scale for comparison.
Figure 4 shows the result of simultaneous scattered light measurement at 90º and forward scatter angles on the solution containing yeast particles. Figure 5 shows the same test on kieselguhr particles, Figure 6 is the result of bacteria size particles, and Figure 7 is protein size particles.
The graphs demonstrate sensitivity to particle size by each of the sensors. The forward scatter sensor has decreasing sensitivity as particle sizes become smaller, and the forward scatter slope is smallest for the small protein sized particles. The 90º scatter signal increases in sensitivity as particle size decreases.
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