Across-The-Pipe Cell Count of Yeast
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A technical poster by John Byrnes and Andrea Valentine, of McNab, Inc.
Presented at the 107th Master Brewers Association of the Americas Anniversary Convention, Cincinnati, Ohio USA
This poster addresses the use of an in-line, across-the-pipe-measuring near-infrared analyzer to measure yeast cell count in brewery applications. One such application involves the monitoring of yeast pitching to ensure in-specification feed. The Model HSA allows the brewer to better control yeast dosage so as to meet specifications and improve uniformity of fermentations and filtering. It eliminates common sampling and dilution errors often found in the laboratory.
Cell concentration levels in the pitching line are ultra-high, with concentration as high as 2 billion cells/ml being typical for concentrated yeast immediately before it is added to wort for fermentation. The HSA may also be used at diluted concentrations in the later fermentation and filtering stages.
The HSA instrument uses a near-infrared radiation signal, and is designed to be mounted directly in the pipe line typically 2 to 6 inches (50 to 150 mm) in diameter, or in the wall of a 15 foot diameter fermentation vessel.
There have been different process analytic methods for measuring or counting yeast levels in pipes in the brewery. One system uses a minute shift in "phase angle"- caused by the presence of the yeast cell wall structure.
Other designs have used turbidity meters to measure the loss of light, which will be somewhat inversely proportional to the concentration of yeast. This method may not be able to distinguish between trub and yeast, and is typically non-linear.
The HSA's extended near-infrared method allows for the measurement of yeast particles with insensitivity to the presence of typical CO2 bubble levels. The HSA, manufactured by McNab Incorporated, is based on selective narrow band NIR absorption. A special absorption band is chosen to measure the concentration of the yeast cells and suppress unwanted interferences. The computation to determine the concentration is based upon the Lambert-Beer Law, where NIR absorption is proportional to the concentration of the yeast.
Yeast concentration was measured reading directly in yeast cells per milliliter. Reference measurements made on a standard laboratory cell counter are shown for correlation.
To ensure accuracy, the yeast counting sensor and its indicator were moved to the laboratory. Yeast samples were prepared and counted by a Coulter counter (4 microns and larger were considered yeast). Particle size effects were accomplished via substitution of known-size particles. The yeast and trub sizes were duplicated with precision-sized plastic beads, and the different outputs were measured according to the bead diameter.
The various concentrations used to establish HSA sensitivity and correlation are from 0 to .02 billion; to 0.2 billion; and to 2 billion cells/ml.
Yeast was introduced into the full diameter pipe and agitated to keep in suspension. Measurements were taken and compared with the reading of the Coulter counter. To test the effect of bubbles, CO2 bubbles were enlarged by use of a vacuum pump and readings taken before and during vacuum.
Figure 1 indicates that the HSA has appropriate linearity and sensitivity over 1.8 billion cells/ml. In effect, this would be a measurement of a highly concentrated yeast slurry—useful for yeast measurement during pitching.
Figure 4 shows the yeast measurements provided by the HSA when vacuum and agitation conditions are altered. The reading remained virtually unchanged whether the conditions were under vacuum, vacuum and agitated, agitated, or static; despite the fact that the CO2 bubbles were enlarged while under vacuum. Had the change in the CO2 bubbles ' caused interference, the readings would have shifted accordingly The one small peak is believed caused by electrical interference in the laboratory The linearity of the graph illustrates the HSA's insensitivity to vacuum-induced changes in bubble size. Hence, the HSA's insensitivity to bubbles is inferred.
Because the HSA instrument employs certain near-infrared frequencies to perform its measurements, the HSA can successfully ignore smaller (<4 pm) non-yeast particles to accurately measure yeast, which features a typical particle size of between 4 to 10 pm.
Figure 5 indicates that the HSA is as much as 100 times more effective in measuring larger-sized particles (4 pm to 10 pm) than smaller ones (< 4 pm).
The data shown in this poster demonstrates that the HSA instrument successfully measures yeast content in various concentrations. Its correlation to Coulter laboratory methods were typically better than 0.97 (1.00 being ideal).
The data also indicate that the HSA is insensitive to CO2 bubble interference, - and that the device is capable of suppressing the detection of small particles during its measurement of yeast.
This instrument may be used for instantaneous reading of yeast cell count in yeast dosing in fermentation stages input to the diatomaceous filter. The HAS follows the earlier Model A-2 design used in yeast-in-beer measurements installed in Europe in the 1970’s.
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