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Measurement of pitching rate

There are numerous methods for the measurement of yeast concentration for pitching into a fermenter (Table 3.4). In the past, the complexity, accuracy and cost of these methods was in accordance with the scale of operations. Today, the advent of cost- effective but sophisticated methods has enabled the ‘up-selling' of more robust approaches to control yeast pitching rate. For the simplest approaches, the achievement of the desired pitching rate is ‘cheap and cheerful. At its simplest, this may be x packs of dried yeast per vessel. Measurement of ‘weight' or mass (variously dry, or wet/centrifuged) of ‘solids' in yeast slurries enables a more controlled approach to yeast pitching. The dry yeast method is more accurate than wet, reflecting changes in cell size/ volume during fermentation (Boulton and Quain, 2001) and during yeast storage (Cahill et al., 1999). However, dry weights are not a practical real-time option unless using a microwave (Rice et al., 1980). However either method is complicated by the distraction of non-yeast solids such as trub and hop material. The quantitative significance of either fraction can be assessed visually (assuming transparent tubes) as distinct bands after centrifugation before weighing or drying. Nevertheless, either approach results in the overestimation of weight and the underestimation of yeast mass. However, whichever approach is used it is strongly recommended that there is correction for yeast viability especially if this is less than 95%. This increases the accuracy of pitching rate and provides a key indication of yeast health and well-being. However, it should be noted that compensation for reduced viability results in the addition of more dead yeast,

Table 3.4 Measurement of pitching yeast concentration

Method

Required equipment

Comments

Wet weight

Centrifuge, analytical balance

‘Cheap and cheerful’. Rapid but (even after water washing) inaccurate because of entrained trub and hop material. Requires correction for viability

Dry weight - oven

Centrifuge, oven, analytical balance

As above but slow - 72 hours at 105°C. Takes 2-3 days

Dry weight - microwave

Membrane filter, microwave, analytical balance

As ‘wet weight’

Viability

Light microscope, vital stain (e.g. Methylene Blue)

‘International method’ (ASBC Methods of Analysis,

2011). Rapid but overestimates viability especially < 80%. Requires a skilled operator

Viable cell count

Haemocytometer, light microscope, vital stain (e.g. Methylene Blue)

‘International method’ (ASBC Methods of Analysis, 2012). Rapid but requires a skilled operator

Electronic particle counters

Proprietary equipment

Requires dilution to ca. 5 x 104 to 1 x 107 cells/ml and filtration to remove non-yeast particles. Yeast must be deflocculated. Rapid analysis, which includes cell size

Biomass probe using radiofrequency permittivity

Proprietary technology

Real time and representative. Quantifies only viable yeast. Options include in-line, mobile skid and off-line laboratory analyser. Requires calibration for each yeast strain

which - through autolysis - will compromise beer quality.

The issue of non-yeast solids is overcome by measurement of yeast count as is or, better still, a viable count in conjunction with a vital stain such as methylene blue, the use of which dates back to 1933 (O'Connor-Cox et al., 1997). Together, they represent the de facto reference method by which other methods (below) are compared. The method requires a skilled and trained operator to juggle the use of a haemocytometer (aka Thoma chamber) and microscope. This is further compromised by the tedium of counting up to 1,000 cells (ASBC Methods ofAnalysis, 2011), which results in operator fatigue when analysing numerous yeast samples. Accordingly, an automated slide-based counter using methylene blue has been developed with both enhanced consistency and reduced analysis time compared with the manual method (Thompson et al, 2015).

Whilst both weight and cell numbers have their advocates, especially from the perspective of simplicity, there are some caveats. Whilst trub adds inaccuracy to the measurement of solids, both approaches suffer from errors associated with the difficulties of representative sampling of thick yeast slurries (40% wet solids, ca 1.5 x 109 cells/ml) from storage vessels. Considerations include vessel scale, homogeneity (thorough mixing), sampling point and sample size. Such sampling errors can then be further compounded by processing steps required for testing, such as the small slurry volumes involved in serial dilution for cell counts.

As would be anticipated, the driver for the improved control and accuracy of yeast pitching came from the bigger demands of large-scale fermentation management. Ideally, such an approach would be in-line, automated, control the pitching process and better still would ‘count' only viable yeast. First up and reported by Riess (1986) was a system using near infra-red turbidometry. Two sensors measured the turbidity of the wort and of the pitched wort, thereby enabling a ‘set-point' to be established and constant yeast count to be maintained. The approach was commercialized and met with some success in achieving a tighter range of cell counts compared with the conventional pitching control system. However, the downsides were the lack of correction for viability together with a lack of correction for non-yeast solids entrained in the injected yeast slurry (Boulton and Quain, 2001). Dark beers were also problematic, as haze measurement was more difficult (Noble, 1997).

The issue was neatly summarized by Harris and co-workers (1987) who observed that ‘an accurate method for the real-time estimation of microbial biomass during laboratory and industrial fermentations remains an important goal. The paper then explored the use of radiofrequency (RF) and ‘concluded that measurement of RF permittivity provides an extremely powerful and convenient means for the assessment of microbial biomass, in situ and in real time. On application of the radiofrequency electrical field, viable cells (with intact plasma membranes) become charged and the measured capacitance reflects linearly the viable yeast concentration. Two years later, Boulton et al. (1989) reported the application of this technology to an automatic yeast pitching system that measured only viable yeast and was independent of any nonyeast solids. However and importantly, because cell size varies, the system required calibration against methylene blue cell counts for each yeast strain in use.

The exploitation of radiofrequency permittivity was undeniably a genuine process innovation and the Aber biomass meter (www.aber-instruments. co.uk) is used in breweries worldwide. Such systems are considered to control pitching rate to better than within ± 2% of the target viable count (Boulton, 2006). Accordingly, demonstrable improvements have been reported in target pitching rate (Maca et al., 1994) and fermenter cycle time (Boulton and Quain, 2001; Carvell and Turner, 2003; Boulton, 2006). Unsurprisingly, there have been range extensions to the use of the technology in controlling pitching rate, including an off-line laboratory analyser and a mobile skid-based unit. Other applications have included the control of yeast cropping together with monitoring propagation and fermentation, most notably in the distribution of yeast in cylindroconical vessels (Boulton et al., 2007).

 
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