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Oxygenation time (h) (c)

Fig. 6.25 Biochemical changes associated with oxygenation of a slurry of lager yeast (c. 20% wet weight to volume) suspended in beer. During treatment the slurry was maintained at 25°C.

production scale. In this patented process (Quain & Boulton, 1987c), sufficient yeast is oxygenated to pitch 1600 hi of high-gravity lager wort. The process is performed in a tank designed to deliver high rates of oxygen transfer and maintain the slurry at a constant temperature (Figs 6.29 and 6.30). During the treatment, the dissolved oxygen tension is maintained at a constant value by a feedback control loop system. Oxygen uptake rates are inferred from the rate of oxygen addition that is required to maintain a constant concentration in the slurry. When the maximum Adot is achieved, the process is judged complete. At this stage the yeast may be pitched immediately into anaerobic wort or chilled and stored until required. After oxygenation, the yeast has a more stable physiology than conventional cropped yeast of 'anaerobic' physiology and therefore it is advantageous to oxygenate as soon as

Oxygenation time (h)

Oxygenation time (h)

-Zymosterol

- Ergosterol

- Episterol

- Lanosterol

- Ergosta-7,22,dienol

- Fecosterol

1 2 3 4 5 Oxygenation time (h)

Fig. 6.26 Changes in the sterol spectrum (a), and relative concentration of each sterol (b) during oxygenation of a lager yeast slurry. Conditions were as described in Fig. 6.23.

1 2 3 4 5 Oxygenation time (h)

-Zymosterol

- Ergosterol -Episterol -Lanosterol

- Ergosta-7.22.dienol

Fig. 6.26 Changes in the sterol spectrum (a), and relative concentration of each sterol (b) during oxygenation of a lager yeast slurry. Conditions were as described in Fig. 6.23.

Oxygenation time (h)

Fig. 6.27 Effect of prolonged oxygenation of a lager yeast slurry on the rate of oxygen uptake, total intracellular sterol concentration and exogenous ethanol concentration. The conditions were as described in Fig. 6.23.

Oxygenation time (h)

Fig. 6.27 Effect of prolonged oxygenation of a lager yeast slurry on the rate of oxygen uptake, total intracellular sterol concentration and exogenous ethanol concentration. The conditions were as described in Fig. 6.23.

possible after cropping. During the oxygenation process the slurry may be supplemented with glucose or maltose. This compensates for depletion of the endogenous glycogen pool, which can occur if the yeast is stored under inappropriate conditions or held for too long in fermenter before cropping.

Some of the changes seen in production-scale oxygenation of pitching yeast are shown in Figs 6.31(a), (b) and (c). In the trial shown, the slurry was supplemented with maltose (3% w/v) added after 30 minutes' oxygenation. The oxygen uptake maximum, inferred as oxygen demand to maintain a constant DOT, occurred after 1 to 2 hours. The peak Adot coincided with maximum sterol synthesis, exothermy and

Viability {"/.initial)

Fig. 6.28 Effect on the viability of yeast during storage, at the temperatures indicated, before and after oxygenation. Yeast (35% wet weight to volume) suspended in beer was oxygenated for 4 hours while being maintained at a temperature of 25°C.

Viability {"/.initial)

Fig. 6.28 Effect on the viability of yeast during storage, at the temperatures indicated, before and after oxygenation. Yeast (35% wet weight to volume) suspended in beer was oxygenated for 4 hours while being maintained at a temperature of 25°C.

Glycol / hot water

Wort main i

Sugar Antifoam

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