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Control of the Yeast Propagation Process�How To Optimize Oxygen Supply and Minimize Stress

MBAA TQ vol. 42, no. 2, 2005, pp. 128-132  |  VIEW ARTICLE

Olau Nielsen. Scandi Brew, Alfa Laval Copenhagen A/S, Rosenk�ret 18, DK-2860 S�borg, Denmark.

Abstract
In a modern propagation plant, the question how to control the process by ensuring optimal air supply and at the same time minimizing any stress that the yeast may encounter during its growth in the propagator must be answered. Tests were carried out in a 10-hL test propagator and later in different full-scale propagators in breweries with net volumes of 100 and 160 hL. The intention was to try to detect any signs of stress as a result of the use of a new aeration device and to find the optimum aeration profile during the propagation. In the most recent experiments, the outlet gas from a 160-hL net propagator was monitored and the oxygen content and gas volume were compared with cell growth and yield factors. Since this control is very informative, but also quite complicated, a more simple control was also sought, and some valuable and easily applied methods were developed. The main findings so far are the following. No stress can be detected as a result of the use of the agitator. The benefit of a high concentration of dissolved oxygen during propagation is limited. On the other hand, it seems that oxidative stress is a smaller problem than is carbon dioxide stress, so the aeration must be sufficient to assume a low concentration of carbon dioxide in the propagator. By means of monitoring the amount of gas in the outlet from the propagator, it is also possible to determine when the aerobic metabolism stops. A simpler but equally reliable way to monitor this is by measuring the yield factor as the number of million cells per milliliter produced per degree Plato consumed. Low yield factor means no aerobic activity. An even simpler but not so reliable method is to observe the foam formation. High foam levels mean large amounts of carbon dioxide caused by a high degree Plato consumption. High degree Plato consumption means anaerobic metabolism�and no aerobic growth.
Keywords: aeration, oxygen, propagation, stress, yeast, yield

 

S�ntesis
En una planta moderna para la propagaci�n de levadura, es necesario controlar el proceso de manera de asegurar que se proporciona la cantidad �ptima de aire a la vez de minimizar el estr�s ocasionado a la levadura durante su reproducci�n en el propagador. Se realizaron pruebas, primero en un propagadpr de 10 hL y luego en propagadores cerveceras de 100 y 160 hL netos, con la intenci�n de determinar el perfil �ptimo de aeraci�n con un nuevo aparato, como tambi�n determinar si este nuevo aparato ocasionaba alg�n estr�s a la levadura. En los experimentos m�s recientes, en el propagador de 160 hL neto, se midi� el volumen y el contenido de ox�geno del gas expulsado del propagador, y se compar� estos valores con la reproducci�n de levadura y factores de rendimiento. Dado que este procedimiento es m�s bien complicado, se desarrollaron otros m�todos m�s sencillos que arrojaron datos valiosos. Hasta ahora no se ha podido constatar ning�n estr�s ocasionado por el agirtador. Se determin� que hay l�mites a cuan alta puede ser la cencentraci�n de ox�geno en el propagador, pero tambi�n se constat� que el estr�s ocasionado por altas concentraciones de ox�geno es menor que el estr�s causado por el gas carb�nico, por lo que es necesario mantener un bajo nivel de gas carb�nico en el propagador. La medici�n de la cantidad de gas a la salida del propagador tambi�n permite determinar cuando el metabolismo aer�bico cesa. Un m�todo m�s sencillo para constatar esto es midiendo el rendimiento, el n�mero de c�lulas por mililitro producidos por grado plato consumido, a diferentes tiempos. Un bajo rendimiento indica el final de la actividad aer�bica. Otra manera aun m�s sencilla, pero menos confiable, es por la observaci�n de la formaci�n de espuma. Mucha formaci�n de espuma es causado por la formaci�n de mucho gas carb�nico como consecuencia de un gran consumo de extracto, lo que a la vez se debe a un metaboilismo anaer�bico y el fin del crecimiento aer�bico.
Palabras claves: aeraci�n, ox�geno, propagaci�n, estr�s, levadura, rendimiento

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