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
