Evaluation of the Oxygen Requirement of Lager and Ale Yeast Strains by
Preoxygenation
MBAA TQ vol. 40, no. 4, 2003, pp. 283-289 |
VIEW ARTICLE
Sofie An Depraetere (1), J. Winderickx (2), and F. R. Delvaux (1). 1. Centre
for Malting and Brewing Science, Katholieke Universiteit Leuven, Kasteelpark
Arenberg 22, 3001 Heverlee, Belgium. 2. Department of Molecular Physiology of
Plants and Micro-organisms, Katholieke Universiteit Leuven, Kasteelpark Arenberg
31, 3001 Heverlee, Belgium.
Abstract
An adequate oxygen supply is essential for yeast growth and its fermentation
performance, because oxygen is crucial for the synthesis of sterols and
unsaturated fatty acids. An alternative to wort oxygenation is the
preoxygenation technique, in which yeast cells can be directly exposed to
oxygen in a controlled process before fermentation. This paper is the result of
investigating the response of two lager and six ale yeast strains to
preoxygenation, whereupon yeast cells were preoxygenated in a membrane loop
reactor. Successful results have been achieved in spite of the different
response of every yeast strain. The buildup of sterols and unsaturated fatty
acids varied considerably among the different yeast strains. Sterol
concentrations increased 1.5- to 4-fold during preoxygenation, and a 1.5- to
6-fold increase in unsaturated fatty acid concentrations was also detected.
Fermentation performance of the preoxygenated yeast cells was good for seven of
the yeast strains. For one ale yeast, the preoxygenation procedure that we used
did not bring the yeast cells to optimal conditions for beer fermentation.
Formation of esters and higher alcohols was noticeably higher in beers
successfully brewed with preoxygenated yeast when compared with that in beers
brewed with untreated yeast. The importance of the yeast strain has been
established, thereby implying the need for specific preoxygenation procedures
for every yeast strain in order to optimize the process. The results show that
the preoxygenation technique has considerable promise and is worthy of further
examination as an interesting alternative to wort oxygenation.
Keywords: fatty acids, fermentation, oxygenation, sterols, yeast
S�ntesis
El ox�geno es crucial para la s�ntesis de esteroles y �cidos grasos no
saturados, siendo esencial disponer de una fuente adecuada de ox�geno para una
sana reproducci�n de levadura y un buen desempe�o durante la fermentaci�n.
Una alternativa a la aeraci�n del mosto es la t�cnica de la preoxigenaci�n,
en la que las c�lulas de levadura son expuestas directamente al ox�geno en un
proceso controlado, antes de ser dosificadas al mosto. Se presenta los
resultados, con dos cepas de levadura lager y seis de ale, a la preoxigenaci�n
en un reactor de membrana de ciclo cerrado. Se han obtenido buenos resultados
a pesar de las diferencias en la respuesta de las diferentes cepas de levadura.
La acumulaci�n de esteroles y de �cidos grasos no saturados vari�
considerablemente entre las diferentes cepas. La concentraci�n de esterol
aument� 1,4 a 5 veces durante la preoxigenaci�n, a la vez que la
concentraci�n de los �cidos grasos no saturados aument� de 1,5 a 6 veces. El
desempe�o fermentativo de las levaduras preoxigenizadas fue bueno para siete
de las ocho cepas. Este procedimiento de preoxigenaci�n no consigui� llevar
a una de las cepas de levadura ale a sus condiciones �ptimas para la
fermentaci�n cervecera. La formaci�n de esteres y alcoholes de alto peso
molecular fue notablemente mayor en las cervezas fermentadas con levadura
preoxigenada. Se ha demostrado la gran importancia que tiene la cepa de
levadura, por lo que habr�a que desarrollar un procedimiento de preoxigenaci�n espec�fico para cada cepa de levadura para asegurar que el
proceso sea el �ptimo. Los resultados demuestran que la t�cnica de la
preoxigenaci�n es bastante prometedora y vale la pena que se examine m�s a
fondo como una interesante alternativa a la aeraci�n del mosto.
Palabras claves: �cidos grasos, fermentaci�n, oxigenaci�n, esteroles, levadura
