Experiment Pitching Rate and Oxygenation

Abstract

Esters are an important component of the aroma of German wheat beers. Common home brewing knowledge lists pitching rate and level of oxigenation as important factors that effect the level of esters that are produced during fermentation. While it is commonly believed among home breweres that lower pitching rates result in higher ester levels, the literature reports that increased pitching rates lead to higher levels of esters. This experiment is designed to evaluate the affect of oxygen levels and yeast pitching rate on the ester production.

Introduction

Esters are formed through a condensation reaction between an alcohol and an acid [Wikipedia]. In brewing, 2 major processes exist in which esters are formed. During intra cellular ester formation, the yeast's metabolism produces esters through enzymatic reactions. But esters can also be formed by a simple condensation reaction between an organic acid and an acohol. But due to the slow reaction rate, this form of esterformation doesn't play a role in primary fermentation and produces significant results only after extended aging (12+ weeks) [Engan 1974 via Hermann 2005]. This ester formation during aging is responsible for the dark fruit notes of aged beers. But more interesting for the brewer is the ester production during the primary fermentation.

During Fermentation pyruvic acid, an intermediate product of the pathway that leads to alcohol, is reduced to oxalacetate and then to acetyl CoA. Acetyl CoA is the basis for a host of compounds including sterols for cell wall construction and esters [Noonan 1996]. All authors agree that increased biomass production (creation of cell walls) reduces the Acetyl CoA that is available for ester production and leads to reduced ester levels in the beer [Narziss 2005, Clone, Walsh, Noonan 1996, Fix 1999]. Differences however exist with respect to ester production and yeast growth. Fix [Fix 1999] writes that any increased activity on the acetyl CoA branch, i.e. yeast growth, will increase ester production while other authors [Narziss 2005, Clone] state that increased yeast growth leads to a decrease in esters since more of the acetyl CoA is used for sterol synthesis.

Sources

[Clone 1] Danstar FAQ: Yeast Growth

[Hermann 2005] M. Hermann, Entstehung und Beeinflussung qualitätsbestimmender Aromastoffe bei der Herstellung von Weißbier, Dissertation, Technical University Munich, 2005

[Walsh] A. Walsh, Ester Formation, www.brewery.org

[Noonan, 1996] Gregory J. Noonan, New Brewing Lager Beer, Brewers Publications, Boulder CO, 1996

[Narziss, 2005] Prof. Dr. agr. Ludwig Narziss, Prof. Dr.-Ing. habil. Werner Back, Technische Universitaet Muenchen (Fakultaet fuer Brauwesen, Weihenstephan), Abriss der Bierbrauerei. WILEY-VCH Verlags GmbH Weinheim Germany, 2005

[Fix, 1999] George J. Fix Ph.D, Principles of Brewing Science, Brewers Publications, Boulder CO, 1999

These are literature sources that I didn't read, but the statements were cited by papers I read:

In [Hermann 2005]:

[Engan 1974] Engan, S.: Esters in Beer, The Brewers Digest, November 1974, page 40-48.