Captain's Log

Stardate ​October 1st 2009

Decided on brewing beer
Focus probably on the fermentation process
Having the yeast break down starches and sugars in hops and malt and turning it into alcohol and other compounds [That should be interesting if you identify and research the specific compounds involved JCB]

10/28/09 Homework Assignment

The role of oxygen in yeast metabolism during high cell density brewery fermentations
**Article Link**
[Full Marks]

Abstract
Different oxygen conditions were investigated to determine the change in growth yield and to appraise the yeast parameters during the fermentation process of beer. Other factors were also affected with the change in oxygen. The goal is to alter oxygen conditions to provide quicker fermentation while keeping the quality of the beer abou the same.

Introduction
Fermentation is the most time consuming process of brewing alcohol, the ability to speed the process saving time and money. Oxygen is used in the manufacture of unsaturated fatty acids and sterol which the yeast requires to grow during the fermentation process. Cropped yeast is used industrial fermentations and may not contain enough sterols or fatty acids, so to acheive high quality the oxygen may be altered. Oxygen is normally provided from wort aeration
but this technique is inconsistent in quality. To gain consistent quality the yeast can be oxygenated in a controlled process beforehand. This study looks at different oygenation conditions for yeast and at the application of preoxygenated yeast, the five specific conditions are listed in Table 1.

Materials and methods
An industrial lager brewing yeast strain was used and the conditions and method at which the preoxygenated yeast was prepared is discussed with preparation in duplicate.
Dead yeast cells were stained and the specific gravity of he fermenting medium was measured. Volatile compounds were determined by headspace GC.
The total fatty acids were analyzed in duplicate via GC as well.
The amount of glycogen and trehalose were determined from a method discussed elsewhere. These were tested in triplicate.
Quantitative polymerase chain reaction was used to find the expression levels of specific genes.

Results
The study discusses the progress of the fermentation rate which was monitored by plotting the sugar density versus time. These results show that the change in oxygen causes a change in yeast growth. The viability of the yeast populations was also monitored. In all conditions the viability remained the same and decreased after a long time.
The FAN uptake levels were measured as well and that levels were enhanced when faster than normal pitching rates were used.
Concentrations of higher alcohols were increased by the pitching rate. No large differences were found etween different oxygen conditions though. Table 3 lists a flavour profile for all conditions.
The fatty acid profiles for all conditions was taken. Pitching rate and oxygen conditions strongly influenced the concentrations of C16:1 and C18:1 fatty acids.
The glycogen conent followed the same trend in the reference sample as in other conditions. No significant differences were found because of oxygen conditions. After fermentation the level was again measured. Trehalose levels were also insignificant in change.
The gene expression levels are discussed as to what each means and why it was monitored. The profiles are plotted in Figure 5 with differing results for each expression based on the condition.

Discussion
The purpose of this study was to determine how oxygen affects yeast growth for performance while keeping the quality of the beer the same.
Yeast growth is critical because without this growth the fermentation process can't even take place. More oxygen is needed in fermentation processes that are pitched with high concentrations of yeast. Which suggests that oygen is the limiting factor in hell cell density fermentations. Yeast preoxygenation was looked at but that alone does not give sufficient fermentation results when compared to the reference. A mixture of wort aertion and preoxygenation however works well. High oxygen levels alone do not give similar results when compared to the reference, suggesting that other factors that limit the reaction also exist. Higher levels of oxygen decrease the amount of numerous flavour compounds that give the fruitiness of lager beer. The total diacetyl conent is much higher in high cell density fermentations by the oxygenation seems to reduce the amount of diacetyl.
Varying oxygen levels produce similar amounts of unsaturated fatty acids. The same goes for the amounts of glycogen and trehalose. The expression levels were monitored and how they affect the ability of the yeast to survive by protecting it from cellular damage.
This sttudy helps support evidence that changes in oxygen conditions before fermentation can increase the stability and give high quality beer. A combination of yeast preoxygenation with wort aeration seems to give the best results. More work is yet needed on the physiological properties of yeast.