Beer BasicsFermentation

June 2008

Today’s Topics• Introduction• Yeast Types

– Ale Yeast– Lager Yeast

• Yeast Attributes– Flocculation– Attenuation– Temperature

• Pre-Fermentation– Wort Aeration– Yeast Pitching Rates– Yeast Propagation

• Fermentation Stages– Lag Phase– Growth Phase– Low Kräusen– High Kräusen– Late Kräusen

• Fermentation By-Products– Esters– Fusel Alcohols– Ketones

• Diacetyl• 2,3-Pentanedione

– Fatty Acids– Sulfur Compounds

Introduction

• As brewers we take malt and perform a mash to convert starches to sugars and dextrins; however, it is yeast that is responsible for converting sugars into alcohol and CO2. So, without yeast we would only have cloying sweet wort and no beer.

Yeast Types

Ale Yeast• Scientific name is Saccharomyces Cerevisiae• Typically ferments at higher temperatures,

normally between 55 – 75 °F• Usually is a top fermenter• Cannot completely ferment Raffinose• Higher fermentation temperatures usually

produce more esters

Yeast Types

Lager Yeast• Scientific name is Saccharomyces Carlsbergensis• Typically ferments at lower temperatures, normally

between 46 – 56 °F; however, some ferment at the low end of ale yeast, i.e. California Common yeast ferments in the range 55 – 60 °F.

• Usually is a bottom fermenter• Ferments Raffinose• Cooler fermentation temperatures usually lead to

cleaner beers, i.e. less esters and less fusel alcohols

Yeast AttributesFlocculation• Flocculation means yeast cells clumping

together during fermentation. Flocculation is a yeast specific characteristic. Some yeast clump together quickly while others do not flocculate.

• Top fermenting yeast that flocculate are typically carried to the surface by CO2 gas. Bottom fermenting yeast tend to flocculate after CO2 levels have diminished and they sink to the bottom.

• Yeast mutation can change the flocculation trait of pure yeast strains.

Yeast AttributesAttenuation• Attenuation is the ability of yeast to metabolize wort sugars• Apparent attenuation is measured by the drop in specific

gravity as yeast converts sugar into ethanol and CO2, without correction for alcohol. Real attenuation corrects for alcohol. For normal strength brews, apparent attenuation is approximately 1.2 times the real attenuation

• All things being equal, lager yeast ferment more completely than ale yeast. Yeast appropriate for brewing can metabolize single and double sugars but the amount of fermentation of triple sugars depends on the yeast strain.

• Yeast mutation can change the attenuation characteristic of pure yeast strains

Yeast Attributes

Temperature• Ale Yeast usually ferments at a higher temperature

compared to lager yeast.• There are variations between different yeast strains.

For example, Dusseldorf alts typically are fermented around 60-65 °F which is on the cool side of ale yeast fermentation temperatures.

• On the other hand, California Commons are typically fermented around 55-60 °F which is on the warm side of lager yeast fermentation temperatures.

Pre-Fermentation

Wort Aeration• It is very important to add oxygen to the wort just prior to

pitching yeast because the yeast require oxygen in the first phase of fermentation to build up metabolic energy.

• Aeration can be achieved by adding oxygen directly or by adding air. In each case, aerating stones are used to assist in dissolving oxygen into the wort.

• Normal gravity worts (12 °P or less) require 4-5 ppm of oxygen, while wort with 15 °P requires 8-10 ppm of oxygen.

• The level of dissolved oxygen in wort goes down as temperature and gravity units go up.

Pre-Fermentation

Yeast Pitching Rates

• According to George Fix, the appropriate level of yeast to pitch for ales is 750,000 yeast cells per milliliter of wort for each degree Plato of wort.

• Mathematically,Ale pitching rate = 750,000*(mL of Wort)*(°Plato ofWort)

• The pitching rate for lagers is twice the amount shown for ales.

Pre-Fermentation

Yeast Pitching Rates

Example – How much yeast should you pitch to make 10 gallons of your favorite lager if the original gravity is 1.048?

The answer is:• =1,500,000*37,853*12• =681,354,000,000• or, around 700 billion yeast cells.

Pre-Fermentation

Yeast Propagation

• Typically liquid yeast packs do not have enough yeast cells to carryout an effective fermentation.

• As an alternative to pitching multiple packs of liquid yeast, you may consider making a yeast starter.

Fermentation Stages

A normal fermentation proceeds in stages:lag phase, growth phase, low kräusen,high Kräusen, and late kräusen.

The stages may overlap and the timing andduration of each stage depends on the type of yeast, specifically, whether you have an ale orlager yeast.

Lag Phase• During this stage of fermentation the yeast become

acquainted with their environment and they access the level of oxygen, amino acids and sugars.

• In this phase the yeast produce enzymes that allow amino acids and sugars to permeate the yeast cell wall.

• The yeast compile food reserves and store the fuel in the form of glycogen, a carbohydrate.

Growth Phase• During the growth phase yeast begin to grow through cell

budding.• Oxygen dissolved in the wort is used by the yeast to generate

sterols. High levels of sterols are required for yeast cell walls to become permeable. Glycogen levels are reduced during sterol synthesis, however, glycogen increases during the main part of a normal fermentation.

• Poor yeast growth is usually caused by low amino acid levels, or low levels of dissolved oxygen in the wort.

• If an appropriate level of healthy yeast are pitched then signs of fermentation (i.e. CO2 formation) should begin within 8 hours of pitching ale yeast and 18 hours for lager yeast.

Low Kräusen Phase

• At this point, the yeast have depleted all the oxygen dissolved in the wort. Consequently, from this point forward the process is anaerobic.

• Yeast metabolism of amino acids and sugars are in full force.

• Fusel alcohols and diacetyl may be produced during this phase. At this stage, lower fermentation temperatures will inhibit the production of fusel alcohols and diacetyl.

High Kräusen Phase

• For ales, most of the sugars have been metabolized by this phase. However, lager yeast will metabolize most sugars during the high kräusen phase.

• During fermentation pyruvic acid is converted to acetaldehyde, which in turn, is reduced to ethyl alcohol.

• Normally, primary fermentation lasts 3-5 days for ale yeast and 6-8 days for lager yeast.

Late Kräusen Phase

• Lager yeast begin to metabolize some of the fermentation by-products that were produced during the low kräusen phase. In particular, diacetyl levels can be reduced by performing a diacetyl rest.

• Most simple sugars have been converted after primary fermentation. During secondary fermentation, yeast slowly convert the more complex triple sugars.

• Secondary fermentation typically last about 1-3 days for ale yeast, but may last up to a month for lagers.

Fermentation By-Products

Esters• Chemically, esters are formed when an alcohol combines with

an organic acid• They typically impart a fruity aroma and flavor to beer.• There are many esters associated with beer fermentation.

Two common esters associated with brewing are Ethyl acetate, and Isoamyl acetate. Ethyl acetate produces a fruity character and can be detected at 33 ppm, while Isoamyl acetate is responsible for the banana characteristic in German Wheat Beers. It can be detected at 3 ppm.

Fermentation By-Products

Esters• Ester formation is positively correlated to wort

gravity, yeast growth, and fermentation temperature. That is, higher gravity worts, rapid yeast growth, and higher fermentation temperatures increase ester production.

• Ester formation is reduced by high yeast pitching rates because the yeast will not grow as fast. Also, wort with insufficient oxygen levels favor ester formation.

Fermentation By-ProductsFusel Alcohols• These alcohols have a more complex molecular structure than

ethyl alcohol.• They provide an initial sweetness followed by a harsh after

taste.• Formed by the metabolism of amino acids, so over

modification during malting or mashing can lead to higher fusel alcohol levels.

• They increase with fermentation temperature, level of amino acids, and wort gravity.

• Wild yeast can produce very high levels of fusel alcohols

Fermentation By-Products

Fusel Alcohols• Some yeast strains produce phenolic alcohols that

typically have a medicinal flavor; however, the clove like character of German Wheat beers is produced from the phenolic 4-vinyl-guaiacol

• Wild yeast can produce phenolic alcohols with very unpleaseant flavors

Fermentation By-Products

KetonesThe two important ketones in brewing are diacetyl and 2,3-pentanedione. In theliterature, these two ketones are classifiedtogether as the vicinal diketone level in beer.

Ketones

Diacetyl • has a very low flavor threshold, .10 mg/L (ppm)• In fresh beer, low levels of diacetyl may impart a caramel

flavor, however, over time it will take on a butter or butterscotch characteristic

• Early in a normal fermentation, during the aerobic stage, yeast will produce diacetyl. Later in the anaerobic fermentation stage, yeast reduce diacetyl to levels below the flavor threshold.

Ketones

Diacetyl • Gram-positive lactic acid bacteria can produce large amounts

of diacetyl• Mutant yeast cells can lose their ability to reduce diacetyl,

leading to elevated levels of diacetyl• Wort that does not contain sufficient levels of the amino acid

valine can lead to higher levels of diacetyl. Fortunately, most all-malt worts contain an over abundance of amino acids.

Ketones

Diacetyl • Higher temperatures early in the fermentation lead to higher

levels of diacetyl and higher temperatures later in the fermentation lead to a greater reduction in diacetyl.

• For lager yeast that typically ferment at lower temperatures, some brewers perform a diacetyl rest during the latter fermenatation stage called the late krausen phase. To perform a diacetyl rest, slowly raise the fermentation temperature to around 60°F and hold this temperature for two days and then slowly lower the temperature back to theoriginal fermentation temperature. Here slowly means no more that 5 °F per day, otherwise you may shock the yeast.

Ketones

2,3-pentanedione• Has a flavor threshold of 1 mg/L (ppm)• Produces a flavor similar to honey• Found in some Belgium ales where honey flavors are

appropriate for the style• Wort that does not contain sufficient levels of the amino acid

leucine can lead to higher levels of 2,3-pentanedione. Fortunately, most all-malt worts contain an over abundance of amino acids.

Fatty Acids

• Tend to add a soapy flavor to beer• They are produced when yeast break down

amino acids• They are suppressed by lower fermentation

temperatures• Usually the yeast will convert fatty acids to

aldehydes then into alcohols

Sulfur Compounds

• Hydrogen sulfide production during fermentation can lead to flavors reminiscent of rotten eggs; however, during normal fermentation, hydrogen sulfide is reduced during the fermentation process. The flavor threshold for hydrogen sulfide is 10-35 ppm.

• Gram-negative bacteria, like Escherichia coli can produce large amounts of sulfer compounds

• For ales, higher fermentation temperatures tend to suppress sulfur compounds