Brewing Science. Yeast

Transcription

1 Brewing Science Yeast

2 Introduc3on As brewers we take malt and perform a mash to convert starches to sugars and dextrins; however, it is yeast that is responsible for conver3ng sugars into alcohol and CO 2. So, without yeast we would only have cloying sweet wort and no beer.

3 The Role of Yeast in Brewing UnwiFngly, ancient brewers domes3cated yeast. Selected yeast that made good beer. Deduced that yeast was important to make beer. Collect the creamy foam or sediment from one brew. Use it to pitch the next brew. Did not know what yeast was. Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company

4 The Role of Yeast in Brewing 1680 Antonie van Leeuwenhoek Observed yeast in beer Cagniard Latour Microbe is responsible for alcoholic fermenta3on Justus von Liebig and Friedrich Wohler Alcohol is produced by a chemical process in which dead and decaying yeast par3cipated. Sa3red Latour s theory in Annalen der Chemie... Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company

5 .small animal which sips sugar through its snout, and excretes alcohol from its gut and carbonic acid from its urinary organ. Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company

6 The Role of Yeast in Brewing Louis Pasteur Yeast was responsible for alcoholic fermenta3on Emil Chris3an Hansen Developed pure culture technique Isolated pure cultures of brewing yeasts Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company

7 Types of Brewing Yeasts Two types of brewing yeasts, originally classified on floccula3on behavior Top- fermen3ng Ale yeast Weiss yeast Bo]om- fermen3ng Lager yeast Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company

8 Top- Fermen3ng Yeast

9 Bo]om- Fermen3ng Yeast

10 Weiss Ale Lager Lab Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company

11 Yeast Types Ale Yeast Scien3fic name is Saccharomyces Cerevisiae Typically ferments at higher temperatures, normally between F Usually is a top fermenter Cannot completely ferment Raffinose Higher fermenta3on temperatures usually produce more esters

12 Yeast Types Lager Yeast Scien3fic name is Saccharomyces Carlsbergensis Typically ferments at lower temperatures, normally between F; however, some ferment at the low end of ale yeast, i.e. California Common yeast ferments in the range F. Usually is a bo]om fermenter Ferments Raffinose Cooler fermenta3on temperatures usually lead to cleaner beers, i.e. less esters and less fusel alcohols

13 Taxonomy Ale and Weiss yeasts - Saccharomyces cerevisiae Polyploid and probably aneuploid. Non- ma3ng Sporulates poorly and poor spore viability Lager yeast - Saccharomyces pastorianus S. cerevisiae S. carlsbergensis S. uvarum Sporulates very poorly - poor spore viability Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company

14 Yeast A]ributes Floccula0on Floccula3on means yeast cells clumping together during fermenta3on. Floccula3on is a yeast specific characteris3c. Some yeast clump together quickly while others do not flocculate. Top fermen3ng yeast that flocculate are typically carried to the surface by CO 2 gas. Bo]om fermen3ng yeast tend to flocculate aeer CO 2 levels have diminished and they sink to the bo]om. Yeast muta3on can change the floccula3on trait of pure yeast strains.

15 Yeast A]ributes A2enua0on A]enua3on is the ability of yeast to metabolize wort sugars Apparent a]enua3on is measured by the drop in specific gravity as yeast converts sugar into ethanol and CO 2, without correc3on for alcohol. Real a]enua3on corrects for alcohol. For normal strength brews, apparent a]enua3on is approximately 1.2 3mes the real a]enua3on 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 fermenta3on of triple sugars depends on the yeast strain. Yeast muta3on can change the a]enua3on characteris3c of pure yeast strains

16 Yeast growth Alcohol and CO 2 Fermenta8on Flavor compounds Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company

17 Carbona3on Lagering Off- flavor reduc3on Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company

18 Malt Mill The Brewing Process Mash Tun Cereal Cooker Brink Fermentation Lauter Tun Brew Kettle Hops Aeration Hot Wort Receiver Lagering Wort Cooler Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company

19 Pre- Fermenta3on Wort Aera0on 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 fermenta3on to build up metabolic energy. Aera3on can be achieved by adding oxygen directly or by adding air. In each case, aera3ng 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.

20 Pre- Fermenta3on 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. Mathema3cally, Ale pitching rate = 750,000*(mL of Wort)*( Plato ofwort) The pitching rate for lagers is twice the amount shown for ales.

21 Pre- Fermenta3on 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.

22 Pre- Fermenta3on Yeast Propaga0on Typically liquid yeast packs do not have enough yeast cells to carryout an effec3ve fermenta3on. As an alterna3ve to pitching mul3ple packs of liquid yeast, you may consider making a yeast starter.

23 Yeast Starter Starter Recipe from The optimal media for cell growth and health require using a malt based media (DME) fortified with nutrients. Gravity should be kept near and cultures should be grown at 70 F. Recipe 0.5 cup DME (100g, 3.5oz), ½ tsp Wyeast Nutrient, 1qt.(1L) H2O Mix DME, nutrient, and water; Boil 20 minutes to sterilize. Pour into a sanitized flask or jar with loose lid or foil. Allow to cool to 70 F. Shake well and add yeast culture. Timing of Starter: Because starters are inoculated at high cell densities, growth is usually maximized within hours. The gravity of the starter should always be checked prior to inoculation into wort to assure proper cell growth. Cultures should be used immediately, or refrigerated for up to 1 week before using. Cell viability will decrease rapidly if culture are left at ambient temperatures for extended time.

24 Fermenta3on Stages A normal fermenta3on proceeds in stages: lag phase, growth phase, low kräusen, high Kräusen, and late kräusen. The stages may overlap and the 3ming and dura3on of each stage depends on the type of yeast, specifically, whether you have an ale or lager yeast.

25 Fermenta3on Stages

26 Lag Phase During this stage of fermenta3on 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.

27 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 fermenta3on. 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 fermenta3on (i.e. CO 2 forma3on) should begin within 8 hours of pitching ale yeast and 18 hours for lager yeast.

28 Fermenta3on Stages

29 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 fermenta3on temperatures will inhibit the produc3on of fusel alcohols and diacetyl.

30 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 fermenta3on pyruvic acid is converted to acetaldehyde, which in turn, is reduced to ethyl alcohol. Normally, primary fermenta3on lasts 3-5 days for ale yeast and 6-8 days for lager yeast.

31 Late Kräusen Phase Lager yeast begin to metabolize some of the fermenta3on by- products that were produced during the low kräusen phase. In par3cular, diacetyl levels can be reduced by performing a diacetyl rest. Most simple sugars have been converted aeer primary fermenta3on. During secondary fermenta3on, yeast slowly convert the more complex triple sugars. Secondary fermenta3on typically last about 1-3 days for ale yeast, but may last up to a month for lagers.

32 Balanced Growth Yeast growth affects beer flavor. Need balance between yeast growth and beer flavor. The brewer needs... Desired flavor profile in desired 3me. Sufficient yeast crop for subsequent fermenta3ons. Oxygen is growth limi3ng nutrient. Control point Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company

33 Yeast Metabolism During Fermenta8on Sugars Oxygen Glucose Membranes CO 2 Ethanol Acetaldehyde Pyruvate TCA Cycle Energy Unsaturated Fatty Acids Sterols Amino Acids Esters Higher Alcohols Organic Acids VDK Amino Acids Sulfur Volatiles Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company

34 Fermenta3on By- Products Fusel Alcohols These alcohols have a more complex molecular structure than ethyl alcohol. They provide an ini3al sweetness followed by a harsh aeer taste. Formed by the metabolism of amino acids, so over modifica3on during mal3ng or mashing can lead to higher fusel alcohol levels. They increase with fermenta3on temperature, level of amino acids, and wort gravity. Wild yeast can produce very high levels of fusel alcohols

35 Fermenta3on 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

36 Higher Alcohols Formed by the decarboxyla3on and reduc3on of α- keto acids. From amino acid anabolism and catabolism. Alcohol Amino Acid α-keto acid Isoamyl Leucine α-keto-isocaproate Amyl Isoleucine α-keto-3-methylvalerate Isobutanol Valine α-keto-isovalerate Propanol Threonine α-keto-butyrate Alcoholic, solventy, and fruity flavor notes Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company

37 Fermenta3on 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 fermenta3on. 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 characteris3c in German Wheat Beers. It can be detected at 3 ppm.

38 Fermenta3on By- Products Esters Ester forma3on is posi3vely correlated to wort gravity, yeast growth, and fermenta3on temperature. That is, higher gravity worts, rapid yeast growth, and higher fermenta3on temperatures increase ester produc3on. Ester forma3on is reduced by high yeast pitching rates because the yeast will not grow as fast. Also, wort with insufficient oxygen levels favor ester forma3on.

39 Esters Closely linked to lipid metabolism - growth. Reac3on of an alcohol and fa]y acid intermediate Acetate esters Ethyl acetate solventy, fruity, sweet Isoamyl acetate banana Phenethyl acetate roses, honey, apple Fa]y acid esters Ethyl caproate apple, aniseed Ethyl caprylate apple Isoamyl decanoate tropical fruits Fruity flavor notes Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company

40 Fermenta3on By- Products Ketones The two important ketones in brewing are diacetyl and 2,3- pentanedione. In the literature, these two ketones are classified together as the vicinal diketone level in beer.

41 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 3me it will take on a bu]er or bu]erscotch characteris3c Early in a normal fermenta3on, during the aerobic stage, yeast will produce diacetyl. Later in the anaerobic fermenta3on stage, yeast reduce diacetyl to levels below the flavor threshold.

42 Ketones Diacetyl Gram- posi3ve lac3c 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.

43 Ketones Diacetyl Higher temperatures early in the fermenta3on lead to higher levels of diacetyl and higher temperatures later in the fermenta3on lead to a greater reduc3on in diacetyl. For lager yeast that typically ferment at lower temperatures, some brewers perform a diacetyl rest during the la]er fermenata3on stage called the late krausen phase. To perform a diacetyl rest, slowly raise the fermenta3on temperature to around 60 F and hold this temperature for two days and then slowly lower the temperature back to the original fermenta3on temperature. Here slowly means no more that 5 F per day, otherwise you may shock the yeast.

44 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.

45 Vicinal Diketones Threonine Pentanedione α-ketobutyrate α-acetohydroxybutyrate Isoleucine pyruvate α-acetolactate Valine Diacetyl Buttery, butterscotch flavor Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company

46 Fa]y Acids Tend to add a soapy flavor to beer They are produced when yeast break down amino acids They are suppressed by lower fermenta3on temperatures Usually the yeast will convert fa]y acids to aldehydes then into alcohols

47 Sulfur Compounds Hydrogen sulfide produc3on during fermenta3on can lead to flavors reminiscent of ro]en eggs; however, during normal fermenta3on, hydrogen sulfide is reduced during the fermenta3on process. The flavor threshold for hydrogen sulfide is ppm. Gram- nega3ve bacteria, like Escherichia coli can produce large amounts of sulfer compounds For ales, higher fermenta3on temperatures tend to suppress sulfur compounds