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Brewing Science Yeast 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. Provided by Ken Woodson & the North Texas Home Brewers Association The Role of Yeast in Brewing • Unwittingly, ancient brewers domesticated 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 The Role of Yeast in Brewing • 1680 Antonie van Leeuwenhoek – Observed yeast in beer. • 1837 - Cagniard Latour – Microbe is responsible for alcoholic fermentation. • 1839 -Justus von Liebig and Friedrich Wohler – Alcohol is produced by a chemical process in which dead and decaying yeast participated. – Satired Latour’s theory in Annalen der Chemie . . . Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company ….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 The Role of Yeast in Brewing • 1866 - Louis Pasteur – Yeast was responsible for alcoholic fermentation. • 1883 - Emil Christian Hansen – Developed pure culture technique – Isolated pure cultures of brewing yeasts Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company Types of Brewing Yeasts • Two types of brewing yeasts, originally classified on flocculation behavior… • Top-fermenting – Ale yeast – Weiss yeast • Bottom-fermenting – Lager yeast Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company Top-Fermenting Yeast Bottom-Fermenting Yeast Weiss Ale Lager Lab Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company 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 Provided by Ken Woodson & the North Texas Home Brewers Association 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 Provided by Ken Woodson & the North Texas Home Brewers Association Taxonomy • Ale and Weiss yeasts - Saccharomyces cerevisiae – Polyploid and probably aneuploid. – Non-mating – 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 Yeast Attributes Flocculation • 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. Provided by Ken Woodson & the North Texas Home Brewers Association Yeast Attributes Attenuation • 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 Provided by Ken Woodson & the North Texas Home Brewers Association Fermentation • Yeast growth • Alcohol and CO2 • Flavor compounds Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company Lagering • Carbonation • Off-flavor reduction Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company The Brewing Process Malt Mill Mash Tun Cereal Cooker Brink Hops Aeration Fermentation Lauter Tun Brew Kettle Hot Wort Receiver Lagering Wort Cooler Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company 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. Provided by Ken Woodson & the North Texas Home Brewers Association 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. Provided by Ken Woodson & the North Texas Home Brewers Association 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. Provided by Ken Woodson & the North Texas Home Brewers Association 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. Provided by Ken Woodson & the North Texas Home Brewers Association Yeast Starter Starter Recipe from https://www.wyeastlab.com/hb_makingastarter.cfm The optimal media for cell growth and health require using a malt based media (DME) fortified with nutrients. Gravity should be kept near 1.040 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 24-36 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. 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 and duration of each stage depends on the type of yeast, specifically, whether you have an ale or lager yeast. Provided by Ken Woodson & the North Texas Home Brewers Association Fermentation Stages 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. Provided by Ken Woodson & the North Texas Home Brewers Association 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. Provided by Ken Woodson & the North Texas Home Brewers Association Fermentation Stages 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. Provided by Ken Woodson & the North Texas Home Brewers Association 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. Provided by Ken Woodson & the North Texas Home Brewers Association 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. Provided by Ken Woodson & the North Texas Home Brewers Association Balanced Growth • Yeast growth affects beer flavor. – Need balance between yeast growth and beer flavor. • The brewer needs... – Desired flavor profile in desired time. – Sufficient yeast crop for subsequent fermentations. • Oxygen is growth limiting nutrient. – Control point Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company Yeast Metabolism During Fermentation Sugars Oxygen Membranes Glucose CO2 Ethanol Acetaldehyde Energy Pyruvate TCA Cycle Unsaturated Fatty Acids Sterols Amino Acids Esters Higher Alcohols VDK Organic Acids Amino Acids Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company Sulfur Volatiles Fermentation By-Products Fusel 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 Provided by Ken Woodson & the North Texas Home Brewers Association 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 Provided by Ken Woodson & the North Texas Home Brewers Association Higher Alcohols • Formed by the decarboxylation and reduction of a-keto acids. – From amino acid anabolism and catabolism. Alcohol Isoamyl Amino Acid Leucine a-keto acid Amyl Isoleucine a-keto-3-methylvalerate Isobutanol Valine a-keto-isovalerate Propanol Threonine a-keto-butyrate a-keto-isocaproate Alcoholic, solventy, and fruity flavor notes Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company 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. Provided by Ken Woodson & the North Texas Home Brewers Association 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. Provided by Ken Woodson & the North Texas Home Brewers Association Esters • Closely linked to lipid metabolism - growth. • Reaction of an alcohol and fatty acid intermediate • Acetate esters – Ethyl acetate – Isoamyl acetate – Phenethyl acetate solventy, fruity, sweet banana roses, honey, apple • Fatty acid esters – Ethyl caproate – Ethyl caprylate – Isoamyl decanoate apple, aniseed apple tropical fruits Fruity flavor notes Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company Fermentation 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. Provided by Ken Woodson & the North Texas Home Brewers Association 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. Provided by Ken Woodson & the North Texas Home Brewers Association 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. Provided by Ken Woodson & the North Texas Home Brewers Association 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 the original fermentation temperature. Here slowly means no more that 5 °F per day, otherwise you may shock the yeast. Provided by Ken Woodson & the North Texas Home Brewers Association 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. Provided by Ken Woodson & the North Texas Home Brewers Association Vicinal Diketones Threonine Pentanedione a-ketobutyrate a-acetohydroxybutyrate pyruvate a-acetolactate Diacetyl Buttery, butterscotch flavor Kindly provided by Tom Pugh and David Ryder of Miller Brewing Company Isoleucine Valine 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 Provided by Ken Woodson & the North Texas Home Brewers Association 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 Provided by Ken Woodson & the North Texas Home Brewers Association