4- Yeast, fermentation & bacteria Flashcards
Yeast
Microorganisms, 1 cell, during active fermentation population be 100 million cells in 1 ml (0.03 fl. Oz.) of liquid
Environmental requirements for yeast
Microorganisms compete for sugar, yeast has 2 advantages:
1- can tolerate acidic environments, distillers may add acid to sugary liquid if level too low
2- produce ethanol (other microorganisms can’t thrive)
Temperature requirements
Below 10 C/50 F yeast dormant, active when warmer, ferment faster the warmer it is, die over 30 C/ 86 F
Distiller provides
Nutrients, manage temp, lower acidity levels
Nutritional requirements
Deficiencies can cause yeast to stop fermenting or die (stuck fermentation), can sometimes restart but other microorganisms start to consume sugars & create unpleasant aromas
Fermentable sugar
Yeast need sugar for energy- fructose, sucrose or glucose
Nitrogen
Vital for growth, essential component of proteins & DNA, many forms, from raw material, sugar cane has low levels so add N supplements
Yeast outputs
Metabolise nutrients to create energy, remain healthy & reproduce, sugar metabolism produces ethanol, CO2 & heat
Ethanol
2nd most important component by volume after water, amount produced can range from 4% abv to 20% abv
CO2
Vented off, high levels can cause suffocation, dense so it can collect in tanks
Heat
Can reach >30 C (86 F), above this yeast die, temp managed
Methanol
Lower boiling point than ethanol, get higher levels from fermenting fruit juices d/t pectin, aromatically unattractive, potentially harmful, level permitted tightly regulated, managed during distillation &/or maturation
Fusel alcohols
Propanol, butanol & iso-amyl alcohol, ‘higher’ or ‘heavy’ alcohols, higher boiling point than ethanol, sugary liquid determines amount made & amount of esters produced, high levels give unpleasant aromas & coarse texture, controlling level is key stylistic concern during distillation
Fusel oils
Aka fusel alcohols, includes fatty acids & esters
Fatty acids
Acetic & butyric acids, not strong acids & doesn’t taste acidic, yeast combines some with alcohols to form esters, some not in spirit because boiling points too high &/or they don’t form esters in significant quantities, role in sour mashing
Esters
Formed when fatty acid reacts with alcohol, ‘esterification’, fruity & floral aromas, esters present at end of fermentation made in yeast cell
Iso-amyl acetate
Made when iso-amyl alcohol combines with acetic acid
Sulfur compounds
Released by yeast during fermentation, unpleasant smelling when conc, levels low in fermented liquid & not easily detected, distilling can make them highly pronounced, remove after fermentation- no way to stop yeast from producing them
Yeast choice
Saccharomyces cervisiae- reliably produces high levels of ethanol & positive aromatic compounds, different ‘strains’ display slightly different chars
Cultured yeast
Desirable strains identified, isolated & stored in secure specialist facilities, small # of cells grown into healthy population, no limit on # of times original isolated cells used to grow new populations
Cultured yeast formats
Main difference is shelf life- some can store for short periods, some have to be used immediately
Preparing the cultured yeast
Add to sugary liquid, yeast rapidly divides & dominates liquid, transferred to large vessel where it dominates too, repeated several times until enough for main fermentation tank, after 12 hours rapid cell division takes place
Choice of yeast
Some choose single strain, most choose small range, other use # of strains at same time, ex. reliable fermenter with one that produces high levels of aromatics, majority use cultured yeasts for consistency
Ambient (wild) yeast
Lives in distillery year round, in air & on surfaces, sugary liquid exposed to air will begin to ferment
2 concerns using ambient yeast
Few use ambient yeast, Mezcal & baijiu use
1- no guarantee that right microorganisms will dominate fermentaion
2- can take longer to get going, other yeast or bacteria can become highly active (atypical or unpleasant aromas)
Sequential & parallel fermentaion
2 approaches: finish 1st step before starting 2nd, or take place at same time
Sequential fermentation
All sugars present at start, can predict level of alcohol at end, important for process efficiency
Raw materials containing sugar
Fruit, sugar cane- no choice but to use sequential fermentation
European tradition
Enzymatic hydrolysis in whisky production typically not complete before fermentation starts, sugary liquid susceptible to spoilage while enzymes processing starch, longer wait= more risk, once most starch converted fermentation can start, enzymes continue while yeast fermenting, not completely sequential but not parallel either
Parallel fermentation
Developed in Asia for baijiu, soju & shochu, only used with raw materials that contain starch, filamentous fungi create amylase enzymes, when combined with starchy ingredients (already contains some sugars) fermentation starts right away using cultured or ambient yeasts, enzymes continue to break down starch at same time sugars created consumed by yeast
Challenges of parallel fermentation
More challenging to manage d/t risk that yeast will consume sugar faster than it can be produced by enzymes, results in yeast starving & stuck fermentation, ensure that quantity of enzymes is more than sufficient
Liquid-state fermentation
Most fermentations, yeast rapidly divide & occupy whole liquid ensuring sugar consumed, takes 24 hours to > week to consume all sugar, then yeast die, some leave alcoholic liquid in tank to allow further flavours to develop d/t bacterial activity
Semi-solid-state fermentation
Liquid with high level of solids, thick consistency, typically grains, baijiu & shochu common
Solid-state fermentaion
Baijiu, fermenting material full of individual bits with gaps between, must be moist, yeast grow through grains by cell division, expands through mass, in sealed containers, several months
Bacteria
Present almost everywhere in air & on surfaces, not able to create alcohol
Malolactic conversion
Malic acid in wine metabolized by LAB producing lactic acid, actively encouraged for distillation, makes wine more stable so can store longer, don’t use SO2 for wine destined for distillation
Malic acid
Key nutrient for many spoilage bacteria, if MLF happens immediately then adds to stability, also protected by ethanol (9-10%) & high acid
Bacterial flavour creation
Bacteria create large # of aromatic compounds, many not desirable, avoid delays & ensure that yeast rapidly dominates
2 exceptions for bacteria
1- some producers of baijiu encourage creation of pungent bacteria aromas
2- some allow bacteria to contribute aromas after fermentation, called long fermentation, ex. Scotch whiskies left to stand several hours once yeast stopped fermenting, gives light aromas but adds to complexity, some rum producers also use for longer & aromas are more pronounced
