4- Yeast, fermentation & bacteria

Question 1

Yeast

Answer 1

Microorganisms, 1 cell, during active fermentation population be 100 million cells in 1 ml (0.03 fl. Oz.) of liquid

Question 2

Environmental requirements for yeast

Answer 2

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)

Question 3

Temperature requirements

Answer 3

Below 10 C/50 F yeast dormant, active when warmer, ferment faster the warmer it is, die over 30 C/ 86 F

Question 4

Distiller provides

Answer 4

Nutrients, manage temp, lower acidity levels

Question 5

Nutritional requirements

Answer 5

Deficiencies can cause yeast to stop fermenting or die (stuck fermentation), can sometimes restart but other microorganisms start to consume sugars & create unpleasant aromas

Question 6

Fermentable sugar

Answer 6

Yeast need sugar for energy- fructose, sucrose or glucose

Question 7

Nitrogen

Answer 7

Vital for growth, essential component of proteins & DNA, many forms, from raw material, sugar cane has low levels so add N supplements

Question 8

Yeast outputs

Answer 8

Metabolise nutrients to create energy, remain healthy & reproduce, sugar metabolism produces ethanol, CO2 & heat

Question 9

Ethanol

Answer 9

2nd most important component by volume after water, amount produced can range from 4% abv to 20% abv

Question 10

CO2

Answer 10

Vented off, high levels can cause suffocation, dense so it can collect in tanks

Question 11

Heat

Answer 11

Can reach >30 C (86 F), above this yeast die, temp managed

Question 12

Methanol

Answer 12

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

Question 13

Fusel alcohols

Answer 13

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

Question 14

Fusel oils

Answer 14

Aka fusel alcohols, includes fatty acids & esters

Question 15

Fatty acids

Answer 15

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

Question 16

Esters

Answer 16

Formed when fatty acid reacts with alcohol, ‘esterification’, fruity & floral aromas, esters present at end of fermentation made in yeast cell

Question 17

Iso-amyl acetate

Answer 17

Made when iso-amyl alcohol combines with acetic acid

Question 18

Sulfur compounds

Answer 18

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

Question 19

Yeast choice

Answer 19

Saccharomyces cervisiae- reliably produces high levels of ethanol & positive aromatic compounds, different ‘strains’ display slightly different chars

Question 20

Cultured yeast

Answer 20

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

Question 21

Cultured yeast formats

Answer 21

Main difference is shelf life- some can store for short periods, some have to be used immediately

Question 22

Preparing the cultured yeast

Answer 22

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

Question 23

Choice of yeast

Answer 23

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

Question 24

Ambient (wild) yeast

Answer 24

Lives in distillery year round, in air & on surfaces, sugary liquid exposed to air will begin to ferment

Question 25

2 concerns using ambient yeast

Answer 25

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)

Question 26

Sequential & parallel fermentaion

Answer 26

2 approaches: finish 1st step before starting 2nd, or take place at same time

Question 27

Sequential fermentation

Answer 27

All sugars present at start, can predict level of alcohol at end, important for process efficiency

Question 28

Raw materials containing sugar

Answer 28

Fruit, sugar cane- no choice but to use sequential fermentation

Question 29

European tradition

Answer 29

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

Question 30

Parallel fermentation

Answer 30

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

Question 31

Challenges of parallel fermentation

Answer 31

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

Question 32

Liquid-state fermentation

Answer 32

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

Question 33

Semi-solid-state fermentation

Answer 33

Liquid with high level of solids, thick consistency, typically grains, baijiu & shochu common

Question 34

Solid-state fermentaion

Answer 34

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

Question 35

Bacteria

Answer 35

Present almost everywhere in air & on surfaces, not able to create alcohol

Question 36

Malolactic conversion

Answer 36

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

Question 37

Malic acid

Answer 37

Key nutrient for many spoilage bacteria, if MLF happens immediately then adds to stability, also protected by ethanol (9-10%) & high acid

Question 38

Bacterial flavour creation

Answer 38

Bacteria create large # of aromatic compounds, many not desirable, avoid delays & ensure that yeast rapidly dominates

Question 39

2 exceptions for bacteria

Answer 39

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