Yeast

Yeast, the brewer’s most important partner.

 


It is thought that there are approx. 950 compounds that make up beer.  Yeast is responsible for 400 of those compounds!

Brewer’s yeast (genus: Saccharomyces) is a type of fungus that consumes fermentable sugars in the wort and excretes alcohol and CO2, a process known as fermentation. 

Ale & Lager Yeast

More than 500 types of yeasts have been isolated, not including the numerous wild strains.  Brewers yeast is generally divided into two strains:

Ale Yeast (Saccharomyces cerevisiae):

·       Thrives at 60-85˚F

·       Top cropping

Lagers (Saccaromyces carlsbergenis):

·       Can ferment as low as 40˚F

·       Bottom cropping

·       Can be used at al temperatures (such as a California common style)

Definitions

 

Yeast pitch: The slurry mass of yeast cells for the inoculation of yeast into fresh wort.

Flocculation:  Aggregation of yeast cells into clumps that sink to the bottom, near the end of fermentation. 

Attenuation: The percentage of reduction in the wort's specific gravity caused by the transformation of contained sugars into alcohol and carbon dioxide gas through fermentation.

Dry vs. Liquid

 

            Dry                   Pros                             Cons

                                    Consistency                   Limited to very few varieties

                                    Long shelf life               Only 80% viable

                                    Simple storage/transport

 

            Liquid              Pros                             Cons

                                    Lots of varieties            Shorter shelf life

                                                                        Doing a starter is recommended (but not required)

Phases

 

1.     LAG PHASE 

·       Yeast adjust to the new environment: Changes in temperature and nutrients

·       Oxygen rapidly used by yeast

 

2.    LOGRITHMIC PHASE 

 

·       Fast yeast reproduction

·       Uptake of nutrients and sugars

·       Heat is generated

·       Volatiles are driven off

 

3.    DECLINING PHASE 

 

·       Increase in acidity, alcohol and carbon dioxide

·       Utilization of glycogen and sterol reserves

·       Reproduction ceases

·       Flocculation

What Makes Yeast Happy

 

PITCHING RATE

 

TEMPERATURE

 

OXYGEN

 

FERMENTABLE SUGARS

·       Crabtree Effect

Flavor-active Compounds from Yeast

 

Factors that affect Fermentation Flavors:

            Yeast:               Strain

                                    Pitching rate

            Fermentation:   Temperature

 

            Flavor-active Compound                        Example of By-Product  & its Flavor / Taste

·       Acids                                        Acetic acid                   Vinegar

·       Alcohols                                    Ethanol                         Alcohol

·       Esters                                       Ethyle acetate               Acetones/solvents

·       Aldehydes                                 Acetaldehyde                Unripe green apple

·       Ketones                                     Diacetyl                        Butter, honey

·       S-Compounds                            Hydrogen Sulfide           Sulfur

·       Phenolics                                  4-Vinyl guaiacol Cloves, spicy

 

Example: Rolling Rock – Acetaldehyde

·       Concentration of acetaldehyde increases by high fermentation temperature, high pitching rates, high pH, or infection.

 

Example: Higher Alcohol (Fusel Oil)

·       Higher alcohols (i.e. boozy, or sherry/port-like) are increased by high fermentation temp (generally), high aeration.  Reduce by pitching at a higher rate and avoid oxygen after pitching.  Unfortunately, once they are there, they can’t be influenced during maturation.

 

Bottle fermentation

Yeast can also be used to carbonate, known as bottle-conditioning.  Sometimes fresh yeast is added (i,e, for high-gravity beers), sometimes residual yeast from primary/secondary fermentation. 

 

~Tiffany (February 2014 Meeting)

© Hop Bombshells 2014