Introduction to Yeast

Question 1

what are the 3 beer components ?

Answer 1

1. Malt - caramel, sweetness
2. Hops - flowery, bitterness, foam
3. Yeast - good flavours and undesirable flavours

Question 2

what is yeast ?

Answer 2

Yeast are unicellular fungi that reproduce vegetatively by budding, and that form sexual states not enclosed in a fruiting body

Question 3

how many genera of yeast have been identified ?

Answer 3

around 100 genera have been identified divided into > 700 species

Question 4

how many generas are used for brewing ?

Answer 4

only a single genera is used for brewing

Question 5

what genus does the most ‘domesticated yeast’ belong to ?

Answer 5

Saccharomyces = ‘sugar fungi’

Question 6

when was yeast discovered and isolated ?

Answer 6

in the 18th century

Question 7

where does yeast live ?

Answer 7

its found in the natural environment:

• water
• soil
• air
• plants
• animals
• man made environments

Question 8

how is yeast used in biotechnology ?

Answer 8

• production of beer
  it is also found in bread, wine, distilled spirits, bioethanol, soy sauce
  its also used as a bio ingredient and in the pharmaceutical industry for a variety of different purposes

Question 9

what name is given to yeast that are safe ?

Answer 9

GRAS (generally recognised as safe)

-yeast can also be pathogenic (not safe, not industrial yeast)

Question 10

what can yeast cells be a model organism to study ?

Answer 10

Yeast can be used as a model organism to study cellular activity and function in higher organisms such as humans

Question 11

how do yeast cells from different species differ ?

Answer 11

they differ in size, shape and colour

Question 12

describe Saccharomyces

Answer 12

they are typically ellipsoidal and around 5-10um in diameter and cream in colour

Question 13

What is the process called whereby yeast produce new cells ?

Answer 13

Budding

Question 14

what is budding ?

Answer 14

its a type of asexual reproduction
after the DNA is replicated, a daughter cell is produced which grows out of the side of the mother cell. In this way yeast divide to produce new individuals which is genetically identical to its parent

Question 15

how can we tell that budding has happened ?

Answer 15

• mother cells distinct from daughters
• cells can only bud a finite number times
• each cell has a finite lifespan and cells age and die

Question 16

how is wort produced ?

Answer 16

it is produced from hops, malted barley and water

Question 17

what is the by-product of the growth of yeast on wort ?

Answer 17

Beer

Question 18

what is the most important step in fermentation ?

Answer 18

its to convert the sugars present (in wort) into ethanol and carbon dioxide

Question 19

what are the two types of yeast inoculum ?

Answer 19

1. Active dried yeast (ADY)

2. Liquid yeast

Question 20

what are some of the features of active dried yeast ?

Answer 20

• produced by specialist suppliers
• long shelf life
• limited choice of yeast strains
• used by small brewers

Question 21

what are some of the features of liquid yeast?

Answer 21

• typically produced on site
• short shelf life
• specific strain used for each product
• used by larger/multinational brewers

Question 22

how is yeast stored ?

Answer 22

its stored under liquid nitrogen at approx -196 degrees celsius

Question 23

what two stages that plant propagation can occur in ?

Answer 23

1. batch propagation

2. fed batch propagation

Question 24

what is a batch process ?

Answer 24

its whereby yeast and all the nutrients required for growth are mixed together in one ‘batch’ and the culture is allowed to grow

Question 25

how is the fed batch process different ?

Answer 25

the fed batch fermentation is a little bit different as it creates an environment where nutrients are 'fed' into the vessel at a slow rate - this is important when a huge quantity of cells are required

Question 26

what is the process called when propagated yeast is introduced to the wort ?

Answer 26

pitching

Question 27

how many cells are used in the pitching process ?

Answer 27

1 x 10^7

Question 28

what happens at the start of fermentation ?

Answer 28

Yeast takes up sugars, nutrients and oxygen present in the wort and uses this to divide. Once the oxygen is removed yeast switches from respiration to the fermentation pathway

Question 29

how many times does the yeast population divide at the start of fermentation ?

Answer 29

3 times

Question 30

what does CO2 production do to the yeast ?

Answer 30

CO2 maintains the yeast in suspension

Question 31

what happens towards the end of fermentation ?

Answer 31

CO2 production slows, vessel is chilled and yeast begins to collect in the cone. The yeast begins to settle out and accumulate in the cone of the vessel. The yeast will then remain in the cone until fermentation is complete in terms of sugar utilisation and flavour production.

Question 32

what is flocculation ?

Answer 32

this is the ability of cells to adhere to each other and to form clumps . Its a reversible process and Yeats cells can clump together and form flocs which can then disassociate so that cells are on their own again.