Unit 2 - Lesson 8 - Yeast

Question 1

What are yeast cells?

Answer 1

Single-celled micro-organisms and member of the fungus kingdom
Can be cultivated or found in nature
Very small and seen at 100x magnification

Question 2

What is the yeast genus & species used in fermentation?

Answer 2

Genus: Saccharomyces - derives from Greek word sugar and mushroom “sugar fungus”

Species: S. Cerevisiae - wine, baking, distilled & brewing

Cerevisiae = Latin for beer

Question 3

What are yeast cells structures?

Answer 3

Nucleus
Mitochondrion
Vacuole
Inner Plasma Memb Rane
Daughter Cells
Cell Wall

Question 4

How small are yeast cells?

Answer 4

5-10 microns and ellipsoid in shape

Question 5

What is the Yeast Cell Structure?

Answer 5

2 outer layers (cell wall and inner membrane)

Inner with nucleus, vacuole, mitochondria and nutrients

Question 6

What is the yeast’s cell wall made of?

Answer 6

Protein and polysaccharides

Question 7

What does the plasma membrane in the yeast cell accomplish?

Answer 7

Allows molecules in and out of the cell

Regulates unfermented materials taken in the cell

Question 8

How thick is a yeast cells wall?

Answer 8

100-200 nano microns

Wall thickens when cell reached post fermentation stage

Question 9

What are bud scars?

Answer 9

Scares withiin a yeast cell indicating age.

Older yeast produces more buds and hence more scars

Question 10

What are 2 indicators of age in yeast?

Answer 10

Bud scars

Shape, size and # of vacuoles which change with age

Question 11

What environment can yeast operate?

Answer 11

Both aerobic and anaerobic

Question 12

How does yeast function under aerobic conditions?

Answer 12

With little other microbiological activity, ample carbs and nutrients yeast will multiply
Used for commercial production

Question 13

How does yeast function under anaerobic environment?

Answer 13

Exothermic - energy release
Assuming no other microbial activity, suitable carbs and nutrients - yeast converts carbs into alcohol, CO2 and flavor compounds

Question 14

How do yeast cells multiply?

Answer 14

Asexually - through budding from a mother cell

Each bud leaves a scar on mother and smaller one on daughter cells

Question 15

How do you determine yeast cells age?

Answer 15

Count the bud scars

Question 16

What is the fluid called within the yeast cell?

Answer 16

Cytoplasm

Question 17

What is a threat at the beginning of fermentation to yeast?

Answer 17

Other microbial contamination

Question 18

When creating commercial yeast in an aerobic environment how is that done?

Answer 18

1. Harvested at various solids concentration through rotary vacuum or dewatering
2. Three yeast products are formed (Cream, Pressed (cake) and Dried)

Question 19

What are the 3 yeast products from commercially grown yeast and their respective water content?

Answer 19

Cream yeast - 18% dry
Pressed Yeast - 25% dry
Dried Yeast - 95% dry

Question 20

What determines yeast product (cream, pressed or dried)?

Answer 20

1. Scale of spirits production
2. Distillery location
3. Auto vs manual control
4. On-site storage capability
5. Availability of required strain

Question 21

What is cream yeast suitable for?

Answer 21

Large-scale production - Delivered in tankers
Requires
1. On-site cream storage
2. Automated delivery
3. Rapid turnover

Question 22

When is Dried Yeast suitable?

Answer 22

Best for smaller scale

1. Limited delivery options
2. Manual reactivation into liquid stream
3. Manual addition
4. Climatic conditions less important vs cream yeast

Question 23

When is pressed yeast suitable?

Answer 23

Small to medium production

Sites require refrigeration storage

Question 24

What are some other forms used in yeast fermentation aside from commercial yeast?

Answer 24

Cereal bassed unfermented liquor
Molasses based liquor
Small-scale yeast propagation
Brewers yeast - not common anymore

Question 25

What is aliquots?

Answer 25

Small volume of yeast rich liquor

Question 26

What are general yeast selection criteria?

Answer 26

1. Raw material used
2. Type and flavor of spirit
3. Local processes used
4. Ambient conditions

Question 27

What are specific yeast selection criteria?

Answer 27

1. Fermentation rate
2. Original gravity & max ABV for wash
3. Consistency of Performance
4. Distillery capability, yeast process & storage conditions
5. Require flavor from yeast strain
6. Reliability and performance in supply chain
7. Commercial cost of yeast

Question 28

How often is yeast reused in a still?

Answer 28

Only once per fermentation cycle

Need to control yeast strain

Question 29

What is the stability of yeast based?

Answer 29

Directly proportion of dry matter. The higher the dry matter, the longer the shelf life.

Question 30

How long can dried vacuum sealed yeast last?

Answer 30

up to 12 months

Question 31

Pressed yeast is usually in the form of ___kg bags and what is the storage temp?

Answer 31

25 kg
3-4ª C
Stored on shelves with good air circulation to prevent hear build up
Shelf life 2 weeks

Question 32

How is cream yeast delivered and what is the shelf life?

Answer 32

25,000L tankers discharged into refrigerated bulk storage

Control temps at 3-4ª C and added to fermenter

Question 33

What is essential in maintaining good quality yeast?

Answer 33

1. Clear stock replenishment timelines

2. Good stock rotation, FIFO

Question 34

How is commercial yeast stored?

Answer 34

In sterile conditions to avoid contamination

Question 35

Why are some distillers using natural yeast fermentation?

Answer 35

1. Reasonably reliable
2. Meet locally available limited processes
3. Produce good flavor at very low cost
   Often in rum, wine and flavored fruit.

Question 36

Why do distillers avoid natural yeast?

Answer 36

Time and taste
Time to get fermentation going is lengthened
Taste will be influenced by the strain of yeast and it is not controlable

Question 37

What process do commercial yeast makers use?

Answer 37

Aerobic