Yeast

Question 1

Yeast

Answer 1

Single called
- eukaryotic
-cell walls like plants
-NIN-photosynthetic
-non-motile
-0.5-10um, much bigger than bacteria

Question 2

Saccharomyces cerevusae

Answer 2

ALE YEAST
-37-44* max growth temp
-18-22* fermentation temp
-floats to surface
-fewer sugars used
-big flavourtown

Question 3

Saccharomyces pastorianus

Answer 3

LAGER YEAST
- 32-40* max growth temp
-7-15 fermentation temp
-sinks to bottom
-more sugars used
-clean/light flavourtown

Question 4

Cell wall and membrane

Answer 4

-cell wall: strength and shape., bud scars from cell division
- plasma membrane: barrier between call and environment, phospholipids and sterols.

Question 5

Nucleus

Answer 5

-genetic material
-16 chromosomes (~6000 different genes)

Question 6

Mitochondria

Answer 6

The powerhouse of the cell

Question 7

Vacuole

Answer 7

Combo recycling plant and warehouse
-big compounds sent here to be broken down, stored, and used later

Question 8

Cytoplasm

Answer 8

• fatty materials and glycogen: jelly that stores all organelles
• main way yeast stores those sweet sweet sugars

Question 9

Consequences of the budding method

Answer 9

Mother and daughter cells are different. Mothers are bigger and daughters are smaller. Yeast cells have finite lifespans; when they have budded their maximum number of times, they die. This means that any yeast population will contain a mixture of young, middle-aged, and old yeast cells. Like any population, the individual members have different capabilities.

We must crop and re-pitch the yeast. In other words, we collect yeast from one fermentation, store some of it, and then re-use it in a subsequent fermentation. This means that we must choose which part of the cropped yeast we are going to keep. As we have briefly mentioned, some yeast strains (mainly lager types), drop out to the bottom of the vessel at the end of fermentation.

The brewer must choose which part of the crop to keep for re-pitching. Brewing yeast cells grow bigger with age; these older cells tend to drop out first. We can work around this by sending the first portion of our crop to waste. This waste predominantly contains older cells and a lot of trub. Similarly, the top of the crop consists mainly of smaller and immature cells. The middle fraction of the crop is the best to re-pitch, as it contains the most typical yeast cells.

Question 10

Yeast storage

Answer 10

-not exposed to air
-nitrogen or co2
-3 days or less
- 2-4 degrees
- low temps: slow down cell activity ie breakdown of glycogen (the chain of cliches that year uses in starvation mode)

Answer 11

here are many other reasons for limiting the number of re-pitchings, especially using bottom cropping yeast. Most re-pitchings are carried out in very big fermenters, using highly concentrated wort. This is very stressful for yeast. Each fermentation cycle increases the chance of a mutation occurring. A mutation occurs when the genes are not copied correctly; new cells may show abnormal behaviour. This can change our fermentation profile or it can alter our beer flavour. For these reasons, brewers dispose of yeast cultures and replace them with a new one before any undesirable changes occur. The maximum number of cycles conducted ranges between 5-20. However, very stressful fermentations have less than ten, and sometimes as few as three.

Question 12

Advantages of dry yeast

Answer 12

• simplifies handling
• no requirement for costly storage plant
• simplified pitching
• long shelf life in sealed packets
• no requirements for wort aeration/ oxygenation

Question 13

Disadvantages of dried yeast

Answer 13

-?limited choice of strains
- continual cost of buying yeast
- short shelf life if packet is open.

Question 14

Zinc in water

Answer 14

Can lead to astringent flavours and be toxic to yeast