Lecture 5: Fermentation

Question 1

Describe Saccharomyces cerevisiae and the products (and byproducts) made with it

Answer 1

Feeds on sugar, but too much sugar will lower Aw and slow down growth

3 common products:
Beer (Ethanol + CO2)
Wine (Ethanol)
Bread (Carbon dioxide)

Question 2

Name five important genera of LAB.

Answer 2

Lactococcus
• Lactobacillus - Cheese and yogurt (Lactic acid)
• Letconostoc
• Streptococcus
• Pediococcus

Question 3

What is the optimal temperature of yeast bread fermentation and why?

Answer 3

30-35 ℃

Yeast starts to die off at 41 ℃, but you need it warm enough to activate it

Question 4

What is the purpose of kneading and proofing of the dough?

Answer 4

Kneading - denatures protein (gluten), stretching it out and making it elastic

Proofing - bread rises b/c of ethanol and carbon dioxide; is yeast working/enzymes consuming

Question 5

How did ethanol leave the bread dough?

Answer 5

Evaporated from bread while baking

Question 6

Why sulfur dioxide (SO2) or potassium metabisulfite is added to crushed grapes for wine production?

Answer 6

To remove all the wild yeast, bacteria, and fungi so you have a clean juice to start with

Sulfur compound helps to preserve the color of the wine

Question 7

What will happen without sulfur dioxide (SO2) or potassium metabisulfite is added to crushed grapes for wine production?

Answer 7

The pigment is very sensitive to light and to oxidation, so would have degraded quality of wine

Question 8

How to treat the CO2 produced by yeast in wine?

Answer 8

• May be allowed to escape
• May be collected, compressed, and sold
• May be retained (sparkling wines, like champagne)

Question 9

What factors affect wine’s sweetness?

Answer 9

• The degree of fermentation - how much sugar is used up during the fermentation process
• Sugar content of the fruit
• Adding unfermented juice or sugar

Question 10

What is the difference of yogurt fermentation and cheese production?

Answer 10

Cheese - curdling of casein in milk, then add lactic acid bacteria & renin, renin (only works in low pH) curdles protein – LAB added to lower pH so renin can work

Yogurt – no renin just LAB – LAB lowers pH & casein gets curdled but the texture is more gel like (cheese is tightly packed curdle) – acid curdles casein in yogurt but renin curdles protein in cheese

Question 11

What is the function of LAB in yogurt and cheese?

Answer 11

LAB lowers pH so curdling happens

• in cheese - LAB lowers pH so renin can curdle protein
• in yogurt - LAB lowers pH AND curdles casein

Question 12

Why is cheese has higher fat content than yogurt?

Answer 12

Cheese is tightly packed casein (which contains all of the fat from milk.
Yogurt also contains whey (cheese does not), so is a larger mix and thus less fat

Question 13

Why is brine used in pickle fermentation?

Answer 13

Cucumbers have a lot of spoilage agents (bacteria, yeast, molds) than most, so can end up with soft pickle or bloating

When making homemade pickles you need to start with 2-3% salt brine, but keep adding more to help lower Aw (b/c spoilage agents can eat lactic acid and raise Aw)

Question 14

Why is brine used in sauerkraut fermentation?

Answer 14

Salt solution is used to control water activity/other bacteria growth
• Because don’t add sulfur compound
• Don’t want to add too much b/c that would bring down Aw and LAB could also not survive

Pull nutrients from inside of cabbage/pickle

Lot of acid, anaerobic environment, and low Aw make harsh conditions that a lot of other bacteria can’t survive in