36 Questions
What is the primary source of biological carbon dioxide production?
Yeast fermentation
How many yeast cells are approximately in one pound of compressed yeast?
15 trillion
What is the purpose of fermentation in yeast cells?
To break down sugars for energy
Who proved that living yeast was necessary for fermentation?
Louis Pasteur
What is the function of amylase in bread baking?
To break down starch into sugar
What is the term used to refer to the many enzymes in yeast that break down sugars?
Zymase
What is the overall process by which yeast breaks down sugars to carbon dioxide?
Glycolysis
What is the proportion of alcohol produced during fermentation compared to carbon dioxide?
Equal amounts
What is the result of yeast fermentation in yeast-raised baked goods?
Production of carbon dioxide and alcohol
What happens to yeast fermentation when the dough temperature rises above 120°F (50°C)?
Fermentation slows down
What is the effect of salt on yeast fermentation?
Salt retards yeast fermentation
What happens to yeast fermentation when a large amount of sugar is added?
Yeast fermentation decreases
What is the ideal pH range for yeast fermentation?
pH 4-6
What is the effect of antimicrobial agents on yeast fermentation?
They slow or stop yeast fermentation
What happens when a large amount of yeast is used in a lean dough?
Dough becomes exhausted of sugars
What type of sugar is fermented slowly?
Maltose
Why is it best to use a small amount of yeast when using a long fermentation time?
To prevent yeasty flavor
What is the purpose of using a preferment in yeast doughs?
To adjust the amount of salt and sugar
Why is active dry yeast not popular with professional bakers?
It releases a substance detrimental to the quality of gluten in dough
What is the ideal temperature for hydrating active dry yeast?
105°F - 115°F
How much active dry yeast should be used compared to fresh compressed yeast?
Half as much
What is the main advantage of using active dry yeast in pizza or tortilla production?
It creates extensible dough
When was instant yeast developed?
1970s
How should instant yeast be added to dough?
Added directly to dough
How much instant yeast should be used compared to fresh compressed yeast?
One-quarter to one-half as much
How long can unopened instant yeast last at room temperature?
1 year
What type of yeast strains grow well in high-sugar environments?
Osmophilic yeast
What happens when regular yeast is used in sweet, rich doughs?
It takes an hour or more to adapt to the environment
What is the name of the traditional, naturally fermented sourdough starter in French?
Pain au levain
How is a sourdough starter typically prepared?
By mixing flour and water and allowing wild yeast to ferment
What is the main advantage of using pure yeast cultures?
They are more consistent
What is the typical color of fresh compressed yeast?
Light grayish tan
How is compressed yeast typically used?
By dissolving it in warm water
What is the main characteristic of active dry yeast?
It has a low moisture content
What is the treatment that active dry yeast undergoes to reduce moisture?
Spray-drying
What is the approximate proportion of dead yeast cells in active dry yeast?
One-quarter pound per pound
Study Notes
Yeast Fermentation
- Yeast fermentation is a process in which yeast cells break down sugars for energy, producing carbon dioxide and alcohol as byproducts.
- Yeast uses the energy for survival and reproduction.
- The rate of yeast fermentation is affected by several factors, including temperature, amount of salt, amount of sugar, type of sugar, pH of dough, presence of antimicrobial agents, and amount of yeast.
Yeast Cells
- Yeast cells are very small, single-celled microorganisms, with approximately 15 trillion cells in one pound of compressed yeast.
- Yeast cells reproduce by budding, and visible scars from previous buddings can be seen on the yeast cell.
Enzymes and Fermentation
- Yeast can be thought of as tiny enzyme machines, breaking sugars into smaller and simpler molecules with every step.
- Yeast lacks amylase and cannot break down starch into sugar, which is why amylase is an important additive in bread baking.
- The overall process of fermentation involves multiple steps, controlled by separate enzymes, and produces carbon dioxide, alcohol, and small amounts of flavor molecules.
Factors Affecting Yeast Fermentation
- Temperature: Yeast is dormant at 32–34°F (0–1°C), active at 50°F (10°C), and slows down at 120°F (50°C), with an optimal temperature of 78–82°F (25–28°C).
- Amount of salt: Salt retards yeast fermentation, and the optimal amount of salt is 1.8–2.5% (baker's percentage).
- Amount of sugar: Small amounts of sugar (up to 5%) increase yeast activity, while large amounts (above 10%) slow fermentation.
- Type of sugar: Sucrose, glucose, and fructose are fermented rapidly, maltose is fermented slowly, and lactose is not fermented at all.
- pH of dough: The optimal pH for yeast fermentation is an acidic 4 to 6.
- Presence of antimicrobial agents: Certain antimicrobial agents slow or stop yeast fermentation.
- Amount of yeast: The more yeast, the faster the fermentation, but excessive yeast can add an undesirable yeasty flavor and exhaust dough of sugars.
Types of Yeast
- Compressed yeast: Fresh, moist cakes or blocks with 30% yeast and 70% moisture, lasts up to two weeks when stored wrapped in plastic and refrigerated, and three to four months when frozen.
- Active dry yeast: Dried granules with less than 10% moisture, sold in vacuum-packed jars or pouches, and lasts several months at room temperature, longer if frozen or refrigerated.
- Instant yeast: Developed in the 1970s, instant yeast can be added directly to dough without first hydrating in water, and is more vigorous than compressed or active dry yeast.
Sourdough Starters
- Sourdough starters are prepared by mixing flour and water, allowing wild yeast and lactobacilli bacteria to ferment, and can be used to leaven bread.
- Fresh starter does not need to be made for each new day's production, and can be saved for the next day's bread by mixing it with fresh flour and water.
- Some bakeshops pride themselves on the number of years they have continued to bake from their original starter.
Learn about yeast cells, their structure, and the process of fermentation. Discover how yeast breaks down sugars for energy and reproduces through budding.
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