Discovering the Science of Yeast in Baking

Table of Contents

A. The Basics of Yeast

     1 - Why Yeast is Important in Baking

           How Does Yeast Work?

     2 - Dry Yeast vs. Fresh Yeast

          Active Dry Yeast and Instant Yeast

     3 - Yeast Production

B. The Science Behind Yeast

     4 - The Composition of Yeast

     5 - The Enzymes in Yeast

     6 - What Yeast Needs to Multiply

     7 - What Harms Yeast

     8 -  The Effects of Temperature

1. Why Yeast is Important in Baking

Yeast is known as a ¨Biological Leavening Agent¨. Put simply, yeast is a natural, living, fungus that people use to cause their bread to rise. Without yeast, bread would be very dense and chewy. 

Now, you might be wondering, ¨but how does it cause bread to rise?¨ Well, let me tell you -- and hang in there cause this gets a little sciency! 

- Plainly put, yeast converts sugars to CO2 and alcohol. 

     - As mentioned before, yeast is a living fungus, and everything living needs to eat. Well, what yeast eats are sugars. Sugars refer to not just the sugar you add to a bread like white sugar or honey, but sugars are also conisdered starches. Sugars and starches are very similar in their molecular composition, which is why yeast consumes both of them. And, most grains contain a large portion of starches, so naturally, when making bread, that is often made from flour (made of finely ground wheat grains), the yeast has a lot of starches to eat from. 

      - Once the yeast has begun eating the sugars (and starches), the yeast begins to multiply and through that multiplication process, CO2 is released. This is what causes bread dough to rise.

Now, you might be wondering why the CO2 doesn't just escape into the air that we breath. This is because of something in the dough called ¨gluten¨.

- Due to the proteins that are also found in flour, with the addition of water (or liquid) to a bread dough, gluten is formed and the gluten traps a lot of the CO2 rather than letting it go. This creates bubbles/pockets of CO2 all throughout the bread. 

- The yeast will continue to eat the sugars and starches and continue to multiply until all of the sugars and starches in the dough are consumed. 

Wow! That was a lot! But, lets move on to some more practical and relevant information for those who want to get into baking. 

2. Dry Yeast vs. Fresh Yeast

While most people are accustomed to thinking about yeast in terms of “active” or “instant”, these are
actually subcategories of a larger classification that can be divided into “dry yeast” and “fresh yeast”.

Fresh Yeast

- Fresh yeast is quite literally what is sounds like, it is yeast that has not been dehydrated and is in its living and most active form. 

     - Fresh yeast comes in the form of a compressed block or sometimes even a liquid or cream like form. 

     - Fresh yeast has a shelf life of 2-3 weeks when refrigerated and wrapped in plastic. 

     - Often times, when using fresh yeast, the yeast might come into contact with bacteria from your fingers or the table and might grow some white spots in the surface of the yeast, this is okay to still use as it is simply from bacteria and has nothing to do with if the yeast is still ¨good¨. Simply remove the white spots and use the remainder of the yeast. 

          - However, if you see black or green spots on the yeast, then you should not use the yeast anymore because that is mold. 

      - As a general rule of thumb, twice the amount of fresh yeast (in weight) should be used when converting from dry yeast. 

          - So, if your recipe says: 5g of dry yeast, use 10g of fresh yeast. 

Dry Yeast

- Dry yeast is becoming more and more popular in the world due to its ease of use. As implied through the name, dry yeast has been dehydrated to extend the shelf life and to be able to fit much more of it in a given space or container. 

     - The biggest difficulty with using dry yeast is to control the moisture content once the container as been opened. 

          - When dry yeast comes into contact with moisture, it activates the yeast and then begins to reduce the leavening capability over time. 

          - For this reason it is very important to store dry yeast in a air tight container and avoid any moisture getting into the container as much as you can. 

          - If the dry yeast is stored too warm it also has a negative impact on the yeast and decreases the length of its shelf life. 

- With all of this said, there are different recommendations for storing dry yeast. Just remember, the key word is ¨dry¨. A refrigerator accumulates a good amount of moisture, but if the yeast is in a airtight container, a fridge or freezer should only extend the shelf life of the dry yeast once the container has been opened. 

     - Different people say different things, so I will just go off of reliable sources, personal experience and tests I have performed at home. 

     - In an unopened container, dry yeast can last up to 2 years - and being frozen or sitting at room temperature has not been shown to change that. 

     - After opening and at room temperature, dry yeast has a shelf life of  2 months. 

     - After opening and in the refrigerator, dry yeast has a shelf life of 4-6 months. 

     - After opening and in the freezer, dry yeast has a shelf life of 9-12 months. 

- As a general rule, use half as much dry yeast as fresh yeast. 

     - If you recipe says 20g fresh yeast, use 10g dry yeast. 

Now, moving on to the types of dry yeast. 

ADY - Active Dry Yeast

- Active dry yeast is used in Europe mostly for beer production, but it is used very often in bread making in the United States and other places in the world. ADY is quite similar to IDY (instant dry yeast), but how to use them is surprisingly different. 

     - Active dry yeast must be ¨proofed¨ by putting it into a warm liquid before adding it to the remaining dough ingredients. 

          - Oftentimes, it is also a good idea to add some sugar to the liquid and just wait 5 minutes or so to see if the yeast is still active. You know if it is active if the yeast floats to the top and starts to cover the surface in a layer of foam; this is often referred to as ¨blooming¨ (see picture below for reference).   

IDY - Instant Dry Yeast

- Instant dry yeast is used all over the world for baking and is very easy to use. The only real difference between ADY and IDY when you look at them is that IDY is made into smaller particles.  

     - Instant dry yeast does not need to be proofed or added to any liquids, it should simply be added with the dry ingredients. 

*This picture depicts a yeast cell multiplying or ¨budding¨. 

3. Yeast Production

Yeast has been used in bread making since the 1300's, however, it wasn't until the 1800's, with the invention of the microscope that yeast was even discovered to be the cause of bread rising. About a hundered years after this discovery, yeast cultivation and reproduction developed. 

- The yeast reproduction process is known as ¨budding¨. 

     - Budding starts with a single cell of yeast, also known as, the ¨mother¨ cell. 

     - The yeast cell consumes sugars and forms a knob or bud. 

     - The bud grows and eventually develops into a genetically identical yeast cell to the mother cell. 

     - The new yeast cell detaches from the mother cell and then the process starts over. 

          - Each yeast cell can duplicate a maximum of 25 times. 

- The ideal pH for yeast to multiply is a pH of 4-6. 

- The ideal temperature for yeast to multiply is around 30* C or 86* F. 

- In the ideal conditions, reproduction of a single yeast cell takes 3-5 hours. 

     - In a factory, the reproduction of yeast takes about 3-4 days since they are working in such high quantities. 

- The reproduction process is the fastest a thin nutritious (sugary) liquid with oxygen. 

- Once the yeast has multiplied the various methods of processing and packaging take place. 

     - Fresh yeast is compressed into blocks, wrapped, and stored in a fridge at around 4* C (39* F), where the yeast will hibernate and then be shipped out to various locations. 

     - Dry yeast is made by placing the newly made yeast on thin sheets where it is dried in a tunnel drier that maintains a temperature of 28-42* C (85-107* F) for 2-4 hours, until the yeast is dry and can thereafter be packaged and shipped out to various locations. 

*There are many steps and terms that I did not mention for the sake of ease and understanding. To learn more in depth about the exact process of yeast production and for exact terms, follow this link. 

4. The Composition of Yeast

- The most commonly used yeast species in food production belongs to a saprotrophic fungus. 

     - This fungus is found in baker's yeast, sourdough, and brewer's yeast (for the production of alcoholic beverages). 

     - The use of a fungus is what makes it possible to convert different sugars to CO2 and alcohol. 

          - The formation of CO2 and alcohol inhibit competing microorganisms, making the fungus (or yeast) better able to multiply.  

The size and shape:

- The shape of yeast is easily described as small, round or oval shaped cells.

     - Yeast is approx. 2-10 mu wide and 4-21 mu long -- (1 mu = 1 millionth).

- In 1 gram of yeast, there is 8-10 million yeast cells. 

The different parts inside of a yeast cell: 

- The nucleus 

     - This is considered to be the yeasts brain and is the core of the cell. . 

     - The nucleus divides during reproduction. 

- The cytoplasm

     - This is most easily described as the gel-like solution filled with proteins and enzymes. 

     - The 5 main enzymes are called: 

          - Zymase, Invertase, Maltase, Protease, and Lipase. 

- The vacuole

     - This contains the liquid nutrients needed for the yeast cell to survive when the yeast is not fed from an outside source. 

          - The vacuole is the reason yeast can live in the fridge or freezer for a certain amount of time. 

- The cell membrane

     - The cell membrane is a porous shell made of various proteins and fats. 

          - The cell membrane is penetrable by dissolved nutrients. 

What yeast consists of in %:

75% - Water

15% - Protein

8 % - Carbohydrates

2% - Lipids, nucleic acid, minerals, salt, B1 and B6, precursors to D and E vitamins, magnesium, iron, phosphorus, potassium, sulfur, calcium, and at least 5 enzymes. 

5. The Enzymes in Yeast

ZIMPL - this is an acronym for the 5 enzymes found in yeast. But first, lets discuss what enzymes are and why they are important.  

A basic overview of enzymes:

- Enzymes are a subclassification of proteins. Each enzyme has a unique function and performs best under different pH values and temperatures. 

     - When a temperature becomes too high for a specific enzyme, the chemical composition of the enzyme begins to break apart, changing the tri-dimesional form, meaning the enzyme can no longer function properly. 

          - Fun fact: It is due to the effects of temperature on enzymes that is it is easier to digest warm food than cold food - the enzyme found in spit (saliva) functions better in warm temperatures. 

- Enzymes are found in all living things; we could not exist without enzymes. 

     - Almost everything we work with in a bakery and in the kitchen stems from living organism so it is important to understand how enzymes work. 

- Each enzyme has a very specific scope and represents a different chemical reaction. 

     - They are like ¨natures toolbox¨. 

          - Enzymes can cut into molecules, put molecules together, and change the order of molecules without the enzyme itself being destroyed. 

ZIMPL

Zymase

     - Divides glucose and fructose into CO2 and ethanol (alcohol). 

          - It is due to this enzyme that bread rises. 

Invertase

     - Divides sucrose (white sugar from cane and beets) into fructose and glucose. 

          - It is due to this enzyme that yeast is able to absorb the sugar we add to dough (it breaks the sugar down into its most simple form (monosaccharides)). 

Maltase

     - This enzyme is also found in flour.

     - Divides maltose to glucose. 

          - Similarly to invertase, this enzyme makes more simple sugars available (glucose) from the break down of starch (maltose) - typically the starch found in the flour.

          - It is due to this enzyme that sourdough does not need to be fed sugar to survive and create natural yeast and why we do not have to add sugar to a bread dough in order for it to rise. 

Protease

     - This enzyme is also found in flour and malt. 

     - Divides proteins into amino acids, going from non-soluble to soluble. 

          - Amino acids are used to build up yeast cells as well as gluten. 

          - It is due to this enzyme that bread has a darker crust and that the crust often has a stronger flavor.

Lipase

     - Divides the fats into 1 glycerol and 3 fatty acids.

          - Free fatty acids are converted into flavor compounds through other reactions. 

          - It is due to this enzyme that different types of bread can have different flavor profiles. 

6. What Yeast Needs to Multiply

There are 4 basic environmental requirements that need to be met in order for yeast to multiply. 

     1. Yeast needs water or a liquid. 

     2. Yeast needs oxygen. 

     3. Yeast needs heat. 

     4. Yeast needs nutrients. 

- If one of these 4 needs are not met, yeast cannot multiply. 

Other contributing factors: 

- Osmotic pressure - the water pressure inside and outside the cell. 

- The pH value is also important. 

- Yeast multiplies most quickly in a thin, nutrient dense, solution that has a good oxygen supply and is around 30 *C (86 *F). 

7. What Harms Yeast

There are 4 things that harm (or inhibit) the yeast cells ability to multiply:

     1. Salt - this draws water from the yeast cell. 

     2. Fats - this encapsulates the yeast cell, making it unable to absorb water, oxygen, or nutrients. 

     3. Alcohol - too high of an alcoholic environment (more than 10%-15%) is toxic and can kill the cells. 

     4. CO2 - this is the waste product of yeast. Too high of a CO2 concentration (around 40%) will suffocate the yeast. 

- Looking to these 4 things, it is understandable why in baking school, as mentioned in The Basics of Bread Making, bakers are taught not to mix salt with yeast or to dissolve the yeast in the fat (oil or butter).

     - This is also the reason why we ¨smash¨, ¨poke¨ or ¨pound¨ the dough after the first rise, it is to get the CO2 out on the dough and let new oxygen in.