WHAT IS YEAST?
Yeast is a type of fungus, a living single-celled microorganism, that converts sugar and starch into carbon dioxide bubbles and alcohol when used in the production of bread, beer, and wine. There are many varieties of yeast, but the most common one used for food production is called Saccharomyces cerevisiae, “saccharo” meaning sugar and “myces” meaning fungus. This yeast can metabolize simple sugars, such as glucose, sucrose and maltose and convert them into alcohol and carbon dioxide through a process known as fermentation, a process which causes the dough to rise, giving bread its characteristic light, airy texture and flavour. In bread making, yeast is responsible for the fermentation component as the yeast cells consume sugars in the dough, break them down as food within their cells, and then release carbon dioxide into the environment as a result of respiration and as a by-product. This carbon dioxide gas forms bubbles within the matrix of the dough and causes the dough to rise when it is trapped by properly formed sheets of gluten. Many of the flavours present in bread come from other metabolites of yeast, such as lactic acid. Yeast also has a very distinctive smell and creates the unmistakable aroma of baking bread. Yeast reproduces by budding, which is when a new bud grows from the side of an existing cell. This bud eventually breaks away from the mother cell to form a separate daughter cell. Each yeast cell, on average, undergoes this budding process 12 to 15 times before it is no longer capable of reproducing. During commercial production, yeast is grown under carefully controlled conditions on a sugar-containing medium typically composed of beet and cane molasses. Under ideal growth conditions a yeast cell reproduces every 90 minutes.
HOW IT WORKS
Activation – First, the yeast is activated by mixing it with warm water or milk and a small amount of sugar. The sugar provides food for the yeast, and the warm temperature helps to wake it up and get it growing.
Fermentation – Once the yeast is activated, it begins to feed on the sugar and produces carbon dioxide gas and alcohol as waste products. This is called fermentation. The carbon dioxide gas gets trapped in the dough, causing it to rise and giving bread its light, airy texture.
Gluten Formation – As the yeast is fermenting, the dough is also being kneaded. This process helps to develop the gluten in the dough, which gives the bread its structure and chewy texture.
Proofing – After the dough has risen, it is shaped and allowed to rise again in a process called proofing. During this time, the yeast continues to produce carbon dioxide gas, which causes the dough to rise even further.
Baking – Finally, the bread is baked in the oven, which causes the yeast to die off and the carbon dioxide gas to escape. This leaves behind small pockets in the bread, giving it its characteristic texture.
TYPES OF YEAST
These are the different types of yeast that can be used in bread making:
Active Dry Yeast – is an older type of yeast which is generally no longer used in bakeries. The yeast is dormant, and is best used after proofing and rehydrating, which involves the yeast being sprinkled over warm water with pinch of sugar and left to stand for 10 minutes until creamy and bubbly.
Instant Dry Yeast – has been developed from special strains of yeast that withstand drying. It comes in smaller granules than active dry yeast, absorbs liquid rapidly, and doesn’t need to be hydrated or “proofed” before being mixed into flour and dry ingredients.
Bread Machine Yeast and Rapid Rise Yeast – are instant yeasts that may include bread improvers such as ascorbic acid.
Compressed Yeast – also known as fresh or cake yeast is made of living yeast pressed into blocks, available in 454 g (1 lb.) blocks in Canada. This yeast is most suited for use in small bakeries, as it has good rising qualities and produces excellent-tasting breads, croissants, and Danish pastries. Fresh yeast should be proofed in tepid water (26°–32°C) without contact with salt or sugar.
Cream Yeast – is ready to be used immediately and can be metered directly into a mixer. Its performance is consistent as it is unlikely to be damaged during storage and transport.
CONDITIONS FOR YEAST
As yeast is a living organism it needs certain conditions to be controlled so that it can operate at its optimum and avoid risks of inactivity:
• Sugars are required as a food source for yeast.
• Warm temperatures are needed. Temperatures close to freezing point will slow yeast activity and temperatures below freezing or above 46°C will kill yeast cells.
• Water is used to disperse the yeast and dissolve other materials. Yeast needs to always be surrounded by water as its natural environment.
• Nitrogen is critical for optimum yeast activity. (patent flour contains but about, 2 mg. of amino-acid nitrogen for every 100 gm) If soluble nitrogen is lacking in bread dough, then improvers containing nitrogen, such as ammonium chloride, can be added.
• Mineral salts encourage yeast activity and accelerate fermentation.
FERMENTATION
In bread making, yeast ferments the carbohydrates present in sugar and flour, breaking them down to simpler sugars and eventually to carbon dioxide gas. The formation of gas causes the dough to rise. Fermentation softens the gluten, a complex protein which makes bread more elastic. Kneading develops the gluten and spreads the gas cells evenly through the dough to create a fine textured product. When bread is baked the initial heat increases yeast action, expanding the gas cells and the loaf rises. Once the dough reaches 46°C the yeast is killed and the loaf cooks to a crisp brown crust with a soft moist and even crumb.
During fermentation, yeast is a key ingredient and serves three primary functions:
• Production of carbon dioxide – This is generated by the yeast as a result of the breakdown of fermentable sugars in the dough. The production of carbon dioxide causes expansion of the dough as it is trapped within the protein matrix of the dough.
• Dough maturation – This is accomplished by the chemical reaction of yeast-produced alcohols and acids on flour protein and by the physical stretching of the protein by carbon dioxide gas. These results in the light, airy physical structure associated with yeast-leavened products.
• Development of flavour – Yeast gives bread and other yeast-leavened products their characteristic flavour. During dough fermentation, yeast produces many secondary metabolites such as ketones, higher alcohols, organic acids, aldehydes and esters. Some of these, alcohols for example, escape during baking. Others react with each other and with other compounds found in the dough to form new and more complex flavour compounds. These reactions occur primarily in the crust and the resultant flavour diffuses into the crumb of the baked bread.
STORAGE
Proper storage of yeast is crucial for maintaining its viability and quality. Here are some best practices for storing yeast:
Temperature – Yeast should be stored in a cool and dry place, ideally between 32°F and 45°F (0°C and 7°C). Temperatures above 45°F (7°C) can cause the yeast to become inactive or die.
Moisture – Yeast should be stored in a dry environment to prevent moisture from activating the yeast prematurely or causing it to clump together.
Air exposure – Yeast should be stored in an airtight container to prevent exposure to oxygen, which can lead to oxidation and decrease yeast viability.
Shelf life – Check the expiration date on the package to ensure that the yeast is still fresh and within its recommended shelf life. Using expired yeast can result in poor performance and inconsistent results.
Freezing – It is possible to freeze yeast to extend its shelf life, but it is important to first seal it in an airtight container or freezer bag to prevent moisture and freezer burn.
By following these best practices, you can help ensure that yeast stays fresh and viable, which will result in better performance and consistent results in your baking or brewing. ADB
Saccharomyces cerevisiae is commonly known as baker’s yeast or brewer’s yeast.