Yeast is a unicellular fungus essential in daily life. This gift of nature is used in baking leavened bread and in food fermentation. Its remarkable abilities transform simple ingredients into delicious human foods, and prevail for thousands of years.
Earliest known evidence of bread-making, a type of coarse grain flatbread, comes from a 14,000-year-old archaeological dig site in Black Desert of Jordan. People may have used the bread as a wrap for roast meat.
Honey mead is one of the first intentional fermentation processes c. 8000 BCE. It's made with wild yeast, moisture and honey, discovered by early humans in abandoned bee hives. The process of baking with yeast comes a little later.
Around 3000 BC, Egyptians are baking leavened bread with wild yeast spontaneously occurring from the surrounding environment. It's noticeable as the white residue on ripe grapes. The deliberate cultivation of yeast in baking starts around 500 BCE.
As human populations grow through history, different cultures develop methods for using the power of yeast. It's not until 1857 AD that its importance in fermentation is officially understood by scientists.
Louis Pasteur's research identifies yeast, specifically Saccharomyces cerevisiae, as a living organism responsible for fermentation. This discovery changes previous assumptions of inorganic chemical reactions.
Science of Yeast: Interactions and Processes in Baking
When the industrious single-celled organism yeast is combined with flour, water, and sugar, it begins to metabolize sugars and other carbohydrates. This process happens in anaerobic conditions.
Fermentation
Activation: When yeast is introduced to warm water and sugar, it activates and becomes "active" yeast. The warmth encourages the yeast to feed on the available sugars.
Sugar Metabolism: Yeast consumes glucose and other fermentable carbohydrates present in the flour. Through the process of glycolysis, the yeast converts sugar into pyruvate. Glycolysis is a series of reactions to extract energy from glucose by splitting it into two three-carbon molecules or pyruvates.
Anaerobic Fermentation: In the absence of oxygen, pyruvate is further broken down into ethanol and carbon dioxide. The reaction is fundamentally glucose + enzymes = carbon dioxide + ethanol / lactic acid, with the formula C6H12O6 (glucose) → 2C2H5OH (ethanol) + 2CO2 (carbon dioxide).
Leavening of the Dough: The carbon dioxide produced during fermentation is trapped in the dough, causing the bread to aerate, expand and rise. The volume and texture depend on several factors, including type of yeast, amount of sugar, dough hydration, and fermentation time.
Flavor Development: Besides producing carbon dioxide, fermentation also develops flavors and aromas essential to good bread. Organic acids, esters, and alcohols contribute to the complexity of the final product.
Baking
Fermentation is the heart of baking. Initially, yeast breaks down sugars through glycolysis into pyruvate. Under anaerobic conditions, the pyruvate is then converted into ethanol and carbon dioxide. The released carbon dioxide is trapped in the dough.
When dough is baked, heat causes the trapped carbon dioxide to expand. As temperature rises, yeast cells die and alcohol evaporates, leaving behind the distinct aroma and flavor associated with freshly baked bread.
Compounds like esters and phenols emerge during fermentation, creating the bread's distinct taste and aroma. Yeast doesn't work alone and often teams up with benevolent bacteria. Sourdough bread has a tangy taste due to lactic acid bacteria working with the yeast.
The golden-brown bread crust is created by the Maillard reaction, which creates melanoidins, responsible for the flavor of browned foods. Melanoidins are brown, high molecular weight polymers formed when sugars and amino acids combine at high heat and low moisture.
The Maillard reaction is common in many foods including seared steaks, fried dumplings, cookies and biscuits, breads, toasted marshmallows and falafel.
Interesting Facts
Different Yeast Strains: There are over 1500 species of yeast, but the species Saccharomyces cerevisiae is dominant in baking and brewing due to its reliability and efficiency in fermentation.
Yeast Reproduction: Yeast can reproduce asexually through a process called budding. In ideal environments, a real yeast colony can double in size every 90 minutes.
Nutrient Powerhouse: Yeast is packed with essential nutrients, including protein, B vitamins, and minerals. For instance, nutritional yeast can provide up to 14 grams of protein per 1-ounce serving, making it a valuable addition to vegan diets.
Dough Types Matter: Different doughs use different yeast types. Sourdough uses wild yeast, creating unique flavors, while commercially produced yeast enhances consistency and speed.
Temperature Sensitivity: Yeast thrives at around 95°F (35°C). Temperatures above 140°F (60°C) kill yeast, while below 70°F (21°C) can slow fermentation, impacting dough rise and flavor.
Proofing Time: Proofing times can vary greatly. For example, sourdough may take several hours to rise, while commercial yeast can double dough volume in as little as 40 minutes, affecting flavor complexity.
Yeast Lifespan: Dry yeast can officially last up to 24 months, while fresh yeast typically needs to be used within two weeks for maximum potency. Personal experience using 10-year-old dry yeast, however, yields hearty yeast activity and microbial enthusiasm.
Food & Drink Made with Yeast Fermentation includes:
Sourdough Bread
Sourdough rises naturally, without the use of commercial yeast. It relies on a 'starter,' a fermented blend of flour and water containing wild yeast and beneficial bacteria, to rise. This process also gives sourdough its characteristic tangy flavor and slightly chewy texture.
Cheese
Some cheeses undergo fermentation processes involving yeast. For example, the rich flavor of certain blue cheeses, such as Roquefort, comes from Penicillium roqueforti, a beneficial fungus. While all cheese is fermented, not all contain active microbes. Cheese like cheddar and parmesan are heated to temperatures high enough to kill these organisms.
Sauerkraut and Kimchi
Fermented vegetables like sauerkraut and kimchi rely on yeast and beneficial bacteria for their unique flavors. These processes not only enhance taste but also contribute probiotics, improving gut health.
Coffee
In conventional wet processing, yeasts are introduced to aid in the decomposition of mucilage and expedite the drying of the beans. This technique involves immersing coffee beans in water tanks, where yeast-induced biochemical reactions generate favorable sensory traits.
Chocolate
Yeasts are essential in chocolate production. To achieve high-quality chocolate, cocoa beans must undergo a proper fermentation process, which involves both yeasts and bacteria. Yeasts are particularly important as they're active in the initial days of fermentation.
Vinegar
Vinegar production begins with yeast fermentation, converting alcohol into acetic acid. Different alcoholic beverages can be transformed into vinegar, creating diverse flavors ranging from red wine vinegar to apple cider vinegar.
In the budding yeast S. cerevisiae, acetic acid is a normal co‐product of alcohol fermentation. Cells under physiological conditions do not normally sense this compound as toxic and can use acetate as a regular carbon source by channeling it into respiratory metabolism.
Kefir
Kefir is a type of fermented milk made using kefir grains, which are a symbiotic culture of yeast and bacteria, resulting in a tangy and probiotic-rich beverage. Kefir grains originate in the Northern Caucasus Mountains.
Beer
Beer relies primarily on yeast fermentation to convert malt sugars into alcohol. Depending on the yeast strain used, beer can vary widely in flavor; for instance, lager yeast produces a crisp taste, while ale yeast creates fruity notes.
Wine & Mead
Yeast converts grape and other fruit sugars into alcohol in wine and mead-making. The choice of fruit variety and yeast strain affects a wine's flavor, aroma and complexity. For example, using Saccharomyces bayanus yeast can yield higher alcohol content and a cleaner taste.
Yeast is a remarkable microorganism forming the foundation for various foods and beverages worldwide. Understanding the science behind yeast fermentation allows bakers to experiment and innovate, leading to diverse and flavorful bread variations.
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