As a brewer looking to start a brewery, it is important to understand the impact of fermentation on beer flavor. Most brewers are aware that fermentation temperature affects the aroma of beer. Different yeasts, when fermented under different environmental conditions, produce a range of fermentation flavors such as esters, phenols, diacetyl, ethanol, and higher alcohols, which contribute unique flavors. Different combinations of these compounds result in completely different flavors and can greatly influence the quality of the beer. Today, beer offers a wide variety of aromas and flavors, and improper fermentation can ruin the beer, leaving it with unpleasant off-flavors.
To enhance the flavors, we can explore other sources of flavor substances and promote their production during fermentation.
Esters are flavor compounds produced during fermentation. They are also commonly found in everyday items such as essential oils and candy.
Types of Esters in Beer
Esters play a crucial role in the taste and aroma of beer. The following are some common esters found in home or commercial brewing:
Isoamyl acetate – imparts a ripe banana flavor. A lower content of isoamyl acetate results in a stronger mellow flavor in the beer, while higher levels enhance the ester and solvent flavors. The best quality beer is achieved when the concentration is controlled between 1mg/kg and 2mg/kg.
Ethyl acetate – gives off a nail polish remover smell at higher levels. When the ethyl acetate content is low, the hop and mellow aromas in the beer are strong, but the overall aroma may lack harmony. The purest aroma in the beer can be achieved by controlling its concentration between 15mg/kg and 25mg/kg.
Ethyl caproate – imparts flavors of pear, apple, and pineapple. When the content is low, the hop aroma is strong, but the body of the beer may lack softness. When the content is too high, the wine aroma becomes overpowering, and the hop aroma diminishes, resulting in a flavor similar to liquor. Generally, the concentration should be controlled between 0.1mg/kg and 0.25mg/kg for optimal beer aroma.
Ethyl butyrate – imparts a tropical fruit flavor.
Ethyl caprylate – imparts flavors of apricot and pineapple.
The Relationship Between Beer Style and Esters
Esters exist in various types, both good and bad, and contribute to the flavor profile of beer. Different esters provide different beer styles. In the modern craft brewing industry, beer styles are classified based on the type of yeast used. In the United States and many other regions, beers that use top-fermenting yeast are called Ales, while those that use bottom-fermenting yeast are called Lagers. It’s important to note that it is the yeast, not the malt recipe, that determines the ester aroma, and specific yeast strains are used to ferment specific beer styles. For example, German wheat yeast produces a banana aroma, while English yeasts are used in traditional British beers and hazy IPAs to bring fruity esters.
Factors Influencing Ester Formation
The fermentation temperature of beer greatly influences the types of ester aromas produced. Higher temperatures generally increase the levels of esters in finished beer, while lower temperatures tend to reduce ester formation. Therefore, to control the content and types of esters in beer, it is crucial to select the appropriate yeast strain and control the fermentation temperature.
Low-temperature fermentation is believed to produce beer with improved taste, aroma, higher ethanol content, and increased beer productivity. Breweries often use high specific gravity wort and ferment it at higher temperatures to save energy, space, and time. Choosing the correct process temperature, especially during the initial stages of fermentation, is crucial for yeast propagation.
The structure and morphology of the fermentation vessel impact the formation of ester aromas. Taller fermenters produce fewer ester aromas compared to shorter, stocky fermenters. This is because taller vessels experience higher hydrostatic pressure and have more dissolved carbon dioxide, which inhibits ester formation. Pressurized and airtight fermentation can increase the content of saturated carbon dioxide in the beer, inhibit active yeast growth, and facilitate ester formation. Stirring during fermentation can also increase the ester content in the by-products of fermentation.
Yeast Strain and Quantity
The formation of esters is influenced by the genetic characteristics of the yeast used. Different yeast strains produce different fermentative metabolites. Yeasts with strong vitality and healthy characteristics have a metabolic process that promotes acyl-CoA synthesis, which leads to ester formation.
Wort Composition and Concentration
Under the same wort and fermentation conditions, adding more yeast reduces the yeast multiplication factor, alleviating competition between fatty acid synthesis and ester synthesis for acyl-CoA. This results in enhanced ester synthesis and increased ester content. Higher wort concentration also leads to increased ester production. Additionally, excessive oxygen and oil can stimulate ester production by yeast.
However, simultaneously increasing the dissolved oxygen and wort inoculum does not lead to a proportional increase in yeast concentration. As a result, fatty acid synthesis decreases, while ester production increases. Beers with higher dilution concentrations have a higher ester content compared to beers fermented at the same concentration, which increases the risk of flavor defects.