What is the effect of adding freshly fermenting beer during krausening?

Adding freshly fermenting beer during krausening primarily serves the purpose of reducing unwanted green flavors that can develop during the initial fermentation stage. Krausening involves the addition of a small amount of young, actively fermenting beer to a beer that is nearing completion of fermentation. This process not only helps to carbonate the beer naturally but also introduces fresh yeast and lively fermentation that can assist in maturing and rounding out flavors.

As the new yeast from the krausen works on the residual sugars, it can help scrub out undesirable compounds that lead to the perception of off-flavors often associated with young beer, such as vegetal or harsh "green" notes. Consequently, this technique can significantly enhance the overall flavor profile and drinkability of the final product by promoting a cleaner and more refined taste.

The other options do not accurately represent the primary effect of krausening. For example, while malt flavors may be influenced by other factors during fermentation, the primary role of krausening is not to emphasize those flavors. Similarly, while bitterness can increase during the brewing process, krausening does not directly aim to enhance bitterness, nor does it decrease carbonation—rather, it helps to carbonate the beer.