What Does Coffee Blooming Mean in Pour-Over Brewing? How to Judge Coffee Bean Freshness? What's the Optimal Bloom Time?

Understanding Coffee Blooming: Beyond Freshness

"Blooming" can be understood as a preparatory step in pour-over coffee brewing. Because coffee grounds, after blooming, can better release their flavor compounds when extracted with hot water. Therefore, blooming has become an essential step in most brewing methods.

In addition to allowing better extraction of coffee beans, the blooming phenomenon—where the coffee bed expands—has been used by many as a measure of bean freshness. How is this measured? For example: during blooming, the larger the expansion of the coffee bed, the fresher the beans; conversely, the smaller the expansion, the less fresh the beans. (The coffee bed on the left shows minimal expansion)

But! Did you know that bean freshness is actually not the only factor that affects the size of the bloom? For example, in the image that FrontStreet Coffee has prepared above, at first glance, the beans on the left might not seem as fresh as those on the right (because the bloom isn't as large), but in reality, they were both roasted on the same day! Not only that, but their grinding times differed only in sequence, and they were bloomed simultaneously! So the question arises: besides freshness, what other reasons cause variations in bloom size?

The Science Behind Coffee Blooming

Before answering this question, to prevent beginners from not understanding why bean freshness relates to blooming, FrontStreet Coffee will first share the specific purpose of blooming and why the coffee bed expands during this process!

What is the specific function of blooming?

Coffee beans continuously accumulate carbon dioxide during the roasting process, and this carbon dioxide gradually releases after roasting is complete. However, before it's fully released, the presence of large amounts of carbon dioxide can hinder hot water from extracting flavor compounds from the coffee grounds, essentially acting as an obstacle. Therefore, we need to release it through the blooming process! When we finish pouring a small amount of hot water, there's a waiting period—this is the moment when carbon dioxide is released. After most has been released, we then pour a large amount of hot water for extraction. At this point, the hot water can better extract the flavor compounds from the coffee because the "obstacle" (carbon dioxide) has been eliminated.

Once we understand this point, it's easy to comprehend why freshness correlates with bloom size! Carbon dioxide should dissipate into the air after being released, but because the coffee grounds are connected by hot water, forming a sealed space, large amounts of carbon dioxide emerging from the coffee grounds have no escape route, so they can only be trapped within the space formed by the coffee bed! Like a balloon, as more carbon dioxide is released, the entire coffee bed expands larger and larger! Since fresher beans contain the most carbon dioxide, they naturally release more gas during blooming, resulting in a larger bloom. However, when beans have been stored for some time, most of the carbon dioxide has dissipated. At this point, blooming will naturally fail to form a large "hamburger" shape due to insufficient carbon dioxide content (it's mostly just the expansion of coffee grounds after absorbing water).

Other Factors Affecting Bloom Size

In summary, the reason bloom size correlates with coffee bean freshness is due to the amount of carbon dioxide content. However! Bloom size should absolutely not be used solely as a measure of bean freshness, because besides freshness, many other factors affect the degree of blooming, such as: extraction efficiency. The reason we can see the "large bulge" rising from the coffee bed in a short time is because the coffee grounds received sufficient "stimulation." Simply put, the coffee grounds had enough contact with hot water, allowing carbon dioxide to be released in large quantities within a short time.

When we don't use the right parameters—for example, when coffee beans aren't ground fine enough, water temperature isn't high enough, or flow rate is too fast, resulting in incomplete contact between hot water and coffee grounds—all these factors prevent the coffee grounds from receiving sufficient "stimulation," thereby failing to release gas quickly. This leads to situations where even very fresh coffee beans cannot produce a large "hamburger" shape during blooming, just like in the image FrontStreet Coffee showed at the beginning! Because the beans on the left used coarser grinding, lower water temperature, and a filter cup with faster flow rate, this caused even very fresh beans to fail to produce a large bloom! When we often mention that light roast beans don't bloom as large as dark roast beans, it's for this same reason! Because light roast beans have a harder texture and require higher extraction efficiency to "stimulate" the coffee beans and achieve massive gas release; otherwise, they naturally cannot produce a large bloom. Additionally, due to the lighter roast degree, their expansion coefficient is insufficient, so even with abundant carbon dioxide content, they cannot bloom as large as dark roast beans.

Therefore, everyone should now clearly understand that bean freshness is not the sole determinant of bloom size; extraction efficiency and roast degree also play crucial roles~

Important Notice :

前街咖啡 FrontStreet Coffee has moved to new addredd:

FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473