What is Tiger Stripe Espresso? Where Does Espresso Crema Come From? How Do Fine Grinds Affect Coffee?

What Are Those "Oil Spots" on Espresso Crema?

Today, FrontStreet Coffee came across an interesting question while browsing online. A netizen posted a photo of their extracted espresso and asked: Why are there "oil spots" on the surface of the espresso crema? (Reference image below)

From the image above, we can clearly see substances floating on the crema surface that resemble "oil spots." However, those who have read FrontStreet Coffee's articles from the past two months or have experience with espresso will know that these brown substances are not actually "oil spots" but coffee grounds. In the coffee community, this phenomenon where coffee grounds float visibly on the crema surface has a specific name—"tiger striping." Yes, this is something FrontStreet Coffee has introduced and shared before. But as FrontStreet Coffee browsed through the comments, we found that many people believe this "tiger striping" in espresso is a phenomenon that only occurs with over-extraction. Is this really the case?

Does "Tiger Striping" in Espresso Mean Over-Extraction?

Not at all! The appearance of "tiger striping" is not caused by coffee over-extraction. On the contrary, it's the presence of "tiger striping" that leads to coffee exhibiting characteristics similar to over-extraction. To help everyone understand the relationship between these two phenomena, we first need to understand how tiger striping forms, and then why it becomes associated with over-extraction!

The Science Behind Tiger Striping

As we all know, espresso crema is formed by tiny bubbles created by carbon dioxide and coffee bean lipids under the high-pressure extraction of an espresso machine. Besides carbon dioxide and coffee bean lipids, crema also contains a certain amount of extremely fine coffee particles—what we commonly call "fines." The reason is simple: the filter holes at the bottom of the portafilter basket are relatively large and cannot block these tiny coffee particles from passing through. These fines are initially difficult to notice because they float on the coffee surface with the crema. However, as they gradually become saturated or as the crema dissipates, the fines lose their buoyancy and sink to the bottom of the coffee. This is why the bottom becomes a "hard-hit area" where we discover coffee grounds. (Exaggerated example below)

Causes of Tiger Striping

As FrontStreet Coffee mentioned, fines are normally mixed within the crema and difficult to detect. However, if we see distinct gradient colors on the crema surface with obvious fines forming "tiger striping," this could be caused by two situations: either too many fines are being released, or the fines are not fully incorporated into the crema! FrontStreet Coffee will briefly list several situations that might lead to tiger striping: When coffee grounds are ground too fine, the quantity of fines increases, which naturally might lead to more fines being extracted during extraction, forming obvious patterns on the surface (the same applies to increased extraction time caused by overly fine grinding); or if we shake too much during puck distribution, causing fines to concentrate in the lower layer, making them easier to be carried out during extraction; also, if the distance between the espresso cup and the portafilter spout is too close, preventing fines floating on the surface from being incorporated into the crema, etc.

The Connection Between Tiger Striping and Over-Extraction

In summary, from the above examples, we can understand that the appearance of tiger striping is not caused by over-extraction, but rather by fines appearing on the crema surface. So why does over-extraction become associated with tiger striping? The reason is simple: our understanding of over-extraction mainly involves coffee showing obvious negative characteristics such as bitterness, harshness, and astringency, while fines themselves carry bitterness! FrontStreet Coffee often mentions that in espresso, an appropriate amount of fines helps increase the coffee's body and complexity. However, fines are also a double-edged sword. When there are too many fines, the bitterness also increases, causing the coffee to exhibit obvious bitter flavors. At the same time, due to their insoluble nature, they also reduce the coffee's clarity, making it taste obviously muddled.

Conclusion: Distinct Phenomena

Therefore, when the content of fines in espresso is too high, it's easy for people to mistakenly think that the coffee has extracted some negative substances due to the bitterness, but in reality, there are still certain differences in the nature of these two phenomena. So, over-extraction and the appearance of tiger striping cannot be confused as the same thing.

- END -

Important Notice :

前街咖啡 FrontStreet Coffee has moved to new addredd:

FrontStreet Coffee Address: 315,Donghua East Road,GuangZhou

Tel:020 38364473