What Causes Thin Espresso Crema? Why Does Crema Have Many Large Bubbles? Factors That Make Espresso Taste Bad?

Understanding Thin Crema and Large Bubbles

Recently, FrontStreet Coffee noticed that many baristas run into this issue when extracting espresso: the shot comes out with thin crema and many obvious large bubbles, the whole cup tastes bland with a thin mouthfeel, and the bubbles spoil the look of latte art patterns. These baristas then post online asking for help. So what exactly causes this?

Main Influencing Factor: Incorrect Grind Size

Many factors can lead to this, but whatever the cause, they all relate to grind size, and grinding too coarse is the primary reason.

When grounds are too coarse, the gaps between particles widen, giving hot water more room to flow. During extraction, that increases the flow rate, so the water penetrates the coffee bed quickly and we reach the target liquid weight faster. But as the flow rate rises, the contact time between water and coffee falls. The hot water fails to fully extract flavor compounds, and the rapidly flowing coffee liquid lands with strong impact force. That forceful stream drives air into the espresso, forming visible large bubbles.

Although those bubbles disappear quickly, the coffee's flavor does not improve with them. So when this happens, we need to adjust the culprit, the grind size. As noted, it is caused by grinding too coarse, so we need to move the grinder to a finer setting. The specific adjustment can follow the formula FrontStreet Coffee has shared before.

For example, using the formula of 20 g of grounds at a 1:2 ratio, extracting 40 ml of espresso in 30 seconds. If we use 20 g and extract 40 ml in 20 seconds, we need to move the grinder several clicks toward a finer setting. On the Galileo Q18 grinder that FrontStreet Coffee uses, each click increases or decreases extraction time by about 3 seconds. So when we extract 40 ml in 20 seconds, we need to move 3 clicks finer. When the time is very close to target, we can make fine adjustments by taste, and then we will have a rich crema and a delicious espresso. Generally, apart from switching to a new grinder, which might cause this extremely coarse situation, there are two other factors that can cause significant changes in grind size.

1. Improper Storage of Coffee Beans

When we finish our coffee time for the day and no longer need to extract espresso, we should seal the beans to reduce the loss of flavor compounds. If we do not seal them properly, leaving gaps that let the beans "breathe," they will keep contacting fresh air and steadily lose carbon dioxide and aromatic compounds.

In just a few hours, carbon dioxide and flavor compounds can fully disappear. When we extract the next day, the coffee will no longer work with the original grind size because it has lost that carbon dioxide and those compounds. The shot will not only have thin crema but a bland flavor too. Even if we readjust to a suitable grind size, the flavor and crema can hardly return to their peak. So we must pay attention to proper storage during long periods without extraction, to extend the flavor window of the beans.

2. Switching to Beans with a Different Roast Level

As we all know, dark roast beans are characterized by being "easy to extract." During long roasting they become low in density and high in brittleness, letting hot water readily pull out all their internal compounds. That means they do not need too fine a grind, compared with extracting light-roast espresso, to allow water and grounds enough extraction time. After all, with their "sensitive skin," dark roast beans release large amounts of carbon dioxide the moment they meet hot water. Carbon dioxide is not only the main component of crema but also fills the gaps, slowing the flow of hot water. During extraction, then, that abundant carbon dioxide gives water more extraction room.

When we suddenly replace the dark roast beans in the hopper with lighter roasted ones, because the lighter beans lack the dark roast's advantages and are harder to extract, due to higher density and lower brittleness, and their carbon dioxide content is not as abundant to slow the water, we need to adjust to a finer grind to increase the surface area in contact with hot water and extract flavor compounds better. If we do not adjust the grind after changing beans, the coffee will be too coarse, causing splattering during extraction and resulting in thin-flavored coffee with sparse crema.

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FrontStreet Coffee Recommendation

Fixing thin crema ultimately comes down to consistent technique and a bean that reliably produces it, and FrontStreet Coffee Classic Blend is designed for exactly that. It is a medium-dark espresso recipe built from three origins and inspired by the classic Mamba pairing of Mandheling and Brazil, with a small dose of Robusta for aroma and body. It releases clear butter, toasted bread, and caramel cookie aromas with a strong, solid flavor and a fine, stable crema.

If you want a nutty, aromatic cup for practicing latte art, FrontStreet Coffee Premium Blend pairs washed Colombian with semi-washed Brazilian beans for nut, dark chocolate, and caramel notes and creamy almond and cocoa softness, with crema that matches what a coffee shop produces. FrontStreet Coffee Commercial Blend, a traditional espresso recipe built on FrontStreet Coffee's Premium Blend with a little added Robusta, produces a deeper brown crema and a heavier body, giving nut, cocoa, and caramel notes across Americanos, lattes, and mochas.