Gaggia Classic crema is a bit "bubbly"

Question

I've got a Gaggia Classic (this particular model) that I've been doing some light mods to (changed 12 bar spring for a 9 bar, lower drip tray and so on). Anyway, lately I've noticed that the crema when I pull a shot ends up getting a bit "bubbly" at the edge rather than creamy/silky. It starts off okay and then when it's finished, the crema has a ring around the edge with lots of bubbles (say the size of a small ball bearing, larger than peppercorns let's say). I haven't changed my method of grinding, tamping, etc. I've only had it 6 months, do I need to descale it, or ...? (please pardon me, I'm really new to all of this and have been watching plenty YouTube videos just to learn how to use it, and think I've nearly got the hang of it). To be honest, it was doing this bubbly crema thing even before I changed the spring for 9 bar (based on this video, I'm sure I did it correctly, even though he went through the specific process way too quickly in my opinion). Any help is greatly appreciated

Answer 1

It starts off okay and then when it's finished, the crema has a ring around the edge with lots of bubbles

This sounds very much like you're pulling a nice shot of espresso but then letting it run way too long. Does the color of the liquid also change and become a lot lighter? If so, you are definitely pulling to much water through your puck. The liquid in your last picture looks quite light and translucent so that really might be it.

In your comments you say

I ground 12 grams of coffee beans, and the weight of just the extracted coffee liquid in the mug seems to be 150 grams

This really is too much for 12 grams of coffee. The ideal ratio is somewhere between 1:2 and 1:3 depending on your preference. So with 12g of ground coffee, I wouldn't go for more than 36g of liquid in your cup.

You might say that you like the less intense taste and less thick consistency if the ratio is higher but you'll have to do this at the expense of the crema.

However, it is important to note, that crema doesn't say anything about the quality of the coffee. Crema "happens" because the water is under such a high pressure, that the CO2 from the coffee is forced into the liquid, resulting in that foam we call crema. You can have excellent coffee with no crema and you can also have absolutely rancid coffee with crema, don't get too hung up on that if you prefer your espresso (or with your amount of water it would, ironically, be more of a [Swiss] Caffè crema) with more water.

You could also try to brew an Americano: brew a regular espresso with a ratio of 1:2 to 1:3 and then add hot water. This has one major benefit: You can use the same grind setting for all of your espresso based drinks.

If you want higher consistency shot to shot (at the expense of a little bit more work) you can buy a scale specifically designed for espresso. That way you can weigh and time your espresso shots and then adjust your grind size accordingly. As a general starting point for your setup, I would recommend a total brew time of 30s with 12g in and 30g out.

If you really wanna get deep into espresso and hear from an industry expert, I can definitely recommend Understanding Espresso, a youtube video series by James Hoffmann, where he goes through all the variables involved when brewing espresso in a slow and understandable manner.