How to Judge Milk Foam Consistency Through Sound

Creating a good latte requires fine, smooth, and highly fluid milk foam. What do you pay attention to when practicing latte art? And do you notice the sounds during the milk frothing process? Actually, the sounds that occur during milk frothing can be very helpful for judging the quality.

The most common milk frothing failures include foam that is too thin, too thick, too coarse, or layered. So how can we judge through the sounds during the milk frothing process?

First, we need to understand what sounds occur during the milk frothing process. The coffee machine's steam wand emits high-temperature water vapor. If all of it is added to the milk, the milk will heat up quickly. When the steam nozzle is in a semi-submerged state (half in milk, half exposed to air), it will inject air into the milk. The milk proteins will wrap around the air, forming milk foam. When frothed to the ideal foam thickness (adjust frothing time according to different types of beverages), lift the milk pitcher upward, bury the steam pipe in the milk, and enter the texturing phase to make the foam fine and dense. Stop when the temperature reaches 55-65 degrees.

So we can derive a theory here: the amount of milk foam (thickness) depends on how much air is injected, and during this frothing process, a sound similar to tearing paper "tsst~tsst" will be made. Actually, we can use these "tsst~tsst" sounds as a reference for how much air is being injected.

Experiment: Sound Duration Effects

Through experiments, using the same milk pitcher and milk amount, we compare the effects of air intake duration on milk foam through the "tsst~tsst" sound.

Short "Tsst" Sound

After starting the frothing process, let the liquid surface rotate, exposing the steam tip, which will make a "tsst~tsst" sound. However, the "tsst" sound only lasts for about 0.5-1 second before lifting the milk pitcher, burying the steam pipe in the milk to enter the texturing phase. At this point, a continuous sharp, piercing sound will be emitted until the frothing is complete. The resulting effect will be relatively thin milk foam, almost to the point of having no foam, with strong fluidity, no change in the liquid surface, making it difficult to create latte art. The texture will also be too light.

Long "Tsst" Sound

After starting the milk frothing, make the liquid surface rotate, exposing the steam tip. The "tsst~tsst" sound begins to appear during the frothing process, and the "tsst" sound lasts for about 7-8 seconds. After that, bury the steam tip to enter the texturing phase, during which a heavy, thick air intake sound will be heard until the frothing is complete. This can clearly show that too much milk foam has been created, and the foam will be rather thick with poor fluidity and relatively coarse texture. This will be more similar to cappuccino foam. The texture will be rather thick, and layering may easily occur. This will be more similar to cappuccino foam.

Perfect Latte Milk Frothing

For milk frothing in a latte, during the frothing process, you first need to find the appropriate angle and position between the steam wand and the milk pitcher. After starting, let the liquid surface rotate, exposing half of the steam tip to allow air to enter for frothing milk foam. When the "tsst~tsst" sound appears for about 2 seconds, you need to bury the steam tip (not recommended to bury too deep or too close to the pitcher wall) to enter the texturing phase. At this time, a stable, subtle air intake sound will be emitted until the temperature reaches 55-65℃. The resulting milk will have fine, smooth, and highly fluid foam, suitable for latte art, with moderate foam texture.

Conclusion

In summary, the duration of the "tsst" sound indicates that the longer the milk frothing time, the more air is injected, and the foam will be thicker; the shorter the frothing time, the less air is injected, and the foam will be thinner. When no air is injected, a continuous sharp, piercing sound will occur.

Left: Thin foam Right: Thick foam

So, first we need to find the appropriate angle between the steam wand and the milk pitcher. Then we can first determine whether air is being injected during frothing by whether there is a continuous sharp, piercing sound, and then determine the thickness of the frothed milk foam by the length of the "tsst" sound. Therefore, when practicing normally, everyone can pay more attention to these sounds. These sounds are very helpful for determining whether the milk foam has been properly frothed. Of course, the most important thing is to practice more.

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