Why You Shouldn’t Brew Coffee With Milk: The Science Behind Poor Extraction and Bad Flavor

 Recently, Front Street noticed that quite a few bloggers have been sharing tips on using milk instead of water to brew coffee, claiming that the resulting coffee tastes great. Because of this, many friends often message Front Street asking what parameters they should use when brewing coffee with milk. Honestly, Front Street already covered this topic last year, and even shared the feasibility of using milk to brew drip bags earlier this year. And the conclusion of all these posts is exactly the same: using milk makes it very difficult to brew good-tasting coffee. The reason is simple — milk is simply not suitable for brewing coffee. (Note: only for brewing)

Why isn’t milk suitable for brewing coffee? Why does Front Street claim it isn’t? Because when we brew coffee with milk, we face a lot of limitations. The biggest challenge is that milk does not have enough extraction efficiency. It simply cannot, like water, extract the flavor compounds from coffee within a short amount of time.

Temperature Limitations:
First, let’s talk about temperature. When discussing milk-based espresso drinks, Front Street often mentions that the ideal temperature for steamed milk is around 65°C (149°F) and should not exceed 70°C (158°F). This isn’t only because lactose is activated within this range, providing greater sweetness, but also because once milk exceeds 70°C, some of its internal components start to break down due to the high heat. As a result, the texture of the milk deteriorates (the difference isn’t huge, but it does become thinner and less sweet).

If we want a milk coffee that tastes great and feels smooth, the milk temperature shouldn’t exceed 70°C. But as everyone knows, at 70°C—whether it’s milk or water—it’s extremely difficult to extract flavor compounds from coffee quickly. So the first major challenge of brewing coffee with milk is that if we want an ideal experience, we can’t use high-temperature milk. But if the milk isn’t hot enough, then we won’t be able to extract enough flavor compounds in a short time.

Concentration Limitations:
Even if we ignore temperature limitations and heat the milk to 100°C to brew coffee, we still won’t be able to produce a rich and flavorful cup. Why? Because milk’s concentration is simply too high.

I often explains that concentration gradients are one of the core forces driving extraction. Hot water can quickly extract coffee’s flavor compounds because water has low concentration, creating a large gap between it and the coffee grounds. This large gradient allows flavor compounds to transfer rapidly into the water. But as the water becomes more concentrated, the gradient shrinks, and extraction slows down.

Milk, however, has an extremely high concentration — far higher than espresso — so when we brew coffee with milk, it can dissolve some flavor compounds, but the extraction speed becomes incredibly slow.

For example, below is coffee brewed by soaking grounds in 100°C milk for about 30 minutes. Even with stirring and other methods to boost extraction, the final result was simply milk with a hint of coffee — and that coffee flavor was under-extracted: astringent, weak, and overall not tasty.

From this experiment, we can see that because of milk’s extremely high concentration, we cannot fully extract coffee’s flavor compounds in a short amount of time. What does this mean? It means you can brew coffee with milk, but it will very likely taste bad — unless you use immersion-style extraction like milk cold brew, where extended time allows for a more complete extraction.

Flow Rate Limitations:
Finally, let’s talk about flow rate. The “coffee” in the previous example, the one soaked in hot milk for 30 minutes, wasn’t originally meant to be immersion-brewed — it was supposed to be a pour-over. So why did Front Street later describe it as milk-soaked coffee?

Simple — milk is so thick that filtration becomes painfully slow. Front Street used 225ml of hot milk to brew 15g of coffee. At first, the milk dripped at a decent speed, but after the two-minute mark, it slowed dramatically, eventually dripping one drop at a time. After the 11-minute mark, it took several seconds for a single drop to fall. By the 30-minute mark, with patience exhausted, Front Street poked a hole in the filter paper with a latte art needle just to get the brew to drain. Of course, the trade-off was that you might taste some grounds. Yet even then, the coffee tasted disappointing — far from enjoyable.

The extremely slow filtering essentially turned pour-over into immersion. And that’s no different from soaking grounds directly in milk. If we want to increase flow rate, the only option is to grind much coarser so milk can pass through faster. But coarser grind means even lower extraction, resulting again in milk that only has a hint of coffee flavor.

That’s why Front Street consistently emphasizes that milk is not suitable for brewing coffee. If we want a rich, flavorful milk-brewed coffee, immersion is still the recommended method. At least immersion allows longer contact time and stronger agitation, making it easier to dissolve the coffee’s flavor compounds.

Lastly, if you really can’t resist the temptation to experiment, then at least avoid using a pour-over kettle to hold milk, or else… well, you’ll find out soon enough~~