Is solubility the key to a better cup of coffee?

Wageningen Food Safety Research scientist, Dr Anja Rahn, explains the importance of solubility when it comes to preparing the perfect cup of coffee.

A typical coffee bean is around 30 to 40% in soluble in water. This means that, when brewed, the majority of the coffee solids remain, leaving the wet coffee grounds familiar to baristas the world over.

However, brewing good coffee isn’t as simple as extracting as much coffee as possible.

Dr Anja Rahn works as a senior scientist at Wageningen Food Safety Research in the Netherlands. She explains that contrary to popular belief, just because you can extract a higher percentage of a coffee bean, it doesn’t mean you always should.

“Anything above 30% may not be desirable from a sensory point of view,” she explains. “For an espresso, you’d be looking at between 20% and 22%.”

This is the reason coffee brewing recipes are so precise when it comes to water: a calculated combination of coffee-to-water ratios and brewing time enables the optimum extraction of the hundreds of soluble compounds found in coffee.

But not all these compounds behave the same way during extraction. Acids and caffeine solubles are smaller, extracting more quickly, which is why under-extracted coffee can taste astringent. On the other hand, lipids, fats, and melanoidins take measurably longer to extract and add a layer of nutty, vanilla, and chocolatey flavours.

So how can baristas use solubility to achieve a better cup of coffee?

coffee solubility

How does a coffee’s solubility impact its extraction?

The full amount of the coffee bean extracted into water is often referred to as total dissolved solids (TDS) and is generally measured using a refractometer.

According to industry standards, the optimum extraction rate for a coffee bean is between 18% and 22%. This range allows for a balanced and layered cup of coffee containing enough TDS for a good – but not heavy – mouthfeel.

Experienced roasters treat coffee intended for filter brewing and espresso differently. Generally, during espresso extraction, water has less time to dissolve coffee solubles. As such, the coffee is often roasted darker to facilitate greater solubility.

Conversely, filter coffees can be roasted far lighter, as this method requires more contact time between the water and the ground coffee. This way, less volatile desirable compounds reach the cup, resulting in a drink outcome from espresso.

Additionally, because some coffee beans are inherently more soluble than others, the brewing process or recipe may need tweaking for each coffee – even for different batches of the same coffee.

However, Anja explains that those with the ability to do so should specify their own roast profiles. “Not every consumer has the same sensory expectations, so I don’t think it should be up to roasters to specify a recipe,” she says. “What’s good for you might not be desirable to other people.”

There are also ways to mitigate the unpredictability of coffee extraction. For example, many professionals agree that first and foremost, adjusting the grind size is the most effective way to control coffee solubility and extraction rates.

Changing the grind size changes the available surface area of coffee. A finer grind means the extraction of compounds happens quicker.

Another crucial factor is the quality of the brewing equipment. High-quality espresso machines are specifically designed to optimise extraction, employing technology that enables flow profiling and precise water temperature control.

Even the best modern home espresso machines are capable of pre-infusing coffee grounds – another variable that can be tweaked to optimise extraction.

Nevertheless, Anja stresses that good quality coffee which has been roasted well is the best route to perfecting extraction.

“Soluble components are only one portion of quality,” she notes. “If you have a good product and a good roast degree then the extraction is sort of like fine tuning – getting the product balanced.”

grinding coffee

What factors can affect coffee solubility?

Because the solubility of different coffees is not uniform, finding the perfect recipe for extracting each coffee requires planning and patience.

Anja points out that each part of coffee’s journey, from seed to cup, can impact its solubility.

“Going right back to the farm level, there are a number of factors that affect how soluble coffee beans are,” she explains. “Even genetically, there are some coffees which are more predisposed to creating sugars.”

She adds that growing conditions, agricultural practices, and growth rates can also impact solubility. For example, bean density is an important indicator of both quality and solubility.

“Things which grow quickly are less dense, and less density leads to more porosity, which in turn leads to a higher rate of extraction,” she elaborates. “This doesn’t mean it’s good coffee, but you’ll certainly extract more from it.”

Post-harvest, coffee processing can further affect solubility. Anja says that washed coffees extract differently from naturals, partially because natural coffees typically contain more sugar. This leads into the roasting process, which can also dictate solubility and extraction characteristics.

“Generally, the heavier the body and the longer you roast, the more you’re going to extract,” she explains.

Some of the soluble components and their characteristics are more desirable than others. As mentioned, sugar can easily be manipulated through agricultural practices, processing methods, and roasting.

When all desirable qualities are considered, extracting a balanced cup of coffee with decent mouthfeel and balanced flavours becomes a very skilled undertaking. But the science behind coffee solubility is continually expanding and evolving.

“I’m working on a project at the moment to show the influence of different stages in the supply chain and how it affects quality,” Anja says. “There’s very little consolidated information in this area – even down to the physiology of the green coffee bean – which is unfortunate, as we should know this by now.”

Is solubility the key to a better cup of coffee?

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