If you ask people to describe the taste of black coffee, the most common response will be "bitter". But people, even with little tasting experience, recognize the acidity, sweetness, and various descriptors in this beverage - floral, fruity, and others.
The drink's sweetness can vary from grain harvesting to minor changes in roasting and preparation -- science has yet to reveal a clear relationship. That's why the UC Davis Coffee Center partnered with the SCA and was supported by the Breville Corporation to conduct a comprehensive scientific study of drip coffee. The aim was to find out what and how affects the natural sweetness of the drink.
The scientists decided to analyze how the chemical and sensory properties of coffee change throughout the brewing process. To do this, fractionation was carried out - during preparation, the drink was divided into fractions (portions). This made it possible to evaluate the taste of coffee at different stages.
#coffee from the Huila region of Colombia was used for the experiment. It was prepared in a drip coffee machine at 91.5°C for 4 minutes. During this time, the capacitance was changed every 30 seconds. The first fraction is from 0 to 30 seconds, the second - from 31 to 60 seconds, etc. As a result, 8 different fractions were obtained. Then they made another of the same coffee, but did not separate it into fractions.
To understand how the sweetness of coffee changes during the preparation process, scientists experimented with different fractions of coffee brewed in a drip coffee maker.
They consulted a group of experts to evaluate the flavor of the resulting samples. The aim was not to evaluate the quality of the beverage, but only to evaluate the intensity of its sensory properties.
To enter the group, the experts applied test triangulation: they determined which of the three coffee samples differed from the other two. After selection, the group went through several weeks of calibration using the SCA flavor wheel.
During the experiment, participants tasted coffee samples in special isolated booths with red lights so as not to base their assessments on the color of the brewed coffee.
Experts say bitter, sour, flower, fruit, etc. They were faced with the task of objectively evaluating 9 samples (8 fractions and a whole) according to 23 taste and aroma characteristics. All drinks were served in random order, each - 3 times. Each fraction was pre-measured for physical parameters including total dissolved solids (TDS). This indicator reflects the true strength of the drink.
In total, the panel of experts evaluated 324 servings of coffee.
On average, experts rated the first fraction as the most painful. The intensity of bitterness decreased continuously from the first fraction to the last fraction.
The reduction in perceived bitterness was associated with the TDS of each fraction, with the early fractions having high TDS, while the later fractions decreased almost tenfold. The bitterer the drink, the higher the concentration of dissolved molecules in it: the subsequent fractions turned out to be less bitter, because they were less concentrated. As the strength of the drink decreased, some sensory properties (acidity, astringency and smokiness) also decreased.
The sweetness of the coffee became more pronounced towards the end of the brew despite the lower TDS.
But the sweetness in later fractions, on the contrary, increased. The density of other identifiers valuable to the private sector has also increased – floral, honey and fruity.
This result is surprising as late cuts have a much lower TDS. The fewer coffee molecules in the liquid, the sweeter it turns out. This is paradoxical to sensory science: If you add more sugar to the water, it will become sweeter, and not vice versa.
Various theories have emerged regarding this.
The first theory is that subsequent fractions have a higher concentration of dissolved natural sugars.
About 10% of the mass of green Arabica beans is sucrose. It does not usually withstand roasting, as it is involved in many complex chemical reactions. During frying, complex carbohydrates are broken down into monosaccharides - simple sugars. They also provide a noticeable sweetness. So one possible explanation is that monosaccharides (fructose or glucose) are extracted more slowly from coffee during the brewing process. Accordingly, in the late fractions they give higher concentrations and therefore more sweetness.
To test this theory, the scientists measured the monosaccharide concentration in each fraction using liquid chromatography and mass spectrometry. The first fraction showed the highest sugar concentration, which decreased continuously in later samples.
A surprising fact: The total concentration of all sugars in any fraction was significantly below the human sensory perception threshold for sugars. This means that experts cannot detect sweetness due to sugar in the samples tested. It turned out that nothing else was responsible for the increase in sweetness in later samples.
It's not sugar from the sweetness in coffee, it's the head.
Another thing is responsible: either some descriptors or a decrease in the concentration of bitter and sour components in the drink.
The second theory is that the human brain identifies certain taste qualities as sweet. This hypothesis is consistent with the observation that sweetness increases with the addition of floral, honey, and fruity descriptors.
A third theory is that the higher concentration of bitter and sour compounds in early cuts suppresses or "masks" sweetness. Therefore, it is more pronounced only in the later fractions, where there are less bitter and sour compounds.
More research is needed to refute or substantiate the second and third theories. So far, experts are more inclined to the third.
The study revealed that the natural sweetness of coffee is not dependent on sugar. It is more pronounced towards the end of the preparation of the drink. Most likely, this is because in the later fractions there are less bitter and sour compounds that mask the sweetness in the initial stages.
These results open up a new way for baristas to get creative: A single drink can have several different flavor profiles. You can choose which fractions to combine to get a different result for the customer's request. For example, bitter and sour early coffees may appeal to those who prefer to add cream and sugar. Light tea, with floral, fruity and sweet notes, and coffee lovers without additives will like a drink from sweeter late fractions.