Cold brew preparation from an expert perspective

The basics of cold brew coffee

Most coffee brewing methods use high temperature water (near boiling point) and short extractiontimes of less than 5 minutes. In the case of cold brew, however, the opposite is true. Thewater needs to be cold and the extraction time long.

Cold brew coffee can be prepared by direct or indirect immersion (using an insertion filter) or, for example, in a French press.

How to prepare cold brew according to your preferences?

  • Temperature,
  • the age of the beans and their degree of roasting,
  • thecoarseness of the ground coffee,
  • thequality of the water,
  • the ratio of water to coffee,
  • and, above all, the extraction time.

All these factors influence the taste of the resulting coffee, regardless of whether it is prepared hot or cold, i.e. as a cold brew. In addition to the factors mentioned above, the coffee beans used also have a great influence on the final taste.

How do I know the brewing time, temperature, grind coarseness and more? We'll have to look into the chemistry of this for a moment. Based on different preparation methods, using different temperatures, grind coarseness and extraction times, we'll see how the substances in coffee evolve during extraction and how to take advantage of this.

How does cold brew extraction work?

The first major difference in the preparation is the preparation time. Cold brew is usually extracted for12-24 hours. So the question might come up, "won't the coffee be over-extracted after 14, 18 or even 20 hours when normally with warm water the preparation takes 5 minutes max?" No it won't. In fact, the overall higher temperature increases solubility and affects the saturated vapour pressure of the compounds.

Due to the low temperature and long contact time, the resulting coffee has quite different physico-chemical and sensory properties. This is also influenced by the speed and method of extraction of the chemicals present in the roasted coffee.

The different chemical compounds present in the coffee have different chemical properties such as polarity and solubility. They also have different extraction rates. In addition, higher temperatures lead to greater evaporation and release of organic water volatiles.

How does cold brew coffee extraction work?

What other physico-chemical and sensory properties does cold brew have that are different? And what are the implications for its taste? We will see the action of all the different factors in a study backed by several Colombian scientists.

During the research, different extraction times were observed - 14 and 22 hours. Thecoffee used for the research was a selection of Colombian coffee from the two regions of Huila and Nariño. Both coffees were sourced from microlots. Two grinding coarsenesses were used during the research - medium and coarse.

Significant differences were found, especially in the flavour profile and in the concentrations of non-volatile compounds such as caffeine, chlorogenic acid (an ester of caffeic acid) and trigonelline, and it is these that we will focus on now.

Acidity in cold brew

The reason why many people like cold brew is its lower acidity, which is confirmed by tests. However, it should also be taken into account that roasting also has an effect on the lower acidity. This is because the total titratable acidity decreases as the roasting level increases.

The choice of the correct roast may therefore be more important than the preparation method. If you stick with a single bean, the brew method can help you achieve the desired acidity or non-acidity.

pH content/acidity

Now back to the research. At first glance, the differences found in pH in research may seem minimal, but in reality, changes in values even in the order of tenths are noticeable.

Thelowest pH values (i.e. higher acidity) for both Nariño (4.81 pH) and Huila (4.88 pH) were observed withcoarse grinding and 14 hours of extraction. As you are well aware, even in the case of hot coffee, a shorter extraction usually leads to higher acidity.

Moreover, the amount of pH depends on the country of origin, the maturity of the coffee, the processing and the whole coffee handling process. This is also shown by research which shows that these particular coffees tested are more acidic than coffees from different parts of the world on average. In fact, when comparing the pH of coffees from different countries, the values range between 4.96 and 5.14 pH.

Hot vs. cold coffee

You've no doubt had the experience of making coffee but not being able to drink it in time. In that case, you must have found that your coffee tastes different. Its flavour profile has changed, the fruit has become more pronounced or, on the other hand, the flowers have become more prominent. This is also true if you prepare the coffee cold straight away.

The same ratio of coffee to water was used in both cases during the test. So what were the results? Both cold-brewed coffees showed higher values in the extraction yield and total dissolved solids categories than their hot counterparts, especially when using a coarse grind and a long extraction time (22 hours).

Thedifferences in gallic acid values can be explained by the presence of several water-soluble polyphenolsin roasted coffee , which exhibit different chemical properties, some of which, such as chlorogenic acids, caffeine and soluble melanoidins, are removed at different contact times.

Flavour profile of cold brew coffee

In this study, the taste profile of coffee beverage was evaluated in terms of general characteristics such as aroma and flavor, acidity, body, taste and aftertaste.

Overall , thecoffees prepared with hot water were more pronounced in both aroma, body or taste. This is no doubt also due to the water/coffee ratio used, which was 1:10. However, it is more common to use a ratio of 1:16 for hot coffee, while for cold brew a ratio of 1:10 is appropriate.

The values for all categories were fairly even, but the overall best impression was made by the two coarsely ground coffees with an extraction time of 14 hours.

For the coffee, individual flavours such asdifferent fruits, nuts, spices and other aromatic foodswere also evaluated as part of the flavour profile. Cold-brewed coffees had higher scores for malt, pepper, almond, cocoa and forest fruits than hot coffee.

Overall, then, it was found that cold brew can exhibit higher levels of caramelisation of sugar, sweetness and bitterness. The body of the cold brew samples were creamier and creamier, while the hot coffees were rated as more sour and pronounced in terms of flavours as well.

What substances are found in cold brew coffee?

Volatile substances are one of the main factors that determine the quality of coffee. To test for volatiles, samples of cold brew prepared from coarsely ground coffee that had been extracted for 14 hours were selected. The most abundant volatile compounds (in order of frequency) in both cold brew coffee samples were furans, pyrazines, ketones, aldehydes, pyrroles, esters and acids.

Only about 5 % of these compounds are responsible for the aroma of the coffee, which would result in approximately 50 odoriferous compounds in roasted coffee. However, the presence of volatile compounds also depends on the chemical compounds of the coffee beans, the roasting process and the extraction technique.

The main volatile compound identified in the cold coffee was furfuralin both coffees . The highest level of furfural was found in Huila coffee. Furfural is related to sweet, bready and caramel flavours.

Thesecond most abundant group was pyrazines. In general, pyrazines were described as nutty, earthy, roasted and green aromas

Ketones were the next most abundant group of volatiles in the cold brew samples tested. Overall, ketones are one of the most abundant compounds in filtered coffee beverages.

Seven ketones were identified in Huila coffee, while only four were found in Nariño coffee. Ketones are described as havingbuttery, caramel, musty, mushroom or fruity aromas.

Acids and pyridines were found only in cold brew prepared from beans from the Huila region. Pyridines are associated with poor quality coffee because they are related to sour, pungent and diffuse odour notes.

Summary of research on cold brew coffee

So, in a nutshell, what all happens during cold brew extraction?

The long extraction time tends to increase the values of

  • total dissolved solids
  • extraction yield
  • and the gallic acid content.

Moreover, the values were even higher when using coarse grinding.

In the preparation of hot coffee, the drivingforce istemperature, which promotes the extraction of the chemicals present in the coffee grounds.

Conversely, when preparing coffee cold, extraction requires more time to compensate for the low temperature. This is evidenced by the very similar values of the substances contained.

Flavour-wise, cold brew is quite complex and different from hot coffee. The flavour is particularly noticeable

  • malt
  • pepper
  • almonds
  • cocoa
  • chocolate
  • flowers,
  • smooth and creamy body.

What is the ideal extraction time?

Although coffee that was coarsely ground and extracted for 22 hours was evaluated as the ideal conditions, higher scores in the overall sensory evaluation were recorded for samples processed using coarse grinding and extraction time of 14 hours.

However, as it depends on the coffee used, you can reflect on the interval of 12-16 hours and coarse grinding.

This is because a long extraction time can lead to over-extraction of substances associated with undesirable notesand to the nullification of all the good flavours that are already in the drink .

And my final tip is toput the cold brewin the fridgefor a while after filtering. This way the cold brew is chilled and thetaste is more subtle afterwards.