Why Does Cold Brew Taste Different?
Cold brewed coffee tastes quite different to the same coffee brewed hot, then chilled. It tastes considerably less acidic, with more ‘brown’ flavours (on the spectrum from chocolate to rubber), and generally few of the complex aromas that distinguish the coffee’s origin, variety, and process.
For this reason we at BH advocate brewing hot, and flash-chilling your coffee, rather than brewing cold. However, customer demand for cold brew shows no sign of slowing, so we set out to investigate why it tastes the way it does — and learned something about acidity in coffee in the process.
Is Oxidation a Factor?
Hot brewed coffee loses flavour and complexity over time, becoming less aromatic and more bitter. This is due to chemical changes that start taking place in coffee as soon as it’s brewed – this includes oxidation of coffee oils, acids, and aromatic compounds, but also the breakdown of chlorogenic acid, and the loss of volatile aromas to the air. Oxidation and the loss of aromas happen faster at higher temperatures, while on the other hand, chlorogenic acid seems to be more stable around between 80–85°C (TR Lingle, 1996).
While oxidation will be happening more slowly in cold brew, the time it takes to brew (at least 8 hours) and the likely longer time before consumption, gives a lot more time for the reaction to proceed. There doesn’t seem to be any published research on how fast these reactions are in brewed coffee at room temperature or below, so we can’t be sure whether these reactions are playing a part in the flavour difference. However, coffee brewed hot, flash chilled, then stored for 8 hours still tastes very different to cold brew fresh out of the brewer — so even if these reactions are affecting the taste of cold brew, it can’t be the whole story.
Acidity in Cold Brew
Two recent studies by chemists at the Thomas Jefferson University in Philadelphia shed some light on one of the major differences between hot and cold brewed coffee, namely the acidity.
Cold brew is often marketed as being less acidic than hot brewed coffee, and it generally tastes less acidic. However, the authors, Megan Fuller and Niny Rao, point out that the pH of cold brewed coffee is very nearly the same that of hot brewed coffee (NZ Rao and M Fuller, 2018). Instead, they found a noticeable difference in ‘titratable acidity’ – a measure of the total amount of acids present in the coffee.
What is Titratable Acidity?
Acids are compounds that release hydrogen ions when dissolved in water. The concentration of hydrogen ions in the water relates to the pH – the more hydrogen ions, the higher the acidity, and the lower the pH. Some acids only release some of their hydrogen ions when dissolved in water.
When you add a base, it reacts with some of the hydrogen ions, which reduces the acidity. These acids can release more of their hydrogen ions in response, which means it takes more of the base to neutralise them. The amount of base needed to neutralise the brew is called the titratable acidity (TA). Incidentally, water chemistry geeks will notice that the relationship between pH and TA is thus very similar to the relationship between alkaline and alkalinity.
Why is Titratable Acidity Important?
Research into acidity in coffee has shown that pH isn’t very closely linked to perceived acidity – in other words, coffee that tastes more acidic isn’t necessarily so. TA, on the other hand, is more strongly correlated with perceived acidity, the authors write.
The differences between the pH and the TA, as well as explaining why the hot brewed coffee tastes more acidic, also implies that hot brewed coffee may include some specific acids that don’t make it into cold brew, the authors say. “Cold brew coffee extracts were found to have lower concentrations of acidic compounds and may be less chemically diverse.”
This could contribute to the more complex taste of hot brewed coffee – especially in coffees with more complexity to start with. In their study, Rao and Fuller found the largest difference in TA between hot and cold brewed coffee in high-altitude coffees from Ethiopia and Colombia. The smallest difference in TA was in lower-altitude coffees from Brazil and Myanmar. “It is a good assumption that the chemical diversity varies with the growing region”, Rao says. “Chemical diversity in coffee will also vary with growing season and agricultural practice”.
What About Extraction?
Given that some acids were not being extracted in cold brewed coffee, is it possible to improve the flavour of cold brew just by increasing extraction, to try and get those acids out?
In a previous study (M Fuller and NZ Rao, 2017), Fuller and Rao discovered that grind size doesn’t have a significant effect on the amount of chlorogenic acid and caffeine extracted in a cold brew. At the slow extraction rates of cold brew, water has time to penetrate into the centre of all the grounds, so the surface area doesn’t affect extraction as much. This means that grinding finer doesn’t help.
The authors speculate that the compounds extracted in hot brew but missing from cold brew are either larger molecules that are only soluble in hot water, or molecules that are tightly bound to the cellulose. If the missing compounds aren’t soluble in cold water, then no amount of time or agitation will extract these molecules into the brew, either. The authors didn’t study total extraction or TDS in their research, though. “We will definitely include this metric in our next study.” Rao says. “Stay tuned.”
M Fuller, NZ Rao, 2017. The Effect of Time, Roasting Temperature, and Grind Size on Caffeine and Chlorogenic Acid Concentrations in Cold Brew Coffee. doi: 10.1038/s41598-017-18247-4 (https://www.nature.com/articles/s41598-017-18247-4)
TR Lingle, 1996. The Coffee Brewing Handbook: A Systematic Guide to Coffee Preparation. P47