Temperature Difference

In the previous two articles, I introduced you to the work of a coffee connoisseur who was possibly one of the first true coffee scientists — a man who lived over two centuries ago. Determined to brew the best cup of coffee, he went about defining coffee quality as a factor of aroma retention (a topic we discussed in Part 1 of this series). He noticed that convection currents facilitate aroma loss, harming coffee quality. To address this complex issue, he simplified the convection process by breaking it down into two components: causes, such as temperature differences, and effects, such as particle motion. This approach enabled him to carefully analyse every decision made during the brewing process, in an effort to reduce these factors and enhance aroma retention. Part 2 of this series explored our scientist’s suggestions for reducing agitation in the brewing method. This article, Part 3, focuses on his recommendations for minimizing temperature-driven convection currents.¹  

The Cold Truth About التخمير البارد

Being a true scientist, our coffee enthusiast didn’t just assume that water at boiling temperature made the best coffee. Instead, he followed the scientific method, as discussed in Part 2, and conducted a series of experiments at various temperatures to reach his conclusion.

‘I thought it possible, though not at all probable, that some lower temperature than that of boiling water, might also be most advantageous in preparing Coffee; but after having made a great number of experiments, in order to ascertain that important point, I found that Coffee, infused with boiling water, was always higher flavoured, and better tasted, than when the water used in that process was at a lower temperature.’

This passage conveys a gentle scepticism about using water below its boiling point to brew coffee. His experiments were not solely motivated by curiosity; rather, they were aimed at exploring the beneficial use of lower temperatures, as observed in the beer brewing industry. He pointed out that ‘it is well known that the heat of boiling water is not that which is the most favourable for extracting from malt those sacc[h]arine parts which it furnishes in the process of making beer.’ This reveals that the scientist was unaware that the barley and other grains used in the malting stage of brewing beer contain amylase enzymes that break down the larger starch granules into their smaller sugars. These sugars then feed the yeast that generate alcohol during ⁩التخمير. 

If malting were carried out at temperatures above 40°C (104°F), enzymes would begin to denature, becoming inactive and rendering the process entirely superfluous. Since coffee is roasted at far higher temperatures than those used during brewing, it can be confidently assumed that all enzymes in green beans have been inactivated by the time this process is being carried out. 

However, the impact of the temperature of the extracting solvent (water) on the quality of the extract (brew) remains highly relevant, particularly in relation to our scientist’s initial hypothesis regarding convection-associated aroma loss. If brewing at a lower temperature had produced the same brew quality, then the temperature-driven aroma loss could have been completely avoided by not heating the water in the first place. Problem solved! This would have substantially improved the aroma retention and overall quality of the brew — but unfortunately, his findings revealed otherwise.

‘I have frequently … made Coffee, in two like coffee-pots, with this single difference, that the water poured into one of them has been boiling hot; while that poured into the other, has been at some lower temperature; and I have constantly found, that the Coffee made with the boiling water has been preferred by all good judges; especially when they have been presented with the two kinds of Coffee at the same time, without being told in what manner they were prepared.’

The passage above suggests that the scientist conducted multiple sensory panels comparing the brew quality between these methods, ultimately identifying a difference. What is particularly lovely to read is that the scientist included the fact that panellists had not been informed of the samples’ identities. This is a roundabout way of communicating to the reader that knowing the identity of the samples could have influenced or biased a panellist’s response. By withholding this information from the panellists, he is attempting to eliminate these biases and seeking an objective response. This technique is now known as a single-blind study. Double-blind studies, where both the test-giver and -taker are unaware of the samples’ identities, are now preferred, to eliminate any potential biases from either party² influencing the study’s outcome.

What is most laudable about this scientist is that his curiosity did not simply end after he replaced boiling water with its cooler equivalent. He appears to have asked himself whether the heat transferred from the boiling water to the grounds played a role in generating coffee’s characteristic aroma. This led him to reheat the التخمير البارد in an effort to replicate the coffee quality produced with a hot extraction. Unfortunately, although it was a clever line of reasoning, it turned out to be untrue.

'I have likewise made Coffee with cold water, and afterwards heated it; but this I have always found to be of a very inferior quality: it is very bitter, and not unfrequently of a sour disagreeable taste, especially when the cold water is a long time in passing through to Coffee in powder, and when they are suffered to remain together over night.’

Although this confirmed that boiling water was essential to brewing the best cup of coffee, he did not immediately assume that cold water was incapable of extracting the aroma components he desired. Instead, unable to rule out that the aroma was being driven out of the coffee when it was heated, he opted for a more scientific and diplomatic interpretation. 

‘The fine aromatic substance is either not extracted by cold water, or it escapes afterwards, while the Coffee is heating.—The fact is, that very little of it can be perceived in the Coffee, after it has been heated … now it is quite certain that boiling water extracts from the prepared grain, more of those particles which give the agreeable taste and flavour to the Coffee, or, in other words, that give it strength, than an equal quantity of water less hot: this fact has been ascertained by many experiments, and is now generally acknowledged.’

During a time when thermometers were likely expensive and thermocouples did not exist, using boiling water instead of water a few degrees below boiling, as recommended by modern filter coffee recipes, was more practical.

‘[T]he temperature of boiling water is preferable to all others for making Coffee, … on account of its constancy.’

It is important to remember that this scientist’s findings reflect his preferred brew method, filter coffee, and not immersion brewing, which is commonly used today for التخمير البارد. The different brew methods are likely to alter the chemical profile of the التخمير البارد. I have not thoroughly researched this subject yet, but one would assume that all water-soluble components, such as caffeine³ and acids, could be extracted from the coffee grounds using room-temperature water. This suggests that the extraction efficiency of the brewing methods is mainly responsible for the difference in coffee quality when the same brew ratio is used. Various parameters can influence this, including the relative انتشار rate of جزيئات. Large جزيئات generally travel more slowly than smaller ones. However, the greatest difference between brew methods is the extraction time. Modern immersion methods measure brew time in hours, unlike the minutes elapsed in brewing filter coffee. This strongly suggests that a desired sensory profile cannot be achieved in just a few minutes.

Although this scientist did not enjoy التخمير البارد prepared using his preferred drip filter brew method, he was not the first to extract coffee using water below its boiling point. We know from Sylvestre Dufour’s writings in 1685 that this practice not only predated our protagonist scientist by over a hundred years, but it also predated the introduction of coffee to the Americas in 1713: