Coffee, oils and Clapham Common

A pour over at Röst Stätte coffee in Berlin. How does the preparation method affect the oils visible on the surface of a coffee?

Have you been making more coffee at home through the Covid-Lockdown times? Each morning, I have taken time to brew a coffee (or several depending on the way I feel that day) and then sit down and notice what is going on in the mug. The way the steam swirls upwards in turbulent patterns, the white mists on the surface of the coffee and the peculiar effects they have on reflected (and refracted) light from the coffee’s surface. And, the oils that appear on the surface of black coffee.

The appearance of these oils is very dependent on the way that you make your coffee. A cafetiere/French press is an immersion method of brewing coffee with no subsequent fine filtration of the grounds. It is therefore quite likely that the oils present in the roasted coffee bean will make their way to the surface of your coffee. If you brew by a pour over method on the other hand, it is thought that the paper filter should take out the oils as you brew. However, even when using a paper filter on a V60, a thin layer of oil can sometimes be seen on the surface of my coffee, visible in the sunlight on those mornings. How much oil is making it through the filter?

Regardless of whether you view the oils as important for the flavour or detrimental to it, there is something quite remarkable about oil patches on the surface of water: they can be a single molecule thick. Of course, you can pile more oil on the surface of the water and then you see the interference effects with light as you see on the surface of polluted rain puddles next to roads, but if there is a large enough area of water, the oil will spread out until it is one molecular layer thick.

How can we know that it is just one molecular layer thick? In one of those experiments that it is probably better to know about rather than to rush out to repeat, a clue came in the 1760s when Benjamin Franklin put a teaspoon of oleic acid (found in olive oil) on the surface of Mount Pond on Clapham Common. As he watched, the surface of the pond, which had been active with many capillary waves blowing over it, was calmed as the oil dispersed across the surface. First the oil remained in a small patch but it then grew, and grew until it reached the other side of the pond.

Mount Pond on Clapham Common. Probable site of Franklin’s 1760s experiment stilling the waves with a teaspoon of oil.

Franklin had been expecting the calming effect of the oil on the water waves, in fact he had been looking for it. On his journey to the UK from the USA he had been watching the wakes behind the ships in the fleet that were accompanying his ship on the journey. Two of the ships showed remarkably calm wakes, a fact that he had remarked upon to the ship’s captain. The captain had responded quite flippantly that it was probable that the cooks had emptied their greasy water over the sides of the ship. Mariners knew that oil and greasy cooking water, calmed the waves around the ships. We can learn a lot by talking to each other and listening to their experience.

The mariners knew that oil calmed the water but why? How? If we think about the oil as a surface layer over the water, it becomes possible to imagine an answer to this. Without the oil, when the wind blows over the water it will act to exaggerate the small perturbations on the surface of the water (caused by water flow, falling raindrops etc) which can then grow into waves. With a layer of oil on the surface, when the wind blows, if the oil is thick, it will act to blow the oil into a thinner layer covering the water surface. If the oil is thin already, it would take a lot of energy to stretch the oil surface to accommodate a growing wave. Either way, rather than exaggerate an existing perturbation on the surface of the water, the wind over an oily surface will tend to drag out the oil film, which will have the effect of calming any perturbations rather than encouraging them.

But how realistic was it that Franklin’s teaspoon of oil could have covered Clapham Common pond? About one hundred years after Franklin, Agnes Pockels and Lord Rayleigh were studying the effect of oil on the surface tension of water. As they did so, they calculated the thickness of thinnest oil layer that they could disperse over the surface of the water bath they were studying. Pockels calculated this thickness as 1.3 nm, Rayleigh at 1.6 nm, either way, a layer that is 10 000 times thinner than a grain in the smallest espresso grind coffee.

And one molecular layer thick.

It was while experimenting with surface tension that Agnes Pockels and Lord Rayleigh (separately) calculated that a layer of oil on water was just over 1 nm thick.

So to return to Clapham Common pond. 1 teaspoon is 5 cubic cm. If the oil formed a layer 1.5 nm thick over the surface of the pond, it would disperse over an area just slightly over 3000 square meters. It is perfectly possible for one teaspoon of oil to disperse over the surface of Mount Pond in Clapham Common. But what is possible is not necessarily advisable so let’s reverse the question and ask how much oil is on the surface of the coffee? Assuming that what is on our coffee is genuinely one molecular layer thick, or about 1.5 nm*. My cup has a radius of 4cm, meaning that the volume of oil on the surface is 0.0075 cubic millimetres. One metric teaspoon of olive oil is 5 cubic centimetres or 4.55 g. If we use the ratio of the volumes to calculate the ratio of the mass, we find that the oil we can see on the surface has a mass of about 7 micro grammes. A tiny amount, but a value consistent with studies suggesting that a small amount of cafestol (associated with the lipids in the coffee) gets through to the brew even in pour overs.

There is plenty to notice in a coffee, what do you see in yours?

*It is of course possible that the oils are actually thicker than this, but the paper filter does result in an oil film that is far from continuous across the coffee surface, suggesting that the oil is already stretched as far as it could be.

Calming the waves at Brutti & Boni

Brutti And BoniBrutti & Boni is a fairly new Italian cafe in South Kensington. Located at the less busy end of Gloucester Road, it was quiet when we popped in to try it a couple of weeks ago. The bright interior has light coming from a roof window at the back of the shop, though it seems that many people opt to sit outside with their espresso in the morning, watching the traffic go past. They serve Caffe Molinari coffee together with a good selection of Italian food items. All in all, a good place to go if you are in the area visiting the Science, Natural History or Victoria and Albert museums and fancy a break and a relaxed coffee nearby.

Inside, the shelves are stacked with various Italian condiments, pasta and olive oil. It was this that prompted me to visit Clapham Common to retrace the steps of Benjamin Franklin. Franklin of course was one of the founding fathers of the USA. He was also a keen scientist, diplomat, printer, in fact the man in some ways defines the word “polymath”. His interests and importance span so many areas that it is difficult to write a two-sentence description of him. Fortunately, for the purposes of today’s Daily Grind, I do not need to. Today, all that is important is that Franklin did some experiments on Clapham Common with oil.

Shelves of olive oil at Brutti & Boni

Shelves of olive oil at Brutti & Boni

Franklin had been investigating the “old wives tales” that a small amount of oil placed onto water ‘calmed the waves’. In fact, the old wives tales can be traced back to Pliny (the Elder) in his Natural History written in around 77AD. Pliny had written of pearl divers and how they sprinkled oil on their faces so that the water above them became calm, allowing them to see the oysters that they were looking for on the sea bed. Franklin himself describes, in his letter to the Philosophical Transactions (1774), an event that he experienced in 1757 while sailing to the UK. Noticing that the wakes behind two of the boats in the fleet were calm, he describes how he asked his ship’s captain about this curiosity. Replying slightly dismissively, as if to someone who is quite ignorant of the workings of the world, the ship’s captain replied that “The cooks… have I suppose been just emptying their greasy water through the scuppers, which has greased the sides of these ships a little”. Obviously it was common knowledge that oil calmed the waves.

So, one day in the 1760s, Franklin took a walk to Clapham Common and to Mount Pond. Emptying about a tea-spoonful of oil (oleic acid) into the pond he watched as the oil produced an “instant calm [on the pond] over a space several yards square, which spread amazingly, and extended itself gradually till it reached the lee [opposite] side, making all that quarter of the pond, perhaps half an acre as smooth as a looking glass.” Oleic acid is the principal component of olive oil. Franklin had effectively calmed the waves on the pond with a mere tea-spoonful of olive oil.

A view over Mount Pond, Clapham Common

A single tea spoon of oil would calm the ripples on Mount Pond, Clapham Common

We can calculate how thin the layer of oil had become by dividing the volume of oil in a teaspoon (5cm³) by the area of half an acre (2023 m²) to get an oil layer that was 2.5 nm thick. To put this in perspective, a coffee bean of width 7 mm would fit nearly 3 million of such oil layers in itself width-wise. Later, more precise, measurements of the thickness of such an oil layer, by Lord Rayleigh and Agnes Pockels, gave 1.6 nm and 1.3 nm respectively. This is approximately the length of a single oil molecule. It seems that the waves on water can be stilled by a single molecular layer of oil. How does this work? Why not let me know what you think in the comment section below.