Crema and the laws of physics

It took me a while to notice that one of the prettiest things about espresso was a little bit confusing.

Watch this video and perhaps you will see what I mean:

In theory it doesn’t make sense. The bubbles contain CO2, which is much less dense than coffee so they should be rising quite quickly and not falling. What would make a bubble sink instead of float? You’ll see the same thing happening if you watch a pint of Guinness settle out – and it was their research that gave me the answer to this question.

What is happening is that plenty of bubbles of crema are rising, but they are rising predominantly in the middle of the shot glass and the pressure they create as they rapidly rise effectively sucks down the smaller bubbles close to the walls of the glass. There is nice explanation of Guinness here (with pictures!)

Another mystery solved…..

Sugar (Part 1): Sucrose

I know this blog is predominantly about coffee but sometimes I find the things that surround us worth a look too. Though we may not approve of customers shovelling spoonfuls of sugar into their drinks, and we certainly don’t approve of espresso that requires a spoonful to go from being mouth-achingly sour to merely unpleasantly astringent, I thought it might be of interest to have a slightly more in depth look at the little white crystals.

The components of sucrose: glucose & fructose


As a building block of other sugars and carbohydrates it is referred to as a simple sugar. Though present in fruits naturally, it is usually found as part of a mixture of other sugars. It is the building block that links together to create starch. If you go looking for it in the kitchen then you’ll find it in corn syrup.

It doesn’t taste as sweet as sucrose, nor can you dissolve as much in water.


You can usually find this on the shelves of the supermarket in crystalline form. It has the same chemical formula as glucose, it is just arranged slightly different which makes it taste much sweeter. It is also a lot more soluble in water, and in water it exists in different shapes which further influence its perceived sweetness.

Perceived sweetness is highlighted again by fructose. In a cold drink it makes sense to sweeten something with fructose rather than sucrose. It is sweeter so you can use less which decreases the calories of the drink. However, put it in a cup of coffee and its perceived sweetness drops to that of sucrose. All sugars suffer variations in their sweetness in relation to temperature but none as drastic as fructose.


Produced via photosynthesis and then extracted from plants like sugar cane and sugar beets. It is made of a unit of glucose bound to a unit of fructose (sorry if this is stating the obvious). We find it the most useful of sugars because it has a high sweetness, and tastes pleasant at higher concentrations – other sugars can get a bit harsh. You can dissolve 2 parts to 1 part water and resulting solution is thicker and more viscous than other sugars.

Broken down again we have another use for it – invert sugars. When you break down sucrose in the presence of acid or an enzyme then the components don’t crystallise – fructose won’t fully crystallise in the presence of glucose and sucrose. The best chocolate syrups for etching don’t really have much in the way of chocolate in them – the fat content would ruin your foam before you had a chance to drag it one way or the other. If you check the back and they are high in invert sugar then they will sit on top of the drink and look pretty for as long as you need.

The profile of sucrose is different again. Sucrose takes a short time to register on the tongue, whereas fructose is more immediate and is stronger. Sucrose lingers longer, whereas fructose’s sweetness is shortlived. (These peaks – in my head at least – mimic the flavour release profiles linked to fat content – with fructose being like a skimmed milk cappuccino and sucrose more like full fat).

Sucrose in coffee

Sucrose isn’t present in roasted coffee, as it all reacts to form other components during roasting. However it is present in green coffee, and is the major free sugar therein. Various comparative studies of arabica and robusta have been done – though often with quite small sample ranges so the data is not absolute. Ranges of up to 8% have been found in arabica, whilst around 4% have been found in robusta 1. Obvious variations in the sucrose can be the result of varietal, ripeness, processing and storage but I am yet to find anything concrete (links welcome).

In the next article I will have a look at a few other sugars, as well as artificial sugars and sugar alcohols. If there is interest then perhaps a short summary of the history of sugars in our diet.

Comments welcome

  1. Tressl, R. Holzer M. and Kamperscroer, H., Proc. 10th Coll. ASIC, 1982, 279-92  ↩︎