[recommended pre-read: Potato Science]
To me, the goal is somewhere between being lumpy at one extreme and gluey at the other. Connotations of ‘fluffy, light, smooth, creamy’.
The starting point is the potato itself. Floury and waxy-type potatoes both work for mash. Floury potatoes are relatively high in starch and low in water, compared to waxy varieties that are low in starch and high in water. Floury tend towards a fluffy result, whereas waxy can be creamier and smoother, but needs more mashing and so risks becoming gluey.
After that decision is made, it’s all down to the cooking and the science. There are three objectives: 1) hydrate (swell) the starch granules so they gellate, 2) retain the starch inside the potato cells (remember it’s escaped starch that causes glueyness), and 3) separate the potato cells from one another to produce the smooth texture of mash. We want the left image, not the right.
As heat is applied, the starch granules start to separate and bond with cellular water and hydrate (and swell). This process continues as the temperature rises, and by about 70C the starch occupies most of the cell – the cell itself is now container of a viscous gel.
This is where we would like to stop, as further heating can swell the starch enough for it to break out of the cell. Unfortunately, we cannot stop now – at this temperature the pectin binding potato cells together has not broken down enough to allow us to separate the cells (by mashing). Pectin doesn’t begin to break down until ~85-90C. So that’s a good time to stop and mash, but it is a compromise between hydrating the starch, and breaking down the pectin, and it’s not ideal.
It’s a dilemma, but one that was solved by the instant mashed potato industry a long time ago, and that was famously brought to the attention of cooks and chefs only relatively recently (1998) by Jeffrey Steingarten. If they had not solved this, they would have been the ‘instant wallpaper glue’ industry instead.
The solution is retrogradation. It’s mentioned in potato science 101. It involves a 2-step heating process. Heat the potato to the upper limit for starch hydration (70C) and then cool it down. As the potato cools, the swelled starch granules reorganise themselves into a new structure that is stable, resists rupture and resists further hydration. The second step is to now simmer the potatoes at ~90C to break down the pectin and enable the potato cells to separate. The stabilized starch granules will not rupture or swell during this second cooking step.
This is relatively simple in a home kitchen, but does require a thermometer!
Some other considerations:
Prepare the potatoes in ~2cm slices. This is a compromise because cut potatoes can lose more nutrients into the cooking water, but thin slices are necessary for the potato to be heated to 70C uniformly throughout.
Mash the potatoes with a ricer. No one uses a food processor, and rightly so because the blade will cut open cells and release starch. A potato masher (or fork) is not too bad, but it is necessary to repeatedly crush already mashed potato in order to get at the last lumps, so is more damaging. With a ricer the potato is crushed just once.
It is one of life’s little ironies that although the potato is essentially fat-free, it always tastes better with a bit of butter (or other fat) added. In mash, it actually has a purpose. It gets in the way of any free starch binding to itself and forming glue – it acts to reduce gumminess. This is why a mash made with waxy potatoes, which need more mashing or greater force, is likely to require more butter. It is most easily incorporated in cold pieces. Finally, the consistency of the mash is adjusted with dairy.