Cooking is an art, but baking is definitely both an art and a science. Few things seem to illustrate the science of baking as much as making a meringue. I’m no subject matter expert, but I’ve read up on the subject from a number of places – biochemistry books, cookery technique books and various online sources (it helps I work at an University so have access to some good libraries!). While I found quite a lot to go from, there was no one single place with this information altogether. This is then is a baker’s/layperson’s explanation of what’s going on for anyone else who is as interested about this as I’ve been. I’ve tried to check everything I’ve written, including the illustration, and have developed this post to be as correct as I can. If something is howlingly incorrect please let me know (and tell me why it’s incorrect) and I’ll do my best to improve it. So, how exactly does watery egg albumen turn into snow white, crisp on the outside and gooey-mallowy in the middle meringue? It turns out there are a lot of chemical and biochemical processes going on and there are a few things you can do (or avoid) to help you to get the optimum meringue. Lower down I’ve explained some of the ingredients and methods that can improve your meringues or cause you problems and looked into any scientific reason behind them. There’s also an explanation of the cooking process – what exactly happens to the meringues as they dry out.
The science-y explanation
Egg white (or albumen) contains almost zero fat, less than 1% carbohydrate (glucose) but around 92% water. What’s left (about 8% of its total composition) is made up of proteins, trace minerals and vitamins. The proteins are the important bit for making meringue. Egg white proteins are long strands, suspended in water that makes up most of the egg white. They lie coiled up individually like tiny balls of wool. This is because each protein hosts two types of amino acid and some are attracted to water (hydrophilic) and some repel against water (hydrophobic) and chemical bonds keep them that way. This means that when the proteins are coiled the water-loving amino acids all sit round the outside closest to the water and the water-hating amino acids hide inside the coiled-up strands to avoid get wet. (I’ve drawn up a very rough representation of what’s happening with the protein strands – please see 1 in the drawing).
The proteins will stay in this form unless they are subjected to physical stress, certain chemicals or heat and the incorporation of air. We’re interested in the physical stress option for a typical French meringue – beating the hell out of the egg white with a whisk. [Italian and Swiss meringue methods introduce heat stress to the mix as well, which causes thermolysis (where the heat causes the proteins to pull apart). Italian meringue recipes include pre-heated sugar syrup and Swiss meringues are made over a bain-mairie (hot water bath).] When you beat egg white you cause the break-up of the chemical bonds that keep the protein strands together. This is called denaturation. By whisking you also start to incorporate air bubbles into the egg white that the hydrophobic amino acids become attracted to and this also encourages the proteins to unravel from their natural curled-up state. These two stress processes cause the coiled-up protein strands to un-curl and turn the egg white from a liquid into a foam. The chemical bonds that hold the protein strands together break and the hydrophobic amino acids start to attach to the bubbles of air you’ve whipped in, holding the air in place and keeping the foam structure fairly intact. (See 2 in the drawing). The final part of this is coagulation, where the protein strands, attached to air bubbles by the hydrophilic amino acids, start to bump together and create chemical bonds with each other, creating a sort of mesh-like structure. This keeps the air bubbles locked in place and supports the foamy composition of the whisked egg white. (See 3 in the drawing). The three states often cited for whipping meringue – soft, firm and stiff peaks – relate to how much stress the proteins have been subjected to. Less stress by whipping (and therefore also less air) leaves the protein strands less untangled so they can’t bond together quite so effectively. This means the foam structure is not so strong, giving softer peaks. The more you beat it applies higher stress and more air so the stiffer the foam will be. This is because you will really straighten out the proteins, so they are fully open to being in contact with other strands and can create new chemical bonds around larger air bubbles. But beware – there is a limit to the stress you can apply and egg white can be overwhipped. Proteins can be stretched too far, become unstable and collapse, releasing the captured water and air. This results in a flat meringue where seconds before it was beautifully fluffy. There is a remedy though – please see below.
Added sugar dissolves into the water molecules in the egg white and this actually increases both the strength and elasticity of the whole mix, and helps support the proteins from stretching too far and collapsing. This, in turn, allows a little more air to be whipped in making the egg whites even fluffier. Sugar needs to be added after the stress process has already started – so never, never add sugar before you start whipping. If you add sugar first it will have the opposite effect to what you want and will prevent the protein strands from uncurling. Because sugar is there to dissolve with the water molecules you should give it a fighting change and use the finest caster sugar you can get. Some recipes even list icing (confectioners) sugar.
Things that can help a meringue
Using fresh eggs
It’s best to use only the freshest eggs that you have for meringues. Fresh egg albumen has a high acidity level, and this level drops sharply as the egg ages. Acid in the egg white will slow down the coagulation process (where the un-curled protein strands bond with each other), which gives you more time to beat in air, it will seem harder but it’s definitely worth it for that perfect meringue. I’ve seen a few places which suggest that you should use older eggs because they are easier to whip up to a foam. In older eggs the chemical bonds in the proteins have loosened, making it easier to beat in air and so get a foam more quickly and with less effort. However, it’s a false economy because once whipped up they will not coagulate fully due to those relaxed chemicals – the bonds won’t reform with any adherence. This means you’re less likely to get really stiff peaks, the meringue will sag and loosen and it will be less likely to have a nice crisp shell as it will stay slightly sticky. So use fresh eggs for the best results. That said, while you should still avoid eggs that are getting close to their ‘use by’ date, you can get away with eggs that are a few days old by employing the next tip…
Adding white wine vinegar, lemon juice or cream of tartar
Some people swear by adding one of these ingredients in the meringue mix. They may not know why they work or may think that it just makes the meringue more ‘glossy’. The real reason that these work as an extra ingredients is because they all increase the acidity level of the mix, mimicking the same effect as using the freshest eggs. It’s better, though, to just to use the freshest eggs as something made with the fewest ingredients is preferable and it also just slightly alters the flavour (especially the lemon juice – but then you might want that for your recipe). However, if your eggs are a few days old, it’s worth putting in a half a teaspoon of white wine vinegar (my preference out of the three ingredients) to help. Having tried all three ingredients my least favourite is cream of tartar as it definitely results in a slightly drier and crispier meringue. I prefer my meringues to be more gooey on the inside, but if your preference is for crispy it may be the extra ingredient for you. If using, you must add the extra ingredient after you’ve incorporated the sugar.
A metal or glass bowl
Any metal or glass bowl is just easier to wash and keep grease-free than a plastic one. However, I have read in several places that there is a specific benefit to using a copper bowl – see below. However, I can’t imagine anyone other than a professional pâtissier using these as they are just so expensive.
Copper molecules actually bind with one of the proteins in egg white. This binding causes a reaction that tightens the chemical bonds between the strands, resulting in a stiffer and less prone to collapse foam.
Things that can cause problems
Any fat present will make the denaturation process more difficult. It’s not impossible to whip a meringue where some fat is present, but it will take a whole lot longer – so long it may not be worth trying! Anything more than the smallest amount of fat and it will be impossible. One way to ensure the bowl is scrupulously clean is to rub it with the cut edge of half a lemon (this also adds a little of one of extra ingredients listed above that can help with the meringue mix).
It’s not the plastic itself that will cause you grief, but the fact that plastic attracts fat and it’s more difficult to complete clean a plastic bowl free of grease than most other materials. So, avoid a plastic bowl if you can to give yourself an easier task. However, if you’ve only got a plastic bowl, clean it with very hot water, wipe it bone dry with a kitchen towel, maybe wipe over a cut lemon and apply a little extra effort. If it’s clean, it will still work.
Dirty utensils (beaters, whisks, spatulas etc)
As with bowls, make sure your utensils are scrupulously clean. Any residue or grease on them will affect the denaturation process and stop you from getting a fully fluffy mix. Wash in the hottest water posible and leave to dry out or dry with a kitchen towel.
The reason you need to separate out the egg yolk intact from the egg white for meringues again is because of fat. Yolks have a high fat content. With the teensiest amount of egg yolk in it’s still possible, but as with the comments for plastic bowls, you’ll need to whip the meringue for a lot longer and a lot harder. Anything other than a minute amount of yolk and you should just start again separating the eggs out – save the original whites and yolks for something else.
Eggs from the fridge
It’s best to have room temperature eggs for making meringue, as the bonds holding the protein strands in curls will already be slightly weakened. Room temperature eggs are already going through a very mild occurrence of heat stress (or ‘thermolysis’ as mentioned earlier) which will lead to denaturation. It just gives you a head start.
The cooking process
Actually, meringues are less ‘cooked’ than actually just ‘dried out’. In your carefully whipped and very fluffy meringue, water and air bubbles are held in the foam. Think of how a sponge looks – a framework of material around pockets of air and water. All you need to do to a meringue is heat it enough to tighten the chemical bonds in the protein strands (to finalise coagulation) and to evaporate the water, leaving the framework intact. Cooking/drying out slowly with a low heat also enables the proteins to coagulate together in an even way (it gives them time to ‘settle’), ensuring the structure of the meringue is perfect. Use a low temperature (a max of about 120°C for a conventional oven or 100°C for a fan oven) to remove the water and ensure the best bake. In fact, you can actually dry out a meringue by putting it in either a very low oven (80/60°C) for a few hours or an overn that was heated and turned off as soon as you put the meringue in; just leave the meringue in overnight or for about six hours in this case. I’ve given a recipe for French meringue in another blog post, plus it has some explanations on how to check if your meringues are ready and what you can do if things have gone a bit wrong.