Hollow Macaron Shells ---the Science, the Solution and why I don't get Stressed about it

Of all the "macawrongs" that bakers experience, hollow shells is perhaps one that many tear their hair out over. While I don't obsess over hollows because I understand why it is fairly difficult although not impossible to achieve full macaron shells consistently (see the happy red shell below), many bakers do. I have typed my response too many times over in the macaron group so I have decided to once and for all, pen it down properly so I don't have to repeat myself 😂.

The blue and green hollow shells looking at the full red shells with envy! 

Before I begin explaining how to overcome hollows, let me explain the fundamentals and science of things. Macarons are meringue based cookies and meringues are UNSTABLE structures that will breakdown over time. Even when you use Swiss or Italian meringues where the egg whites are semi cooked to produce a more stable meringue than the French meringue, they will eventually breakdown with time. Hollows happen when the meringue in your piped batter starts to breakdown before the internal structure of the macaron shell is set by baking.

That leaves us with two strategies to combat hollows:
1. Anything that helps to set the internal structure as soon as possible by baking
2. Anything that helps to keep the meringue stable for a longer time during drying stage (if you are drying) and during the initial baking stage (piped batter in oven but structure not set yet).

Not resting at all is what some bakers do to fulfil option 1. But this is not always possible for some bakers as the membrane formed on the surface of the piped batter may not be strong enough to withstand the internal expansion of the batter in the oven. This results in cracked shells. I believe many would rather have hollow shells than exploded shells. Whether or not you are able to get away without resting depends a lot on how your oven transfers heat to your macarons, the humidity of environment, type of meringue, ingredient ratio and whether things like cornflour /cornstarch is added. The longer you rest your shells before baking, the sturdier the membrane on the surface of the piped batter is. But if the meringue starts to break down during this resting stage, a hollow will start to form under that membrane. So there is a tradeoff you have to consider.

What can we do to minimize the resting time then?

-Reduce humidity of environment. If you live in an arid area, good for you! You probabably don't need to rest your shells or rest for long. For those of us in the tropics (like me), you may have to dry your shells in an aircon room, under a fan or use a dehumidifier to speed this up.

- Add cornflour /cornstarch to your dry ingredients or French meringue (if using French method). This helps a membrane to form more quickly and your shells dry faster. It also makes the membrane sturdier and less prone to cracking when you bake. How much to add? I personally don't add more than 1tsp per egg white recipe as I find that it alters the delicate texture of macarons to become a lot chewier. But this can be fixed by maturing for longer time with filling or brushing the bottoms of the shells with unwhipped heavy cream before filling. Some bakers add up to 10g cornflour per 100g dry ingredients. That is way too much for me but it works for them.

- Oven drying. There are 2 ways to go about this. The first one is a technique I use in macaron class to speed things up. I partially dry the shells at 60°C oven for 5-10min using top and bottom heat only. And then either pop it straight into another oven preheated to baking temperature or let it continue to rest at room temperature while the oven preheats to baking temperature. The second method is a method that I use to get super full and shiny surfaced shells. But it is very much oven dependent. I preheat the oven to baking temperature, pop the freshly piped tray of macarons in and bake with the door ajar (about 10cm wide) for 3-5min before closing the door to bake the rest of the way. You need to keep a close eye on this one and do some trial and error as to what temperature works best for your oven. I don't recommend this method if your oven tends to have high temperature overshoot or unstable oven temperature during baking.

Using a higher initial baking temperature also ensures the meringue starts setting quickly before it starts breaking down in the oven. When the initial temperature is too low or when heat conduction to the base of the macaron shell is poor (due to use of silicone mat, poor heat conduction of baking tray), the hollow forms near the base of the shell instead or near the top, resulting in a concave at the base of the shell.

How do we go about option 2?

- Add cornflour or cornstarch to dry ingredients or meringue (for French method). Besides helping to shells to dry faster and form a sturdier membrane, this also helps to stabilise the meringue. Some bakers use it as a stabiliser for the meringue in chiffon cakes.

- Add egg white powder to egg whites for making meringue. When the egg whites are whipped to form the meringue, protein molecules in the egg whites unravel and form a foam structure. By adding more protein in the form of egg white powder, you are adding in more building blocks to the foam structure, making it sturdier and less prone to collapse. I personally don't really like this although many bakers swear by it, because I find that the macaron batter is thicker and resulting surface of piped batter is less smooth. It is great for round macarons, but not so good for someone like me who makes intricate characters and shapes most of the time. 
(Update 07/03/22: After revisiting this option again for a few rounds of bakes involving character macarons, I find that it helps to improve meringue stability without affecting texture too much if not too much egg white powder is added. About 1/2-3/4 tsp per large egg white is sufficient.) 

- Add Cream of tartar. This is an acid that helps to stabilise the meringue. I use it in macarons and chiffon cakes all the time. You add it to the egg whites when they are beaten to frothy stage for French method and before beating the egg whites for Swiss and Italian methods. Some people find that adding cream of tartar ruin their macarons but some find it helpful. You have to try it out for yourself to see if it works. I add about 1/8tsp per egg white.

- Use Swiss or Italian method instead of French. When I have projects that require long hours of piping with several coloured batter for character macarons, I use the  Swiss or italian methods because of the better meringue stability.

- Ensure that your meringue is of good quality. If you are using French method, beat your meringue until it is really stiff and dry but not curdled looking. For those of you using stand mixers, the meringue should be able to ball up inside the whisk. Take your time to whip up the meringue at medium to medium-high speed on the mixer as it will be more stable this way. You should take about several minutes to beat to stiff peaks.

If you are using Swiss method, make sure that the sugar is all dissolved in the egg whites during double boiling stage before you transfer to stand mixer to beat till firm peak. Any undissolved sugar will cause the meringue to be unable to whip up to firm peak within a few minutes of medium-high speed beating. Heat up your egg whites slowly to make sure the sugar has enough time to dissolve and don't forget to keep whisking. I take about several minutes to do this. 

For Italian method, make sure that your syrup temperature reaches 115-118°C before pouring into egg whites slowly and beat the meringue at high speed until it is cool enough (around body temperature) before you stop the mixer to check. Don't stop beating too early.

If your meringue is under-whipped for all three methods, it will break down earlier.

I hope this is helpful information for you and you don't fret as much about hollows because the recipients don't really care, as long as the macarons are yummy! Most of the time even if you get hollows, they aren't super huge and they will disappear with time after maturing with filling. Don't take the joy out of baking by stressing over little details like this!

With love,
Phay Shing