If you’re into decorating cookies you’ve probably had to decide more than once whether you were going to use a Royal Icing or a Sugar Glaze/Glacé Icing* to decorate your cookies. Both can be used for decoration purposes, though they behave and look slightly different. One (Glacé Icing) is made with just two ingredients: powdered sugar + water. The other (Royal Icing) also includes egg whites.
The addition of this one single ingredient drastically changes the properties and behavior of the icing! It’s some pure science at play, involving complex 3D structures. In other words, our curiosity was peaked, so we’re diving in!
*Whereas Royal Icing is quite a standardized name, Glacé Icing isn’t. Some may call this cookie icing, powdered sugar icing, or sugar glaze, to name just a few. What we’re referring to is an icing made with just powdered sugar and water (and maybe some color or flavor) that can be used to decorate cookies.
Royal Icing & Glacé Icing
We’ve discussed glacé icing quite extensively before. It’s a fascinating product because in just a little bit of water, you’re added heaps and heaps of icing sugar until you’ve reached the consistency you’re looking for. It’s pretty simple to make, if you have a bowl and a fork, you’re good to go. All you do is mix the icing sugar with the water, which can be done by hand pretty easily.
How to make royal icing
Royal icing is made using a whisk, we chose to use the whisk on a stand mixer, but a hand whisk or any type of electric mixer works just fine. To make it, egg white powder (or liquid egg whites) and icing sugar are first gently mixed (to avoid creating a dust storm) with water. Then, at a higher speed, the mix is aerated and whipped up into a foamy light mix.
Depending on your use case, you might add a little extra water to make it slightly thinner, or add a little extra sugar, to thicken the icing. If you’re looking to create very fine, intricate decorations on cookies you probably want a slightly thicker icing. If you want the icing to flow and fill large surfaces, you’d want it a little thinner.
Royal vs. Glacé (Powdered sugar) icing
Despite there only being minor differences between the two icings (the egg white powder and whisking), they do behave quite differently!
First of all, royal icing sets noticeably quicker than glacé icing. During drying the icing loses some of its moisture. This causes the icing to thicken and firm up. If you use a very thick layer of icing you will notice it takes a lot longer to set. It will take longer for the moisture to evaporate out of the icing.
Icing has fully set when you can stack the cookies on top of one another, without the icing being affected. For royal icing it’s dried within 1-2 hours, depending on the consistency of your icing as well as the temperature and humidity around you. Glacé icing on the other hand can take as much as 24 hours to dry, again, this depends on its consistency.
Want to be updated on new food science articles? Subscribe to our weekly newsletter
‘Crackability’ is a term we just came up with ;-). It’s a measure for how easy the icing on top of your cookie cracks. Royal icing is very firm and sturdy. It doesn’t crack easily. Glacé icing on the other hand is a lot more prone to crack (we’ll get to why that is below). So if your cookies will have to whether some form of transport or shaking, using a royal icing is probably your best bet.
Aside from being less prone to cracking, the overall strength of royal icing is typically a lot higher than that of glacé icing. It isn’t as susceptible to deformations (e.g. when pressed upon). Also, when breaking a cookie covered with royal icing, you tend to need more force to break the cookie, due to the added strength of the icing.
Amount of colorant
When decorating cookies with icing, you’d probably want to add a splash of color. Even though both icing can be colored in a similar manner, the amount of colorant you need differs significantly between the two!
When you make royal icing you’ve introduced a lot of air bubbles within the icing. Air bubbles scatter light in all directions. As a result, an aerated icing will be lighter in color than a non-aerated one, despite containing the same amount of colorant. Those two cracked hearts in the photo above, they were both made with the same concentration of colorant! The royal icing turned pink whereas the glacé icing a dark red.
Of course it’s important for the icing to look good. However, taste and overall eating experience are just as important! The difference in strength of the icings also results in a different eating experience. The weaker glacé icing dissolves a lot more quickly in your mouth. You could even lick the dried icing off the cookie if you’d want to. This is not possible for the royal icing. It doesn’t dissolve as easily and that also makes it less sweet.
Why glacé and royal icing behave differently
So why do these two icing show such differences? Both the aeration process as well as the presence of egg whites play a role here. Luckily, a pair of scientists looked into this using advanced microscopy techniques in the early 2000s.
Glacé icing is a jumble of sugar crystals
Scientists showed that in a glacé icing, you’ve essentially got a lot of small sugar crystals, packed together in a little bit of liquid. Most sugar crystals haven’t dissolved, nor have they bonded together. The individual crystals simply sit close together, with pretty large empty spots in between as well.
Because all these crystals haven’t formed a cohesive structure, it’s quite soft and prone to cracking. When you create a crack, this crack can easily continue to travel further. The crack will easily find a spot where it can travel in between two crystals. Since these crystals don’t ‘hold onto’ each other, they can’t stop the crack.
The same is true for the overall strength of the icing. Because those individual crystals don’t form a network, the icing is as strong as its weakest component, the liquid binding the crystals together. This is by far not as strong as the individual sugar crystals themselves.
Royal icing under the microscope
When scientists look at Royal Icing under the microscope, the image they get is quite different. Even though it was made using the same sugars, the particles were a lot smaller in size with a more irregular shape. This irregular shape makes it easier for the sugar particles to ‘lock’ into each other and hold on tight. There aren’t as many voids between the particles either.
Of course, the royal icing also contains small round air bubbles. These round air bubbles contributed to the strength and resistance to cracking. By being round, it is harder for a crack to travel through them linearly. Air bubbles would often serve as the end point for a crack, preventing further propagation.
Using some advanced microscopic images, scientists thus discovered that the microstructure of Royal Icing is noticeably different from that of glacé icing. Both the presence of egg whites and whipping the mix seem to have enabled sugar crystals to organize themselves in such a way to produce a strong material.
Egg whites are great at holding onto air
So why do you need both aeration & the egg whites? Well, the two build on each other. As we know, egg whites are great at holding onto air. The proteins in the egg whites have hydrophilic as well as hydrophobic regions. The hydrophobic regions prefer to sit in air, as opposed to water. The hydrophilic areas prefer water. This way these proteins position themselves in between air and water, stabilizing the air bubble that would have otherwise escaped quite easily.
So for the air bubbles to contribute to the strength of the icing and help prevent cracking, the presence of egg whites is essential!
Difference in sensitivity to water
Another difference we noticed when making the two types of icing is the impact of a little additional water to the recipe. Whereas the same amount of extra water would greatly impact the flow behavior of the glacé icing, this was less so for the royal icing. The flow properties of the glacé icing changed very rapidly, and a little more or less water had a huge impact. However, for the royal icing, once a certain consistency was achieved, a little more or less water didn’t have as much of an impact.
This is most likely again due to the difference in microstructure of the two. Since the glacé icing consists of roughly stacked sugar crystals with very little cohesion and structure, a little extra water can make the crystals flow along sid each other a lot more easily. However, in the royal icing a more cohesive structure had formed, thanks to the egg whites. This structure is less affected by a little extra water.
Royal icing is a type of meringue
Looking at the royal icing recipe we realized that royal icing is actually a type of meringue! Meringues are made by whipping egg whites with sugar into a light and airy consistency. Royal icing using the exact same ingredients and processes, so is in fact a meringue.
So why does it behave so differently? That is mostly due to the very high sugar content. The large amount of sugar thickens the meringue considerably and makes it more dense. This is a good thing. You don’t want royal icing to contain too many, nor very large air bubbles. These air bubbles would only cause the icing to break more easily.
In summary, these are the main differences between the two types of icing. All because of two small differences!
|Royal Icing||Sugar/Glacé Icing|
|Ingredients||Water, icing sugar, egg white (powder)||Water, icing sugar|
|Process||Whip the ingredients to aerate||Just mix the ingredients|
|Properties||Hard and sturdy||Softer, more prone to cracking|
|Eating experience||Harder, doesn’t melt in the mouth||Sweeter, dissolves rather easily|
J. Buckman and C. Viney, Note. The Effect of a Commercial Extended Egg Albumen
on the Microstructure of Icing, Food Sci Tech Int 2002;8(2):109–11, link