Cookie icing science – Science guide of cookie decoration

Every Christmas, Easter, Halloween, Valentine’s day or any other big holiday decorated cookies will start popping up all over the internet. Decorated pumpkins, heart, eggs and trees with beautiful colours. Here at FoodCrumbles we’re not really good in cookie decorating, but we consider ourselves decent cookie decoration analyzers. Because what if the icing seems too runny or thick? Or if doesn’t look as good once stored or if you miss one ingredient in a recipe. Or, you’re just like us, you don’t really like cookie decorating, but just want to spice up your cookie a little. Or, you daily decorate thousands of cookies on your equipment, how will you develop your icing?

Some cookie icing science should help solve all your troubles!

Making cookie icing

There are roughly two types of cookie icing: icing to decorate a cookie with a lot of detail and icing to just cover a cookie. Both require a very different consistency. Here we’ll focus on the icing you’d use to make a detailed  drawing on a cookie. This type of icing requires very specific flow characteristics. It should be liquid enough to make a detailed drawing, but solid enough to hold up nicely.

Icing sugar & Water

The simplest cookie icing contains just two ingredients: icing sugar and water. You don’t tend to see cookie icing recipes with granulated sugar instead of icing sugar. This is because you want to produce a smooth silky icing without any clumps. Icing sugar is sugar, but milled into a very fine particle size. This way icing sugar dissolves in your mouth really easily, without you even tasting any lumps. It also makes it a lot easier to dissolve the sugar in water. It will take a lot more time and effort (and probably some heat) to dissolve the granulated sugar in water.

So why use sugar at all? This is because sugar can make a nice thick viscous fluid when mixed with water. A higher sugar concentration will lead to a thicker liquid. With sugar this can be controlled quite easily. A little more sugar will make it more viscous whereas a little less sugar will make it more runny.

Why egg whites are included in cookie icing

A lot of icing recipes will call for egg whites (fresh or pasteurized) or meringue powder. Even though you can make a fine icing without these ingredients, they will influence the appearance of your icing. When using egg whites, the recipe will ask you to whip the egg whites. This is done to incorporate air into your icing, just like meringues. The proteins in egg whites are great at stabilizing this foam and holding on to the incorporated air bubbles.

Whereas egg whites are pretty fluid and thin, whipped egg whites are a lot firmer and stiffer. As a result, an icing with whipped egg whites is also a lot firmer than one without. The blog big bake theory has a very nice image showing this difference.

Corn syrup

Various cookie icing recipes call for the incorporation of corn syrup. It is often just a small amount and its main role is to give the icing some extra gloss. You will mostly see it used when eggs are left out of the icing since these icing tend to dry more quickly.

Flavours & Colours

Adding flavours and colours will dilute the cookie icing slightly, making it more runny. However, they are generally used in such small amounts that they shouldn’t drastically affect your consistency.

Best cookie type for decorating

Once you have your icing, you should think about the type of cookie you’d like to use for decorating. In theory, you can decorate every cookie of course, but in reality, your cookie will have to meet several requirements:

  1. Have a flat surface: for optimal decoration results, a flat surface works best. This will make it easier to make a detailed drawing on the cookie surface. You get a flat surface by using little leavening agent and by using a cookie dough that can be rolled out flat.
  2. Be sufficiently dry: a very moist cookie isn’t suitable for decoration. This has to do with moisture migration. Icing has a pretty low water activity due to the high sugar content. This is becomes even lower over time since part of the moisture will evaporate concentrating the sugar even further. If a cookie on the other hand is very moist, some water might migrate into the icing. This will make it wetter and thus harder to set properly.
  3. Not be too sweet: icing is sweet, so if your cookie is already very sweet, it will make it even sweeter, better to have a less sweet base, although tastes can differ of course!

When choosing a cookie, be original. You will see that almost all of the cookie decorators use the same type of pretty plain standard cookies to decorate such as shortbread or gingerbread.

What happens during and after decorating the cookie

First of all, make sure your cookie has cooled down. The consistency of your cookie icing depends quite strongly on temperature. At a higher temperature it will be more fluid. This might cause the icing to be too runny.

In industry you will see icing being applied to warm or hot cookies since that speeds up drying. However, if you want to do so, you will have to make sure

Once the icing has been applied it will start drying immediately. As long as the icing is in a vessel, bowl or bag, it won’t dry out too much. The surface area is quite small compared to the overall volume. However, once you’ve applied the icing, the surface area will be a lot larger. This makes it a lot easier for the moisture to escape from the icing. As a result, the icing will thicken and harden out.

Drying icing at a large scale

When you decorate cookies at home drying the cookie is a matter of patience. You make sure the cookies don’t touch each other and you’re fine. However, imagine drying far larger quantities. If you’re got several hundred it becomes a matter of stacking them smartly and maybe helping the drying along with a fan to wave off moisture.

If you have thousands though, things become more complicated. The cookies will not dry quick enough at room temperature and thus extra heating is used. However, you don’t want the icing to crack so generally the maximum drying temperature is 80C. Above this temperature moisture evaporation goes too fast and might create bubbles and other inconsistencies.

Rheology of cookie icing

Whereas at home you can easily adjust your cookie icing recipe if it doesn’t work well, that’s not as easy in a factory. So they need a way to easily determine whether an icing is suitable for use. Control of recipes and simple trial-and-error will work, but a more scientific approach would be to analyze the flow properties in more detail. The field of research that studies this types of behaviour is rheology.

Another reason to use this type of analysis is because of industrial equipment. The cookie icing will have to flow through this equipment and often the supplier of this equipment will advise what type of flow properties are required for it to work optimally. By knowing this, a new icing can be developed in a faster way. Since you know the range of flow properties that will work, you’d develop an icing that meets these criteria. That way, the chance of success in a larger trial is a lot larger than if you would make an icing based on look and feel.

Fixing cookie icing

Based on what we discussed so far you should have a good feel for why certain things are done when making cookie icing. Based on that, we’ll end with a few basic tips for making cookie icing.

  1. The icing is too runny: you’ve added too much water, adding less water will make a thicker more viscous consistency
  2. The icing is too thick: you haven’t added enough water, add a little extra to help it along
  3. The icing is not airy enough: you haven’t whipped the egg whites long enough, you can often continue whipping them even after the rest of the moisture and sugar

Sources

Sweet sugar belle, on the use of a fan to dry your icing, link

Sally’s baking addiction, on decorating cookies with icing, link

Biscuit, cookie and cracker manufacturing, Manual 5, Secondary processing in biscuit manufacturing, chapter 7, link

Biscuit, cracker and cookie recipes for the food industry, Duncan Manley, 2001, chapter 10.3, link

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