Ever made your own stock, stored it in your fridge overnight and ended up with a stock jelly instead of liquid? Ever made a wobbly sweet, soft pannacotta? You might not realize it, but despite the fact they taste and look so different, that stock jelly and pannacotta have one big similarity: gelatin.
Gelatin is a well known so-called ‘gelling agent’, you can make very delicate and pretty gels with gelatin. But using (and choosing) gelatin requires some tips and tricks which is why this post is completely dedicated to the science of gelatin.
What is gelatin?
Gelatin is a protein. Proteins are a group of molecules which all have a similar basic structure. Proteins can be found in a lot of foods such as eggs, milk and meat. The gelatin proteins are pretty special though. Most proteins denature when they are heated, let’s say to above 70C. That means that they will irreversibly unravel and bond with one another (think of frying an egg). Gelatin on the other hand doesn’t do that. It can be heated safely up to at least 100C without it breaking down.
How does gelatin work?
At these higher temperatures gelatin does unfold and disperse in for instance water. However, this isn’t irreversible. Instead, when the gelatin dispersion is cooled down again the gelatin will fold back up and get entangled in other gelatin molecules. This entanglement and rolling up causes a gel to form. The water inside the gel cannot move as freely anymore and is caught in between the molecules.
Where does gelatin come from?
Gelatin is made from collagen. You might have heard from collagen when making a soup using bones (e.g. oxtail soup) or when making a slow cooking beef stew. Collagen makes up most of the connective tissue in animals, there’s a lot of collagen in skins for instance, but also in the meats that need a long and slow period of cooking.
The most common sources of commercially available gelatin is pig gelatin. Nevertheless, both beef and fish gelatin exist as well. It is interesting to note though that the different sources of gelatin will also give a different functionality. Their structures differ making them more or less strong.
Gelatin is made by hydrolyzing collagen. Hyrolysis is a general term describing a type of chemical reaction in which a water molecule comes in and cleaves a chemical bond. In this case the hydrolysis causes the transformation of collagen into gelatin. Unfortunately I wasn’t able to find a lot of descriptions of this chemical reaction going into more detail.
As was mentioned at the start of this post, it’s easy to make your own gelatin. Simply take meat bones and boil them for quite a while. The collagen will come out of the meat and react into gelatin. Gelatin is water soluble and will sit in the water.
In factories they use a similar process but use the addition of an acid (pH < 7) or an alkaline ingredient (pH>7) to speed up the process. The addition of a component helps to hydrolyze and extract the collagen from the product. The exact production process depends a lot on the source of gelatin (e.g. a skin vs. bones or pig vs. beef). During the production process manufacturers can adjust certain parameters to make different qualities of gelatin.
How to use gelatin
You can buy gelatin in all sorts of shapes and sizes. Consumer gelatin can be found in sheets or powders. Larger scale batches are generally powders supplied in large bags.
Gelatin can disperse in warm water, but it is very important that gelatin is not added to warm water immediately. Instead, gelatin always has to be soaked in cold water for a longer period of time before using. Also large scale manufacturers will generally have such a step. During this soaking phase the gelatin will hydrate. The hydration prevents the gelatin from clumping up when it’s added to hot water. I suspect the reason is similar to why you should never add corn starch to hot water immediately, but always mix it up with some cold water first.
The more gelatin sits in a solution, the stronger the gel becomes. This is not always desired. A pannacotta for instance should be a little wobbly. The quantity will thus depend on what you’re aiming for. But, it also depends on te gelatin itself.
Recipes tend to call for 3-4% of gelatin for creating a strong gel. However, since there are a lot of different types of gelatin, the exact quantity will depend on the type of gelatin used. The strength of gelatin is generally expressed in bloom (g). The bloom value is measured by pressing down on a gel of the gelatin and determining the weight that can be put on the gel before it breaks.
Why doesn’t it work?
Gelatin is a protein so for the gel to work it is important that the protein remains intact. As we discussed, gelatin happens to be a protein which is quite resistant to heat. Nevertheless, it’s still possible to break down gelatin and make the gel unstable.
Certain fruits (probably the most well known example is the pineapple) contain enzymes (proteases) which can break down proteins. Thus they can also break down gelatin, resulting in a weaker gel. Enzymes can be deactivated by heat (enzymes are proteins as well), so boil that pineapple before making a pineapple jelly.
Besides completely breaking down gelatin, ingredients can also just interfer with the gelatin strands that form a gel. Acids and salts tend to weaken the gel whereas milk and sugar tend to strengthen the gel. Both of them are important to take into account when developing a recipe because more or less gelatin might have to be added.
Madehow.com to learn more about the gelatin production process.
PBGelatins describe their production process in more detail with a nice compact overview.
This article is a little harder to get through but contains a lot of fact and figures and more in-depth information on gelatin.
Scientific article looking into the various sources of gelatin.
McKenzie for an explanation on bloom strength.
On Food and Cooking, Harold McGee; one of my favorite sources for all things related to food and science!