If you think of chemistry, you might be thinking of a lab, with people wearing white lab coats, where lots of complicated, maybe even dangerous chemicals are being mixed and tested by experienced chemists.
You might almost forget that your kitchen is full of chemistry and that in some cases all you need for some spectacular chemical reactions is a pot, a stove, and some sugar! Once that sugar starts to caramelize when heated up, chemical reactions are going haywire. An amazingly complex set of chemical reactions occurs which transforms your humble sweet, white sugar, into a slightly bitter, complex, brown flavor bomb! The chemistry you’ll see right in your own kitchen might even be more complex (and probably less well understood) than the chemistry those scientists do in the lab.
So sit back for a ‘real’ chemistry lesson as we dive into the (unknown) science of caramelization!
- What is caramelization?
- Caramelization reaction mechanism
- Influencing caramelization reactions
What is caramelization?
When we talk about caramelization in this article, we’re talking specifically about the caramelization of sugar. To caramelize sugar all you need are sugar + heat. Once the sugar is hot enough, caramelization will set in. During caramelization sugar changes from white or colorless into yellow, orange, brown even black. At the same time, the flavor of the sugar changes drastically, from purely sweet, to a more complex profile that might still contain some sweetness but also bitterness and so-called ‘caramel’ flavor. The flavor is so distinct, it has its own name!
Related to the Maillard reaction
A close relative of caramelization is the well-known Maillard reaction. Whereas caramelization only requires sugar to occur, the Maillard reaction needs both proteins and sugars. Whereas caramelization only occurs at high temperatures (at least above 110°C (230°F), but more often well above 150°C (300°F)) the Maillard reaction can take place at considerably lower temperatures. However, since they both use similar components, once your product is hot enough they can both occur simultaneously, depending on the conditions.
Temperature of caramelization
Caramelization requires high temperatures to get going. The temperature at which this occurs depends on various factors. The first, and most important, is the type of sugar. Regular sugar (sucrose) and glucose start caramelizing at 160°C. Maltose, quite a common ingredient in corn syrup, on the other hand only caramelizes at 180°C whereas fructose can caramelize at 110°C.
Caramelization reaction mechanism
Caramelization reactions are surprisingly complex and, just like the Maillard reaction, not completely understood. There are simply too many things happening at once. That said, there are some things we know, and for us to dig into those we have to start by looking at our starting components: sugars.
What are sugars?
Caramelization needs sugars to occur. However, there are a lot of different types of sugars. All sugars belong to a group of molecules called carbohydrates. They are all made of the same building blocks, monosaccharides, that are connected together in various ways and sizes. The smaller carbohydrates, those made up of just one (the monosaccharides) or two (disaccharides) ‘building blocks’, are what we refer to as sugars.
Common monosaccharides are dextrose (glucose), fructose, and galactose. The most common disaccharide is sucrose, which is ‘regular’ sugar. Sucrose is made up of one glucose and one fructose molecule. Other common disaccharides are lactose (found in dairy) and maltose (used for beer).
Larger carbohydrates such as starches can be broken down into sugars. Starch is simply a long chain of glucose molecules, so when it breaks down, you’re left with individual sugars!
Initiating caramelization: heat
To kick off caramelization reactions, your sugars need to be hot enough. The heat is required to initiate the reactions. The temperature at which caramelization starts varies by sugar type. Fructose kicks off first at 110°C (230°F), whereas maltose needs to be 180°C (356°F). Galactose, glucose and sucrose all start to caramelize around 160°C (320°F).
Chemical reactions abound
Once the sugars are hot enough a lot of chemical reactions will all happen simultaneously. There are a few recurring patterns though.
Step 1: Enolization
Often caramelization starts by reordering within the sugars themselves through a reaction type called enolization. During such a reaction an oxygen atom with the sugar molecule that was initially bound to a carbon atom with two connections, now becomes connected to one carbon and one hydrogen atom. This slight shift in structure then enables other reactions to occur.
Step 2: Dehydration
Next up, the sugar molecule will likely lose a water molecule through a reaction called dehydration. There are several ways in which this can happen and it can also lose more than one.
Step 3: The Wild Wild West
After these first two relatively simple and common reactions (which might not even always occur!) it truly becomes the wild west out there. A lot of different reactions will and can occur during this time. During these reactions different types of molecules will form.
The molecules that turn your caramel brown will be large molecules, made up of a lot of smaller molecules that have reacted together. This process is called oligomerization. Three types of molecules are often mentioned to be formed during this process (caramelan, caramelen, and caramelin). However, despite this being cited often, the proof for these molecules being formed actually isn’t very strong, nor well understood. Instead, it’s more likely that a wide range of different molecules are formed.
The more aromatic molecules on the other hand are a lot smaller (hence they can evaporate and reach your nose). Common examples of these molecules are diacteyl (essential for a buttery smell), as well as hydroxymethylfurfural (HMF), hydroxyacetylfuran (HAF) or furanones such as hydroxydimethylfuranone (HDF) and dihydroxydimethylfuranone (DDF).
Upon analyzing caramels over 1000 components have been found, again showing just how complex these reactions can be and what a wide variety of components can be formed!
Influencing caramelization reactions
Both temperature as well as sugar type impact how the complex series of reactions occurs. But there’s more. The pH-value of the sugar solution also has a big impact. A more acidic or alkaline environment speeds up caramelization. It can also cause caramelization to start happening at a lower temperature compared to the ‘normal’ caramelization temperature of that sugar.
We’ve tested this more extensively previously when making flavorful sugar syrups (that had been caramelized).
Making caramel colors
Even though caramelization reaction mechanisms aren’t completely understood in detail, we know enough to consistently produce caramels from sugar. The food and coloring industry uses this expertise to make a range of caramel colors that can be added to foods to color the food. The food then doesn’t need to caramelize itself, instead, the color can simply be added. These colors don’t have a strong flavor though and are really mostly used for color since they are quite strong.
In Europe these colors are labelled as an E-number, E150 with four different varieties (a, b, c and d).
Ready to start doing some chemistry in your own kitchen? Grab some sugar and a pan and you can get going. Or, start making some caramels, caramel popcorn, or sugar syrups to bring your knowledge into practice!
E.H. AJANDOUZ, L.S. TCHIAKPE, F. DALLE ORE, A. BENAJIBA, AND A. PUIGSERVER, Effects of pH on Caramelization and MaillardReaction Kinetics in Fructose-Lysine Model Systems, Journal of Food Science, Vol. 66, No. 7, 2001, link
Benjamin Caballero, Paul Finglas, Fidel Toldra, Academic Press, 2015, Chapter: Caramel: Properties and Analysis (by N. Kuhnert), link
Food-info.net, Caramelization, link
Shozaburo Kitaoka and Kiroku Suzu, Caramels & Caramelization Part I The nature of caramelan, Agr. Biol. Chem., Vol. 31, No. 6, p. 753.755, 1967, link
Nor Shuhada Binti Shoberi, THE ROLE OF pH, TEMPERATURE AND CATALYST TYPE IN CARAMEL MANUFACTURING PROCESS, 2010, link