Never knew that a little bit of a staple white powder could cause so much bubbling and flowing. Resulting in a super crunchy, slightly sweet/slightly bitter snack. No clue what we’re talking about? It’s honeycomb, made from sugars & baking soda. When making honeycomb you boil sugar and sugar syrup until it’s a nice brown. At that point you add baking soda to the extremely hot sugar mixture. This causes the sugars to bubble and puff in a spectacular manner.
Even though the addition of baking soda most definitely is the most spectacular part of making honeycomb, there’s a lot more chemistry involved. The transformation of pale sugars into a beautiful brown consistency involves a lot of chemical reactions. This browning is called caramelization and is the topic of this post. Some hard core chemistry today.
What is caramelization?
Caramelization is a browning reaction in food, in other words, the reaction creates molecules with a brown colour. Caramelization is, just like the Maillard reaction, an example of a non-enzymatic browning reaction. This means that the browning occurs without the interference of enzymes. Enzymes can catalyze certain chemical reactions as they do for instance for browning of bananas. During caramelization sugars are transformed into brown colours and aromas.
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
When using regular sugar the first step in the reaction mechanism is the decomposition of this disaccharide into two monosaccharides. Part of these sugars will decompose into smaller molecules, often aromas. A lot of aromas are formed during the process. Diacetyl is a very common one, other examples of less well known compounds are: alsohydroxymethylfurfural (HMF), hydroxyacetylfuran (HAF) or furanones such as hydroxydimethylfuranone (HDF) and dihydroxydimethylfuranone (DDF).
A large portion though will participate in reactions to form large brown molecules. These monosaccharides will first react to form a disaccharide. These will continue to react into larger molecules. This is called oligomerization. In this process three main coloured structures are formed:
- Caramelan (C12H12O9)
- Caramelen (C36H18O24)
- Caramelin (C24H26O13)
Chemists still don’t fully understand how these are formed, nor how the molecules actually look like.
Influence of acids and bases on caramelization
As I wrote in the previous paragraph, honeycomb should be heated to 150°C to get a nice brittle texture. However, you might now realize that this is still below the caramelization temperature, which is correct. However, there are other ways to speed up caramelization. One way is the modify the amount of acids or base present. As you’ve learned in my special on baking soda, baking soda can react as a base. Adding baking soda to something will make it more alkaline (or less acidic). By making it less acidic and clearly more alkaline, the caramelization will be sped up quite a bit! Once I added baking soda to my honeycomb the colour clearly darkened to a perfect golden honey colour.
The great thing about baking soda is also that it gives a lift to your product! The baking soda will form gas (see my other post on the topic) and create air bubbles in the sugar syrup. This is great because a solid sugar solution that’s been heated to this temperature will be very hard and tough to eat. But because of all the air bubbles it brakes easily and eating is no problem at all!
Last but not least, if you don’t use a pure sugar but a mixture (as is the case for corn syrup), impurities can also speed up the reaction. My honeycomb started turning a light brown at 150C, even before I added any baking powder.
Why does baking soda work?
Ok, if you’re like me, you are asking yourselves a question now. I just learned that baking soda can form a gas (carbon dioxide) by reacting with an acid. However, we don’t add any acid to the honeycomb, why does it still work?! Great question, I like your thinking. I haven’t (yet) been able to find a conclusive answer. But here’s what I think happens.
First of all, baking soda (sodium bicarbonate) is not a very stable compound, it can react pretty easily for form a gas. When the temperatures are so very high (as is the case with honeycomb) this reaction might can ahead without the acid, it’s probably more stable for the component to be a gas any way. Second, at these high temperatures sugar starts decomposing slightly before caramelisation occurs and impurities will be present from the corn syrup. I think that at least some acidity will be present which will help catalyze the reaction even further.
Honeycomb requires only a few ingredients and steps. Have fun!
- 100g sugar
- 45g corn syrup
- 1/2 tsp baking soda
- Mix the sugar and corn syrup in a pan and add a small layer of water (the exact amount doesn't really matter, if you add more you will simply be cooking longer to boil it all off again).
- Boil the mixture to a temperature of 150C (using a thermometer really is the best way).
- Take the mixture of the heat and mix in all the baking soda in one go. Do this fast and watch out, the very hot(!) sugar syrup will start bubbling a lot.
- Immediately pour onto a heat resitant surface (I use a baking tray covered with baking paper) and leave to cool.
- Once it's cooled down it should break into shards easily.
Interested in learning more about honeycomb? The guardian wrote a great article about honeycomb, with a lot of in depth details. The recipe I used is inspired by theirs. There are also a lot of scientific articles written about caramelisation and the like, for those interested, have look here and here.
Sciencegeist on the chemistry of caramel reactions.
Food-info.net on caramelization