Week 3 of the Great British Bake Off was bread week, and Paul Hollywood set the bakers a technical challenge of making a batch of the German buns called “dampfnudel”. These buns are steamed so that the tops are soft and white but the bottoms are caramelised and brown. Caramelisation is a common process in cookery, adding colour and depth of flavour to sweet and savoury ingredients alike, but what is the chemistry uniting caramel ice cream with French onion soup?
1. Caramel is made by heating sugar and boiling it for a length of time – exactly how long determines the final product. Generally, caramel is sweet, brown and may be hard, soft or liquid; this variety means that it can be found on its own as a sweet or cooked into puddings and sauces. Caramels which have been heated particularly strongly take on a dark brown colour and are not sweet, tasting more bitter and burned, as perhaps unintentionally demonstrated by some of the bakers in the Bake Off tent this week. Even further heating burns the caramel, turning it black as the sugars are slowly degraded to carbon.
While dark caramel may not sound especially appetising, it is probably eaten more often than any other kind of caramel, as it is very widely used as a food colourant, most notably in cola. This is possible, because in small amounts the colour of the caramel is easily visible but the flavour is masked by stronger tastes in the food or drink. Caramel colouring may also have the extra benefit of absorbing light, which would otherwise cause flavour molecules in drinks such as beer to react, spoiling the drink. Beer bottles are traditionally made from brown glass for the same reason.
The caramelisation process is described as a “non-enzymatic browning reaction” because the action of heat alone turns the food brown. Enzymatic browning can occur to certain foods at room temperature, usually on exposure to oxygen – for more details see our post on apples. When caramel is produced on an industrial scale, it may be done under acidic conditions to accelerate the reaction. Since the acid is not incorporated into the final product, it behaves as a catalyst.
Pure sugar is not the only food which will undergo caramelisation; so will certain fruit and vegetables if they are cooked in the right way – this tends to involve frying for a fairly long time but on a low heat to avoid burning. Caramelised onions are a popular ingredient for soup and savoury tarts, changing from white to brown just as pure sugar does, as are caramelised apples for puddings. Onions, apples and many root vegetables contain a high proportion of carbohydrates – also called “polysaccharides” as they are polymers made up of many “monosaccharide” sugar molecules – which can be broken down and then undergo caramelisation.
2. The chemical reactions involved in caramelisation are extremely difficult to describe with any precision, largely because boiling a pan full of sugar is not a sophisticated or subtle process! Nevertheless, it is possible to give an overview of the types of reactions involved and how an apparently simple heating process turns crystalline, white sugar into aromatic golden caramel.
Granulated sugar is made from sucrose – a disaccharide composed of the monosaccharides glucose and fructose. The first thing to happen when the sugar begins to boil is that the sucrose splits into its two constituent monomers. These can immediately react together in random combinations to give polysaccharides in condensation polymerisation reactions, meaning that many small molecules combine to give long chains and in doing so release water. The water quickly boils out of the mixture so the reaction is not reversible. This paper gives a detailed account of the chemical composition of caramels made from various sugars, though perhaps the most important conclusion is that “caramel is composed of several thousands of compounds formed by a small number of unselective and chemoselective reactions”.
The rich flavours of caramel come not only from the polysaccharides but also from a number of small molecules which are formed as side-products of the thermal degradation of sugars. A technique called gas chromatography-olfactometry has been used to separate and identify many small molecules in caramel and describe how each one smells in this study. In this technique, the chemical compounds are separated by gas chromatography and each component is then sniffed by a human assessor, as described in this review article. Among the most prevalent flavour compounds are diacetyl, which has a strong smell of butter, maltol, smelling simply of caramel, and 5-acetoxymethyl-2-furaldehyde, with a more burnt smell. Both the polysaccharides and the small molecule content of caramel are complex and diverse so the smells of different caramels cannot be easily attributed to any individual molecules.
Contributors: Harry Morgan