Kitchen chemistry: Deep-frying crisps
By Mahir Mohammed, MChem Chemistry. Originally published in Resonance.
Crisps are a British favourite. They have been scientifically proven by Dr Tony Goldstone, a neuroscientist at Imperial College London, to aid relaxation - something I have verified myself. There’s never been a better time to be a crisp fan, as we’ve just reached the 70th anniversary of Walker’s Crisps - but just what chemical and physical factors affect the crunch and flavour of these golden wonders?
When potatoes are fried in oil, a multitude of interesting and exciting reactions occur. Firstly, the water content of the potato hydrolyses the deep fat oil. The oil contains triacylglycerols, with ester linkages that are susceptible to nucleophilic attack by water. This hydrolysis produces fatty acids and alcohols. One such fatty acid is linoleic acid, which contains certain weak C-H bonds that are easily broken to produce highly reactive radicals, which go on to initiate a polymerisation reaction (figure 1) in which many single molecules join together to form long chains. The result is a set of volatile compounds.
There is a deep link between flavour and smell; therefore, it is not unexpected that the volatile compounds contribute greatly to the flavour of the food. One in particular is 2,4-decadienal, a compound found in meat, potatoes, butter, and crisps, among other delicious savoury delights! This explains the presence of the vegetable oil ingredient on crisp packets; in reality, the compounds in crisps are a collection of derivatives that originated from the vegetable oil used to fry them.
The unsaturated nature of dienals (figure 2) is what gives rise to their volatility. As compounds containing double bonds do not pack together efficiently, the intermolecular forces of attraction are relatively weak leading to a low boiling point. This low boiling point makes the compounds volatile and contributes greatly to the flavour of the food.
Let’s consider the change in physical properties going from the potato to the crisp. Deep-frying potatoes is a violent process that forces the residual water out of potato cells (this also explains the bubbles surrounding the food when it’s dunked in the deep fryer). The potato cells are now hollow, and biting into a crisp quickly releases this air, resulting in the satisfying ‘crunch’ sound.
The hard outer surface of the crisp, in contrast to the soft outer surface of the potato, is formed by cross-linking of starch polymers. Potatoes contain a lot of starch (a type of carbohydrate), which is really just a glucose polymer. These polymers can be cross-linked through the radical reactions of deep-frying. As the surface of the potato is now more cohesive and able to withstand greater weight, the surface becomes harder – illustrated in a video by AkzoNobel.
Hopefully, you now have an appreciation of the fascinating chemistry that goes into making each and every crisp in your lunchtime packets!