Where’s the physics in making the humble potato crisp that we all know and love?
The manufacturing process is based on a simple kitchen process – take a whole potato, wash, peel, slice, fry for three minutes, and season. But there’s much more science involved than at first glance. British consumers eat 10 million packets of Walkers crisps a day so ensuring that every crisp is the same, great quality – every chip, every bag, every day – is a formidable challenge.
The challenge starts with an inherently bio-variable natural raw material – the potato. Only specially selected British chipping potatoes are used by Walkers, such as Lady Rosetta, Hermes and Marquees, chosen for their great flavour and appearance, disease resistance and crop yield as well as storage capability – the fresh crop season is only four to five months long, so all-year crisp production requires potatoes that keep in good condition in temperature-, humidity-, CO2– and light-controlled long-term storage.
Great flavour requires in-depth understanding of potato biochemistry, such as enzyme inactivation to avoid an unappealing earthy flavour, the role of sugars and Maillard reactions for colour formation, and lipid oxidation in the frying oil. Meanwhile, getting the right texture requires deep understanding of the starch transitions during the dehydration process.
Starch in potatoes starts in its native crystalline form, and, during dehydration, passes through its rubber melt phase, and finally through a glass transition. If the correct starch transformation during dehydration is achieved, we get the light, crispy texture that consumers expect. The soft-matter phase of starch (ie, the melt phase) is critical – understanding internal forces of steam-vapour pressure versus starch yield stresses, the sensitive time/temperature glass transition region, and oil-uptake kinetics are all required to create the desired texture.
Once we understand the physics of the flavour and texture generation, then the dehydration process can be designed to deliver the correct time-temperature profile, slice agitation in the fryer oil and take-out conditions as the slices leave the oil.
Potato slicing is one of the most critical operations. To reduce stress cracking, which can cause in-bag breakage, considerable computer simulation effort has gone into the slicer blade and slicing process design (eg the blade bevel angle or the slicer rotation speed). Slicing also impacts surface roughness, which is a factor in oil uptake during frying.
Sophisticated 3D imaging techniques are used, such as high-resolution X-ray CT scanning, to understand where the oil in a fried potato slice is located relative to the potato starch cells (which impacts mouth feel and perception of succulence) and pore-size distribution (which affects the texture).
What the consumer sees in and eats from a packet of Walkers crisps is simply great texture and flavour, every crisp, every bag, every time – but the scientific understanding behind this simple pleasure is anything but simple.
- This an extract from the IOP report The Health of Physics in UK Food Manufacturing, due to be launched at PespiCo on 21 October. Follow the launch event on Twitter.