Here’s the initial version of my latest piece for the Under the Radar column of BBC Future.
If humans ever voyage to Jupiter, the journey is sure to be arduous and full of danger. But there’s a consolation: chips cooked at the planet’s surface will be crispier.
Perhaps that’s too glib a conclusion to draw from recent work investigating the effect of high gravity on chip frying (that’s French fries or frites outside the UK), not least because gaseous Jupiter of course doesn’t really have a surface and no one plans to go there. But the gastronomic preferences of future astronauts are the genuine motivation for experiments conducted by chemists John Lioumbas and Thodoris Karapantsios of the Aristotle University of Thessaloniki in Greece, and reported in the journal Food Science International. That’s why their work is supported by the European Space Agency.
You see, astronauts sometimes lament the drabness of their pre-prepared space meals, and have even expressed cravings for chips. Some thought has already gone into methods of food preparation in space (if you don’t want potato peelings floating around, it has to be done in a hands-free self-contained system), as well as developing novel sources of fresh food, such as the culturing of artificial meat. But aside from these logistics, there’s also the problem that in zero gravity some of the basic physics of cooking is different.
The wish for decent grub in space is understandable, but also highlights one of the conundrums of human spaceflight. The quest to send humans into space is generally presented in heroic terms as a bold adventure that might bring benefits for all humanity. But once you consider what it really entails, you’re confronted with some pretty prosaic, even bathetic, questions of detail. How will they cope with the boredom and confinement? Will the toilet facilities work? (To judge from the International Space Station, not necessarily.) And will a good fry-up raise their morale? Such questions sit uneasily with the “Columbus” narrative, and arguably might force us to ask whether space is such a good place to put humans anyway.
But back to the deep-fryer. You might wonder why, if we’re talking about chips in space, Lioumbas and Karapantsios are cooking in increased gravity rather than zero gravity. The answer is that they want to map out the whole landscape of how gravity influences the cooking process, to get some idea of the overall trends and patterns as the tug of gravity changes. They are now working on the same questions in microgravity experiments – gravity much weaker than that of the Earth.
For frying and boiling, the key issue is convection. The rate at which foods heat up in water or oil is affected by the way heat circulates in the liquid. This depends on the convection currents created by buoyancy, as hot and therefore less dense liquid rises from the bottom of the pan. This convection won’t happen in zero gravity, because a difference in density doesn’t produce a difference in weight if everything is weightless anyway: there’s no buoyancy. Conversely, in increased gravity convective effects should be more pronounced.
The researchers wanted to know how these differences affect the way chips fry. While achieving low gravity is difficult unless you go into space (or want to brave the free-falling ‘Vomit Comet’ aircraft used by space agencies, which is enough to put anyone off their chips), artificially increasing the force of gravity is relatively easy. You simply attach the apparatus to the arm of a rapidly spinning centrifuge, the rotation of which produces a centrifugal force that mimics gravity.
So that’s what Lioumbas and Karapantsios did. They fixed a deep-fat fryer containing potato sticks in half a litre of hot oil onto the end of the 8m-long arms of the Large Diameter Centrifuge at the European Space Research and Technology Centre in Noordwijk, the Netherlands. This device could generate the equivalent of a gravitational force up to nine times that at the Earth’s surface (that is, 9g).
The researchers monitored the temperature just below the surface of the potatoes, where the crust of the chip forms, and also examined the thickness and profile of the crust under the microscope. Convection currents are created both within the pan as a whole and from the rising of bubbles that grow on the potato surface as the oil begins to boil. As the g-force rises, these bubbles become smaller and more numerous and they rise faster. However, when it reaches 3g, the bubbles are so small that they get stuck to the potato by capillary forces, and so further increases in gravity make little difference.
What’s more, while the crust steadily thickens up to 3g, still stronger gravity has less of an effect on the thickness. Instead, Lioumbas and Karapantsios find that the crust then starts to separate from the softer core of the potato, as superheated steam from the moist potato flesh blows a bubble between the two. But who wishes to eat chips with bubbles in?
So the researchers conclude that if you want chips (or anything else) to deep-fry faster, making them crispy in a shorter time, there’s nothing to be gained, and in fact some disadvantages, from centrifuging to a force greater than 3g. That much is not really a lesson for space cooking, where in general gravity will be much lower than Earth’s (and anyway, on the International Space Station can’t you simply order a takeaway from Pizza Hut?). But it could be worth knowing for the food industry, where centrifugal ‘flash-frying’ might be considered worth a try.
Reference: J. S. Lioumbas & T. D. Karapantsios, Food Research International 55, 110-118 (2014).