If humans ever voyage to a planet far bigger than Earth, the journey is sure to be arduous and full of danger. But there’s a consolation: french fries cooked at the planet’s surface will be crispier.
That’s one way of interpreting new research investigating how unusual gravity changes the physics of deep-frying. It may be a slightly glib way of reading the results, 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. That’s why their work is supported by the European Space Agency.
Preparing food in space presents unique challenges. Apart from the obvious difficulties of floating crumbs, liquids and peelings, the basic physics of cooking is different. For example, in zero gravity there’s no convection in hot fluids to redistribute the heat, so they experience highly localised heating unless you stir. Preparing drinks like percolated coffee is a challenge, because there’s no gravity to pull the water down through the granules. And if you wanted to cook with a naked flame – perhaps unlikely inside a space station – the shape of the flame would be compact and round, rather than elongated and tear-shaped.
Given the constraints, plenty of thought has already gone into methods of improving food preparation in space, to enhance tastiness and healthiness while keeping fuss and waste to a minimum. Fully sealed food preparation units have been proposed that dispense with the need for a human chef to do any chopping or squirting altogether.
Still, astronauts sometimes lament the drabness of their pre-prepared space meals, and have even expressed cravings for fries.
That’s part of the reason why Lioumbas and Karapantsios decided to study deep-frying in space. In their initial experiments, they decided to cook in increased gravity rather than zero gravity, because 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, convection is an essential part of the process. The rate at which foods heat up in water or oil is affected by the way hot liquid circulates. On Earth, the hot liquid at the base of a pan rises because it’s less dense than the cooler liquid above. Yet this convection won’t happen in zero gravity. 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 fries), artificially increasing the force of gravity is relatively easy. You simply attach the apparatus to the arm of a rapidly spinning centrifuge.
Lioumbas and Karapantsios fixed a deep-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 of up to 9g – nine times that at the Earth’s surface.
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.
The pair found that the crust steadily thickens up to 3g, becoming crispier – and the chips fry faster. Beyond 3g, however, 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 fries (or anything else) to deep-fry faster, making them crispy in a shorter time, cooking in a bit of extra gravity helps, but there’s nothing to be gained, and in fact some disadvantages, from centrifuging to a force greater than 3g. This could be worth knowing for the food industry, where centrifugal “flash-frying” might be worth a try.
And crucially, by confirming the importance of convection in the frying process, the work suggests that fries cooked in zero g could end up a soggy flop, with barely any crust. That has yet to be confirmed in subsequent experiments, but for now it looks as though fries in space are off the menu.