Where the Oil Goes: Fat Uptake in Deep Frying

Most of the oil a fried food ends up carrying goes in after it leaves the fryer, not while it’s in the oil. While the food is still steaming, its escaping water keeps oil at the surface; once it comes out and cools, a pressure difference pulls oil inward through the small pores of the crust. How much gets pulled in depends largely on the kind of crust the frying built.

This is the second post in the lab’s frying arc. Why Fried Things Go Soggy: Moisture Migration followed water moving into a fried shell from a sauce. This one follows oil moving into a samosa shell or a pakora, because the job maida or besan is doing in the fryer includes building the crust that decides how much oil comes along with every bite.

Steam keeps the oil out, for a while

Hot oil turns the water inside a food to steam, and that outward rush of vapor is what keeps the surrounding oil from getting past the surface. As long as water is actively leaving, it wins the contest for the food’s pores.

That protection has a limit. Once a piece has been in the oil long enough that most of its water has escaped, oil starts working its way in deeper, before the food is even out of the fryer. Overfrying doesn’t only risk burning a pakora; it opens the door the steam was holding shut.

Most of the oil goes in after the fryer

The obvious assumption is that a food soaks up oil while it cooks. Research that used high-speed imaging on frying dough found otherwise: most of the oil is absorbed during the cooling period right after the food comes out, with only a smaller share taken up while it’s still submerged.

The driver is a pressure effect, not a passive soak. As the food cools, a pressure difference builds between its inside and the air around it, and that difference creates suction. The suction pulls oil in through the crust’s small pores in particular, so the crust isn’t just the crispest part of the food; it’s the part doing the pulling.

The crust decides how much

How much oil that suction can move depends on the crust frying built. A hotter frying temperature produces a harder crust with small pores, and those pores draw in more oil by capillary action (liquid being drawn into narrow spaces) once frying stops. A cooler frying temperature leaves a softer structure that takes up less oil.

So the crust a contender builds in the oil is quietly deciding how much oil it will pick up on the rack. Two shells that spend the same time in the fryer can end up carrying different amounts of oil for reasons that have nothing to do with how long they fried.

What a gum in the coating can do

If the crust’s pores are the pathway, narrowing them is the lever food scientists pull. A study on fried samosa added hydrocolloid gums (water-binding thickeners) such as xanthan gum and carboxymethylcellulose, or CMC, to the coating, and found the oil content dropped as more gum was added.

Xanthan gum gave the biggest reduction in that study, cutting oil uptake by roughly half compared with an uncoated control, with CMC close behind. The likely reason is a film the gum forms on the product, which both limits how much oil gets in and acts as a barrier to moisture leaving the coated food.

What draining catches, and what it doesn’t

A fry basket that strains food as it lifts it out, or a draining rack such as the Japanese abura-kiri, lets oil clinging to the surface run off by gravity. That’s a real mechanism, and it’s why those tools exist.

But draining catches a different pool of oil from the one described above. Surface oil can run off; oil that the cooling suction has already drawn inward through the crust’s pores can’t. By the time a piece looks drained, some of that inward pull has already happened.

What this means for the lab’s Method Showdowns

This predicts that the fryer isn’t the only place a contender’s final oil content gets decided. For the Papdi Chaat crisps, Samosa pastry and Pakora batter Showdowns, a reading taken the moment a piece leaves the oil would catch it partway through, not at its endpoint.

It also predicts that a contender that builds a harder, more porous crust at a given oil temperature should carry more oil once cooled than one that builds a softer structure, whatever their first-bite texture. And it reframes what a gum added to a coating would be doing: not just holding a shape, but narrowing the same pores the cooling suction is going to use.

The short version

• Escaping steam keeps oil at the surface while a food is frying, but that protection fades once most of its water has gone.
• Most of the oil goes in during cooling, pulled through the crust’s small pores by a pressure difference, and a hotter fry builds a crust that pulls in more.
• A hydrocolloid gum in the coating cut oil uptake in a fried-samosa study, with xanthan gum cutting it by roughly half, while draining only removes the oil still on the surface.

Caveat: these mechanisms come from food-science research on fried dough, deep frying in general and a fried-samosa coating study, not from any measurement of a keto besan or almond-flour fritter. How much oil a specific keto contender picks up while cooling, and whether a keto-friendly thickener would do the barrier job xanthan gum or CMC did in that study, is what a kitchen test would have to measure. This is directional kitchen math, not a result and not medical advice.

Next: Acrylamide in Fried Snacks: What It Is and Where It Comes From — the frying arc moves from what goes into a fried crust to a compound that forms in it.

Sources

• Deep frying — Wikipedia: how steam escaping a food resists oil penetration during frying, how that resistance breaks down with overfrying, and the fry basket and abura-kiri as tools built to drain surface oil.
• KU Leuven researchers capture how oil is absorbed in our deep-fried snacks — KU Leuven: most oil absorption happening during post-frying cooling rather than during frying, the pressure-driven suction effect pulling oil through crust pores, and how frying temperature changes crust porosity and oil uptake.
• Scientists find ways to reduce oil intake in deep fried foods — Lightsources.org: independent confirmation that most oil uptake happens during the cooling of fried dough rather than during frying.
• Effect of hydrocolloids on the oil uptake and quality characteristics of fried samosa — Journal of Food Science and Technology (PMC): hydrocolloid film formation as the likely mechanism behind reduced oil uptake, the moisture-barrier effect of hydrocolloids, and the oil-uptake reduction from added xanthan gum and CMC.