There are two questions here:
I'd say the brief answers to these questions are (1) sometimes a bit, especially when not using "heavy duty" foil, and (2) perhaps responsible for a small part of the effect in some situations, though how much likely depends on the foil type/quality and how it is used. More details below.
First, aluminum foil can be porous when it is sufficiently thin. This is usually due to small holes left in the manufacturing process. According to Gordon Robinson's Food Packaging: Principles and Practice (3rd ed.):
Aluminum foil is essentially impermeable to gases and water vapor when it is thicker than 15 µm, but it is permeable at lower thicknesses due to the presence of minute pinholes. For example 12 µm thick foil has a WVTR [water vapor transmission rate] of ≤0.01 g m-2 day -1, and 8-9 µm thick generally of 0.07-0.1 g m-2 day -1 (Lamberti and Escher, 2007). These values are still far below those of most plastic films used for food packaging....
Other sources give slightly different numbers. The Wiley Encyclopedia of Packaging Technology lists:
0.001 in. (25.4 µm) and thicker is impermeable; 0.00035 in. (8.9 µm) has a WVTR of 0.02g/100in.2 (0.065m2); 24 h at 100°F (37.8°C)
How thick is common household aluminum foil? According to this link:
The first source above lists the minimum thickness for impermeability to be about 0.0006" (15 µm, though this may only be for gases, not liquids), while the second source says 0.001". Other sources seem to indicate liquid impermeability requires a minimum thickness of around 0.001". Regardless, it seems likely the "standard duty" foil has some permeability in most cases. On the other hand, "heavy duty" foil is likely completely impermeable (or very nearly so).
But how much is this permeability? At room temperature, it would only allow something like a hundredth of a gram through per day in a pan, probably less. At baking temperatures, permeability would increase, but you still would likely only see a fraction of a gram of liquid getting through during a typical baking period. And for heavy duty foil, the amount should be zero or negligible... at least theoretically.
To move on to the second question, so where does this liquid come from? I myself have seen such liquid even with a double layer of foil or with heavy duty foil.
Other answers have suggested one likely culprit, which is condensation. Ovens often tend to be humid places when uncovered foods are cooking, and food that is in contact with the bottom surface of a pan (and the foil) is generally still cooler than the oven (often the coolest part of the food is on the bottom), which means that humidity will condense on the surface of the pan/foil. It's usually a lot faster for molecules (even larger ones, which often "piggyback" on evaporating water) to move this way in a hot oven than through microscopic pores in the foil.
The other likely issue is imperfections created during the handling of the foil. A perfectly smooth layer of foil removed carefully from the roll may theoretically be impermeable, but bending, crumpling, and otherwise shaping the foil may expand or fracture some tiny pinholes (or other thin sections). All it takes is a few tiny holes (which may not even be visible) to significantly increase the permeability.
Actual leaking may be due to poor manufacture in modern foils or to the severe crumpling done in experiments like this one. (EDIT: See discussion in this thread, which responded to that link with several other experiments. The conclusion was that is was likely design that caused the leak, and some brands of heavy duty foil do seem impermeable to water.)
In any case, it's clear that aluminum foil -- when used practically in home culinary applications -- can sometimes allow some liquid to flow through at a greater rate than the theoretical porosity would suggest.
It’s almost summertime, which means backyard cookouts and long days spent in the sunshine grilling up burgers, hot dogs, and steaks. An outdoor grill infuses your meat with delicious smoky flavor, and washing it down with a cold beer or lemonade makes for the perfect meal. If you’re like most Americans, you probably use aluminum foil for easier cooking and clean-up; but is it actually safe for grilling? While this kitchen staple is convenient for barbecues, it has some potential downsides when it comes to the environment — and possibly, your health. Keep reading to learn about the risks of grilling with aluminum foil.
First, a little about this useful product’s history. Aluminum is a thin-rolled sheet of alloyed aluminum — 98.5 percent aluminum, to be exact, with the balance primarily from iron and silicon to give strength and puncture resistance. In the production process, the molten alloy is rolled thin and solidified between large, water-cooled chill rollers. Aluminum foil was first produced commercially in the US in 1913 for product packaging. (It was used to wrap Life Savers, candy bars, and chewing gum.) Now, you can find a roll in just about any household, as it’s extremely helpful in covering and storing foods. (Click through for 16 genius uses for aluminum foil.) It also makes a great pet toy, if you crumple it up into a ball and drop it near your cat. But should you use it on your grill?
Aluminum foil is great for convenience, but it’s not so nice to planet earth. That’s because it’s only recyclable when it isn’t coated in food residue, which is the opposite of its condition after grilling. You could, of course, rinse your foil and try to wipe it perfectly clean — but that isn’t convenient or efficient, which is why we used the foil in the first place. The result is that the food- and grease-covered aluminum we drop in the recycling bin not only ends up in a landfill, but it also contaminates the other recyclables in your bin.
For a more sustainable — and no less convenient — alternative, the pros at The Daily Meal recommend a reusable grill basket. Similar to the fry baskets at McDonald’s, they keep food contained so that it doesn’t fall through your grill’s grates. The baskets help with cleaning up, too, since you won’t need to scrape bits of meat off the grill and can simply toss the basket in your dishwasher. One to try: Weber’s Stainless Steel Grill Basket (Buy from Home Depot, $34.98).
Because aluminum is one of the most abundant metals on earth, small amounts are naturally occurring in most foods — including fruits, vegetables, meats, fish, grains, and dairy products, according to Healthline. Some of the aluminum you unknowingly end up ingesting also comes from processed food additives, like preservatives, coloring agents, anti-caking agents, and thickeners. Commercially produced foods with food additives are more likely to contain aluminum than home-cooked foods — so you’ve already gotten a healthy head-start by making a meal yourself.
The good news: Your body only absorbs very small amounts of most aluminum you ingest, so it’s rarely considered harmful. Still, cooking with aluminum foil could increase the aluminum content in your food. Studies show that aluminum foil, cooking utensils, and containers can all leach aluminum into food. (A study published in the journal Meat Science, for example, found that cooking red meat in aluminum foil could increase its aluminum content by between 89 and 378 percent.) The amount of “leaching,” or aluminum that passes into your food when cooking with aluminum foil, will be greater if you:
However, as Healthline concludes, there is currently no strong evidence linking the use of aluminum foil to an increased risk of disease.
While cooking with aluminum foil can increase the amount of aluminum in your food, the amounts should be small enough that they pass harmlessly through your body. Researchers have deemed cooking with aluminum foil safe. Still, if you want to help save the earth — the US is quickly running out of landfill space — you should buy a grill basket. Then, check out our stories on best practices for grilling vegetables, juicy steak, chicken, and fish or other seafood.
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