Calcium sulfate is a naturally occurring mineral used in food as a firming agent, coagulant, stabilizer, and calcium supplement. Its chemical formula is CaSO4, and in nature it exists as gypsum, a soft mineral found in sedimentary rock deposits worldwide. Food-grade calcium sulfate is purified and added to a wide range of products, from tofu to canned vegetables to flour, where it serves different functional purposes depending on the food.
How It Works in Food
Calcium sulfate plays surprisingly varied roles depending on what it’s added to. In tofu production, it acts as a coagulant that curdles soy milk into solid curds. In canned vegetables, it keeps the texture firm so they don’t turn to mush during processing. In baked goods, it can function as a leavening agent or dough conditioner. It also shows up as an anticaking agent in powdered products, a pH regulator in processed foods, and a nutrient supplement that adds dietary calcium.
The FDA classifies it under more than ten functional categories, including drying agent, stabilizer, thickener, and fermenting aid. That versatility is why it appears on so many ingredient labels across very different types of food.
Its Role in Tofu
Tofu is where calcium sulfate plays its most iconic role. When added to hot soy milk, its calcium ions bind to soy protein molecules through what food scientists call “ion bridging.” The positively charged calcium acts as a connector between negatively charged protein groups, pulling them together into a solid network. This is the same basic principle behind cheese curds forming in dairy, just with soy protein instead of casein.
Calcium sulfate produces a distinctly smooth, fine-textured tofu compared to other coagulants. Research published in the journal Foods found that tofu made with calcium sulfate has a more uniform internal network with a smoother surface than tofu made with magnesium chloride (nigari), which is the other common coagulant. Calcium sulfate tofu also tends to be harder and firmer than most other calcium-salt tofus, and it retains more isoflavones, the plant compounds in soy that many people seek out for health benefits.
If a block of tofu lists calcium sulfate on the label, that’s also a signal it contains meaningful calcium. Some tofu products deliver a significant portion of your daily calcium needs specifically because of this coagulant.
Canned Vegetables and Other Uses
When vegetables are canned or processed at high temperatures, their cell walls break down and they lose their structure. Calcium sulfate counteracts this by reinforcing plant cell walls. The calcium ions cross-link with pectin, a structural carbohydrate in plant tissue, creating a firmer texture that holds up through heat processing. The United Nations food additive database lists it as a firming agent for canned and bottled vegetables including mushrooms, root vegetables, pulses, legumes, and seaweeds.
In brewing and winemaking, calcium sulfate adjusts the mineral content and acidity of water. Brewers have used gypsum (its natural mineral form) for centuries to harden water for certain beer styles. In flour and baked goods, it can improve dough handling and act as a mild leavening aid.
How Well Your Body Absorbs the Calcium
One practical question about calcium sulfate in food is whether your body can actually use the calcium it provides. A study in the American Journal of Clinical Nutrition tested absorption in young women and found that calcium from a calcium sulfate source was absorbed at a rate of about 23.8%, compared to 25.0% from milk. The difference was not statistically significant. The study also found no increase in calcium lost through urine from the sulfate component, meaning the sulfate doesn’t interfere with calcium retention.
This puts calcium sulfate on roughly equal footing with dairy as a calcium source. For people who avoid dairy, tofu made with calcium sulfate and calcium sulfate-fortified foods can be genuinely useful sources of this mineral, not just token amounts on a nutrition label.
How to Spot It on Labels
On ingredient lists in the United States, you’ll see it written as “calcium sulfate.” In Europe, it goes by its E-number designation, E516. You may occasionally see it listed as “food-grade gypsum,” particularly on tofu packages imported from East Asia. Its natural mineral forms are called anhydrite (the dry form) and gypsum (the form that contains water molecules), but those names rarely appear on consumer food labels.
It shows up most often on tofu, canned vegetables, enriched flour, some canned beans, certain cheeses, and baked goods. If you’re scanning a label and wondering why it’s there, it’s almost always serving as either a firming agent, a coagulant, or a calcium fortifier.
Safety Profile
Calcium sulfate has been used in food for centuries and has a clean safety record. The FDA lists it as Generally Recognized as Safe (GRAS) under Title 21 of the Code of Federal Regulations, section 184.1230. In Europe, the European Food Safety Authority re-evaluated E516 and concluded it poses no safety concern at reported use levels. EFSA found low acute toxicity, no concerns about cancer risk or genetic damage, and noted that typical dietary exposure falls far below the amount that would cause even mild digestive effects like loose stools (which only occurs at very high doses, around 300 mg per kilogram of body weight).
Because the safety margin is so wide, neither the FDA nor EFSA sets a numerical limit on acceptable daily intake. Both agencies classify it with an ADI of “not specified,” which in regulatory language means the substance is considered safe enough that setting a cap isn’t necessary.