Fish oil is made by cooking whole fish or fish parts, pressing the liquid out, and then spinning it in a centrifuge to separate the oil from the water. That basic process, called wet rendering, produces the world’s supply of crude fish oil. But crude oil is just the starting point. Most fish oil you’d buy as a supplement goes through several additional rounds of purification and concentration before it reaches a capsule or bottle.
Raw Materials: What Goes In
Fish oil can be produced from whole fish or from trimmings left over after fish processing. Small, oily species like anchovies, sardines, mackerel, and menhaden are the most common whole-fish sources because they’re abundant, reproduce quickly, and have high fat content. Trimmings from larger fish, including heads, frames, and viscera from salmon or pollock processing, are also widely used. The Food and Agriculture Organization notes that using these byproducts transforms what would otherwise be waste into a nutrient-dense product.
Wet Rendering: The Core Extraction
Nearly all commercial fish oil is made through wet rendering, a method with three main stages: cooking, pressing, and centrifugation.
Large fish are first chopped into smaller pieces, while small fish (generally under 40 cm) go straight into the cooker. The cooker uses indirect steam to heat the fish mass, which does three things at once: it coagulates the protein, ruptures the fat cells, and releases oil along with water that was physically or chemically bound inside the tissue. Think of it like cooking ground meat in a pan and watching the fat pool up as the proteins tighten.
The cooked mass then passes through a strainer and into a twin-screw press, which squeezes out most of the liquid. What’s left behind is called presscake, a solid containing 60 to 80 percent of the oil-free dry matter (mostly protein and bone). This presscake gets dried into fish meal, a major co-product used in animal and aquaculture feed. The liquid that comes out, called press liquor, contains water, dissolved proteins, and the oil.
To isolate the oil from that liquid, manufacturers use centrifuges. A first pass through a decanter centrifuge removes fine solid particles. A second centrifuge then separates the oil from the water based on differences in density. The speed of separation depends on temperature and viscosity, so the liquor is typically kept warm. Industrial centrifuges can process anywhere from 500 to 25,000 liters per hour. The result is crude fish oil, a dark, strong-smelling liquid that still contains impurities.
Refining: Cleaning Up the Crude Oil
Crude fish oil contains free fatty acids, pigments, trace metals, and environmental contaminants like PCBs and dioxins. Refining removes these in stages.
Neutralization is the first step. The oil is mixed with an alkali solution, typically sodium hydroxide (lye), which reacts with free fatty acids and converts them into soapstock that can be washed away. This step also pulls out some phospholipids and trace metals.
Bleaching follows, using absorbent clays that bind to pigments and residual impurities. The clay is filtered out, leaving a lighter-colored oil.
Deodorization strips out the volatile compounds responsible for fish oil’s strong smell and taste. This is essentially steam distillation under vacuum: steam is passed through the oil at high temperatures (around 220°C) for roughly 60 minutes. The vacuum keeps the pressure low so the odor compounds evaporate without requiring extreme heat that would destroy the beneficial fats. Research on croaker oil found that optimal deodorization used 5% steam by oil mass at 220°C for 60 minutes.
Removing Contaminants
Standard refining handles most impurities, but persistent pollutants like PCBs, dioxins, and heavy metals need more aggressive treatment. Short-path distillation (also called molecular distillation) is the industry standard for this. The oil is spread into an extremely thin film inside a heated evaporator, typically running between 170°C and 230°C, under deep vacuum created by oil diffusion pumps and cold traps cooled to minus 25°C. Because the oil film is so thin, each molecule spends only seconds at high temperature. The contaminants, which are heavier, separate from the lighter omega-3 fatty acids.
This step is critical for supplement-grade oil. The finished product must meet strict limits for oxidation markers: the industry standard set by the Global Organization for EPA and DHA Omega-3s (GOED) caps peroxide value at 5 mEq/kg, p-anisidine value at 20, and a combined oxidation score (called TOTOX) at 26.
Concentrating Omega-3s
Crude fish oil from anchovies or sardines typically contains about 18% EPA and 12% DHA. Many supplements advertise concentrations of 50%, 60%, or even 90% omega-3s. Reaching those levels requires additional processing.
The most common method is converting the oil’s natural triglyceride form into ethyl esters. This involves reacting the oil with ethanol, which breaks the fatty acids free from their glycerol backbone and attaches them to ethanol molecules instead. Once in ethyl ester form, the individual fatty acids can be separated more easily because they have slightly different boiling points and molecular weights. Manufacturers use molecular distillation or a process called supercritical CO2 extraction to isolate the EPA and DHA fractions and discard the less desirable fatty acids.
Some products are sold in ethyl ester form. Others go through one more step called re-esterification, where the concentrated EPA and DHA are reattached to a glycerol backbone, converting them back into triglycerides. This final form is closer to the oil’s natural structure and may absorb slightly better in the gut, which is why some brands market “re-esterified triglyceride” or “rTG” on the label.
Preventing Oxidation During Production
Omega-3 fatty acids are highly vulnerable to oxygen. Exposure causes them to break down into compounds that taste rancid and may reduce the oil’s health benefits. Manufacturers combat this at every stage.
During processing, antioxidants like tocopherols (a form of vitamin E) are added to the oil. During storage and bottling, nitrogen blanketing is the standard practice: the headspace in tanks and bottles is flushed with nitrogen gas to displace oxygen. A blanketing regulator mounted on the storage tank maintains a set nitrogen pressure, automatically adding more gas whenever the pressure drops. This keeps the oil in a nearly oxygen-free environment from the moment it’s refined until the bottle is sealed.
Capsule encapsulation serves a similar protective function. Soft gelatin or fish-gelatin shells limit the oil’s exposure to both air and light.
What Happens to the Rest of the Fish
Fish oil production is not a one-product operation. The presscake left after pressing is dried and ground into fish meal, which is one of the most protein-dense feed ingredients available for livestock, poultry, and farmed fish. In some operations, trimmings and byproducts that aren’t suitable for meal are turned into fish silage by adding acids or enzymes that break the material into a liquid mix of hydrolyzed proteins, fats, and minerals. Fish silage is easily digestible for both land and aquatic animals, and its production helps ensure that very little of the fish goes to waste.