You flip over the bag, scan the ingredient list, and see “chicken meal,” “fish meal,” or “animal fat” near the top. Most pet owners take that at face value — a protein source, a fat source, a nutritional building block. What the label does not tell you is how those ingredients were made, and why that process matters for the safety of what ends up in your pet’s bowl.

The answer starts at the rendering plant, a step in the pet food supply chain that almost no one talks about.

What Is Rendering, Exactly?

Rendering is an industrial cooking process that converts raw animal byproducts — slaughterhouse trimmings, fatty tissue, bones, offal, and in some cases entire carcasses condemned for human consumption — into shelf-stable ingredients: protein meals and animal fats.

Here is how it works in broad strokes. Raw material is ground to a uniform size, then loaded into large cookers. In dry rendering, the material is heated at high temperatures (typically 115°C to 145°C / 240°F to 295°F) until moisture evaporates and fat separates from protein and bone. In wet rendering, steam or boiling water is added to accelerate fat separation. The resulting fat is skimmed off and sold as tallow or “animal fat.” The solids are pressed, dried further, and milled into the protein concentrates that show up on labels as “chicken meal,” “poultry by-product meal,” “meat and bone meal,” or “fish meal.”

From a nutritional standpoint, rendering is efficient. It concentrates protein, preserves calcium and phosphorus, and produces a stable, affordable ingredient. What the industry rarely highlights is what else gets concentrated in the process.

How PFAS Enter the Picture

PFAS — per- and polyfluoroalkyl substances, also called “forever chemicals” because they do not biodegrade — are persistent environmental contaminants. They accumulate in animal tissues through a process called bioaccumulation, meaning the higher up the food chain an animal is, the more concentrated its PFAS load tends to be.

Fish are a particularly significant vector. Aquatic ecosystems have been heavily contaminated with PFAS from industrial discharge, firefighting foam runoff, and agricultural inputs. When fish absorb PFAS from contaminated water and then consume PFAS-carrying prey, the chemicals build up in their fatty tissue. That fatty tissue becomes fish meal or fish oil. Rendering concentrates it further.

Independent testing has confirmed the concern. Scientists analyzing 100 commercial cat and dog foods found 34 different PFAS chemicals, with fish-based recipes frequently registering concentrations above the European Food Safety Authority’s intake thresholds for humans. Fish-based wet foods sourced from Asia, the U.S., and Europe were among the worst offenders in studies focused on cats.

PFAS have also been detected in pet food packaging — the grease-proofing agents used to coat the insides of paper bags are a documented source. In January 2025, the FDA announced that 35 food contact notifications related to PFAS-containing packaging substances are no longer effective. And in February 2024, FDA confirmed that all PFAS-based grease-proofing agents had been voluntarily withdrawn from the U.S. market. Progress — but the ingredient-level contamination pathway through rendering remains an open question for ongoing research.

What does PFAS exposure mean for your pet’s health? In people, long-term PFAS exposure is linked to liver damage, certain cancers, immune disruption, and altered cholesterol levels. Early research in companion animals suggests associations with liver and kidney disease in dogs and cats, thyroid disruption, and respiratory issues, though the science is still developing.

The Microplastics Problem in Rendered Ingredients

PFAS are not the only contaminant the rendering process can concentrate. Microplastics — plastic fragments smaller than 5 millimeters, including nanoplastics smaller than 1 micrometer — are now found throughout the food chain, and rendered pet food ingredients are not immune.

Fish meal is a well-documented source. Research has found microplastic abundances ranging from roughly 210 to over 1,150 particles per kilogram in commercial fish meal samples, with fiber-shaped polyethylene and polypropylene fragments being most common. Fish meal produced from dried fish tends to carry higher microplastic loads than meal from fresh fish, because drying concentrates whatever plastic particles were present in the raw material.

A study published in Environmental Science & Technology found both polyethylene terephthalate (PET) and polycarbonate microplastics in U.S. commercial cat and dog foods, as well as in corresponding pet feces — indicating that pets are ingesting and passing these particles. More recently, microplastics have been detected in the internal tissues of dogs and cats, including lungs, kidneys, liver, and blood.

There is also an ironic packaging angle: the shiny inner lining of many kibble bags can shed microscopic plastic particles into the food it is supposed to protect. Highly processed foods — and kibble is among the most processed foods in any species’ diet — tend to carry higher microplastic loads than minimally processed alternatives.

The health effects of microplastics in companion animals are not yet fully characterized, but animal studies across species suggest potential for inflammatory responses, organ accumulation, and cellular disruption. Nanoplastics in particular are small enough to cross cell membranes and reach the brain.

Why the Rendering Step Matters More Than the Label

Labels tell you what ingredient was used. They do not tell you the quality of the raw material that went into rendering, the contamination load of that raw material, or how the rendering process itself may have concentrated contaminants alongside nutrients.

This is not an argument that rendered ingredients are inherently unsafe — they have been the backbone of commercial pet food for decades, and rendering does destroy pathogens and extend shelf life. The argument is that rendering is a concentration point in the supply chain, and that contamination entering the process can exit in a more concentrated form.

For fish meal in particular, the combination of aquatic PFAS accumulation and microplastic saturation in marine and freshwater environments means the raw material arriving at the rendering plant may already carry a significant contamination burden. High-heat processing does not destroy PFAS. It does not eliminate microplastics. It can, in fact, reduce moisture and volume in ways that increase the concentration of both on a per-gram basis.

What Pet Owners Can Do Right Now

Diversify protein sources. Rotating between multiple proteins reduces overexposure to any single contamination pathway. If fish-based food is a staple in your pet’s diet, consider the sourcing region and the manufacturer’s quality testing practices.

Look for brands with third-party contaminant testing. Some manufacturers test finished products for heavy metals, PFAS, and other contaminants and publish the results. That kind of transparency is worth rewarding.

Consider the packaging. While the FDA has moved to phase out PFAS in food contact materials, transition timelines vary by manufacturer. Brands moving to uncoated or alternative packaging materials are worth noting.

Ask harder questions. Supply chain transparency is a legitimate due diligence question, not just a marketing talking point. Where does the fish meal come from? Does the manufacturer test finished products for PFAS? Do they test for microplastics?

Stay current on testing data. Organizations like the Environmental Working Group (EWG) and independent researchers continue to test commercial pet foods. Following those reports gives you the best available real-world data as the science evolves.

The Bigger Picture

The rendering process is not going away. It is fundamental to the economics of commercial pet food, and it serves legitimate waste-reduction functions within the broader agricultural system (and businesses love it for the economics). But understanding it as a contamination concentration step, not just a protein concentration step, changes the questions a thoughtful pet owner should be asking.

Ingredient labels were designed for a world where the primary concern was nutritional composition. The emerging science on PFAS and microplastics in the food supply points toward a world where contamination chemistry matters as much as protein percentages. The supply chain most pet owners never consider is where that chemistry is actually determined.

Your pet cannot read the label. You can. And now you know what the label leaves out.

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