Recycled Restaurant Grease in Pet Food, by Chelsea Kent

Once upon a time, “by-products” from human food meant bruised fruit or half eaten steak. Today, food production involves machinery lubricants, flavor enhancers, chemical stabilizers, food coloring agents, nitrates, sulphites, benzoates, and much more. Farmers used to scrape food straight off the dinner plate into the dogs’ bowl, or throw the cat a chicken egg that was already broken from the coup. Today, high-heat processed kibble and canned pet foods are able to use rancid, expired, chemical-laden and otherwise contaminated “by-products” of human food production.  They are now able to advertise these “foods” as “sustainable” and “palatable” for pets – with no consideration for health ramifications.


French fries, chicken strips, cheese sticks and other fried foods are fried in oils. These are unsaturated oils are from canola, soybeans, cottonseeds, corn, sesame, sunflower seeds, and safflower. These oils are high in calories, increasing the calories of 100 grams of potato from 93 to 319 when fried. (1) Trans-fats are produced in high levels when unsaturated fats are hydrogenated (heated to very high temperature). Science links trans-fats to heart disease,(2) Diabetes,(3) and Obesity.(4) Higher cooking temperatures creates more oxidation and more trans-fats. Hydrogenated vegetable oils contain more than 5% trans-fat (5). In order to decrease the risk of cardiovascular disease, the FDA recommends 0% trans-fats from hydrogenated oil sources in the diet (6).


Some say that trans-fat do not harm health and only improve palatability and acceptance of feeds.(7) Yet, comprehensive research on the topic shows that trans-fats, even in mild or moderate levels, do cause health damage in dogs and cats. In dogs, Pancreatitis, Cushing's Disease and Kidney Diseases are shown to be caused by elevated VLDL-associated triglycerides from trans-fat consumption. In cats, disorders associated with these fats include Diabetes Mellitus and Nephrotic (Kidney) syndrome. Scientists state, “while cats and dogs are spared the risk of atherogenesis and coronary artery disease, …many of the dyslipoproteinemias seen in dogs and cats appear to be similar to those observed in humans… (including) hypothyroidism, diabetes mellitus, and pancreatitis.”(8)


Used cooking oil is dumped in bins outside the back doors of restaurants nationwide. This oil can legally be used as an ingredient in your pets’ food:

AAFCO Official Publication, 2020, pages 386-387 (9):

"33.21 Yellow Grease, Feed Grade, is the rendered product from the tissues of mammals and/or poultry blended with used cooking or frying oil from human food preparation, consisting of animal and/or vegetable fats or oils..." and

“33.24 Used Cooking Oil, Feed Grade, is the product of used cooking or frying oil from human food preparation, consisting of animal and/or vegetable fats or oils, collected from commercial human food facilities and then heated to reduce moisture..."


Next to Hero’s Pets there are 3 bins for fried sludge, one behind each restaurant (usually in the smoking area). They sit outdoors, with no safety screens on them to prevent wild animals, rodents or insects from falling in. They are exposed to the elements and picked up only once every 6 months. The bin closest to Hero’s Pets has a sticker on it that says, “Darling Ingredients.” JBS,(10) Cargill,(11) Griffin Industries,(12) and Darling Ingredients (13) are just a few of the rendering companies that supply pet and livestock feed with this sort of recycled waste. 


We looked into Darling Ingredients.(14) Their website states that they are an, “animal food manufacturing company.” The Chairman and Chief Executive Officer of Darling Ingredients since 2003, Randall C. Stuewe, was the Keynote Speaker at AAFCO's 2020 Mid-year meeting. (15) Their website will show you green, clean looking imagery with cute puppies and beautiful children.

The sector of Darling Ingredients that handles pet feed ingredients and biodiesel ingredients is called Dar Pro Ingredients. Their slogan is, “Transforming your grease and meat by-products into valued feed and fuel ingredients.”(16) They also claim, “DarPro Solutions, part of the Darling Ingredient Family, repurposes used cooking oil and meat byproducts across the United States into renewable biofuels, animal feed ingredients and other household and industrial resources.”(17)

From DarPro Ingredients you can click on “Pet Food Ingredients.”(18) Click “Feed Grade Proteins & Fats” to see that they offer Yellow Grease, Used Cooking Oil, and other ingredients for animal feed use. These ingredients would be illegal to use in human foods.(19)

The Safety Data Sheet created by Darling Ingredients for Yellow Grease states, “Recommended Use: Feed/Pet Food Ingredient, raw material for the manufacture of Chemicals/Biofuels.” And “Synonyms: RCO (Recycled Cooking Oil), Restaurant Grease, Kitchen Grease, Rendered Animal Fat.” And “not to be landfilled. Do not flush to sewer.” (20)


Are there additional contaminants in this type of product?

Yes. Obviously every manufacturer works to ensure that their product does not pose any immediate threat to life or health. They could no longer sell their products if they did. In the case of Yellow Grease it is "made safe" through a process of:

- Water removal

- Sifting of large particles (bone, hair, metal, undecomposed parts of insects, rats, feral cats, etc)

- Heating at high temperatures to sterilize (which causes further oxidation and production of more trans-fats)

- Adding carcinogenic ethoxyquin(21)

- Adding phosphoric acid

- Adding carcinogenic t-butylhydroquinone. T-butylhydroquinone is more toxic than arsenic. T-butylhydroquinone toxicity = 700mg/kg. Arsenic toxicity = 763mg/kg. (22)(23)(24)

Employees of some rendering facilities state that their ingredients are production areas are contaminated with “buckets of dead rats… live rats, a lot of rat droppings… (and) maggots.”(25)

Also, high-temperature cooking results in higher concentrations of acrylamides(1) which are linked to: (26)

- Kidney cancer

- Reproductive tract cancers

- Excessive perspiration

- Muscle weakness

- Tremors

- Organ damage (liver, lungs, kidneys, central nervous system)

- Thyroid neoplasms


Of course, there are plenty of accepted foods and chemicals that are able to match aspects of the toxic profile of Yellow Grease as an ingredient. However, a human is able to choose that risk. Hopefully nobody is knowingly choosing those foods or chemicals as an exclusive food source. In the case of most pets, there is rarely rotation away from any single contaminant or ingredient. Especially since so many high-heat processed brands of food use the same sources for ingredients.


"Yellow grease" is already used to make Biofuels. However, the Energy Information Administration (EIA) states, “Due to competition from these other industrial sectors (feed livestock, soap manufacture, makeup, clothes, rubber and detergents)… less than a third of yellow grease could be spared for biodiesel production annually."(27)

Biodiesel is nontoxic, renewable, and biodegradable. It can be used in cars, railways, aircrafts, as heating oil, for cleaning oil spills, and in biodiesel generators. Biofuels have the ability to essentially eliminate smog, ozone and sulfur emissions. Biodiesel also use reduces poisonous carbon monoxide, unburned hydrocarbons and particulate matter. Biodiesel indisputably improves environmental health.(28)


Yellow Grease is far from the worst waste ingredient used in high-heat processed pet food. However, we should use it as sustainable fuels rather than disease-inducing ingredients in pet food. It is the responsibility of pet food consumers to reject products made with this profitable waste. Every company is trying to market their products as "good," "beneficial," and "safe." Manufacturers of high-heat processed, shelf-stable junk foods use attractive marketing to do so. Unregulated marketing such as the word "supreme" or "natural" is ubiquitous. Pretty stock photos offset the public perception of feed grade products. The public's love and support of "green" things is leveraged with misleading advertising. It is an incredibly important lesson in the conditions we live in as consumers.


The world we live in now requires, perhaps more than anything, that we become wise to the intents and purposes behind marketing and branding. Since the beginning of Hero's Pets we've been digging for hidden truths. The grease dumpsters that you can find behind every restaurant in America provide a little insight into the gap we're trying to close between perception and reality every single day.


  1. Healthline – Why Are Fried Foods Bad For You?
  2. New England Journal of Medicine – Effect of Dietary trans Fatty Acids on High-Density and Low-Density Lipoprotein Cholesterol Levels in Healthy Subjects -
  3. NCBI PubMed – Health Effects of trans fatty acids -
  4. Oxford Academic, American Society for Nutrition, Journal of Nutrition – Consumption of Trans Fatty Acids is Related to Plasma Biomarkers of Inflammation and Endothelial Dysfunction -
  5. NCBI PMC Toxicological Research, Analysis of Trans Fat in Edible Oils with Cooking Process -
  6. AccessData, Trans Fat can increase the risk of developing cardiovascular disease -
  7. Timely Topics in Nutrition, American College of Veterinary Nutrition, Facilitative and functional fats in diets of cats and dogs -
  8. Timely Topics in Nutrition, Lipoprotein-mediated transport of dietary and synthesized lipids and lipid abnormalities of dogs and cats -
  9. AAFCO OP 2020, page 386-387, “Yellow Grease”
  10. JBS Rendering -
  11. Cargill -
  12. Griffin Industries -
  13. Darling Ingredients -
  14. Darling Ingredients main page –
  15. AAFCO 2020 Mid-Year Meeting, Albuquerque – Darling Ingredients Keynote Presentation -
  16. Dar-Pro Grease Use Statement -
  17. DarPro Solutions -
  18. DarPro Pet Food Ingredients -
  19. DarPro Feed Grade Proteins & Fats -
  20. Darling Ingredients SDS, Yellow Grease -
  21. Ethoxyquin MSDS -
  22. t-butylhydroquinone MSDS -
  23. Arsenic MSDS -
  24. NIH PubMed – Antioxidant for Yellow Grease -
  25. Truth About Pet Food, Pet Food and Rendering Plants -
  26. Wiley Online Library, Acrylamide (MAK Value Documentation) -
  27. Wikipedia – Yellow Grease -
  28. Iowa State University – The effect of biodiesel oxidation on engine performance and emissions -