The Most Dangerous (hidden) Ingredient In Almost Every Pet Food

This ingredient could potentially be the actual cause of diseases like DCM (heart disease), cancer, obesity, and diabetes in pets

If you’re like most people, you chose your pets food with some level of care. Perhaps, you trust it because you had a conversation with someone that seemed like an authority on it. Perhaps you trusted the advertising or labeling. Perhaps you trust it due to familiarity – your family has used it for years. Most certainly, you wouldn’t knowingly choose a product that contains “trash” that you know to be dangerous and damaging to health. However, you may be surprised to find that even if you’re buying an “expensive,” “designer,” or “natural” food – you might be doing just that. Especially mass-produced products that are capable of selling to thousands of locations nationwide at reasonable costs, the following ingredients may be dangerously deceiving you and damaging your pets’ health:

  • Aspic
  • Gravy
  • Any kind of “Meal”
  • Any kind of “Fat,” or “Animal Fat” (including pork fat, beef fat, turkey fat, chicken fat, etc)*
  • Any kind of “Oil,” “Vegetable Oil,” or “Vegetable Fat,” “Hydrolyzed Fat,” or “Hydrolyzed Oil.” (including safflower oil, canola oil, sunflower oil, corn, olive, rapeseed, etc)*
  • Fish Oil (16)

Flip over your pets’ packages of food and see if those are listed.  You might think, “Sure, I see it… but that doesn’t seem that bad.”  Here’s how you may be getting tricked, and how your pets health may be suffering as a result of it.  This ingredient may one of the primary culprits for heart diseases, cancers, and obesity in American pets.
(WARNING: gross and upsetting facts and imagery about pet food below. Please read through our references to show that the manufacturers of these products are very, very clear that this disgusting trash is used primarily in your pets’ high-heat processed kibble and canned pet food!)

We first want to acknowledge that humans have been feeding garbage to dogs and cats since they were first domesticated. It wasn’t such a bad idea before factory farming came along. On a family farm pet food was usually nutritious organs, excess vegetables, broken eggs and milk that soured yesterday. Food went straight from the dinner table to the bowl. You could see every ingredient. In modern times, food industries grew and so has the processing. Human food manufacturers, expired grocery meat, restaurant trash, factory farms, and breweries all contribute waste products to the pet food industry, often weeks after discarding.
Waste has to be rendered. Rendering plants do a great service by grinding up our trash, cooking it at high-heat, and stabilizing it with chemicals.  They care very little about the history of the waste before it gets to them.  Their job is to “recycle” it into something that can be used elsewhere.  As a matter of fact, in many cases it is illegal to discard these wastes in landfills because they are too toxic. You can see this on Safety Data Sheets that make statements like, “not to be landfilled. Do not flush to sewer … Recommended Use: Feed/Pet Food Ingredient, raw material for the manufacture of Chemicals/Biofuels.”(1) Additionally, millions of pounds of food and agricultural wastes in landfills results in an increased rodent and pest population – thus spreading disease.

Now you’re probably thinking, “BIOFUELS?! That’s great that our waste is being used a Biofuel!”  Sadly, the Energy Information Administration (EIA) states, “Due to competition from these other industrial sectors (livestock and pet feed, soap manufacture, makeup, clothes, rubber and detergents)… less than a third of yellow grease could be spared for biodiesel production annually.” (2) Yellow Grease is what we want to talk to you about because it is very likely to be in your pets high-heat processed kibble or canned (even many “natural” and “high quality” brands).

“Yellow grease” starts in a restaurant fryer. It is often delivered by the same company that picks it up for recycling/rendering. (3)(4) It’s delivered in the form of a pretty golden oil, like Canola or soybean to be used in restaurant fryers at temperatures as high as 600*F. (5)(6) These oils may be Hexane extracted, steam distilled, refined, high-heat bleached (7) and treated with TBHQ, (8) ethoxyquin (9) or silicone anti-foaming agents. (10)(3) Oils/Fats are used for frying French fries, chicken strips, potato chips, shrimp, cheese, onions, clams… meat and veggies alike are dipped and dropped into the oxidized grease, which is eventually discarded in dirty grease bin out back when they are too old and dirty to use anymore. The grease dumpsters are tucked in back, often under lock and key, where customers don’t have to see or smell a rancid bin of congealed grease while eating their delicious fried lunches.

It gets a little technical (and more graphic) from here on…. (Thank you to everyone who contributed questions and criticism on Facebook which helped us flesh out the details on this topic.)

These oils are high in calories, increasing the calories of 100 grams of potato from 93 to 319 when fried. (11) This makes them a valuable commodity to the pet and agricultural food industry.  Oils/Fats are an inexpensive source of calories that can be added to animal feeds to make them more affordable to consumers.

At this point you’re probably thinking what most people are:

  1. There’s no way that MY pets food uses this stuff
  2. There’s no way this stuff is even legal in pet food
  3. If it IS used in pet food, they must do something to it to make it safe – I mean, they can make clean water out of poop, right?

We’re going to address exactly these things and you’re not going to like the outcome – but it’s imperative if you want your pet to avoid heart disease, obesity and cancers.



The Chicago Tribune posted an article regarding oils used for frying at McDonald’s, stating, “the company’s 13,700 U.S. restaurants alone use more than 75 million pounds of oil each year to prepare the chain’s french fries, chicken McNuggets, chicken strips, and fish fillets.” (12)

There are over 660,700 restaurants in the United States. (13) If we assumed that each used as much oil/fat for frying as each McDonald’s location uses annually then there is still over 3.6 TRILLION pounds of used cooking oil/yellow grease available for “recycling” in the United States annually. As stated above, the EIA says that 2/3rds+ of that (or 2.4 trillion+ pounds) is used in the manufacture of 6 other industrial products, with pet and livestock feed topping the charts for use. If each sector used an equal amount of recycled oil, pet food alone could use 397,866,788 pounds a year.

Mars Petcare (Pedigree, Iams, Greenies, Eukanuba, Royal Canin, Cesar, Nutro, Sheba, FreshPet, California Naturals, Crave, Innova, Karma, Kumpi),

Nestle Purina PetCare  (Purina, ONE, Alpo, Felix, ProPlan, CatChow, Fancy Feast, Chef Michael’s, Bakers, Friskies, Dog Chow, Beneful, Gourmet, Merrick, Whole Earth Farms, Zuke’s),

J.M. Smucker (9Lives, Canine Carry Outs, DAD’s, Gravy Train, Kibbles’n Bits, Meow Mix, Milk-Bone, Milo’s Kitchen, Natural Balance, Nature’s Recipe, Pup-peroni, Rachael Ray Nutrish, Snausages, Ol’Roy)

Hills Pet Nutrition (Science Diet),

Diamond Pet Foods (Ainsworth, American Nutrition, Mulligan Stew, Wellness, CJ Foods, Castor & Pollux, Dr. Foster’s & Smith, Nature’s Variety, Rotations, 4Health, Apex, Chicken Soup for the Pet Lover’s Soul, Taste of the Wild, Diamond/Diamond Naturals, Kirkland, Nature’s Domain, Country Value, Solid Gold, Premium Edge, Professional, Elmira Pet Products, Happy Paws, Happy Tails, K9 Premium, Nutram, Sigma 7, Petcurean, By Nature, Dogswell, Pet Time, Pure Vita, Natural Planet Organics, Life’s Abundance, and

Blue Buffalo (General Mills) (14) account for 6 of the top 10 pet food manufacturers in the world. (15)

We’re not saying which companies use used cooking oil/yellow grease in their products, or that these companies are the only ones – but these companies alone could each use 60 million pounds a year and there’d still be over 37.8 million pounds leftover for everyone else annually.



Per FDA instruction, AAFCO (American Association of Feed Control Officials) is responsible for publishing legal ingredient definitions for manufacturers to use as guidelines when producing pet and agricultural feed products. (16)
“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. It must contain, and be guaranteed for, not less than 90% total fatty acids, not more than 2.5% unsaponifiable matter, and not more than 0.5% insoluble impurities, and not more than 1% moisture. Maximum free fatty acids must also be guaranteed. This product may not include recovered trap grease or material recovered from sanitary sewer sources.  If an antioxidant(s) is used, the common name or names must be indicated, followed by the words “used as preservative.” If the product contains tallow (from cattle) containing greater than 0.15% insoluble impurities, then it must be labeled with the BSE* caution statement “do not feed to cattle or other ruminants.”

*BSE – Bovine Spongiform Encephalopathy, aka Mad Cow


“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. It must contain, and be guaranteed for, not less than 90% total fatty acids, not more than 1% unsaponifiable matter, and not more than 1% moisture. Maximum free fatty acids must also be guaranteed. This product may not include recovered trap grease or material recovered from sanitary sewer sources.  If an antioxidant(s) is used, the common name or names must be indicated, followed by the words “used as preservative.”


We know what you’re thinking: “Right, but ‘yellow grease’ and ‘used cooking oil’ aren’t on the label to my pet’s food so they can’t use it in there.” Actually, this is where you’ve been misled 

All of the ingredients that were listed at the beginning of this article have AAFCO definitions that are either vague enough that virtually anything could be used and approved as those ingredients, or their definitions almost exactly match the definition of Yellow Grease or Used Cooking Oil.  Here’s one of the many examples:


“33.1 Animal Fat is obtained from the tissues of mammals and/or poultry in commercial processes of rendering or extracting. It consists predominantly of glyceride esters of fatty acids and contains no additions of free fatty acids or other materials obtained from fats. It must contain, and be guaranteed for, not less than 90% total fatty acids, not more than 2.5% unsaponifiable matter, and not more than 1% insoluble impurities. Maximum free fatty acids and moisture must also be guaranteed. If the product bears a name descriptive of its kind or origin, e.g. “beef,” “pork,” “poultry,” it must correspond thereto. Rendered animal fat derived from only pork raw materials can be labeled as white grease. Rendered animal fat derived from only cattle raw materials can be labeled as beef tallow. Tallow containing greater than 0.15% insoluble impurities must be labeled with the BSE caution statement “do not feed to cattle or other ruminants.” If an antioxidant(s) is used, the common name or names must be indicated, followed by the words “used as preservative.”

Therefore, by definition yellow grease and used cooking oil meet the requirements necessary to be labeled as “animal fat,” and all of the ingredients listed at the top of this article.

For more examples straight from the AAFCO 2020 OP see this link.

Right behind Hero’s Pets there are 3 bins for fried sludge, one for each restaurant (usually in the employee smoking area). The bin closest to Hero’s Pets belongs to Darling Ingredients. (17) We decided to look into them to see if they sell to pet food manufacturers because we have proof that they collect used cooking oil, leave their oils outside in the smoking section of the nearest restaurant for months, and have no protective cover to prevent rodents, insects or wild animals from falling in… and it stinks like death. (18)

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. (19) 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.” (20) 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.”

From DarPro Ingredients you can click on “Pet Food Ingredients.” (21) 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. (22)

The above are Google Maps satellite view images of the DarPro rendering plant in Denver. It’s at 5701 York street near the Purina Factory. Now we know why it often smells over there.


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 animals, etc)
– Heating at high temperatures to sterilize (which causes further oxidation and production of more trans-fats)
– Adding carcinogenic ethoxyquin (9)
– Adding phosphoric acid
– Adding carcinogenic t-butylhydroquinone. T-butylhydroquinone is more toxic than arsenic. T-butylhydroquinone toxicity = 700mg/kg. Arsenic toxicity = 763mg/kg. (8)(23)(24)(25)
– Silicones to reduce foaming (10)(26)

Employees of some rendering facilities state that their production areas are contaminated with “buckets of dead rats… live rats, a lot of rat droppings… (and) maggots.” (27)

What are the health ramifications of this ingredient?

Trans-fats are produced in high levels when fats are hydrogenated (heated to very high temperature). Science links trans-fats to heart disease, (28) diabetes, (29) and obesity. (30) Hydrogenated vegetable oils contain more than 5% trans-fat. (31) In order to decrease the risk of cardiovascular disease, the FDA recommends 0% trans-fats from hydrogenated oil sources in the diet. (32)

Some say trans-fats don’t harm health in dogs and cats and only improve palatability and acceptance of feeds. (33) Yet, research 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.” (34)

Also, high-temperature cooking results in higher concentrations of acrylamides (11) which are linked to: (35)
– Kidney cancer
– Reproductive tract cancers
– Excessive perspiration
– Muscle weakness
– Tremors
– Organ damage (liver, lungs, kidneys, central nervous system)
– Thyroid neoplasms

How do I find out if the brand of pet food that I’m using uses rendered ingredients or “yellow grease”?

You may reconsider buying pet food from a manufacturer that sources their fats or oils from any of the following companies that sell and/or pick-up and recycle yellow grease:

*Darling moved their webpage for selling yellow grease as a pet feed ingredient after the publication of this article. The website is now branded “Rothsay” but is linked off the original Darling Ingredients/DarPro web link (Reference 18 – click “homepage” on error page, click “our ingredients block, click “feed solutions,” see left wing of the page – lists DarPro, Sonac, Rothsay, and Bakery Feeds).

There may be renderers that are not on this list.  Ask the manufacturer of the brand that you purchase who they source their oils, fats, gravy’s, aspics, etc from and research the company online to determine if they provide the service of rendering used cooking oil/yellow grease for use in pet food/feed.


Also, try not to buy products from feedlot or factory farmed animals who have been fed this same grease. Not only do the animals suffer from the toxicity of it but it can impact the quality of their tissues, fats, milk and eggs. Animals can actually store these oils in their fats when they don’t have the large quantities of vitamins D and E necessary to convert them to normal fats. You and your family could still be consuming these ingredients by eating feedlot meats. (49)


Well that’s all very depressing.  What do I do now?

Don’t demonize rendering plants.  Rendering is a necessity – a way for us to live in a modernized society without being overloaded with trash.  But if we as consumers demand that manufacturers use oil for biodiesel rather than animal feed we’d have a much cleaner environment and better health. As a matter of fact, if we stuck to this demand for pets and agricultural livestock we could have almost 398 million more pounds a year available for biofuels!

Of course, there are plenty of accepted foods and chemicals as toxic as Yellow Grease. A bucket of fried chicken might have a comparable impact on your body. However, a human is able to choose that risk. Hopefully nobody is knowingly choosing those foods or chemicals as an exclusive food source for their pet. 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. That means it can keep building up throughout the pet’s life.

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 could dramatically reduce smog, ozone and sulfur emissions. Biodiesel use also reduces poisonous carbon monoxide, unburned hydrocarbons and particulate matter. Biodiesel indisputably improves environmental health. (50)


Increased popularity of Biofuels creates a competitive market for these ingredients – thus decreasing their use in pet foods. This change can help consumers recognize the value of fresh feeding – when the price of buying “trash” in the form of kibble pellets competes with the price of buying fresh, whole foods.  Plus, it will give the sustainable fuel industry the demand and money to get these fats for use as fuel. And it will remove toxic ingredients from your pets’ foods, your own meat and dairy products, and your soap and detergent products.

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. Beautiful 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.

Check soaps and detergents for grease ingredients. These are made with chemical stabilizers which can permeate through the skin and cause irritation and illness according the MSDS sheets.

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.
Tell your friends. For the sake of their own health and their pets’ health please pass this information along. There is something we can do about it if we all change our buying habits.

*“Chemically, oils and fats are the same, differing only in melting point…” (51)


  1. Darling Ingredients SDS, Yellow Grease
  2. Wikipedia – Yellow Grease
  3. Mahony Environmental – Fresh Oil Services
  4. Mahony Environmental – Used Cooking Oil Recycling
  5. Mahoney Environmental
  6. USDA FSIS Deep Fat Frying and Food Safety
  7. Canola Oil refining, bleaching, deodorizing
  8. t-butylhydroquinone MSDS
  9. Ethoxyquin MSDS
  10. Antifoam – Dimethylpolysiloxane
  11. Healthline – Why Are Fried Foods Bad For You?
  12. Chicago Tribune, Oil Makes Grade On Fries
  13. Statista – Number of Restaurants in the US
  14. Dog Food Brands Share Manufacturing Facilities
  15. Most productive pet food manufacturers
  16. AAFCO OP 2020 pages 386 (Yellow Grease), 387 (Used Cooking Oil), 336 (Aspic), 341 (Gel), 337 (Carriers), 340 (Fat), 341 (Gravy), 341 (Grease), 342 (Meal), 343 (Oil), 383 (Animal Fats, all varieties), 373 (Vegetable Fats and Oils, all varieties), 383 (Hydrolyzed Vegetable Fats and Oils, all varieties), 399 (Fish Oil)
  17. DarPro Solutions
  18. Darling Ingredients main page
  19. AAFCO 2020 Mid-Year Meeting, Albuquerque – Darling Ingredients Keynote Presentation
  20. Dar-Pro Grease Use Statement
  21. DarPro Pet Food Ingredients
  22. DarPro Feed Grade Proteins & Fats
  23. Arsenic MSDS
  24. Mahony Environmental – The turnkey solution for cooking oil management
  25. NIH PubMed – Antioxidant for Yellow Grease
  26. Silicone for use in feed
  27. Truth About Pet Food, Pet Food and Rendering Plants
  28. New England Journal of Medicine –Effect of Dietary trans Fatty Acids on High-Density and Low-Density Lipoprotein Cholesterol Levels in Healthy Subjects
  29. NCBI PubMed – Health Effects of trans fatty acids
  30. Oxford Academic, American Society for Nutrition, Journal of Nutrition – Consumption of Trans Fatty Acids is Related to Plasma Biomarkers of Inflammation and Endothelial Dysfunction
  31. NCBI PMC Toxicological Research, Analysis of Trans Fat in Edible Oils with Cooking Process
  32. AccessData, Trans Fat can increase the risk of developing cardiovascular disease
  33. Timely Topics in Nutrition, American College of Veterinary Nutrition, Facilitative and functional fats in diets of cats and dogs
  34. Timely Topics in Nutrition, Lipoprotein-mediated transport of dietary and synthesized lipids and lipid abnormalities of dogs and cats
  35. Wiley Online Library, Acrylamide (MAK Value Documentation)
  36. Baker
  37. BHT
  38. Cargill
  39. Farmers Union Industries/Midwest Grease/Performance Pet/ Redwood Farms Meat Processors/Central Bi-Products/ Northland Choice/ Artex
  40. A. Wintzer & Son
  41. JBS
  42. Kaluzny Bros/ Mahony Environmental
  43. Mendota Agri products
  44. Sanimax
  45. SRC Companies
  46. Valley Proteins
  47. West Coast Reduction
  48. Aspen Oil
  49. Meat Contamination
  50. Iowa State University – The effect of biodiesel oxidation on engine performance and emissions 
  51. Wikipedia – Frying