Food Science - WOF

Sharon’s Editorial Comment:
I learn a lot hanging out on “food service”, “restaurant management” and “food processing” industrial sites. For me, it’s like being in the belly of the whale, or den of thieves, or pit of vipers….well, you get the idea. They’re not my favorite people. The attached article is fascinating to me because of the primary topic – Warmed Over Flavor (WOF) resulting from the oxidation of fat in fish, poultry, pork and beef due to any number of variables including high heat, storage, type of packaging used, lighting conditions, quality of the product, as well as what the “product” was fed during its life (in beef, grass-fed diet offers a completely different fat composition than grain-fed diets), etc.

Test Question: Is oxidation of our food –
a) good and healthy for our bodies
b) harmful to every major system of our body including endocrine, digestive, heart, respiratory, and neurological

Because the attached article is written by the food industry FOR the food industry, their primary concerns are that of making a nutritionally dead, off-tasting, off-smelling piece of grain-fed meat that has an objectionable Warmed Over Flavor (WOF) into an aromatic, tasty morsel. You’ll not find a word about nutrition as being their primary number one concern. Of course, they’re aware that WOF is indicative of something-gone-wrong with food-not-the-way-it’s-meant-to-be. Not their problem.

I am lacking in logic to the point that I think food processors owe us anything. Their accountability is to their share-holders – not to me and not to you. Their business, plain and simple, is to make something look good, smell good, and taste good, whether or not it actually is.
And as long as people live for the short-term goal of fixing their immediate cravings, or not having enough time to prepare proper food, the food industry will thrive, as will the pharmaceutical industry. We, I fear, will not unless we get back to real food made with our own hands.

Fighting Warmed-Over Flavor

By Lynn A. Kuntz

Convenient, precooked foods save time and fit the labor requirements of many of today’s foodservice establishments. Unfortunately, this convenience often comes with a price. No, not the added costs incurred in formulating these products, but a loss in flavor quality commonly known as “warmed-over flavor.”

Warmed-over flavor, or WOF, is a common phenomenon in cooked refrigerated or frozen meats. It can occur in processed convenience entrées or in last Thursday’s meatloaf special. These products develop stale and/or rancid flavors and odors, while more desirable fresh-cooked notes dissipate. According to John Bauman, vice president culinary business unit, Wild Flavors, Inc., Cincinnati, the off-flavors are often described as “cardboard,” “rancid,” “ice box” and even “freezer-burn.” Since most diners do not look forward to this effect, solutions to this problem are imperative.


The main culprit behind WOF is fat. Meat fats, especially those high in polyunsaturated fatty acids (PUFAs), are prone to oxidation. Because of their high polyunsaturated fatty-acid content, certain meats exhibit a greater tendency to develop WOF. Fish is most at-risk, then in descending order: poultry, pork, beef and lamb.

“It’s a lipid oxidation problem, a very early problem,” explains Marilynn Schnepf, Ph.D., associate professor, Department of Nutritional Science and Dietetics, University of Nebraska, Lincoln. “It starts with the unsaturated fatty acids at the cell membrane. When you cook the meat, it disrupts that membrane and the process can begin within several hours, or overnight. Regular lipid oxidation is something that generally takes days to develop.”

When meat’s fatty acids oxidize, they form reactive free radicals that promote even more oxidation in a chain reaction. During this reaction, fat molecules can split into smaller molecules, including pentanal, hexanal and 2,4-decadienal. Even at concentrations as low as parts per billion, these compounds give rise to the off-odors and flavors described as warmed-over. In the literature, some of these compounds are also described as bitter. These fat-soluble flavor compounds migrate into the melted fat where they volatilize upon reheating.

“Those short-chain aldehydes can taste pretty bad. When we check for warmed-over flavor, we look for breakdown products,” says Schnepf. “Hexanal is a good marker we find correlates well with warmed-over flavor development. Pentane is another compound that develops, and while it doesn’t contribute much flavor, it’s a marker, too.”

Heat, light and certain enzymes can initiate oxidation, and metals, oxygen and enzymes can promote it. “Heating causes changes in the chemical structure of proteins and fats, rendering them helpless against oxygen and free radicals,” explains Roger Maehler, Ph.D., president, Heller Seasonings & Ingredients, Inc., Bedford Park, IL. “The primary cause of WOF is believed to be ferrous iron catalyzed lipid oxidation. The ferrous iron dissociates from the heme complex during the initial cook.” The high temperatures cause the proteins in meat to release free iron (normally chemically bound in the blood) and oxygen, and promote free-radical formation. Researchers believe that the iron ions then help catalyze fat oxidation.

Heat is not the only culprit. Scientists believe that salt, added for flavor and functionality, promotes iron-catalyzed oxidation and its resulting off-flavor development. In addition to iron, other trace metals promote oxidation, especially copper. These may be introduced into the meat via water, cooking and processing equipment, or with ingredient addition. Light, especially ultraviolet and blue-purple fluorescent, also increases the chances of WOF. Certain compounds, such as chlorophyll and riboflavin, increase a product’s sensitivity to light.

In addition to oxidation, other processes play a role in flavor deterioration of precooked meats. “As technology has gotten better, we’re finding that warmed-over flavor is not strictly limited to oxidation,” says Chad Anderson, sales manager, Red Arrow Products Company LLC, Manitowoc, WI. “There’s a loss of desirable meat notes and something that people often describe as stale. Meat is one of the most complex food systems, so it would be a mistake to think that only one thing is causing the problem.”

For example, researchers at Rutgers University, New Brunswick, NJ, have found that certain meat enzymes remain active through cooking and subsequent refrigeration. They analyzed top-round steaks to find the substance these enzymes break down that might be responsible for the lack of fresh-cooked flavor. They identified a peptide that directly stimulates taste receptors to create a umami note, so its disappearance may contribute to the dull flavor impact also associated with WOF.

The off-flavors develop rapidly and many foodservice situations can accelerate flavor deterioration. “One cooking cycle will weaken the flavor stability of a product, and off-flavors can be detected within a day or two of refrigerated storage,” says Tricia Cox, senior food technologist, Heller Seasonings & Ingredients, Inc. “Reheating can further produce off-flavors. Prolonged holding at high temperatures, such as in a steam table, will produce considerable WOF. Exposure to oxygen and light will expedite the formation of WOF notes.”

Whatever the cause, oxidized flavors show up in cooked products after less than 48 hours of refrigeration. While freezing can delay their onset, it doesn’t prevent it, Schnepf points out: “Freezing also can disrupt the membranes and it doesn’t protect it from oxygen, so you have the same principles. Plus, the rate of lipid oxidation is a U-shaped curve with water activity, so if you have a low water activity (as a result of freezing a product’s water), once it’s started, you can accelerate the reaction.”

Part of the Anti-WOF Package

Films that act as oxygen barriers can help decrease warmed-over flavor and packages of the future may become part of the product itself. Marilynn Schnepf, Ph.D., associate professor, Department of Nutritional Science and Dietetics, and other researchers at University of Nebraska, Lincoln, have investigated the use of edible films and natural antioxidants to control warmed-over flavor in meats. These films have been composed of modified corn starch, soy protein, wheat gluten and corn zein.

In addition to the reduced oxygen exposure afforded by the films, Schnepf “has used rosemary oleoresins and mixed tocopherols incorporated into the films to further delay WOF. Using the films to deliver the antioxidants has some problems, however. The antioxidants used are fat-soluble and many of the edible films are hydrophilic, so the release and delivery needs more work. In hamburger, it’s more effective to mix in the antioxidants, but it might have some application in whole-muscle products.”

Still, using the rosemary and edible film kept the marker compounds for WOF in a 9-day-old treated pork patty down to the levels more typically found in a 3-day-old untreated product. So, with further work, this technology may someday become part of the arsenal for addressing WOF problems.

Prevention strategies

Since science has identified heat, light and assorted naturally occurring or added components as causing the WOF problem, science should find the solution. That’s more easily said than done. However, looking at the causes of off-flavors leads to methods that, while not completely solving the problem, offer improvement.

First, there are steps that can be taken on the operator level. “Reducing the time from cook-top to plate, and incorporating antioxidants into the product are two steps that foodservice operators can take to minimize WOF,” Cox recommends. “Holding products on a steam table is about as bad as it gets in a foodservice operation for creating WOF.” Since products experience elevated temperatures for long time periods, this accelerates oxidation and off-flavor development.

To make matters worse, in combination with WOF notes, “Steam tables will cause degradation of the entire flavor system,” says Bauman. “Not only do you have WOF if it’s a precooked meat, but steam tables have the potential to give foods that ‘overcooked’ taste. If you’ve developed a well-balanced flavor to solve the problem, the extra heating can drive off water and volatiles and ruin the whole thing.”

While the interval of holding is — in theory — much less under warming lights, this too may contribute to the problem. The product undergoes a double whammy: exposure to heat and light. Because of the short time to consumption, this should not cause serious problems during normal operating hours, but those who convert yesterday’s end-of-the-night entrées into tomorrow’s soup de jour might factor that into the equation. “Many of the fast-food chains make an effort to ensure that their burgers stay under the lights for 10 minutes or less,” says Schnepf. “That’s so they don’t lose that fresh-cooked flavor.”

The actual cooking methods, on premises or, for precooked products, at the processors, can also influence WOF. Grilling requires high temperatures, so while one might logically assume this would exaggerate the problem, that’s not true. “Cooking processes that use very high temperatures, like grilling, seem to inhibit WOF through the formation of Maillard-browning intermediates,” says Maehler. “Similarly, conditions that favor browning, such as addition of glucose or smoke intermediates, may inhibit WOF.”

When the subject of WOF comes up, often it’s in relation to products like meat loaf or precooked burgers. The explanation is simple — grinding helps disrupt the membrane, plus it introduces oxygen throughout the matrix. Grinding meat also facilitates iron release from the myoglobin, which enhances lipid oxidation.

“You’ll notice that ground products tend to have the most problems,” says Bauman. “Products like hamburgers, chicken nuggets and precooked pizza sausage are all good candidates for warmed-over flavor development. Chicken nuggets, in particular, are known for developing off-flavors. You mix in thigh meat and skin that has high levels of fat, maybe even add soy…well let’s just say some unusual things happen.”

Choice of meat will play a factor in oxidation. Fish and other meats with high levels of PUFAs exhibit more of a propensity toward off-flavor development. Chicken has less tendency to develop oxidized flavors than turkey because the higher level of vitamin E in chicken fat retards oxidation. Bauman points out that the problem will be worse in chicken thigh meat than in chicken white meat because the dark meat contains more fat. Fresh meat will show less of a tendency to develop WOF than older product. This reduces the time the meat is subject to enzymatic oxidation, a process that produces catalysts that promote oxidation even after enzyme inactivation by heat.

Because nitrates and nitrites inhibit oxidation, the WOF problem crops up less frequently in cured meats than fresh cooked products. “Nitrite is probably the best in terms of synergy,” says Anderson. He points out that if there’s a choice between developing a cured vs. fresh precooked product, less problems will occur in the cured meat.

Also, even though WOF is most closely associated with meat, other foods may develop similar problems. “Any products containing heme, iron and fat are susceptible to developing WOF,” says Cox. “Meats will suffer the most, but other food products that can develop flavor off-notes are gravies, macaroni and cheese, and heavy cream sauces.” Luckily, this defect normally doesn’t arise in vegetable products; Schnepf theorizes that this might be the influence of these foods’ naturally occurring antioxidants.

Food science steps in

Foodservice establishments aren’t the only ones that can help fight WOF. “Foodservice operators should also work closely with the processors of their precooked products to take steps at the manufacturing level that would help to minimize the formation of WOF in the products during distribution and storage,” recommends Maehler.

The primary way that food technologists try to reduce the problem is by adding antioxidants, or compounds or ingredients that naturally exhibit antioxidant properties. Antioxidants delay the onset of fatty-acid oxidation by acting as free-radical acceptors or hydrogen donors that inhibit autooxidation.

A number of ingredients act in this way. Synthetic compounds butylated hydroxyanisol (BHA), butylated hydroxytoluene (BHT), tertiary butylhydroquinone (TBHQ) and propyl gallate (PG) are widely used antioxidants in the food industry. However, their usage levels are strictly regulated.

Since chemical soup isn’t always customer-friendly, a wide variety of natural compounds provide oxidation protection in food-ingredient form. According to Cox, the most common natural antioxidants used to fight WOF are vitamin E, rosemary and carotenoids such as beta-carotene and lycopene. In addition, many other spices, fruits and vegetables contain compounds with antioxidant properties and might add benefits to the mix. “Reducing exposure of these antioxidants to oxygen and light will enhance their effectiveness in minimizing WOF development,” she adds.

Natural WOF Fighters on the Menu

Fruits and vegetables that contain naturally occurring compounds that reduce oxidation.

(ORAC units per 100 grams, or approximately 3 1/2 oz.)

Prunes 5,770
Raisins 2,830
Blueberries 2,400
Blackberries 2,036
Strawberries 1,540
Raspberries 1,220
Plums 949
Oranges 750
Red Grapes 739
Kale 1,770
Spinach 1,260
Brussels Sprouts 980
Alfalfa Sprouts 930
Broccoli Flowers 890
Beets 840
Red Bell Pepper 710
Onions 450
Corn 400

Source: USDA Human Nutrition Research Center on Aging, Tufts University, Boston.

Common spices with antioxidant properties include rosemary, marjoram, oregano, sage, thyme, mace, allspice, cinnamon and clove. Rosemary appears to be the most effective because it contains antioxidant phenolic compounds including carnosic acid, rosmanol, rosmariquinone and rosmaridiphenol. Eugenol, carvacrol and thymol are phenols found in cinnamon, cloves, sage and oregano. Because carnosic acid and rosmanol are odorless, manufacturers can develop spice-based ingredients with reduced flavor impact and increased protection against oxidation.

Yeast extracts also exhibit antioxidant properties due to glutathione, Maillard-reaction products and sulfur-containing amino acids found in these ingredients. Researchers at the University of Minnesota, St. Paul, have identified other food ingredients that have a positive effect on oxidative WOF. These include wild rice, due to its phytic acid, and waxy, hulless barley bran, a rich source of tocotrienols.

Another natural possibility for fighting WOF is honey. Research has shown that the darker the honey, the greater its potential to reduce oxidation in foods. In a study conducted at Clemson University, Clemson, SC, the National Honey Board reports that adding liquid or dried honey to injection and tumble-processed turkey breast reduces oxidative lipid breakdown. In addition, the reducing sugars in honey contribute to Maillard browning, a process that generates WOF-reducing compounds.

ORAC might be okay

Foods that score high in an antioxidant analysis called ORAC (oxygen radical absorbance capacity) may offer some protection from WOF. ORAC is a test-tube analysis that measures a food’s total antioxidant power. This has led to investigations as to natural foods that might reduce oxidation and therefore the incidence of WOF. For example, the results of the Boston-based Tufts University ORAC research has led the California Prune Board, Pleasanton, CA, to approve funding to complete a research project studying the antioxidant effectiveness of prune ingredients in a meat system, according to Jim Degen, consultant to the California Prune Board. The research, conducted by Jimmy Keeton, Ph.D. and Ki Soon Rhee, Ph.D. at Texas A&M University, College Station, may be useful in retarding lipid oxidation in meat products that contain fat, endogenous pro-oxidants and salt in their formulations.

The study will compare the antioxidant effectiveness of dried plums at two levels (3% and 6%) in fresh and cooked pork sausage patties and compare them to patties with BHA/ BHT (at levels of 0.02% of the fat content) and a control. Sausage patties will be evaluated after refrigerated and frozen storage for chemical composition, color, oxidative rancidity, descriptive attribute sensory panel for texture and flavor, consumer acceptability and other attributes.

According to Degan, if dried-plum ingredients prove to be effective antioxidants in meat products, they could allow for a “cleaner” label declaration and retard the loss of quality due to lipid oxidation or warmed-over flavor, while potentially providing health benefits to the consumer. As an added benefit, dried plums might also provide valuable functional properties (moisture retention) and serve as a partial fat replacement. Preliminary results are expected by the end of November and the study is scheduled for completion in early 2001.

Stop for a smoke

As mentioned, cured meats resist oxidation because they contain nitrates. Smoking, another popular meat-preservation method, also generates compounds that have antioxidant properties. Research at Red Arrow shows that certain smoke flavorings can reduce the occurrence of WOF and extend shelf life when added to fresh, precooked and processed meats. According to Anderson, adding smoke flavors can reduce oxidation by 20% to 30%.

The phenolic compounds inherent in these smoke flavors act as antioxidants. According to Red Arrow research, the flavors also contain certain carbonyl compounds that react with meat’s amino groups to inhibit WOF production. The water-soluble reaction components were found to be most effective. Mesquite smoke flavorings contain the highest levels of the effective intermediate agents. Company researchers found that application of smoke flavor at approximately 0.03% to 0.50% of the finished product could inhibit oxidation.

The company also offers some smoke flavors that provide color without a high level of flavor. “These lack the phenols associated with smoke flavor,” says Anderson. “But they do provide browning by contributing carbonyls and aldehydes. We’ve found that the carbonyls do suppress bacterial growth, so if there is any flavor interactions as a result, this could be an added benefit.”

Other oxidation answers

Typically, antioxidants go into the product during processing, but scientists are experimenting with other methods of increasing meat’s antioxidant capacity while still on the hoof. “For meats, vitamin E administered prior to slaughter can minimize the potential for WOF development,” says Maehler. This effect was seen in pigs with diets supplemented with vitamin E in several studies. The vitamin E, or alpha tocopherol, migrates to the cell membrane where relatively high levels of oxidation-prone PUFAs are found.

Another method to reduce oxidation is adding chelates. “Using inhibitors like phosphates that can complex the iron is an option,” notes Maehler. If the product contains metal ions, chelating agents such as citric acid, ascorbic acid and some phosphates (sodium tripolyphosphate, pyrophosphate or hexametaphosphate) can stabilize the metal ions and keep them from promoting oxidation reactions. Ascorbic acid also functions synergistically with other antioxidants, and at low levels acts as a reducing agent.

“Phosphates are fairly effective,” says Anderson. “They increase the water-holding capacity of the meat’s protein fibers and can chelate the metal ions. The problem is that, depending on what is going on in the system, you may be better off reducing the water level instead of increasing it when you’re talking about off-flavor development.”

WOF can be slowed by physical means, such as oxygen exclusion through vacuum tumbling and stuffing before cooking. Covering cooked products with liquid or sauces prior to freezing can also provide some protection from oxygen. “If you cook a lot of burgers ahead of time, one strategy restaurants use is to submerge them in a broth to preserve the flavor,” says Schnepf.

Packaging is another physical means of reducing off-flavor development. Because light can accelerate oxidation, eliminating light exposure through packaging might help. Vacuum packaging can control oxygen interactions at the meat surfaces and reduce oxidation. Schnepf notes that this technique, coupled with nitrogen flushing or other MAP techniques, can offer substantial shelf life. Other packaging technologies, such as oxygen scavengers, can be added either as stand-alone into the package or incorporated into the packaging film.


Flavor masking or other methods using flavors that modify the perception of off-notes can be effective tools in the fight against oxidative off-flavors.

“A food system can be quite complex, so there is no magic bullet, and often, methods used successfully in the laboratory do not always work at the production level,” warns Cox. “However, there are some options that have been tested and are being used, including a potato extract that seems to work well in protein products and protein-specific flavors which mask the off-flavor associated with oxidation.”

Beer flavoring is another ingredient that helps eliminate flavor problems that occur during the reheating of meats, according to technologists at Butter Buds Food Ingredients, Racine, WI. At lower levels, it functions as an enhancer, while at higher levels it creates stronger beer flavor notes. Its anti-WOF effect may be a function of the typical yeast notes found in beer, because yeast-based flavors can improve and enhance flavors, mask bitterness, increase aroma and also provide some protection against oxidation.

Still, masking agents that have been developed to mask other off-flavors, such as metallic notes in high-intensity sweeteners or beany notes in soybean products, can prove effective in masking WOF as well, according to Bauman. His company markets a line of products on a natural, water-soluble extract, that affect the taste receptors located in the mouth.

“The product doesn’t have much flavor on it’s own,” says Bauman, “although you can detect a slight flavor if the level is too high. But we do co-process it with other flavors, especially meat notes, to tailor it to specific applications, such as masking warmed-over flavors. In vacuum-packaged refrigerated products where you are looking for a 60-day-plus shelf life it holds up well and is stable in retort products.” Because the flavor can also mask certain desirable notes when used at too high of a level, he recommends that customers discuss its use and allow the company to provide formulation assistance to determine the optimum level.

Merely disguising the oxidative notes is another option. “Highly flavored systems, like a spicy Mexican seasoning, can distract the consumer from any off-notes; there’s some heat and other things that distract the palate,” says Bauman.

In developing a flavor system designed to fight WOF, it’s important to consider the overall flavor profile of the finished product, along with the prevention of off-flavor development, notes Maehler: “Both temperature and hold time during the cooking process will influence overall taste. For optimal flavor, this usually means adding spices and flavorings at a slightly higher level than would be needed for immediate consumption.”

Once an appropriate flavor system is developed, it is incorporated into the product by means of a mix-in seasoning, rub or marinade, depending on the type of product developed. For example, in a whole muscle product, “marination is recommended to enhance protection and minimize the possibility of off-notes developing within the internal area of the product,” Cox says. “However, since the surface of the product is more prone to oxidation, topical application would certainly help to minimize the formation of WOF notes.”

WOF seems to be a common adversary in the trend toward pre-prepared foodservice entrées. In the end, it looks like a combination approach, from both the operator and the processor, using cooking, processing, storage and formulation, is best-suited to win the war.

  Textile help