Pink Ring in Non-Cured Meats

by Robert Hanson

Prized quality attribute or serious color defect?

If you took a piece of cooked beef out of a production oven, sliced it, and saw a distinct pink ring around the outside edges, would you be pleased or alarmed? The answer, of course, depends on what product you were trying to make. In roast beef, for example, pink ring is generally considered a serious color defect because consumers might associate it with undercooking (Cornforth et al., 1991). For barbecue meats, on the other hand, pink ring is often a prized quality attribute because it enhances the product's "pit-smoked" appearance.

What causes the pink ring, and how can it be controlled? Let me summarize the results of a recently completed study on surface pinking that discusses what causes the pink ring, and how it can be controlled. This study was the result of a joint research project by ALKAR, the Institute of Gas Technology, Maxon Corp. and Utah State University. I will also tell you how this research can be used to either prevent or create pink ring in non-cured meats.

What Causes Pink Ring?

Before surface pinking can be controlled, we must first understand what causes it. It is widely understood that non-cured meat products often develop surface pinking when cooked in direct gas-fired ovens (Cornforth et al., 1991). When sliced, this pinking is observed as a ring around the outside edge of the product cross-section (See photo below). In beef, the ring is a reddish-pink color, and in pork, chicken, and turkey it is bright pink. Depending on the cooking conditions, this ring may be just thick enough to be barely visible or it may be clearly visible to a depth of one-half inch.

It is also generally known that for ovens heated with direct-gas, indirect-gas, steam coils, or electric-heating elements, only the direct gas-fired ovens cause pink ring. Even though natural gas burns very cleanly, apparently the residual combustion byproducts are concentrated enough to cause pink ring. But which combustion gases are responsible? What concentrations are enough to cause visible pinking?

To answer these questions, the above-mentioned joint research project was conducted to determine the threshold levels of combustion gases that would cause pink ring. ALKAR and IGT conducted a series of cooking trials in an electric-heated batch oven. Controlled levels of three pure gases known to cause pinking - carbon monoxide (CO), nitric oxide (NO), and nitrogen dioxides (NO2) were injected into the oven during cooking.

To spare you the suspense, although CO and NO are commonly cited as the most likely cause of pink ring, we discovered that the real culprit was NO2 (Cornforth et al., 1998). The trials showed that NO2 is a potent pinking agent. Even extremely low concentrations caused pink ring in non-cured meats - 2.5 ppm for beef and 0.4 ppm for turkey. By comparison, the recirculated air in gas-fired batch ovens generally contains approximately 10 ppm of NO2 more than enough to cause surface pinking. The usual suspects, CO and NO, also cause surface pinking, but only at concentrations much higher than typically found in gas-fired batch ovens.

Preventing Pink Ring

If we know what causes pink ring, how is it prevented? To answer this question, you need to know a few things about NO2. Nitrogen dioxide gas is highly water-soluble. The pink ring in meat is created when NO2 is absorbed into the moist surface and reacts to produce nitrous acid. The acid diffuses inward, creating pink ring via the classic meat-curing reaction of sodium nitrite brines. Cornforth et al. (1998) identified the resulting pink-ring pigment as nitrosyl-hemochrome - the same pigment responsible for the pink color of cured meats.

Pink ring doesn't show up in cured products such as hams, bologna, and wieners because they are already cured with sodium nitrite, and therefore the NO2 from the combustion gases has little or no effect. To prevent pink ring formation, the key point to remember is that NO2 is water-soluble. When non-cured meats are cooked in gas-fired ovens, one way to prevent pink ring is to use a high-temperature drying step (180-200°F) to rapidly dry the product surface at the beginning of the process. The dry product surface will impede absorption of NO2, thereby reducing pink ring. The trade-off, of course, is that high-temperature drying will also increase shrinks and skin development.

Any process or product conditions that help keep the surface moist such as high-humidity oven conditions, low-velocity air, or high-moisture product formulas will promote NO2 absorption and pink ring. Even if non-cured products are cooked in an oven that is not direct-gas fired (eg. electric, steam-coil, or indirect-gas), other sources of NO2 may still cause pink ring. For example, pink ring has been found in non-cured meats that were cooked in mixed loads of cured and non-cured products, presumably because NO2 from the cured meats was released into the oven air. Another example is that gas-fired make-up air units located in the same room as steam-heated ovens have reportedly caused pink ring. Apparently, enough NO2 from the make-up air units was drawn into the oven intakes to cause pinking.

Creating It: Barbecue

Although pink ring is undesirable in many meat products, it is highly prized in one resurgent market niche - barbecued meats. Meat for traditional barbecue is usually made from non-cured meats such as pork picnics, beef flats, and beef briskets. A distinct pink ring is a desirable trait and is often referred to as "smoke ring." As it turns out, however, the pink ring comes from the fire, not the smoke.

In the past, the only way to achieve the distinct "pit-smoked" color and flavor and the "fall-off-the-bone" texture of traditional barbecued meats was to use old-fashioned "pit" smokehouses. These pit smokers are sometimes called "gravity" smokehouses because they often rely on free-convection airflow for circulation or have only a very small circulation fan. They are usually heated using either burning logs alone or a combination of burning logs and a small gas burner. Because of the minimal air recirculation, pit-smokers generally have long cooking times, uneven product temperatures, and unpredictable product shrinks.

However, in spite of these shortcomings, attempts to duplicate the distinctive color, flavor, and texture of pit-smoked barbecue meats using modern ovens and conventional smoke generators usually came up short. To make pit-smoked products in a modern smokehouse, we combined our new findings on NO2 with prior experience in cooking and smoking to develop a process that duplicates the color, flavor, and texture of barbecue meats made in a pit-smoker. The "fall-off-the-bone" texture was the easy part - the product had to be cooked hot enough and held long enough to break down the connective tissue and tenderize the meat. But to produce the distinctive flavor, color, and deep pink ring, the key once again was to understand a few characteristics of NO2.

To create a pit-smoked product, you need the following conditions in an oven: (1) a moist product surface, and (2) lots of NO2 (much more than a gas-burner alone can produce). Although pit-smokers are often inconsistent and unpredictable, they do generally satisfy both of these requirements. First, the product surfaces stay moist because there is little or no airflow to dry the product. Second, the burning logs supply lots of NO2. Combustion engineers will tell you that NO2 is a product of incomplete combustion, and therefore dirty combustion such as burning logs will generate large amounts of it.

Once we understood what made the pit-smoker process work, we could duplicate those conditions in a modern smokehouse. The oven conditions and fan speeds were controlled to keep the product surface moist during smoking. To generate large amounts of NO2, we had to duplicate the log-burning process. We found that the smoke from smoldering sawdust in a conventional smoke generator did not create the desired pit-smoked flavor and pink ring. Therefore, we developed and tested a new log burner that was specially designed to work with a modern smokehouse.

Extensive pilot-plant trials and a successful field installation of the new ALKAR barbecue oven showed that we can make traditional barbecue meats in a modern smokehouse. These products duplicated those made in old-fashioned pit smokers but with a shorter cooking time and more consistent and predictable quality, temperatures, and yields.

Will Rogers once said, "It ain't what you don't know that hurts you, it's what you do know that ain't so." For years, we thought we knew that NO and CO caused pink ring in cooked, non-cured meats but the culprit turned out to be NO2. Now that the characteristics of NO2 and pink ring are better understood, we can use this knowledge to either prevent a costly color defect or create a desirable product attribute. Either way, we're money ahead. Mp

Robert Hanson is currently manager of technical development at ALKAR where his duties include the research and development of cooking and chilling processes used in batch ovens and continuous systems. Hanson has a bachelor's degree in Agricultural Engineering Technology and a master's degree in Meat Science.

Pit-Smoked Beef:

Pink Ring: Pinking is observed as a ring around the outside edge of non-cured product cooked in direct gas-fired ovens.