Maintaining visual appeal and flavour quality remains a challenge for meat manufacturers due to oxidative rancidity and microbial spoilage. Colour is the main aspect that defines the quality of fresh meat products and it is the most important criterion consumers use in selecting fresh meat.
Equally important to red colour in fresh meat is brown or grey colour in cooked meat and pink colour in cured meat. It is estimated that nearly 15% of retail beef is discounted in price because of surface discolouration, which is equivalent to annual revenue losses of one billion1.
A survey conducted by Carr and Miller in multiple retail stores revealed that poor colour was both the most common reason pork packages were deemed unacceptable (63.3% of responses) and the most common reason pork products were removed from display (43.3% of responses). They described poor colour as grey (50%), greyish green (36.7%), light green to brown (10%) or too dark (3.3%)2.
Whereas colour is a crucial factor for a first purchase, a fresh and consistent flavour will convince the customer to buy the same meat product again.
Myoglobin Chemistry and Colour
Myoglobin is the heme protein primarily responsible for meat colour. Myoglobin concentration varies between and within animal species and is affected by factors such as age, sex, diet, genetics and environmental factors. The concentration of myoglobin per kilogram of lean meat is the highest in beef, followed by lamb, pork and poultry3.
Myoglobin is present in three different forms, which are in equilibrium with each other (Fig.1). Stable oxymyoglobin (MbO2) is formed in an oxygen rich environment and has a bright red colour which is considered a mark of quality. Fresh cuts of meat consequently are red on the outside (up to a depth of about 1 cm). Inside the thicker pieces, oxygen cannot penetrate well and a higher presence of myoglobin (Mb) brings about a more purple colour. Over time a continuous oxidation will transform Mb into metmyoglobin (MMb+) which has a brown colour due to the oxidation of the heme iron from Fe2+ to Fe3+. Consumers perceive this colour change to brown as a cue that the meat is no longer fresh.
Fig. 1: Schematic of myoglobin redox interconversions on the surface of meat5.
The formation of brown coloured MMB+ is reversed at low oxygen presence, such as found inside the meat or in non-oxygen permeable packaging. However, this means the pigment will remain present as purple Mb, but as soon as a package is opened, it will transform quickly to bright red MbO2.
Nitrite is used during the curing of meat. The resulting nitric oxide (NO) forms a bright red and highly stable complex with the heme group of myoglobin and even with oxidized MMb+. This colour is even stable towards heating, which is not the case for fresh uncured meat that will turn brown after heating4.
Meat containing up to 30% metmyoglobin still has an intense red colour and concentrations of 30-45% metmyoglobin will have a muted red appearance. Relative metmyoglobin percentages between 45-60% cause a brownish-red colour and at 60-75%, the meat is more brown than red. The colour is brownish grey when metmyoglobin exceeds 75%3.
Off-odour and off-flavour in meat products is caused by either microbial spoilage or oxidative rancidity. Lipid oxidation of meat products is the primary cause of quality loss in frozen products and Modified Atmosphere Packaging (MAP). Hydroperoxides are the primary byproducts of lipid oxidation (in the absence of light) and they degrade to form alkoxy radicals which undergo further reactions to form complex mixtures of secondary oxidative byproducts with distinct flavours and aromas. These secondary oxidative byproducts include molecules such as alkanes, alkenes, aldehydes, ketones, alcohols, esters, lactones, epoxides and acids.
In cooked meats, aldehydes are the most significant breakdown products and they are responsible for the phenomena known as warmed over flavour (WOF), which yields cardboard-like, painty and rancid flavours. When meat is minced, ground or cooked, the cell membranes are disrupted, further exposing the lipids to oxidation. According to research, subcutaneous fat can produce about 50 volatile components throughout development of WOF, yet intramuscular fat can generate more than 200 volatile components6.
Light oxidation in refrigerated display cases
Meat products with a high fat content, such as ground meat, sausages and bacon, are susceptible to photo-oxidation, a photochemical reaction between light and fat lipids which leads to rancidity, noticeable by a bad smell and a yellowish tinge. Because of their high fat content, bacon and sausage have a much shorter shelf life than other meats. Even when frozen, their shelf life is on average only three months long. Ground meat and sausages are more susceptible to spoilage because the fats are evenly dispersed throughout the meat.
The chlorophyll in herbs and spices commonly added to sausages and processed meat absorbs light, accelerating the rate of lipid oxidation. Similarly, vegetable-based food dyes added to cured or processed meat will also absorb light and increase the rate of oxidation. The rate of spoilage can vary depending upon the salts and chemicals used, which can speed up the decomposition of the meat or its oxidation.
Antioxidants delay the onset of lipid oxidation by donating hydrogen atoms to quench free radicals, forming a stable antioxidant radical that is unable to participate in the propagation reactions. They are also key players in maintaining redness in fresh and frozen meat, since lipid oxidation byproducts interact directly with myoglobin and make the heme iron more susceptible to oxidation. Several natural plant extracts are known for their antioxidant activity. Two well-known botanicals are rosemary (Rosmarinus officinalis) and green tea (Camellia sinensis). Rosemary is commonly associated with two antioxidant molecules, carnosic acid and carnosol. Carnosic acid and carnosol are free-radical sequestering antioxidant molecules. The antioxidant activity of green tea extracts is generally attributed to the water-soluble flavan-3-ols collectively known as catechins. The reducing agent ascorbic acid is effective in delaying discoloration in fresh meat because it helps maintaining the heme iron in its reduced state. Juice from the acerola cherry (Malpighia emarginata) is a rich source of ascorbic acid, so it is a clean-label ingredient that helps to delay discolouration in
fresh meat. Chelators such as citric acid are also helpful because they bind metal ions, so they are unavailable to initiate oxidative reactions.
Kemin’s natural plant extracts FORTIUM® R, NaturFORT™ can conserve the appearance, taste and quality of meat products with a balanced impact on their flavour, colour and odour profiles. With natural plant extracts customers benefit from proprietary flavour extracts developed with effective antioxidant properties for delaying oxidation and increasing shelf life.
1Smith, G.C, Belk, K.E., Sofos, J.N. Tatum, J.D, Williams, S.N. (2000). Economic implications of improved color stability in beef. In E.A. Decker, C. Faustman & C.J. Lopez-Bote (Eds.), Antioxidant in muscle foods: Nutritional strategies to improve quality (pp. 397-426). New York: Wiley Interscience.
2Carr, M.A. and Miller, M.F. (1998) Improvement of retail case life of pork-phase I. Des Moines, Iowa, National Pork Producers Council, pp. 101-107. NPPC Publications, Washington, District of Columbia
3Feiner, G. Meat products handbook: practical science and technology. CRC Press: Boca Raton, Florida, 2006.
4Food Chemistry H.-D. Belitz, W. Grosch, P. Schieberle, 4th revised and extended.
5Mancini, R.A., Hunt, M.C. (2005). Current research in meat color. Meat Science. 71, 100-121.
6Pegg, R. B. and Shahidi, F. (2012) Off flavors and rancidity in foods. In L.M.L. Nollet (Ed.), Handbook of meat, poultry and seafood quality (pp.127-139). Second Edition. Ames, Iowa: Wiley & Sons.