Meat

The main responsible of the red color of meat is myoglobin. The color in fresh meat is determined by the relative amount of the three differents forms myoglobin can assume:

Deoxymyoglobin (Fe2+) (purple)	+ O2 - O2	Oxymyoglobin (Fe2+) (bright red)
Deoxymyoglobin (Fe2+) (purple)	autoxidation	Metmyoglobin (Fe3+) (brown)

The ratio deoxy/oxy depends on the partial pressure of oxygen. The reddish-purple colour of vacuum packaged meat results from deoxymyoglobin. In presence of oxygen, oxymyoglobin is formed and meat assumes a bright red color. A slow but continuous autoxidation leads to the formation of metmyoglobin which causes brown or tan colouring.

Fresh meat has an enzymatic system to reduce metmyoglobin back to myoglobin, thys system seems to be similar to that which reduces methemoglobin to hemoglobin in erytrocites. Fresh meat has then the ability to change between all three states. But as meat ages, it loses the ability to change back to oxymyoglobin or deoxymyoglobin and eventually stays the brown/tan colour of metmyoglobin. The reduction of metmyoglobin can be easily reversed at low oxygen partial pressure for this reason packaging of meat is conveniently accomplished with material non-permeable to oxygen. In this condition all myoglobin is present as deoxymyoglobin and when the package is opened it will be transformed in oxymyoglobin and meat will assume a bright red color.

Metmyoglobin is unable to bind oxygen but can form complexes, having spectra similar to oxymioglobin, with NO, N₃^-, CN^- and thus they can impart a "fresh" color to meat (see meat curing).

The color of meat is also influenced by the age of the animal, the species, sex, diet, and even the exercise it gets. The level of myoglobin increases with age and this results in a darker color. The same happens in exercised muscles and thus the same animal can have variations of color in its muscles.

Changes in color occuring while meat is frozen or stored in the refrigerator are normal (see what said above) and do not affect meat safety. These changes are minimized by using freezer-type wrapping and by expelling as much air as possible from the package.

When spoilage occurs in addition to the color change, the meat will have an off odor, be sticky or tacky to the touch, or it may be slimy. If meat has developed these characteristics, it should not be used.

Meat color

Meat aging

Meat aging is the process where meat becomes more tender as it is stored under refrigeration temperatures. Tenderization is useful to reduce shrinkage of the tissues and loss of moisture. The tenderness improvement comes from the action of meat naturally occurring enzymes (neutral protease and collagenase) breaking down the muscle proteins. Meat improves in tenderness very rapidly from day 1 to day 7, less rapidly from day 7 to day 14, and very slowly after day 14.

For most cuts, 14 days of aging is the optimum time necessary to achieve tenderness. For some "white table cloth" hotels and restaurants, longer aging times will be used to maximize tenderness and to enrich flavor.

Papain (INS 1101ii), Bromelain (INS 1101iii), Ficin (INS 1101iv) are cysteine proteases used as additives to accelerate the process of meat tenderization.

The term "meat curing" refers to methods used to preserve meat, e.g., salting, smoking.

Salting

The addition of salts in opportune concentrations reduces water activity thus inibithing the growth of microrganism responsible of spoilage. Salts also reduce the activity of enzymes presents in the muscle. Salted meat changes upon cooking to grayish-brown (denaturated metmyoglobin).

Nitrates and Nitrites

Sodium or potassium nitrite is most commonly used in modern curing formulas. The natural enzymes present in meat tend to reduce nitrite to nitric oxide, which combines with myoglobin to give the typical "pink" cured color when heated. This is a slow process when it is allowed to occur naturally so reducing agents are sometimes added to accelerate these reactions.

Nitrates and Nitrites are used as additives mainly to preserve the red color of meat but they also have antimicrobial activity (generally used in a mixture with NaCl). They are important for the protection of non-sterilized meat against the infection by Clostridium botulinum (at 100 mg/kg). At 50 mg/kg they confer a characteristic taste to the meat. They are toxic only at high doses (formation of methemoglobin) but in presence of amines can lead to the formation of carginogenic nitrosamines.

 

Nitrosamines

Secondary aliphatic or aromatic amines and N-substituted amide react with nitrous acid to yield Nitrosamines and nitrosamides, a class of powerful carcinogenics.

The true reactive agent is nitrosonium ion (NO⁺) or similar molecule such as nitrous anhydrate (H₂O⁺ - NO) and nitrosyl halides (XNO). These molecules are product from the decomposition of nitrous acid at low pH. Notes: Primary and tertiary amines can yield nitrosamines only after the decomposition of the product of reaction with nitrous acid and succesive rearrangements.