Objectives:
(1) To demonstrate the relationship between myoglobin and meat color.
(2) To show how the chemical state of myoglobin is related to meat color.
(3) To identify factors associated with the discoloration of fresh meat.
Reading material: Principles of Meat Science (3rd Edition), chapter 6, pages 126 to 131.
Heme pigment found in myoglobin
Hemoglobin — transports O2 from lungs to cells
Myoglobin — stores O2 in cells
Meat color is impacted by the following factors:
(1) Quantity of myoglobin
(a) Age within species — myoglobin loses its affinity for oxygen as age increases.
| Age class | Myoglobin content |
|---|---|
| Veal | 2 mg/g |
| Calf | 4 mg/g |
| Young beef | 8 mg/g |
| Old beef | 18 mg/g |
(b) Species differences — age related as well as differences between “red” versus “white” muscle fibers.
| Species | Color | Myoglobin content |
|---|---|---|
| Pork | Pink | 2 mg/g |
| Lamb | Light red | 6 mg/g |
| Beef | Cherry red | 8 mg/g |
(c) Type of muscle
| Type of muscle | Name | Myoglobin content |
|---|---|---|
| Locomotive | Extensor carpi radialis | 12 mg/g |
| Support | Longissimus dorsi | 6 mg/g |
(2) Chemical state of myoglobin
| Bonds | Compound | Color | Name |
|---|---|---|---|
| Fe++ Ferrous (covalent) | :H2O | Purple | Reduced myoglobin |
| :O2 | Red | Oxymyoglobin | |
| :NO | Cured pink | Nitric oxide myoglobin | |
| :CO | Red | Carboxymyoglobin | |
| Fe+++ Ferric (ionic) | -CN | Red | Cyanmetmyoglobin |
| -OH | Brown | Metmyoglobin | |
| -SH | Green | Sulfmyoglobin | |
| -H2O2 | Green | Choleglobin |
(3) Metmyoglobin Reducing Activity (MRA)
reduced myoglobin —> oxymyoglobin —> metmyoglobin —> reduced myoglobin —> etc.
Differs according to muscle: some high in MRA (longissimus dorsi), some low in MRA (biceps femoris)
- Usually caused by highly oxidized meat or meat that is exposed to high-oxygen atmospheres
- Meat will turn brown around 130°F internal temperature
- Reason why color is not a good indicator of cooked meat
- Usually caused by high pH meat
- Product will stay pink (uncooked color) even with degrees of doneness that should ensure appropriate cooked endpoint are followed.
(5) Vitamin E feeding of cattle
Vitamin E can be fed to livestock to increase the alpha-tocopherol concentration in muscle. Alpha-tocopherol is an antioxidant and retards the conversion of reduced myoglobin and oxymyoglobin to metmyoglobin.
(6) Activity of bacteria
| Pigment | Catalyst | New pigment |
|---|---|---|
| Oxymyoglobin |
oxidation + bacteria —————————-> |
Metmyoglobin (-OH) |
| Metmyoglobin |
bacteria —————————-> |
Choleglobin (-H2O2) |
| Metmyoglobin |
bacteria —————————-> |
Sulfmyoglobin (-SH) |
(7) Curing
| MyoglobinOxymyoglobinMetmyoglobin | plus NO———> | Nitric oxide myoglobin | plus heat———> | Nitrosylhemochromogen |
(8) Wrapping film
Vacuum packages (polyvinylidene chloride-polyvinyl chloride); barrier to O2
Retail film (polyvinyl chloride); permeable to O2
Review of Material — What the student should know:
(1) What causes meat to have a particular color.
(2) The role of bacteria in color development and maintenance.
(3) The influence of oxygen on meat color.
(4) The chemistry behind myoglobin and meat color.
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