Meat Color

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

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)

(4) 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.

(5) Activity of bacteria

Pigment	Catalyst	New pigment
Oxymyoglobin	oxidation + bacteria ---------------------------->	Metmyoglobin (-OH)
Metmyoglobin	bacteria ---------------------------->	Choleglobin (-H₂O₂)
Metmyoglobin	bacteria ---------------------------->	Sulfmyoglobin (-SH)

(6) Curing

Myoglobin
Oxymyoglobin
Metmyoglobin

plus NO
--------->

Nitric oxide myoglobin

plus heat
--------->

Nitrosylhemochromogen

(7) 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.

Links to related sites on the Internet

Myoglobin HyperBase at W.M. Keck Center
Geometric Aspects of Protein Structure and Function