Ghent University

Laboratory for Animal Nutrition and Animal Product Quality

Member of the Food2Know network – www.food2know.be

Kleurvorming en kleurstabiliteit van

vers vlees

S. De Smet

Colour perception

of meat

• Quantity of pigment (myoglobin) and chemical

state

• Reflection characteristics of the surface

• Spectrum of the light used

PSE Normal

Myoglobin = heme

protein

Predominant meat pigment (80-95% of the meat pigment)

Stores O2 for aerobic metabolism

Water-soluble

Present in cytosol

Purplish in color

Protein portion: globin

Non-protein portion: heme

(porphyrin ring with iron)

Myoglobin chemistry

~ meat colour Heme group in protein

Protein can denature

and detach Hist 93

Hist 64

6th position on

iron for ligand

binding

Heme iron

Myoglobin chemistry

~ meat colour

• Quantity of myoglobin

– Differences between species and muscles (‘red’ vs. ‘white’

muscle fibers)

• Chemical state of myoglobin

– Ferrous Fe++ (red or purple) vs. ferric Fe+++ (red or brown)

• Ligand

– None, water, O2, NO, CO, Zn…

• Physical state of protein

– Native (red purple or brown) vs. denatured (pink or brown)

Myoglobin content in

meat ~ species

Myoglobin (mg/g wet tissue)

Tuna 0.5-1

Poultry 1-2

Pork 1-3

Veal 1-3

Lamb 4-8

Young beef 4-10

Mature beef 16-20

Fresh Meat Myoglobin

Redox Forms

Deoxy-

myoglobin

Fe++

Oxy-

myoglobin

Fe++

Met-

myoglobin

Fe+++

Melvin Hunt (USA) & Chris Raines (USA)

Melvin Hunt (USA) & Chris Raines (USA)

“Practical” Meat Colour

Triangle

Deoxymyoglobin

No oxygen - Fe++

Oxymyoglobin

Oxygen - Fe++

+ Oxygen

MRA: Reduction and

oxygen consumption

Metmyoglobin

Oxidized - Fe+++

Oxidation and

oxygen consumption

1. Cut

2. Bloom

3. Vacuum pkg.

Mancini & Hunt (2005)

Meat Science 71: 100-121

Myoglobin redox

interconversions on the

surface of meat

Mancini & Hunt (2005)

Meat Science 71: 100-121

Denatured metmyoglobin

(Cooked meats)

Cooking temperatures denature the various forms

of myoglobin to create denatured metmyoglobin

(purple) (red) (oxidized brown)

(cooked brown)

Heme iron detaches from

globin and oxidizes Insoluble and catalyzes

oxidation

Myoglobin Oxidation

Pathways

Objective measurement

of colour and colour

stability

• Colour

– CIE L*, a*, b* values

– Hue, saturation

• Colour stability

– Decline in a* value and

saturation in time

– % pigment composition

based on reflectance

spectra

Myoglobin

reflectance spectra

Factors affecting

meat colour

• Animal genetics and production

• Temperature and chilling

• Muscle differences

• Muscle chemistry: OC & MRA & NADH

• Meat pH, meat quality and lipid oxidation

• Postmortem age of the meat

• Ground versus whole muscle

• Oxygen partial pressure and exposure

• Packaging type and atmospheres

• Processing ingredients and heating

(Source: D Gerrard, adapted from Briskey, 1964)

Pork Quality Development

Myoglobin & O2

Partial Pressure

Oxymyoglobin

Deoxymyoglobin

0 5 10 15 20 %O2

0

% M

b F

orm

100

0 20 40 60 80 100 120 140 Oxygen Partial Pressure, mmHg

Metmyoglobin Air

Melvin Hunt (USA) & Chris Raines (USA)

Colour development at

meat surface

~ D

~ p

d oxymyoglobin

deoxymyoglobin

metmyoglobin

oxygen

bright red

brown

purple red

~ 1/A

d ~ p (partial O2 pressure)

D (O2 diffusion rate)

1/A (residual activity of mitochondria = respiration)

high O2 quick formation of oxymyoglobin

low O2 slow formation of metmyoglobin

Vac Pkg or

Fresh Cut DMb Exposure,

mins to O2 DMb

OMb

DMb

OMb Exposure, hrs to O2

OMb MMb Extended O2

Exposure DMb Partial Discolouration

OMb MMb DMb

Discoloured in O2

MMb DMb

Melvin Hunt (USA) & Chris Raines (USA)

Layering of Mb in meat with

and without exposure to 02

Vacuum packed

or Fresh Cut DMb

DMb

OMb Exposure, hrs to O2

Exposure, mins to O2

DMb OMb

Discoloured in O2

MMb DMb

OMb MMb Extended O2

Exposure DMb

DMb

OMb MMb Partial

Discolouration

Melvin Hunt (USA) & Chris Raines (USA)

Factors affecting

meat colour

• Muscle differences: huge variation in colour shelf life

– Myoglobin content

– Oxygen consumption (OC)

– Metmyoglobin reducing ability (MRA)

– Metabolites (e.g. NADH)

• Postmortem age of the meat

– OC, MRA and NADH decrease

– Colour shelf life decreases

– Ability to bloom increases

~ muscle fiber type

~ perimortem conditions

Oxygen

consumption (OC)

• Meat Enzymes & Mb compete for O2

• More OC = Less O2 penetration

• More OC = Slower bloom

• More OC at higher temperature

• More OC earlier postmortem

• More OC in more “red” type muscle

No OC, little or no MRA

Metmyoglobin reducing

activity (MRA)

• Enzymes (and non-enzymatic) iron

reduction delays MMb accumulation

• MRA differs between muscles

• MRA decreases postmortem

• More MRA at higher temperature

• Repeated use decreases MRA

MRA must conserve for colour shelf life

Metmyoglobin

reduction

• Transfer of one electron from a donating

source to the ferric heme of metmyoglobin

• NADH: an essential component but

declines early postmortem !

• The location of the NADH pool as well as

mechanisms that regenerate NADH in

postmortem muscle are unclear

Improve colour

stability

If we do not add substrates, we must rely on

natural muscle colour chemistry

Saturate muscle with antioxidants in feed

Effective rapid chilling and “normal” pH decline

Minimize post-mortem storage

Minimize processing to conserve MRA

Colour stability :

effect of vitamin E

Concentrate

supplementation

Richardson (2001)

Natural supply from

grass

Titration of vitamin E vs

colour stability and

TBARS in beef

100

80

60

40

20

0

0 2 4 6 8

-tocopherol ( g/g fresh muscle)

% m

etm

yoglo

bin

3-3.7 g/g

TB

AR

S (

g M

DA

/g m

eat)

Richardson (2002) after Arnold et al.

(1993) and Liu et al. (1995, 1996)

8

6

4

2

0

Meat colour

• Most important factor that determines

consumer buying decision

• Knowledge of myoglobin chemistry and

factors that affect colour important to

understand problems

• Thanks for your attention