Myoglobin and Meat Color - by Chris Raines (@itweetmeat)

My Oh Myoglobin !

Dr. Chris Raines craines@psu.eduFDSCI 400 350 AS&INovember 18, 2009 16 Meats Lab.

• Understand function of myoglobin (Mb) in meat vs. its function in muscle

• Know interrelationships among various Mb redox forms

• Integrate principles of colorimetry and spectrophotometry with food/meat

• Learn how meat color can be managed via environmental control

Myoglobin (Mb) Basics – Today’s Objectives

Myoglobin FDSCI 400 C.R. Raines

• Globular protein with 8 α-helices and a hydrophobic core

• Responsible for carrying O2 in muscle tissue

• Has an almost ‘instant’ binding tenacity for O2

• Primary pigment of red meat

What is Myoglobin (Mb)?

Myoglobin FDSCI 400 C.R. Raines

Which would have the most Mb?

Myoglobin FDSCI 400 C.R. Raines

Why managing color is so important

Myoglobin FDSCI 400 C.R. Raines

All meat in this photo is equally ‘fresh’

Why the differences?

[Mb] in muscle related to its function

• Different muscle fiber types for different muscle metabolism, ‘use’

• Oxidative and glycolytic• Red (oxidative), White (glycolytic), and

Intermediate (AC/DC) fibers• Muscles different colors due to fiber type

makeup and reflects [Mb]

Myoglobin FDSCI 400 C.R. Raines

Colors define products

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Undesirable color costs money

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• Discoloration costs U.S. meat industry > $500M (combined effect) annually

• Attributed to:– Mismanagement (handling, formulation)– Exhausted enzymes (MMb reductase)

• Includes:– Lost contracts– ‘Reduced for quick sale’ items

Mb general structure

• Mb contains heme (haem) iron, thus more Mb, more FE

• This is why red meat is ‘redder’ than white meatMyoglobin FDSCI 400

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Myoglobin FDSCI 400 C.R. Raines

Species differ in [Mb]

Fresh Pork Fresh Beef

Mb general structure

• Fe state and ligand contribute to overall color by ‘shifting the plane’ and how light is reflected

• O2 is the most widely understood ligand to Mb Myoglobin FDSCI 400

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• Deoxymyoglobin (DMb)– Mb in its native state– Appears purple– Ferrous (Fe2+)

• Oxymyoglobin (OMb)– Mb has been oxygenated (bloomed)– Appears red– Ferrous (Fe2+)

• Metmyoglobin (MMb)– Mb has been oxidized– Appears brown– Ferric (Fe3+)

Mb redox forms

Myoglobin FDSCI 400 C.R. Raines

• Carboxymyoglobin (COMb)– Mb has been exposed to CO– CO binds to Fe more strongly than O2

• Greater binding affinity by Mb

– Appears red– Ferrous (Fe2+)– Very similar to OMb– ≤ 0.4% U.S. legal in U.S. (red meat)– Not regulated in fish (cold smoking)

Mb redox forms

Myoglobin FDSCI 400 C.R. Raines

DEOXYMYOGLOBINDMb Fe++

No O2 present

OXYMYOGLOBINOMb Fe++

Atmospheric O2 present

METMYOGLOBINMMb Fe+++

Low O2 partial pressure

CARBOXYMYOGLOBINCOMb Fe++

CO present

- O2, - e-

+ CO

+ O2- O2,

enzymatic reduction

(MRA)

+ O2, - e-

Interrelationship of Mb redox forms

Myoglobin FDSCI 400 C.R. Raines

Oxygenationaka

‘Blooming’

Oxidation

Know and love this diagram

• Metmyoglobin reductase– A pesky little co-factor– Referred to as MRA (metmyoglobin

reducing ability or activity)– Reduces MMb to DMb– MRA can depend on meat ‘age,’

temperature, pH, [Mb], added ingredients, among other factors

Reducing power

Myoglobin FDSCI 400 C.R. Raines

DMbDMb

OMb

Freshly cut Bloomed, freshBloomed, after

storage

MMb

DMb

OMb

Development of Discoloration

Myoglobin FDSCI 400 C.R. Raines

• Electron balance shifts, enzymes get tired, and MMb develops on surface of intact meat

Development of Discoloration

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• Maintaining the ‘bloomed layer’ is key to keeping meat looking fresh

Omb & COMb

DMb

Another color, form of Mb

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OMb OMb Dinytrosylhemochrome

+ Heat

Cured Meat Color

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Nitrosylhemochrome

Visible light spectrum

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Objective color measurements

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• Spectral data (400 – 700 nm)• ‘Colorspace’ values:

– L* - lightness– a* - redness– b* - yellowness

• Also very important to corroborate spectral data with ‘visual’ color, just like a sensory panel

The painter’s color wheel

Myoglobin FDSCI 400 C.R. Raines

Colorimetry :: L*

Myoglobin FDSCI 400 C.R. Raines

• L* is an assessment of ‘lightness’ – much like from black to white

Colorimetry :: a* and b*

Myoglobin FDSCI 400 C.R. Raines

b* a*

?

Where ‘art’ meets ‘science’

Myoglobin FDSCI 400 C.R. Raines

• Equal parts of complimentary colors yield brown color (did you ever make a ‘color wheel’?)

• a* and b* can be included in algorithms to calculate discoloration OMb:DMb relationships

• Discoloration ratio = a*/b*– Development of MMb

• Chroma (hue angle) = tan-1(b*/a*)– ‘true-red’

• Saturation index = (a*2 + b*2)1/2

– ‘Vividness’

Reflectance of Mb forms

Myoglobin FDSCI 400 C.R. Raines

525 nm

1 – DMb2 – Mmb3 – OMb

Reflectance of Mb forms

Myoglobin FDSCI 400 C.R. Raines

• DMb, OMb and DMb have same reflectance values at 525 nm– Isobestic point

• DMb and an absorbance peak @ 630 nm• 630/525 nm measurements to calculate %Mb

that is MMb

Mb denaturation, MMb-like color development

Myoglobin FDSCI 400 C.R. Raines

Denatured MMb D(O)Mb

Mb denaturation potential indicator of doneness

Color anomalies

Myoglobin FDSCI 400 C.R. Raines

• Premature browning– When meat looks more ‘done’ than the final

cooked temperature suggests– Problematic in oxidized ground beef

• Persistent pinking– When meat looks less ‘done’ than the final

cooked temperature suggests– High pH (and high enzymatic activity) meat– Meat packaged in CO

• Why meat thermometers are best way to correctly assess ‘doneness’

Color management :: Lighting

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4 meats × 7 lights = 28 colors

Color management :: Packaging

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Oxygen-permeable overwrap plastic

Modified atmosphere

packaging (MAP)

Why managing color is so important

Myoglobin FDSCI 400 C.R. Raines

Why the differences?

Summary

Myoglobin FDSCI 400 C.R. Raines

• The primary pigment in meat is myoglobin (Mb)• DMb = purple, OMb = red, MMb = brown• L* = lightness, a* = redness, b* = yellowness• Algorithms relate a* and b* to ‘real’ meat color• 2 reasons to understand meat color chemistry

are premature browning and persistent pinking• 3rd reason is lost value• Packaging helps manage meat color

Questions?

Myoglobin FDSCI 400 C.R. Raines

craines@psu.edu

350 Ag Sciences & Industries16 Meats Laboratory…and points between

meatisneat.wordpress.com@iTweetMeat