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A lecture on myoglobin and meat chemistry Chris Raines did in my Food Chemistry class. He was a great teacher and a generous guy. I think he would be glad if his work was available to everyone
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My Oh Myoglobin !
Dr. Chris Raines [email protected] 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
Myoglobin FDSCI 405 C.R. Raines
Undesirable color costs money
Myoglobin FDSCI 405 C.R. Raines
• 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
C.R. Raines
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
C.R. Raines
• 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
Myoglobin FDSCI 400 C.R. Raines
• Maintaining the ‘bloomed layer’ is key to keeping meat looking fresh
Omb & COMb
DMb
Another color, form of Mb
Myoglobin FDSCI 400 C.R. Raines
OMb OMb Dinytrosylhemochrome
+ Heat
Cured Meat Color
Myoglobin FDSCI 400 C.R. Raines
Nitrosylhemochrome
Visible light spectrum
Myoglobin FDSCI 400 C.R. Raines
Objective color measurements
Myoglobin FDSCI 405 C.R. Raines
• 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
Myoglobin FDSCI 400 C.R. Raines
4 meats × 7 lights = 28 colors
Color management :: Packaging
Myoglobin FDSCI 400 C.R. Raines
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
350 Ag Sciences & Industries16 Meats Laboratory…and points between
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