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Lecture 13

ANSC 2340 Lecture 13: Meat Colour and Connective Tissue

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University of Guelph
Animal Science
ANSC 2340
Ira Mandell

1. Meat Colour and Connective Tissue (ANSC 2340) Color of Meat:  Defined as the total impression seen by the eye  Influenced by: o Viewing conditions  Appearance characteristics of the meat  Concentration and state of heme pigments  How the meat is illuminated  Retail stores and taste panel testing???? o Individual variation amongst humans in ability to detect color o Structure and texture of the muscle  Influence reflection and absorption of light  PSE versus DFD Major Pigments in Meat  Hemoglobin: pigment of blood o Found in RBC; fo2 O and 2O transport  Myoglobin: pigment of muscles o Found in sarcoplasm  For storing2O in muscle fibers  Made up of a globular protein and a non-protein heme component o Similar in structure to hemoglobin  Hgb is 4 times the size of myoglobin o Oxidation state of the heme ring has major influence on meat color (Fe – +3 ferrousvs Fe —ferric) o * Catalases & cytochrome oxidase — both enzymes include heme units in their structure  Will make up 80 to 90% (50 to 90%) of total pigment in meat depending on hemoglobin o How well has animal been bled out?  Catalases & cytochrome oxidase o Both enzymes include heme units in their structure  Overall concentration of pigments influence how red the meat is- o Higher myoglobin levels in fibers with strong aerobic metabolism        Red or white fibers? Species Differences in Myoglobin Content: Other Factors Influencing Myoglobin Content  Males (intact males) have more myoglobin than females or castrates: Bulls often have dark red meat  Game animals have more myoglobin than intensively reared animals o Why?  Chicken breast versus the pigeon breast  Wild ruminants versus farm ruminants  Pale muscles have a higher contribution of pigment from hemoglobin from residual blood  Diet effects: low Fe content decreases redness in meat: o milk-fed veal – creating an Fe deficiency anemia  Underwater mammals: high myoglobin concentrations due to physiological adaptation to being under water for long periods of time Different Forms of Myoglobin:  Pigments can react with compounds or ions to cause color change o Chemical state of Fe in heme ring: Fe or Fe o This can also happen with Hgb with lethal consequences for humans & animals  Myoglobin is purple in color o Color of fresh cut meat; this form of myoglobin also called deoxymyoglobin (iron is in a ferrous state – Fe+2) +2 o In uncut meat, it is in reduced form Fe (ferrous) and can only react with water  Myoglobin is purple in color o If we cut the meat: Top 2 to 6 mm will react wi2h O over a 15 to 60 minute period; meat is allowed to bloom  Bloom period required before beef carcasses can be graded (enable the deoxymyoglobin to react with 2 to form oxymyoglobin)  Lean color important for beef quality grading  Beef graders will l downgrade carcasses that produce DFD beef  Form oxymyoglobin which is red in color  Get the desirable red color that consumers prefer  Purple state à bright red state o Cherry red colour associated with fresh cut meat and greater meat quality  Oxymyoglobin o Stability (how long it stays in this state) dependent on sup2ly of O  Enzymes in meat use up O2  O2is used up at high pH & temperature  Due to environment effects on enzyme activity  If meat is kept near freezing it minimize the rate of enzyme activity and O utilization (keep O2 at high conc.) 2  Results on meat color? Stays red  Storage at 0 vs 5 degrees C? put off surface browning from 2 to 7 days  Color & shelf life means money  Surface browning: bright red will turn to metmyoglobin à results in a brown colour (meat may be discounted or removed)  how long it remains as oxymyoglobin depends on O2 supply  Metmyoglobin o If only small amounts 2f O available, Fe becomes oxidized and changes to a brown color called metmyoglobin o Going from Fe to Fe (from ferrous à ferric state is what produces metmyoglobin) o Problem with merchandizing meat  After 2 to 3 days in air  If > 20% of surface pigment oxidized: decreased sales in grocery stores o See this with packages of meat kept in the meat case for extended period of time (NOT POSTMORTEM AGEING OF MEAT)  Consumers assess the freshness of beef based on whether cheery red colour is present – stores will discount meat which has discoloration  Metmyoglobin o Amount influenced by use of Vitamin E in beef diets  Feedlot producers use to feed high levels of Vitamin E for last 100 days on feed  delay the conversion of oxymyoglobin to metmyoglobin  Practice has changed with modified atmosphere packaging  This type of packaging allows the processors to inject gas or gasses (ex. carbon monoxide) that will help preserve colour and prevent deterioration  3 compounds bound to Hgb in the live animal that can be lethal: o carbon monoxide, cyanide, nitrates o Affects its ability to carry oxygen PINK COLOUR IN HAMS: achieved by treating meat with nitrites and heat Impact of Light Scattering on Color:  Light scattering and pH o As pH declines post-mortem, more light scattering occurs; light bouncing off the object and being reflected back o In the case of PSE meat with its low pH, see protein denaturation including myosin o This increases light scattering which helps contribute to the meat looking pale to an observer  Reflectance = light hitting an object and part or all of that light is reflected off the object  Scattering: light is interacting with the object requiring absorption and light is emitted back  Refraction: the bending of light as it passes from one substance to another; light here is absorbed by the object   More light scattering with low pH meat o Affects light path going thru meat  Scattered light more visible to the observer  What causes light scattering?  Reflection of light is responsible for light scattering  Besides protein denaturation, there are changes in fiber membranes, orientation + overlapping of myofilaments, fluid compartments which increases light scattering  More light scattering with low pH meat o Affects absorbance of light by myoglobin o Myoglobin is the dominant pigment in meat  Little myoglobin in pork to begin with due to species effect as pork is not as red in color as beef or lamb  if less light is available to be absorbed by myoglobin with PSE pork, this will contribute to the paleness PSE vs DFD Meat: Color  PSE: paleness also caused by large proportion of free water which is located between muscle cells due to loss of water holding capacity o Thus, increased extracellular water impacts:  Light reflectance (with more “free” water, increase in # of surfaces to reflect light)  Absorption (decrease light absorption)  Color intensity (increase paleness)  DFD meat: High WHC due to high concentrations of intracellular water (amount of denaturation and drip loss) o This helps minimize light reflection and maximize light absorption so the meat looks darker FIBROUS CONNECTIVE TISSUES Collagen:  Most abundant protein in the body (25%)  Why are we interested in collagen? o o Principal structural protein of connective tissue  Connective tissue also contains elastin  FQ videotape mentioned the presence of elastin in meat  Collagen found in tendons, ligaments, bones, cartilage, muscle, skin, blood vessels  Tendons: join muscle to bone  Ligaments: connects bone to bone  Industrial uses of collagen  Leather, glue, cosmetics, food processing o Double-muscled animals have low amounts of connective tissues Why are we interested in collagen regarding muscle?  Structural component: role with muscle? o Form and shape:  Holds muscle fibers together  Protects muscle fibers from being damaged from strong muscle contractions o Supports BV + nerves o Contractile force transmission  Meat quality component: o Responsible for background toughness of meat  When muscle contracts, the contractile force transmitted via connective tissue sheaths to the tendon and then to the bone Collagen (x2)  Where do we find collagen in muscle? o Form of collagen: sheets or cables that are visible to the naked eye o Muscle proper: epimysium  Thick layers (sheaths) located on muscle surface o Fasiculi: perimysium  Thick layers located surrounding bundles of muscle fibers  Myofiber: endomysium  Microscopic strands located around the muscle fiber  Relationship of collagen to gristle  High amounts of collagen in skin – collagen is responsible for leather strength from tanned skins Origin of Collagen:  Tropocollagen o High MW protein o Structural unit of collagen fibril (known as protofibril)  Synthesized in fibroblasts  What are the building blocks? o AA (non-essential; dispensable AA)  this contrasts to muscle proteins which are made up of essential (indispensable) AA and non-essential AA  essential AA: must get from the diet, body can’t synthesize it in adequate quantities or at all o Glycine: Occurs at every 3 AA in the molecule  enables packing of tropocollagen o Proline + Hydroxyproline: 25 to 33% of total AA  Uniqueness of hydroxyproline? Only see it in collagen and elastin  Hydroxyproline content used for? Determining the amount of proteins in CT  Glutamic acid, Alanine, Hydroxylysine o = other common amino acids in tropocollagen  Definition essential AA = amino acids our bodies either can’t synthesize or can’t synthesize in adequate amounts  Think about this: if collagen makes up 25% of proteins in the body, doesn’t it make sense that collagen is a high concentration of non-essential AA? Tropocollagen  Structural unit of the collagen fibril  AA synthesized into 3 polypeptides strands o Specific name of the strand? α chains (19 types)  Arrangement of the strands? Triple helix o Significance? Enhances stability  Once tropocollagen synthesized:(protofibrils) o Aligned in a 1/4 stagger  Produces unique striations o Assembled into collagen fibrils o Synthesis of collagen fibers; fibrils bound to fibers o Fibers laid down in criss cross lattice Collagen (x3)  11-12 different types formed  5 found in muscle  Characteristics of the collagen o Ability to stretch? inelastic o Color?  On an individual fiber basis: colourless  As a sheath or in tendon: white  Strength related to structural+ functional roles o # of fibrils in a fiber dept. on load + stress factors for the collagen fiber o # of fibers, present dependent on tissue load Strength of Collagen:  Presence of intramolecular crosslinks (Xlinks) between PP(polypeptide) strands (alpha chains) within in the tropocollagen molecule  Presence of intermolecular Xlinks (between adjacent tropocollagen molecules)  The two types of Xlinks responsible for collagen having high tensile strength  Effect of diameter o Increase intrafibrillar Xlinks with increasing diameter  Increase toughness o Interfibrillar (intermolecular), noncovalent Xlinks with small diameter fibers  More labile (can be broken down) Effect of Age on Collagen   Young animal o Produces large amounts of new collagen as it grows and develops; this collagen is needed for normal animal structure  Few Xlinks in this collagen and when they are present, they are soluble or reducible  Can be easily broken down  These Xlinks are susceptible to heat— heat in cooking can so
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