Meat Color, Water-Holding Capacity, and Lipid Oxidation Flashcards

Flashcards covering key meat science concepts including meat color, myoglobin biochemistry, packaging systems, water-holding capacity, postmortem pH decline, PSE/DFD conditions, lipid chemistry, and oxidation mechanisms.

What five primary quality traits are evaluated in fresh meat?

The five primary quality traits are color, texture, firmness, water-holding capacity (WHC), and fat quality/quantity.

How are hue, chroma, and value defined in meat color evaluation?

Hue is the basic color related to wavelength (e.g., red, blue, green, yellow); Chroma is the purity, intensity, or saturation of color; Value is the overall light reflectance or brightness.

What do the $L^*$, $a^*$, and $b^*$ color measurement values represent?

$L^*$ measures lightness/whiteness ($0 = \text{Black}$, $100 = \text{White}$); $a^*$ measures the red-green axis (positive = red, negative = green); $b^*$ measures the yellow-blue axis (positive = yellow, negative = blue).

What are the typical normal fresh meat colors for beef, pork, lamb, and poultry?

Beef is bright cherry red, pork is grayish pink, lamb is brick red, and poultry is gray-white.

What is the structural composition of myoglobin and what binds at its six binding sites?

Myoglobin consists of a globin protein, a heme ring, and an iron atom. Four binding sites connect to the heme ring, one to histidine in the globin protein, and the sixth site interacts with compounds like $O_2$, water, carbon monoxide ($CO$), or nitric oxide ($NO$).

How do deoxymyoglobin, oxymyoglobin, and metmyoglobin differ in iron oxidation state, oxygen condition, and visual appearance?

Deoxymyoglobin has reduced iron ($Fe^{2+}$), little to no $O_2$ bound, and a purplish-red/dark red color; oxymyoglobin has reduced iron ($Fe^{2+}$), $O_2$ bound, and a bright cherry red color; metmyoglobin has oxidized iron ($Fe^{3+}$) and a brown color.

What is blooming in fresh meat and how long does it take to develop?

Blooming is the process in which freshly cut meat exposed to air binds oxygen to form oxymyoglobin, producing a bright red surface color within roughly $30\text{--}45\,\text{minutes}$.

What pigment gives cured meat its stable color after heating?

Nitrosohemochrome, which forms after sodium nitrite is reduced to nitric oxide ($NO$), binds to myoglobin, and the meat is heated.

How do High-$O_2$ MAP, CO MAP, and vacuum packaging affect meat color?

High-$O_2$ MAP supports oxymyoglobin and a bright red appearance; CO MAP uses $0.4\%$ $CO$ to form carboxymyoglobin; vacuum packaging maintains low oxygen so meat remains largely deoxymyoglobin until opened.

What are the three forms of water in muscle, their approximate percentages, and their retention characteristics?

Bound water ($4\text{--}5\%$) is tightly held to charged protein groups and not released during processing; Immobilized water ($16\text{--}17\%$) is physically trapped in myofibrillar spaces and contributes to juiciness; Free water ($\sim 79\%$) is held by capillary forces and easily lost as purge/drip.

Why is water-holding capacity (WHC) at its lowest near the isoelectric point ($pI$) of muscle proteins?

At the $pI$ (approx $pH\,5.0\text{--}5.2$), net charge on proteins is approximately zero, eliminating electrostatic repulsion. As a result, protein structures pack tightly together, leaving minimal space to retain water.

What is the steric effect in muscle and what proportion of water-holding capacity does it explain?

The steric effect refers to physical restrictions on space within the myofibril caused by muscle contraction and actomyosin formation during rigor. It accounts for about two-thirds ($2/3$) of overall WHC.

How do PSE and DFD meat compare in stress pattern, rate of pH decline, ultimate pH, and surface appearance?

PSE (Pale, Soft, Exudative) results from short-term stress causing rapid pH decline while warm, producing pale, exudative meat with poor WHC; DFD (Dark, Firm, Dry) results from prolonged stress depleting glycogen, producing a high ultimate pH, high WHC, and a dark, dry surface appearance.

What are the genetic mechanisms and effects of the Halothane/PSS gene and the RN gene in swine?

The Halothane/PSS gene involves a mutated sarcoplasmic reticulum calcium-channel protein that causes excessive $Ca^{2+}$ release and rapid pH decline; the RN gene alters AMP-kinase regulation, leading to higher glycogen stores and an excessively low ultimate pH.

What is an isobestic wavelength, and what is the specific value emphasized for fresh myoglobin pigments?

An isobestic wavelength is a wavelength at which two or more pigments reflect equal amounts of light; for the three major fresh myoglobin pigments, it is approximately $525\,\text{nm}$.

What causes iridescence in cooked or cut meat, and how does it differ from myoglobin discoloration?

Iridescence is a shiny, rainbow-like appearance caused by light diffraction on muscle microstructure when cut perpendicular to fibers (e.g., semitendinosus), rather than a chemical change in myoglobin redox state.

How is fatty acid shorthand notation (such as C18:1 n-9) decoded?

C18 represents 18 carbon atoms; :1 represents 1 double bond; n-9 indicates the location of the double bond starting from the omega/methyl end.

How do chain length, degree of saturation, and cis double bonds influence fat melting point?

Longer chain length and higher saturation increase the melting point due to tighter molecular packing; cis double bonds create kinks that disrupt packing, lowering the melting point and softening the fat.

What is the difference between lipolysis and lipid oxidation?

Lipolysis is the enzymatic hydrolysis of triglycerides by lipases into free fatty acids; lipid oxidation is the oxidative reaction of unsaturated fatty acids with oxygen, resulting in rancidity and off-flavors.

What occurs during the three stages of lipid oxidation (Initiation, Propagation, Termination)?

Initiation forms a free radical via electron/H removal; Propagation occurs when a radical attacks unsaturated fatty acids to create new lipid radicals; Termination occurs when radicals combine or react with antioxidants to stop the chain reaction.

What is Warmed-Over Flavor (WOF) and why does reheating make it noticeable?

WOF is a rapid oxidative flavor deterioration (often described as a 'wet dog' odor/flavor) in cooked, refrigerated meat; reheating volatilizes accumulated oxidation breakdown products so they are readily perceived.

What strategies can meat processors use to slow lipid oxidation?

Processors can limit oxygen (vacuum packaging), keep meat cold (refrigeration/freezing), control catalysts (curing with nitrite), use antioxidants (stabilize radicals), use chelators (bind metal catalysts like phosphates), and reduce unsaturation in feeds.

How do sodium phosphates and sodium chloride (salt) work to improve WHC in processed meat?

Sodium phosphates raise the pH away from the isoelectric point ($pI$); sodium chloride alters the protein charge/environment relationship and promotes extraction and swelling of Salt-Soluble Myofibrillar Proteins (SSP).

Which meat raw materials are classified as High Binding vs Filler based on connective tissue content?

High Binding materials (low connective tissue) include bull meat, cow meat, beef chucks, boneless pork shoulders, lean trim, and beef rounds; Filler materials (high connective tissue) include ox lips, pork stomachs, skin/snouts/lips, livers, backfat, tripe, and spleens.