Muscle to Meat Conversion and Quality Parameters

A set of flashcards covering muscle physiology, the biochemical conversion of muscle to meat, rigor mortis, and factors affecting meat quality such as pH, PSE, and DFD conditions.

What are the two primary myofibrillar proteins found in skeletal muscle?

Actin (thin filaments) and myosin (thick filaments).

What are the components that make up the thin filaments (actin) in muscle?

Two chains of polymerised F-actin coiled together, associated with tropomyosin and troponin.

What is the physical structure of a myosin protein?

A double globular head and two $\alpha$-helical polypeptide chains.

In living muscle, what event initiates contraction?

A nerve impulse causing $Ca^{2+}$ release from the sarcoplasmic reticulum.

What specific action does $Ca^{2+}$ take upon binding to troponin?

It alters its shape and repositions tropomyosin to expose the active site on actin.

What name is given to the permanent complex formed between myosin and actin during rigor mortis?

The actomyosin complex.

Why is ATP required for muscle relaxation?

It is used to pump $Ca^{2+}$ back into the sarcoplasmic reticulum.

What percentage of available binding sites are typically used during normal living muscle contraction?

$$\sim 20\%$$

What is the biochemical cause of the rigid actomyosin complex in rigor mortis?

ATP depletion prevents relaxation while $Ca^{2+}$ leakage from the sarcoplasmic reticulum causes myosin heads to link to all available actin sites.

How does energy metabolism change in muscle tissue immediately after slaughter?

It switches from aerobic to anaerobic pathways due to the interruption of blood supply and $O_2$ delivery.

During anaerobic glycolysis post-mortem, what is pyruvate converted into?

Lactic acid (LA).

What is the typical pH shift in normal pork muscle from slaughter to its ultimate state?

From $\sim 7.2$ at slaughter to an ultimate pH of approximately $5.4-5.5$.

What are the two main reasons the post-mortem pH fall might cease?

(1) All glycogen is depleted or insensitive to breakdown, or (2) glycolytic enzymes are inactivated by the low pH.

Which four meat quality attributes are most influenced by the ultimate pH?

Textural quality, water-holding capacity (WHC), resistance to microbial spoilage, and colour.

What are the characteristics of PSE pork?

Pale, Soft, and Exudative, occurring with a low ultimate pH of $\sim 5.1-5.2$.

Which stress-sensitive pig breeds are associated with the development of PSE pork?

Landrace and Piétrain.

Why does PSE pork exhibit a wet surface?

Proteins near their isoelectric point bind water poorly, resulting in poor water-holding capacity and the production of an exudate.

What causes the pale colour seen in PSE pork?

Myoglobin is oxidised to metmyoglobin and the open, denatured muscle fibre structure scatters light.

What are the characteristics of DFD meat?

Dark, Firm, and Dry, occurring with a high ultimate pH of $\sim 5.9-6.8$.

What causes the development of DFD conditions in pigs or cattle?

Vigorous exercise or stress before slaughter depletes glycogen reserves, resulting in insufficient lactic acid production.

What is the specific term used for DFD beef?

"Dark cutting meat".

Why does DFD meat have a reduced shelf life?

The high pH favours bacterial growth, which enhances susceptibility to microbial spoilage.

How can both PSE and DFD conditions be prevented pre-slaughter?

Through careful handling by avoiding excitement, exhaustion, or prolonged transport of the animals.

What is the rigor mortis onset time for poultry?

$$5\,min-1\,h$$

What is the rigor mortis onset time for beef and lamb?

$$6-12\,h$$

Under what conditions does cold shortening (CS) occur?

If muscle is cooled below $10\,^\circ C$ (specifically in the range of $0-15\,^\circ C$) before rigor mortis is complete.

At what temperature and for how long is meat typically held during the conditioning (ageing) process?

Just above freezing ($\sim 1-5\,^\circ C$) for several days to weeks.

What is the function of calpain enzymes during meat conditioning?

Active at $\sim\text{pH}\,5$, they attack minor "gap" proteins such as titin to increase tenderness.

What role do cathepsins play in the tenderisation of meat?

They may weaken collagen cross-links in the non-helical telopeptide region of tropocollagen.

How does electrical stimulation (ES) help prevent cold shortening in beef and lamb?

It accelerates rigor mortis onset to within $1-2\,h$, allowing for rapid cooling without the risk of severe contraction.