AGEING OR CONDITIONING OF FRESH MEAT

Table of Contents

Ageing/conditioning:

The holding of unprocessed meat above the freezing point (-1.5^oC) in absence of microbial spoilage is known as ageing or conditioning of meat. It is done to improve the tenderness, juiciness and flavor of meat. Structural weakening of the myofibrils, the intermediate filaments and the intramuscular connective tissues, the endomysium and perimysium contribute to the tenderization of meat during post-mortem ageing. It is a practice in the west to age beef and mutton for tenderization and flavor development.

Mechanism of ageing:

Ageing is mainly associated with changes in protein fraction of meat like

Protein denaturation
Proteolysis
Other chemical reaction like breakdown of ATP

I. Denaturation: Denaturation may be defined as a physical or intermolecular rearrangement which does not involve hydrolysis of the chemical bonds linking the constituent amino acids of the polypeptide chains of proteins. It is generally accompanied by an increase in the reactivity of various chemical groups, a loss of biological activity (in those proteins which are enzymes or hormones), a change in molecular shape or size and a decrease in solubility. Proteins are liable to denature if subjected, during postmortem conditioning, to pH levels below those in vivo, to temperatures above 25°C or below 0°C, to desiccation and to non-physiological salt concentrations.

II. Proteolysis: Denatured proteins are particularly liable to attack by proteolytic enzymes. The increase in tenderness, observed on conditioning, was found many years ago to be associated with an increase in water-soluble nitrogen due to the production of peptides and amino acids from proteins. Proteolysis is brought about by cathepsins and calpains mainly. Collagen fibres appear to swell during conditioning. Lysosomal enzymes can attack the cross-links in the non-helical telopeptide region of collagen.

III. Other chemical reactions: ATP is largely broken down to inosinic acid, inorganic phosphate and ammonia during ageing. Later on, degradation of inosinic acid to phosphate, ribose and hypoxanthine occurs which improves flavor. The breakdown of protein and fat during conditioning also contributes to flavor by producing hydrogen sulphide, ammonia, acetaldehyde, acetone and diacetyl. But prolonged conditioning, e.g. 40-80 days at 0°C is associated with loss of flavor as there may also be reactions of sugars like ribose with free amino acids produced during proteolysis and breakdown of dipeptides like carnosine and anserine which are progressively hydrolysed to β-alanine and histidine.

Although conditioning enhances the water-holding capacity of proteins to some extent, the loss due to denaturation changes and to post-mortem pH fall predominates, and meat exudes fluid post-mortem.

Two enzyme systems which are active during ageing:

I. Cathepsins (lysozomal enzymes) named as A, B, D and L forms. The B, D and L are substrate specific. Cathepsins are lysozomal enzymes released by breakdown of lysozomal membranes during post mortem when pH changes from 7 to 5.5 (ultimate pH). The active sites of the enzymes are:

1. Troponin T (pH<6)

2. Cross-linkages of non-helical telopeptides of collagen

3. Mucopolysachharides of ground substance

4. Actin and myosin at pH<5 and temperature >35^oC

Cathepsins act both on cytoskeletal and connective tissue proteins.

II. Calpains or Calcium activated sarcoplasmic factors (CASF) or calcium-activated neutral proteinases (CANP), calcium-dependent proteinases (CDP) (enzymes found in Sarcoplasm).

The release of Ca++ ion in sarcoplasm initiates the activities of these enzymes.

There are two types of calpains.

a) m-calpains: They activate at milli molar Ca⁺⁺ ion concentration.

b) µ-calpains: They activate at micro molar Ca⁺⁺ ion concentration. Only µ-calpains are active during post mortem because milli molar Ca⁺⁺ ion concentration is not achieved to activate m-calpains.

The active sites of calpains are:

1. Troponin T (pH>6)

2. Desmosin (Z line protein)

3. Connectin (gap filament)

4. M line and tropomyosin

It also weakens the binding of α-actin to Z line.

Effects of ageing:

1. Proteolysis: It increases the water holding capacity of meat proteins associated with increased juiciness and tenderness.

2. It improves the flavor by:

a) Breaking down ATP to hypoxanthine (flavor precursor) via a series of reactions.

b) Breaking down fat and proteins to hydrogen sulphide, ammonia, acetaldehyde and diacetyle (flavoring agents).

Procedure for commercial application in beef:

Carcasses should be chilled i.e. cooled for 1 to 2 days at 0.5 to 3.0^oC prior to holding or ageing.
The sides and quarters should be held for 10 to 12 days at 2 to 3^oC.
Before cutting for retail sale, the quarters should be held at ordinary temperature or at 4 to 7^oC for 24 hours.

In commercial practice, the conditioning is limited to 2 to 3 weeks at about -1.1 to 0^oC.