LPT 321: Meat Science (1+1)

PHYSICO-CHEMICAL CHANGES DURING FROZEN STORAGE OF MEAT

• Freezer/frozen storage is a highly effective means of preservation.
• It is a form of low temperature dehydration.
• However, it has been shown that even under ideal frozen stored condition a certain amount of deterioration in quality of meat so not uncommon.

Alteration of the state of proteins

• The physical state of the muscle plasma (globulin and albumen proteins) is considerably altered.
• When meat is frozen below –2°C the formation of ice crystals so raises the concentration of these proteins that they become insoluble and do not regain their solubility when the meat is thawed.

Weep or drip

• Weeping denotes the presence of a watery, bloodstained fluid, which escapes from frozen meat when thawed and consists mainly of water, together with salts, extractives, proteins, peptides, amino acids, lactic acids, purines, vitamins of B-complex and damaged blood corpuscles.
• The latter are responsible for the pink coloration of the fluid and are readily recognizable on microscopic examination.
• Weeping is an undesirable feature and most insurmountable disadvantage of freezer stored meat.
• This is caused partly by the rupture of the muscle cells and tissues by large crystals of ice, and partly by the permanent irreversible change in the muscle plasm, which prevent frozen muscles from reabsorbing water on thawing.
• The size of the ice crystals in frozen meat bears a direct relationship to the damage done to the cells and therefore to the amount of weeping or drip.
• Slow freezing produce large ice crystals, which mechanically rupture the thin sheath of the muscle fibres; rapid freezing incurs less time in the zone of maximum ice formation and the smaller crystals will cause little or no damage.
• The amount of drip is greater in beef than in mutton, lamb or pork, but the better the original quality of a beef carcass the less on the average will be the drip from the meat after thawing.
• Quarters of frozen beef defrosted at 10ºC for 3 days and cut into large wholesale joints lose about 1-2% of their weight during the following day while smaller joints of the retail trade lose 1.5 to 2.5%.
• The rate of thawing in both meat and fish is of less importance than the rate of freezing, for if large crystals of ice have already formed the damage done to the muscle is irremediable.
• The drip is minimized if thawing is very slow.
• One method employed for beef is to subject the meat to a temperature of 0ºC with 70% RH, gradually increasing the temperature to 10ºC and RH to 90%; the fore quarter requires 65 h for complete thawing and the hind quarter 80 h.
• The faster the rate of breakdown of ATP in muscle the more rapid is the onset of rigor mortis and the greater the release of fluid from the muscles.
• If the rate of breakdown of ATP could be slowed, i.e. rigor mortis is delayed; less free fluid would be available for drip formation on subsequent freezing and thawing.
• Again, meat which has a high pH prior freezing has a low drip when thawed, and a useful diminution of drip from butcher’s cuts can be brought about when pH of meat before freezing is 6.1 – 6.3.

Rancidity

• Oxidative process in general is slowed, but in case of prolonged storage of meat in freezer, oxidative changes occur, with fat breaking down into free fatty acids and glycerine.
• The storage temperature and exposure to light are critical factors governing the pace of the onset of rancidity.

 An enhancement in tenderness of meat and a depression in juiciness is the main changes on sensory attributes caused by freezing of meat