Mechanism of drip formation
Muscle consists of bundles of muscle fibres (75-92 % of muscle
volume), connective tissues, blood vessels, nerve fibres, and
extracellular fluid. The fibres are the structural units (Fig. 1). They
may stretch from one end of a muscle to the other. Within each fibre,
there are usually at least 1000 sub-units called myofibrils that extend
the entire length of the fibre. Each myofibril is made up of
sarcomeres. The sarcomere is the repeating structural unit of the
myofibril. It is the basic unit in which events of the muscle’s
contraction-relaxation cycle occur. Sarcomere length is not constant
in pre-rigor muscle. The dimensions are dependent upon the state of
contraction at the time the muscle is examined. When a muscle is at
rest, the typical length of a sarcomere is 2.5 microns. Sarcomeres
contract or shorten as muscles enter rigor. They shorten little if the
muscle enters rigor at 10-15°C. Increased shortening occurs both at
lower temperatures (cold shortening) and at higher temperatures
The causes of drip in meat
Newsletter 02/6 December 2002
Meat Technology Update
Figure 1. Diagrammatic representation of muscle
structure (Judge, M. et al. 1989).
(rigor, or heat shortening). Severely cold-shortened sarcomeres
can be less than half their normal length.
Once an animal is slaughtered, oxygen supply to the muscles
ceases. Thereafter, the main biochemical activity is the conversion
of glycogen to lactic acid and other products – a process called
glycolysis. Glycolysis will continue until a pH is reached (ultimate
pH) where the enzymes that facilitate it become inactivated or the
muscle glycogen is used up. As post-mortem glycolysis proceeds,
the muscle loses the ability to extend and contract as it becomes
set in rigor mortis.
In living muscle most of the volume of a muscle fibre is occupied by
myofibrils. There is little space between them. The water and the
associated protein are located within the myofibril. About 4 to 6 h
post mortem the fibre bundles have shrunk away from one another
leaving gaps. After 24 h there are also gaps between individual
fibres. Post mortem then, myofibrils shrink laterally, and it is likely
that the drip originates by being expelled from the sarcomeres,
particularly shortened ones, as they shrink. The exudation of drip
from the meat surfaces does not take place abruptly at the onset of
rigor; rather it is a gradual process. It appears that the fluid
expelled from the myofibrils at onset of rigor initially accumulates
within the muscle, from where it gradually migrates to its exterior.
Shortening of the myofibrils and fibres hastens this migration.
• The vacuum packaging process;
• temperature of storage of vacuum packs;
• time of storage; and
• whether they are frozen after ageing.
The existence of drip loss from meat as an ongoing phenomenon is
evidenced by the widespread use of soaker pads in vacuum packs
and retail packs. Drip loss – particularly from meat that has been
frozen or vacuum-packaged, or both – is of considerable commercial
significance.
From this brief discussion of some of the factors that contribute to
drip, it is clear that too little known about how to vary these factors in
order to minimise drip loss. Current research will provide some
information, however additional investigations are required.