Lean fresh meat has a water content of approximately 70%.
One of the major challenges of meat processing is concerned
with preventing loss of liquid from lean meat tissue during
storage. When meat is cut, a red solution of proteins that is
known as drip (or weep or purge) oozes from the cut
surfaces. For primals, the volume of drip produced in the first
48 hours or so after boning is typically quoted to be about 1 to
10 mL per kg of meat (0.1 to 1%), while for steaks or chops; it
can be 10 times greater.
In meat that is stored for extended periods (e.g. vacuumpackaged
primals), the drip loss gradually increases with
time. A ‘normal’ amount of drip in commercial vacuum packs
of chilled primals is usually regarded to be 1 to 2%. Drip of
more than 1% is unusual for the main seam-boned primals,
but it could be up to 2% for pieces of meat that are subject to
cutting and trimming to achieve a specification. The amount
will be greater in smaller packs where the surface area is
proportionately greater per unit weight. In some of the
scientific investigations upon which information for this article
is based, the pieces of meat were small (for instance eighths
of striploins in one trial) to ensure that different treatment
effects were tested with practical quantities of meat. For
some of these trials, drip losses of 5 to 10% are quoted.
This article briefly discusses the mechanism of drip formation in meat
tissue and its eventual accumulation at the surface. It also considers
a number of factors that people have demonstrated or implicated as
contributors to drip loss.
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.
Pre-slaughter factors that affect drip
From the very few studies where it has been investigated as a
factor in drip loss, diet does not seem to affect the eventual loss of
drip from vacuum packs or retail packs. Animal age may be a
factor, but the evidence is inconclusive however, with drip being
higher in vacuum-packaged muscles of older cattle in one study;
but lower in another study.
Acute stress prior to slaughter has been shown to increase drip
loss and cooking loss in meat that has had no ageing. Moderate
stress of cattle before slaughter is also believed to be a cause of
drip, although there is no direct evidence of this. There is evidence
that when glycogen levels in muscle are low, drip loss is higher -
and low glycogen levels are often related to stress. Investigations
in this area at the Cooperative Research Centre (CRC) for Cattle
and Beef Quality are ongoing.
If animals are stressed to the point they produce high pH meat, drip
loss could actually be reduced. The proteins in high pH meat have
an increased ability to hold water and, while water may be expelled
from sarcomeres, some of it can be bound to proteins. While high
pH meat produces less drip, obviously this is not an acceptable
option for reducing drip in vacuum packs.
Post slaughter factors
Electrical stimulation does not appear to affect drip loss provided
the meat goes into rigor at the same temperature as non-stimulated
meat.
If meat – electrically stimulated or not – goes into rigor at too high a
temperature, drip loss can be increased. Biochemical changes and
physical disruption in muscle that is in rigor is more pronounced if the
temperature stays high for too long. A combination of low pH and
high temperature can promote re-arrangement or denaturation of
proteins. It is known that protein denaturation leads to greater drip
loss and pale meat. Rigor shortening, or heat shortening, can occur
when meat enters rigor at a high temperature (see below). This also
contributes to drip loss. With heavy sides, if the chilling rate is slower
than desirable, electrical stimulation can worsen the problem of drip,
because the sides do not cool sufficiently before they enter rigor.
It is important to stay within the Meat Standards Australia (MSA)
‘abattoir window’ to optimise tenderness. It may benefit drip as well
but an appropriate study has not yet been done.
There is no published evidence that the use of organic acids (e.g.
acetic or lactic) as food safety intervention treatments has any
detrimental effect on drip. In fact in one study there was less drip
loss from striploins that were treated with a mixture of acetic and
lactic acids than from those that weren’t treated. However in some
commercial trials in Australia with vacuum-packed pork and beef
treated with hot (55ºC) acetic acid solution, there were indications of
elevated drip loss in the packed product. The extra drip may have
been due to denaturation of the surface tissue or it may simply have
been due to inadequate drain times after treatment.
Chilling and time to boning
If carcases that aren’t electrically stimulated are cooled too slowly,
some muscles can shorten significantly due to rigor shortening. It is
known that drip loss increases if rigor shortening has occurred.
If carcases that aren’t electrically stimulated are chilled too quickly,
severe cold shortening of some muscles will occur. Drip losses will
increase. If they are restrained by, for instance, aitchbone hanging
(Tenderstretch), muscles can be prevented from cold shortening.
Little muscle shortening and drip loss occurs if the muscles are
cooled to, and retained in, the range 10°C to 15°C. Minimum
shortening occurs at around 12°C.
From published data, spray chilling of beef sides does not appear to
adversely affect drip loss from primals. However, one Kansas study
with sub-primals (insides) found a slight increase in drip (2.0%
compared with 1.7%) after 15 days when spray chilling was
employed.
Hot or warm boning (accelerated boning) per se doesn’t appear to
affect drip loss. In studies where hot boning has been associated
with increased drip loss, cold-induced muscle shortening has
probably occurred because electrical stimulation wasn’t used.
If the meat temperature at boning is high (some deep muscles will
have temperatures near 20°C after normal overnight chilling), it is
important that after the cuts have been packaged, a good rate of
chilling is continued until the stipulated temperature for shipping is
reached.
There is no information on whether there is an influence of the
period of chilled holding (one, two, three or more nights) on drip
loss.
Rapid fall in pH is an indicator of rapid onset of rigor. As already
mentioned, if carcases go into rigor too quickly – before the meat
temperature has fallen – subsequent drip loss may be increased. It
is important that pH does not fall too rapidly. Researchers at the
CRC for Cattle and Beef Quality who have developed (and
continue to refine) the desirable abattoir window for rates of pH and
temperature decline, have not specifically focused on drip loss.
However, there is good reason to believe that if steps are taken in
an abattoir to adhere to the MSA guidelines (the window) then drip
loss will not be excessive. An example is to optimise electrical
stimulation and other electrical inputs on the slaughter floor.
Current research funded by MLA is helping to better determine the
effects of the inputs on various quality traits including drip.
Cutting style
and differences between cuts
The amount of drip is likely to be higher for cuts where much of the
meat surface is freshly cut than from those that are seam-boned.
Investigators have reported much higher drip losses from retail
portions or sub-pri