30 Chapter 2
1979 ). During the immediate postmortem
period, tissues metabolize glycogen via
anaerobic pathways, lowering pH. ATP is
rapidly consumed, but as reducing equiva-
lents are consumed, it is not regenerated.
Without the plasticizing effect of ATP, actin
and myosin cross - link, the sarcomere short-
ens, fi bers contract, and rigor results. During
the rigor process, muscle cells undergo both
longitudinal and lateral contraction, usually
within 24 hours. WHC decreases during the
postmortem period. Rigor mortis occurs in
beef when the pH drops to 5.9 (Honikel et al.
1981 ). Factors that affect the rate of pH
decline, such as Halothane gene status of pigs
and residual glycogen in the tissues, affect
tenderness, WHC, and color. Factors that
affect the ultimate pH (ryanodine gene status,
stress that alters muscle glycogen content)
also affect these characteristics.
The peak solubility of actin and myosin
occurs between pH 5.7 and 6.0 (Scopes
1964 ). It decreases dramatically as pH drops
from 6.0 to 5.6. These proteins are almost
completely insoluble below pH 4.9.
Sarcoplasmic proteins are soluble between
4.8 and 5.2, regardless of temperature;
however, at or above 37 ° C, even high pH
will not prevent them from precipitating onto
myofi brillar proteins. This decreases WHC
as well as other quality characteristics of
meat. The minimum water - holding capacity
of meat occurs around pH 5.0, which corre-
sponds to the isoelectric point of actomyosin.
In addition, toughness is negatively corre-
lated with initial pH and rate of pH decline
(Zamora et al. 1996 ). Two - thirds of the WHC
losses occurring during rigor are due to loss
of ATP, with the remainder due to pH decline.
The rate of pH decline is partially genetic, in
that pH decreases more rapidly in meat from
some breeds, because of the fi ber - type distri-
bution in the muscle tissue, than it does in
meat from other breeds. Brewer et al. (2002)
reported that carcasses from Duroc and Large
White pigs experienced postmortem purge
losses of 5 – 6%, while those from Pietrain,circulatory competency after harvest requires
that the tissues shift to anaerobic metabolism,
resulting in the accumulation of metabolic
byproducts, including lactic acid, in the
muscle. The pH declines from about 6.8 to
5.7. Endogenous thiol proteinases (cathep-
sins B and L) become activated near pH 5.4.
They are redistributed (intracellularly) during
aging (Spanier et al. 1990 ; Spanier and Miller
1993 ). Proteolytic enzyme activity is temper-
ature - dependent; some (cathepsins B and L)
retain high activity levels even at cooking
temperatures (70 ° C). Pigs with defects in the
ryanodine receptor gene (rn+) undergo exces-
sive (not necessarily rapid) pH decline,
resulting in abnormally acidic conditions in
the meat, which affects water - holding capac-
ity, tenderness, and color (Leach et al. 1996 ;
Bidner et al. 2004 ).
Water - holding capacity (WHC) is the
ability of meat to hold onto its own or added
water when force (heat, pressure) is applied.
Water is the major component (about 75%)
of muscle tissue. Most exists in layers around
polar molecules and between layers of cel-
lular materials. The majority is located in the
intermolecular spaces between the salt - solu-
ble proteins (actin, myosin) of muscle tissue,
which varies depending on various intrinsic
and extrinsic factors (Offer and Knight 1988 ).
Its movement is restricted in a number of
ways that are dependent primarily on the
myofi laments. Some of the factors that alter
the spatial arrangement of the myofi laments
include alterations in net charge induced by
pH changes, screening of charges by anions/
cations, presence of divalent cations (Mg ++ ,
Ca ++ ), denaturing conditions that alter protein
conformation (rapid pH decline while the
carcass temperature is still high), and pres-
ence of plasticizing agents such as ATP and
enzymes (ATPase).
In pre - rigor meat, Mg - ATP = serves to
prevent cross - linking between the contractile
proteins, actin and myosin (Fig. 2.1 ). This
maintains the interfi lamental space such that
water can move in (Siegel and Schmidt