1. Tracing processes of rigor mortis and subsequent resolution of chicken breast
muscle using a texture analyzer
Chunbao Li, Peilei Shi, Chang Xu, Xinglian Xu, Guanghong Zhou *
National Center of Meat Quality and Safety Control, MOST; Key Lab of Meat Processing and Quality Control, MOE, Nanjing Agricultural University, Nanjing 210095, PR China
a r t i c l e i n f o
Article history:
Received 30 April 2009
Received in revised form 26 October 2009
Accepted 28 October 2009
Available online 3 November 2009
Keywords:
Rigor mortis
Thaw rigor
Struggle at slaughter
Texture analyzer
a b s t r a c t
Rigor mortis is an important change affecting meat palatability. However, there seems no efficient way to
continuously and automatically track the whole process of rigor mortis and subsequent resolution. This
study is to explore a method to realize the traceability of the onset and development of rigor mortis of
muscles using a texture analyzer. A penetration analysis was proven feasible to track the changes of mus-
cle within 48 h postmortem. Chicken breasts were penetrated using a 50 mm probe holding until
172,800 s (48 h) immediately after bleeding. The results confirmed that ambient temperature had a sig-
nificant effect on the process of rigor mortis and its subsequent resolution, that thaw rigor occurred for
frozen muscles before rigor, and that struggle at slaughter accelerated the rigor process. The established
approach would give us more accurate details on postmortem physicochemical changes in skeletal
muscle.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Rigor mortis is one of the most important physicochemical
changes in skeletal muscles occurring at a relatively earlier post-
mortem period and then maintaining for a certain period, which
results in an increasing toughness of meat (Lawrie and Ledward,
2006). The rigor process usually includes two distinct phases: a
delay period and a rapid phase (Bate Smith and Bendall, 1949).
After maximum rigor, the muscle undergoes a longer period of
resolution.
Since 1930s, numerous scientists have focused on methods to
determine the onset and process of rigor mortis, including elastic-
ity (Bate Smith, 1939), ultramicroscopic observation (Suzuki,
1976), tensile and adhesive properties (Currie and Wolfe, 1980),
myotonometry (Vain et al., 1992), isometric tension (Hertzman
et al., 1993), NMR and NIR (Tornberg et al., 2000) and sonoelastic-
ity (Ayadi et al., 2007). The above studies have given us a profound
understanding on rigor mortis of skeletal muscles, especially of
‘‘red” muscles, and a technological guidance to control meat qual-
ity (Lawrie and Ledward, 2006). However, samples of single muscle
fibers, single muscle bundles or imparted muscle samples were
usually applied in these studies, and thus it is difficult to give a
more accurate depiction of rigor mortis of an intact muscle.
Thaw rigor is another type of physicochemical changes occur-
ring for pre-rigor, frozen skeletal muscles (Kushmerick and Davies,
1968), which affects meat quality (Dransfield, 1996), especially of
fish (Cappeln and Jessen, 2001; Ersoy et al., 2008). However, few
data are available to give a more accurate depiction of thaw rigor
of an intact muscle.
The objective of the present study is to provide a feasible meth-
od to track continuously and automatically the processes of rigor
mortis and its subsequent resolution of fresh chicken breasts and
thawed muscles frozen before rigor.
2. Material and methods
Due to its fiber structure, muscular tissue is an anisotropic
material. It also demonstrates viscoelastic properties postmor-
tem, since it undergoes several changes, e.g. actomyosin complex
formation and subsequent resolution of rigor mortis. Therefore,
the force necessary to maintain a constant penetration during
texture analysis will vary with time (Fig. 1). The relationship
between force and time is of a complex nature and will depend
on, e.g. fiber orientation, and different viscoelastic factors. It
can, however, be represented schematically in mathematical
terms as
F ¼ fðtÞ
The present study aims at providing a tool to investigate the rig-
or process on a macroscopic level by identifying different regimes
in the relationship between force at constant penetration (F) and
postmortem time (t).
0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfoodeng.2009.10.040
* Corresponding author.
E-mail address: ghzhou@njau.edu.cn (G. Zhou).
Journal of Food Engineering 100 (2010) 388–391
Contents lists available at ScienceDirect
Journal of Food Engineering
journal homepage: www.elsevier.com/locate/jfoodeng
2. 2.1. Sampling
Sixteen broilers (Sanhuang, 10 male, 6 female) were bought
from a live free market (Weigang, Nanjing), which is 700 m away
from the laboratory. The animals were individually handled at
two-day intervals by severing carotid, trachea and esophagus
according to the requirements of Jiangsu Administrative Measures
for Experimental Animals (Jiangsu Directive 2008-45). The left and
right breast muscles (Pectoralis) were cut out intact. Both muscles
were loosely wrapped in plastic film immediately to avoid mois-
ture evaporation. Then, two experiments were performed.
Experiment I: The left breast muscles were used to track the
changes of rigor mortis at ca. 4 or 15 °C.
Experiment II: The right breast muscles were used to track the
changes of thaw rigor at 4 or 15 °C. The muscles were immediately
frozen at À18 °C just after slaughter. After 48 h, the frozen samples
were completely defrosted by running water (temperature: ca.
10 °C) within 2.5 h.
2.2. Penetration test
Changes of rigor mortis or thaw rigor and corresponding resolu-
tion were tracked under a texture analyzer (TAXT2i, Stable Micro
Systems Ltd., Godalming, UK). Meat samples wrapped in plastic
film were placed flat on the platform of the machine and then pen-
etrated by a 50 mm-diameter probe. In Experiment I, the time
intervals were less than 30 min (actually, 20–25 min) between
bleeding and the beginning of penetration. In Experiment II, the
intervals were 135–150 min from the beginning of defrosting and
the onset of penetration.
Meat samples were penetrated by a trigger force of 0.02 N at an
acquisition rate of 0.10 pps (point per second). The test parameters
were set as follows: pre test speed, 2.0 mm/s; test speed, 1.0 mm/s;
post test speed, 2.0 mm/s. The penetration distance or deformation
(e) was 2.0 mm. The probe was held at 2.0 mm till 172,800 s.
(48 h). Then, a curve was obtained showing the change of rigor
mortis and its subsequent resolution.
2.3. Statistical analysis
By the initial setting of the acquisition rate of compression of
0.10 pps, 17,280 pairs of force–time data were extracted from each
curve using the software Texture Expert English 1.22. The data
were separated into delay, rigor and resolution stages.
The ‘‘delay stage” was designated as the period when the initial
force kept relatively constant. The initial force (F0) and the delay
period (t0) were applied.
The ‘‘rigor stage” was designated as the period when the force
continued to increase till the maximum. The maximum force (Fmax)
and corresponding time (tmax) were used.
The ‘‘resolution stage” was designated as the period when the
force declined till constant.
The above parameters at 4 and 15 °C were compared using
ANOVA. At each stage, the force (FðtÞ) was regressed as a function
of postmortem time (t).
3. Results and discussion
3.1. Feasibility of the method
As shown in Fig. 2, the change of the force (F) with time could be
separated into three stages. At the first stage, designated as ‘‘delay
stage”, no significant change occurred. At the second stage, desig-
nated as ‘‘rigor stage”, F value increased rapidly with time. At the
third stage, designated as ‘‘resolution stage”, F value decreased
gradually till constant. The most importantly, the rigor and subse-
quent resolution curve results from 17,280 pairs of force–time
observations, which overcomes the difficulty in the previous meth-
ods to observe at large intervals (usually several hours). Moreover,
the muscle sample is intact during the whole process, which would
Fig. 1. Mechanism for measurement of rigor mortis using a texture analyzer. (a) At
a pre-rigor state, the muscle has normal sarcomere length (L1/n, n is the number of
sarcomeres in a whole myofibril) and fiber diameter (d1). At uniaxial and constant
penetration, penetrating force (F1) is low and relatively constant. (b) At a rigor state,
sarcomere shortens (L2/n < L1/n), concomitant with the increase in muscle fiber
diameter (d2 > d1) and the force (F2 > F1). (c) At a resolution state, sarcomere length
increases (L3/n > L2/n), concomitant with the decrease in muscle fiber diameter
(d3 < d2) and the force (F3 < F2).
C. Li et al. / Journal of Food Engineering 100 (2010) 388–391 389
3. reflect more accurately the change of the whole muscle. This over-
comes the shortcomings in previous studies based on small muscle
sample or simple muscle fibers (bundle) in those muscle fibers do
not shorten or relax at the same time (Klont et al., 1998). Therefore,
it is feasible to apply this method to track the change of postmor-
tem rigor for intact muscle more accurately.
3.2. Rigor change of chicken breast at 4°C and 15°C
Fig. 2 depicts typical change of force at uniaxial, constant pene-
tration of chicken breast as a function of time during rigor mortis.
The chicken Pectoralis muscle had a similar change profile at 4 and
15 °C, including a delay stage, a rigor stage and a resolution stage.
The initial and the maximum forces were similar at the two ambi-
ent temperatures, but the delay and rigor periods were apparently
longer at 4 °C than those at 15 °C (Table 1). It indicates that the
chicken breast muscles undergo contracture to the same extent
at both temperature points, but the contracture rate is slightly low-
er at 4 °C, which is similar to the results of Krompecher (1981).
This is because of lower glycolytic activity at low temperature
(Rhee and Kim, 2001). The force value increased linearly with time
at the rigor stage and decreased exponentially at the resolution
stage at both temperature points (Table 1). The rate of rigor reso-
lution (it equals the difference between maximum force and the
value at complete rigor resolution divided by the time it took) at
4 °C was lower than that at 15 °C. Therefore, high temperature
could not only speed up the processes of rigor, but also accelerate
the rate of rigor resolution of chicken Pectoralis muscle.
3.3. Thaw rigor
As shown in Fig. 3, the force value of thawed chicken breast de-
creased rapidly within the earlier testing period and then had a
slightly gradual decline till constant. The process is just like the
change during the rigor resolution of fresh meat. This indicates that
the muscle could undergo the rigor process at the freezing period
or at the thawing period. In fact, it takes at least several hours
for muscles to be completely frozen when the glycolysis of muscle
continues and thus leads to partial shortening (rigor). Another pos-
sibility is that the muscle that does not deplete the glycogen before
complete freezing would continue to undergo glycolysis and then
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 5 10 15 20 25 30 35 40 45 50
postmortem time (h)
Force(N)
15 4
Fig. 2. Typical curves of force at uniaxial, constant penetration of chicken breast as
a function of time during rigor mortis.
Table 1
Mean ± standard deviation and mathematical models for rigor mortis of chicken breasts at 4 °C and 15 °C*
.
4 °C (n = 7) 15 °C (n = 7) P value
F0 (N)a
0.82 ± 0.10 0.81 ± 0.12 0.12
t0 (h)b
0.18 ± 0.02 0.14 ± 0.02 0.04
Fmax (N)c
1.27 ± 0.12 1.27 ± 0.11 0.21
tmax (h)d
4.14 ± 0.20 3.21 ± 0.15 0.008
Modelse
From 0.5 to 0.68 h (delay stage)
F % 0.82
From 0.68 to 4.14 h (rigor stage)
F % 0.1151t + 0.8271 (R2
= 0.973)
From 4.14 to 48.5 h (resolution stage)
F % 3.2435tÀ0.5705
(R2
= 0.995)
From 0.5 to 0.64 h (delay stage)
F % 0.81
From 0.64 to 3.21 h (rigor stage)
F % 0.2172t + 0.6279 (R2
= 0.979)
From 3.21 to 48.5 h (resolution stage)
F % 2.9123tÀ0.7353
(R2
= 0.970)
*
Three stages including delay, rigor and resolution, were separated according to the data extracted from the curves. The difference between the two temperatures was
regarded as significantly different if p < 0.05.
a
F0 (N): initial force.
b
t0 (h): the delay period.
c
Fmax (N): the maximum force.
d
tmax (h): the time at maximum rigor.
e
F: force at any time; t: postmortem time.
0
2
4
6
8
10
12
0 10 20 30 40 50
postmortem time (h)
Force(N)
15 4
Fig. 3. Typical curves of force at uniaxial, constant penetration of chicken breast as
a function of time during thaw rigor.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 5 10 15 20 25 30 35 40 45 50
postmortem time (h)
force(N)
female male
Fig. 4. Effect of struggle at slaughter on curves of force at uniaxial, constant
penetration as a function of time during rigor mortis of chicken breast from one
male and one female kept at 4 °C.
390 C. Li et al. / Journal of Food Engineering 100 (2010) 388–391
4. rigor (designated as ‘‘thaw rigor”). Therefore, the so-called ‘‘thaw
rigor” could result completely or partially from the muscle short-
ening at the freezing period and/or partially from the muscle short-
ening at the thaw period, which is determined by muscle glycogen
level before freezing, freezing rate and muscle size (Cappeln and
Jessen, 2001).
In the present study, the initial force values of muscles frozen
before rigor were higher than those of fresh muscles, probably
due to a stronger muscle shortening during defrosting.
3.4. Effect of struggle at slaughter on rigor process of muscle
In the present study, although great efforts have been taken
to lessen animals’ struggle at slaughter, struggles still happened
on two animals, one male and one female. Unexpectedly, the
breast muscles did not have the delay and rigor stages (Fig. 4).
This could be because the muscles had depleted all glycogen at
struggle which led to immediate rigor at slaughter (Berri et al.,
2005).
4. Conclusion
A new approach was proven feasible to track more accurately
and continuously the process of rigor and its resolution within a
relatively long period. Using this method, it was confirmed that
chicken breasts had different rates of rigor mortis at two ambient
temperatures, that frozen muscles before rigor underwent ‘‘thaw
rigor” when defrosting, and that struggle at slaughter would result
in muscle shortening.
Acknowledgements
Thanks are given to Mr. Carl Brunius from Swedish University
of Agricultural Sciences for his kind revision suggestion. This
study was funded by 30901126, 200803071024 and the Ear-
marked Fund for Modern Agro-industry Technology Research Sys-
tem (China).
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