410 Part III: Muscle Foods
time of Love’s review, the advent of aquaculture has
made attainable, in theory at least, just such a utopi-
an vision. As every food processor knows, the quali-
ty of the raw material is among the most crucial
variables that affect the quality of the final product.
In fish processing, therefore, the animal’s own indi-
vidual physiological status will to a large extent dic-
tate where quality characteristics will fall within the
constraints set by the species’ physical and bio-
chemical makeup. It is well known that an organ-
ism’s phenotype, including quality characteristics, is
determined by environmental as well as genetic fac-
tors. Indeed, Huss noted in his review (Huss 1995)
that product quality differences within the same fish
species can depend on feeding and rearing condi-
tions, differences wherein can affect postmortem
biochemical processes in the product, which in turn,
affect the involution of quality characteristics in the
fish product. The practice of rearing fish in aquacul-
ture, as opposed to catching wild fish, therefore raises
the tantalizing prospect of managing the quality
characteristics of the fish flesh antemortem, where
individual physiological characteristics, such as those
governing gaping tendency, flesh softening during
storage, and so on, are optimized. To achieve that
goal, the interplay between these physiological para-
meters and environmental and dietary variables
needs to be understood in detail. With the ever-
increasing resolving power of molecular techniques,
such as proteomics, this is fast becoming feasible.
Antemortem Metabolism and Postmortem
Quality in Trout
In mammals, antemortem protease activities have
been shown to affect meat quality and texture (Kris-
tensen et al. 2002, Vaneenaeme et al. 1994). For
example, an antemortem upregulation of calpain
activity in swine (Sus scrofa)will affect postmortem
proteolysis and, hence, meat tenderization (Kris-
tensen et al. 2002). In beef (Bos taurus),a correla-
tion was found between ante- and postmortem activ-
ities of some proteases, but not others (Vaneenaeme
et al. 1994). As discussed in the above section, post-
mortem proteolysis is a matter of considerable im-
portance in the fish and seafood industry, and any
antemortem effects thereon are surely worth investi-
gating.
In a recent study on the feasibility of substituting
fishmeal in rainbow trout diets with protein from
plant sources, 2DE-based proteomics were among
techniques used (Martin et al. 2003a,b; Vilhelmsson
et al. 2004). Concomitantly, various quality charac-
teristics of fillet and body were also measured (De
Francesco et al. 2004, Parisi et al. 2004). Among
the findings was that, according to a triangular sen-
sory test using a trained panel, cooked trout that had
been fed the plant protein diet had higher hardness,
lower juiciness, and lower odor intensity than those
fed the fishmeal-containing diet, indicating an
effect of antemortem metabolism on product tex-
ture. Furthermore, the amount and composition of
free amino acids in the fish flesh was significantly
affected by the diet, as was the postmortem devel-
opment of the free amino acid pool. For example,
while abundance of arginine was found to decrease
during storage of flesh from fishmeal-fed fish, it
increased during storage of plant protein–fed fish
(Table 18.2). The diets had been formulated to have
a nearly identical amino acid composition, and
therefore these results may be taken to indicate
altered postmortem proteolytic activity in the plant
protein–fed fish as compared with the fishmeal-fed
ones.
In the proteomics part of the study, the liver pro-
teome was chosen for investigation, since the liver is
the primary seat of many of the fish’s key metabolic
pathways. This makes a direct comparison of the
proteomic and quality characteristics results diffi-
cult; nevertheless, some interesting observations can
be made. The study identified a number of metabol-
ic pathways sensitive to plant protein substitution in
rainbow trout feed, for example, pathways involved
in cellular protein degradation, fatty acid break-
down, and NADPH metabolism (Table 18.3). In the
context of this chapter, the effects on the proteasome
are particularly noteworthy. The proteasome is a
multisubunit enzyme complex that catalyzes prote-
olysis via the ATP-dependent ubiquitin-proteasome
pathway, which in mammals, is thought to be re-
sponsible for a large fraction of cellular proteolysis
(Craiu et al. 1997, Rock et al. 1994). In rainbow
trout, the ubiquitin-proteasome pathway has been
shown to be downregulated in response to starvation
(Martin et al. 2002) and to have a role in regulating
protein deposition efficiency (Dobly et al. 2004).
Correlating the findings of these two parts of the
study, it seems likely that the difference in texture
and postmortem free amino acid pool development
are affected by antemortem proteasome activity,