268 Chapter 14
and selective breeding may be tools to
decrease phage specifi city, which would
increase the range of bacteria susceptible to
control. Further steps may also include devel-
opment of multistrain phage cocktails with
complementary activity aimed at different
cell structure targets. In addition, little infor-
mation is available on lactic acid bacteria
spoilage control with phage. This bacterial
group contains common spoilers of vacuum -
packaged products. Most current research
focuses on Pseudomonas spp. and B.
thermosphacta.High Pressure Processing
High hydrostatic pressure (HHP) treatment
involves placing packaged meat in a pressure
vessel and applying isostatic water pressure
of 100 to 900 MPa. HHP processing is
considered nonthermal, since temperatures
increase only 3 ° C for every 100 MPa applied
(Aymerich et al. 2008 ). Equipment for HHP
is commercially available, including manu-
facturers Avure Technologies (United States)
and Nicolas Correa Hyperbaric (Spain). HHP
kills bacterial cells through a combination of
actions, with the bacterial membrane the
primary site of damage. Gram - negative
bacteria are more susceptible, followed by
Gram - positive bacteria and spores (Hugas
et al. 2002 ). Linton et al. (2004) reported that
the microfl ora of chicken mince became less
diverse and shifted to Gram - positive bacteria
after HHP treatment. Regarding cell shape,
rods (elongated) are more susceptible than
cocci (round). It is generally believed that
HHP does not signifi cantly change the
sensory quality of meats, although cooked
color (at 150 MPa), oxidation of ferrous myo-
globin (at 400 MPa), and lipid oxidation has
been reported in fresh and marinated meats
(Hugas et al. 2002 ). Results of studies
showing prevention of meat spoilage with
HHP treatment are summarized in Table
14.3.adjusting themselves to regain ability to
infect mutated bacteria (Weitz et al. 2005 ).
Another major drawback of phage technol-
ogy is the diffi culty of getting phages to kill
mesophilic and psychrotrophic microorgan-
isms in a food product stored at refrigeration
temperatures. Phage activity requires actively
growing and dividing hosts, which for many
mesophiles generally does not occur in a
chilled stored product. Although psychotro-
phs can grow at chill temperatures, their
growth rate is slow. For phage to be effective
against spoilers their activity must be certain
at low storage temperatures.
Another issue is that large initial bacterial
populations (3 to 5 log 10 CFU/g) are needed
for phage activity (Greer 2005 ; Hudson et al.
2005 ). Since phage attack on bacteria initi-
ates upon contact, the probability of this
encounter increases if populations of both
counterparts are large. If spoilage bacterial
numbers are low on a product due to the
effectiveness of other control measures, then
phage treatment may not be valuable.
However, in the case of refrigerated meat
spoilage, where generally large population
numbers are present, phage technology may
have merit.
The nature of the environment can play a
signifi cant role in phage attack, with viscous
product physically reducing the possibility
of phage - host interaction. Other drawbacks
include the possibility of phage - mediated
virulence gene transfer between bacteria.
Lysogenic conversion of lytic phage to
temperate phage could result in host protec-
tion from other phages. As a last argument,
the average consumer associates viruses
with disease, and their perception of adding
viruses to foods has yet to be investigated.
To summarize, the biggest challenge in
utilizing phage technology in modern meat
spoilage control might be high phage speci-
fi city, which makes it diffi cult to achieve the
desired results of shelf life extension for
naturally contaminated meat products.
Genetic manipulation of available phages