Biomedical Engineering Reference
In-Depth Information
changes were minimal and were in agreement with the 2.5 kGy threshold dose for
beef as reported by Sudarmadji and Urbain. 57 If products do not become recontam-
inated during storage, spoilage will result from a secondary flora consisting of
radiation resistant survivors including the Moraxella-Acinetobacter group, Brocho-
thrix thermosphacta , yeasts, and lactic acid bacteria.
In order to minimize microbial contamination, packaging prior to radiation
treatment has been proposed. 22,150 For this purpose MAP using CO 2 flushing is most
commonly used although vacuum packaging is also considered a form of MAP; the
packaging material provides a physical barrier to entrance of recalcitrant microor-
ganisms and the carbon dioxide inhibits the growth of aerobes including Pseudomo-
nas . 151 Using low permeability films (for example, <100 ml of O 2 /m 2 per 24 h atm
measured at 25°C and 98% RH) slow growing, psychrotrophic Gram-positive lactic
acid bacteria predominate following radiation, and souring due to acid production
replaces putrefaction by Pseudomonas. Oxygen impermeable films also limit lipid
oxidation which results in the development of product rancidity. 11 This is of particular
importance since oxidative reactions occur more quickly when fat-containing meats
are processed with ionizing energy. In this regard peroxide values are known to
accumulate more rapidly in radiated compared to non-irradiated fat-containing
meats. 150,152 The development of off-odors, which in some cases has been reported
to dissipate within minutes of treatment, 153 and flavors, which are often dose-related,
have been described by various researchers as being metallic and/or slightly tal-
lowy. 152 In addition, fat sections on the surfaces of the meat may appear distinctly
bleached in comparison to the lean. 152 Additional improvements to minimize flavor
changes were achieved by decreasing the temperature during treatment. Chemical
and biochemical reaction rates decrease concomitantly; however, overall dosages
employed during radurization are often insufficient to inactivate many enzyme sys-
tems. 11 Combining freezing and packaging is ideal although not always possible or
practical. As previously stated, when radiation is applied to frozen foods, reactive
intermediates generated from water radiolysis are immobilized and therefore are
prevented from interacting with each another and/or with the food. 11 It should be
remembered, however, that freezing of foods during treatment also enhances micro-
biological resistance and therefore additional survivors may be expected especially
if marginal doses are utilized.
Urbain and Giddings 154 reported that aerobic mesopilic bacteria levels in vac-
uum-packaged beefsteaks following a 2.5 kGy treatment remained below 10 4 cfu/g
even after 21 days when stored at 4°C. The researchers further reported that treating
steaks with phosphates prior to treatment increased overall acceptability. A subse-
quent report indicated that decreasing the dose to 1 kGy still resulted in an acceptable
product in comparison to the non-irradiated control. 155 Niemand et al. 156 reported a
doubling in the shelf life of vacuum-packaged sirloin samples when stored at 4°C
following radurization using a 2-kGy dose. The extension in shelf life from 4 to
approximately 10 weeks was accompanied by a 99.9% reduction in aerobic plate
count and the elimination of various resident microflora including pseudomonads,
Enterobacteriaceae , and enterococci. 13 The more radiation-resistant lactic acid bac-
teria, principally lactobacilli, and to a lesser extent Leuconostoc spp. and pediococci,
survived and progressively increased with storage time and ultimately resulted in
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