Thermal Food Processing

Thermal Processing of Poultry Products 223
Bartlett and Hawke 121 evaluated the heat resistance of L. monocytogenes
strains Scott A (a clinical isolate) and HAL 975E1 (an egg isolate) in the following
five liquid egg products: liquid whole egg (LWE), 10% NaCl whole egg (LWEN),
10% sucrose whole egg (LWES), 10% NaCl egg yolk (EYN), and 10% sucrose
egg yolk (EYS). The presence of salt decreased the water activity (a w) of the
products to a greater extent than sucrose; all water activity values were >0.91
except for salt yolk (a w = 0.867). Survivor curves were constructed and D values
calculated based on data from a 0.2-ml submerged sealed test tube procedure.
The relative heat resistance of L. monocytogenes in the five products was as
follows: heat resistance in salted egg yolk > salted liquid whole egg >> sucrose
egg yolk ≥ sucrose liquid whole egg ≥ liquid whole egg. These thermal resistance
trends were very similar to those reported by Cotterill et al. 97 for S. Oranienburg
in liquid egg products. Based on current U.S. conventional egg pasteurization
requirements, Bartlett and Hawke 121 predicted that process lethalities against L.
monocytogenes Scott A would range from 0.2 log cycle (for salt egg yolk) to 1.8
log cycles (for sucrose LWE). Similar thermal resistance trends were reported by
Palumbo et al., 71 who determined D values for six pooled strains of Salmonella
spp. and for five pooled strains of L. monocytogenes inoculated in plain liquid
egg yolk (EY) and in various EY products containing added salt or added sucrose.
Both pathogens were more heat resistant in EY + 10% NaCl than in EY + 10%
sucrose or in plain EY. Based on D values derived by using a submerged capped
test tube procedure, the lethality of USDA-mandated conventional egg pasteurization
processes was estimated to range from 0.3 log cycle (in EY + 10% NaC1)
to 6.1 log cycles (in plain EY) for Salmonella spp., and from 0.2 log cycle (in
EY + 10% NaCl) to 3.3 log cycles (in EY + 10% sucrose) for L. monocytogenes. 71
In summary, the above four publications indicate that the margin of safety
provided by conventional pasteurization requirements for LWE, plain yolk, and
products containing added sucrose is not large, especially if L. monocytogenes is
present in the raw bulk tank at levels of >100 CFU/ml. In NaCl-supplemented
LWE or yolk, conventional pasteurization would be inadequate to inactivate even
10 CFU/ml of L. monocytogenes. These findings take on additional significance
in light of a recent USDA document that details the criteria for approval to produce
and market liquid egg products with an extended shelf life (i.e., >4 weeks at
4.4°C). 122 In order to gain regulatory approval to produce such products, companies
must pasteurize the product at 60°C for 3.5 min. If alternative thermal
processes are used, the company must provide laboratory data demonstrating that
the thermal process yields a 7 log reduction in the viable L. monocytogenes
population. As noted previously, the data of Foegeding and Leasor 80 and Bartlett
and Hawke 121 clearly demonstrate that a 3.5-min process at 60°C represents only
a 1.7- to 2.7-D Listeria inactivation process. Thus, the proposed 7-D lethality
requirement appears to be unduly harsh and may not permit production of liquid
egg products of acceptable organoleptic and functional quality. At the time of
this review, no studies documenting the thermal resistance of Listeria spp. in
liquid egg white or in liquid egg substitutes were available in the published
scientific literature.