Food Protection Trends - May/June 2019 - 203

Statistical analyses
All experimental procedures were replicated three
times. E. coli population data and temperature data were
analyzed using liner mixed modeling with a compound
symmetry covariance structure, a compound symmetry with
heterogeneous time variances structure, or an unstructured
covariance matrix combined with a PROC MIXED
procedure in the Statistical Analysis Software 9.4 (SAS;
Cary, NC). These covariance structures were chosen on the
basis of Akaike's information criterion (AIC) to obtain the
best covariance structure for microbial population data from
each product. This was considered a four-factor, repeatedmeasures experiment and was analyzed accordingly. A Type
III test for fixed effects was also conducted.
Least squares means (LSMEANS) of microbial populations were calculated by use of the LSMEANS statement in
SAS and were used to compare the significance of effects on
variables and variable interactions at a significance threshold of P ≤ 0.05. To reduce variability in temperature data,
five values near each time point were averaged. Effects of
temperature data variables and variable interactions were also
compared at a significance threshold of P ≤ 0.05.
RESULTS
Temperature data analysis
The effects of cover type, storage location, and depth
variables (main effects), as well as variable interactions,
were analyzed for statistical significance. To perform the
cooling study, temperature was decreased dramatically over
time during the cooling process, and time was therefore
not included as a main effect; rather, all main effects and
interactions were analyzed at six individual time points (0, 2,
4, 8, 12, and 24 hours). The significance of effects of specific
temperature variables will be discussed below.
For taco meat, no variable had a significant effect at time
point 0 hours. At the 2-hour time point, effects of cover
type and storage location by product depth were significant.
Uncovered pans were significantly cooler than single-covered or double-covered pans. The 2-inch depth in the freezer
was cooler than both the 3-inch depth in the freezer and the
2-inch depth with an ice bath in the refrigerator. However,
the 3-inch product depth stored in the refrigerator with an ice
bath was significantly the coolest at the 2-hour time point.
At 4- and 8-hour time points, the effects of storage location, storage location by product depth, and cover type
were significant for taco meat. The freezer cooled 2-inch
product depths more rapidly than 2-inch product depths
in the refrigerator at 4 and 8 hours of cooling. The 3-inch
product depths cooled more rapidly in the refrigerator for
the first 4 hours, but by time point 8, products at 3-inch
depths were at lower temperatures in the freezer. At 8
hours of cooling, the 2-inch product depth in the refrigerator cooled less rapidly than the 3-inch product depth in
the refrigerator.

Uncovered pans of taco meat cooled more rapidly than
single-covered or double-covered pans at 4 and 8 hours.
Storage in the refrigerator or freezer was the only significant
factor for cooling at the 12- and 24-hour time points. Pans
removed from the freezer and placed in the refrigerator after
the 8-hour time point remained at a lower temperature than
those stored in the refrigerator with an ice bath.
No variable had a significant effect at the 0-hour time
point for chili con carne with beans. At 2 hours of cooling,
storage location, product depth, storage location by product
depth, and cover type all had a significant effect. The 2-inch
product depths in the freezer were cooler than 3-inch depths
in the freezer. The 2- and 3-inch product depths stored in
the refrigerator with an ice bath were cooler than 3-inch
depths in the freezer. Uncovered product depths were also
significantly cooler than single- or double-covered product
depths at the 2-hour time point.
At the 4-hour time point, the effects of depth, storage
location by depth, and cover type were significant for the
cooling of chili con carne with beans. The pans stored in
the freezer at 2-inch product depths cooled more quickly
3-inch depths stored in the freezer. The 3-inch product
depths cooled more quickly in the refrigerator than in the
freezer after 4 hours of cooling. The uncovered pans cooled
more rapidly during the first 4 hours than single- or doublecovered pans.
At the 8-hour time point, the effects of location, storage
location by depth, and cover were significant, or nearly
significant (P = 0.0518) at 12-hours, for chili con carne with
beans. At these time points, the 2-inch product depth in the
freezer cooled most rapidly. The uncovered pans were cooler
than the double covered pans after 8 and 12 hours of cooling.
At the 24-hour time point, the effects of storage location
and depth by cover type were significant for chili con carne
with beans. Pans in the refrigerator at the 24-hour time point
were cooler by a small (1.3°F), but statistically significant,
amount. The 3-inch product depths stored uncovered
were the lowest in temperature at the 24-hour time point,
with the double-covered 2-inch pans the second coolest
in comparison, though the difference between the 3-inch
uncovered pans and 2-inch double-covered pans was not
significant (P = 0.0535).
At time point 0, the effect of product depth was significant,
as 3-inch product depths were observed to have a significantly higher temperature than 2-inch product depths for the
low-sodium marinara sauce. At 2 or 4 hours of cooling, there
were no significant effects on cooling. However, the effect of
depth was approaching significance at 2 hours (P = 0.0681)
and at 4 hours (P = 0.0533), with the 2-inch product depths
cooler than the 3-inch depths. The effects of storage location
and depth were significant at the 8-hour time point. Freezer-cooled pans had cooled to lower temperatures at this time
point, and 3-inch product depths continued to be significantly higher in temperature than 2-inch product depths. Storage

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