1.2.11 Microbiological study after storage.
Description
This article is from the Food Preserving FAQ, by Eric Decker ericnospam@getcomputing.com with numerous contributions by others.
1.2.11 Microbiological study after storage.
Levels of Clostridium-like organisms and C. sporogenes PA 3679 in bread
samples baked at 177 C and stored for 90 days at room temperature (RT).
Uninoculated products baked at 177 C for 30, 40 and so min (Table 4)
initially contained undetectable levels of Clostridium-like organisms.
When stored for 90 days at RT the levels of Clostridium -like organisms
were still undetectable (Table 4).
Therefore, with respect to C. botulinum and other sporeformers, these
products would be safe for human consumption if the initial spore levels
are low and if they are stored at room temperature (23 to 25 C) or lower
for no longer than 90 days.
Clostridium sporo genes PA 3679 were detected in inoculated products
stored at RT for 90 days and counts were 2.29, 1.71 and <1 log10 CFU/g
in products baked at 177 C for 30, 40 and 50 min, respectively, (Table 4).
If present in home-canned quick breads at the inoculation level (3.23
log10 CFU/g), endogenous spores will resist low temperature baking. The
significance of this finding on product safety needs to be further
investigated.
Levels of C. sporogenes PA 3679 in bread samples baked at 177 C and stored
for 90 days in the incubator at 35 C
Uninoculated samples. Clostridium-like organisms counts obtained for
uninoculated products stored in the incubator at 35 C were significantly
higher (P>0.05) for the 30 min baking treatment (2.19 log10 CFU/g) than
counts obtained for similar products stored at RT (Table 4). Several gassy
jars (about 50%) from the same treatment (177 C for 30 min) were found and
had to be autoclaved and discarded before 90 days. Counts were determined
only using the remaining jars and may not totally reflect actual levels of
C. sporo genes PA 3679 in these products. For uninoculated products baked
at 177 C for 40 and 50 min, when stored for 90 days in the incubator at 35
C the levels of Clostridium-like organisms had remained at non-detectable
levels. These data show that 35 C was more favorable to repair and
germination of injured endogenous vegetative cells or their spores.
This incubation temperature ranges in the optimum growth temperature range
(35 to 42 C) for C. sporo genes PA 3679. Product prepared and stored under
these conditions may not be safe to consume. Gombas (6) reported that low
or inefficient heat treatments often result in survival of spores of
clostridia, and their subsequent germination and growth in food systems.
Inoculated samples. As expected, most inoculated products stored in the
incubator at 35 C were gassy and spoiled even faster than similar products
stored at room temperature. Several gassy jars, from the 30 (about 80%)
and 40 (about 75%) min treatment, which could not be kept safely until
end of storage period were autoclaved and discarded. Therefore, counts
were determined only using the remaining jars and may not totally reflect
actual levels of C. sporo genes PA 3679 in these products.
In inoculated products, counts of C. sporo genes PA 3679 were <1 log10
CFU/g after baking at 177 C for 50 min (Table 4). Counts of C. sporo genes
PA 3679 remained at levels of <1 log10 CFUIg when stored for 90 days at
room temperature and 35 C, respectively, (Table 4). The ability of spores
to repair is related to factors, such as composition, pH, and a of the
medium (6). These factors are also dependent upon the intensity of heat
injury, structure of spores and storage conditions, particularly storage
temperature and sodium chloride (NaCl) (6,7). Heat resistance of spores
can be affected by various factors including spore structure,
composition and pH of the sporulation medium (6,7,10).
TABLE 3. Counts (log10 CFU/g) of viable vegetative cells of C. sporogenes
PA 3679 in bread samples before and after baking.
Samples Before baking After baking
Baking temp. 177 C 191 C 204 C
Baking time (min) 30 40 50 45 50 55 40 45 50
Uninoculated <1 <1 <1 <1 <1 <1 <1 <1 <1
Inoculated
3.23a
1.99c 1.77c <1
2.99a 2.21c 1.75c
1.83c 1.99c l.71c
Eight samples studied; two replicates; FDT plating in duplicates.
a.b.c Means with same superscripts are not statistically different
(P>0.05).
TABLE 4. Counts (log10 CFU/g) of viable vegetative cell of C. sporogenes
PA 3679 of bread baked at 177 C and stored for 90 days at RT and at 35 C.
Uninoculated
Bake Time Before bake After bake After storage
RT 35 C
30 min <1 <1 <1 2.19c
40 min <1 <1 <1 <1
50 min <1 <1 <1 <1
Innoculated
Bake Time Before Bake After Bake After Storage
RT 35 C
30 min 3.23a 1.99b 2.29b 1.73b
40min 3.23a 1.77b 1.71c 2.59b
50 min 3.23a <1 <1 <1
Eight samples studies; two replicates~ plating or FDT in duplicates.
a,b,c Means with same superscripts are not significantly different
(P>0.05).
From a food safety standpoint, results of this study showed that
inadequate heat treatment (177 C for 30 min) of this type of product
coupled with favorable storage conditions (35 C for 90 days) could
lead to a health risk from consumption of these foods. The significance
of the survival of inoculated C. sporo genes PA 3679 for all baking and
storage treatments evaluated needs to be further investigated. Baking at
a temperature of 177 C, even though resulted in highly desirable product
appearance, did not result in a safe product (totally free of inoculated
Clostridium after storage) for human consumption, especially when
baked products were stored under conditions (35 C ), which favor spore
germination. High baking temperatures (191 and 204 C) were not usually
desirable from a consumer acceptance standpoint because these temperatures
affected the texture and appearance of the products. During these
treatments, excessive crust formation occurred after 55 min baking and
this would affect consumer's acceptability, even though desirable to
enhance the microbial quality of the products. The standard procedure for
home-canned quick bread (5) recommends baking at 191 C for 50 min. This
treatment resulted in non-detectable levels of sporeformers in
uninoculated breads after 8 h of storage at room temperature. An extended
storage study of this and other temperature-time combinations will be of
critical interest to determine safer baking and storage procedures for
this type of product.
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