add flavor to foods as seasonings in
different cuisines. However, their microbiological quality is a great concern because most spices are grown in countries
without standard Western hygiene and
sanitation conditions. 32 Cumin (
Cumi-num cyminum L.) is one of the most
popular spices because of its distinctive
flavor and aroma. 33 Cumin seeds are
susceptible to contamination with bacteria such as Escherichia coli, Salmonella,
Clostridium perfringens, and Bacillus cereus
because of cross-contamination with
soil, water, and manure in the field,
possibly causing public health risks
and foodborne pathogen outbreaks. 34
Therefore, it is necessary to develop an
efficient spice decontamination method
to ensure food safety and maintain food
In one of our studies in RF processing of cumin seeds, we observed huge
variations in heating rate and microbial
inactivation when the two batches of
samples were equilibrated to the same
aw. 7 After the same RF heating time,
the population of both Salmonella and
Enterococcus faecium for the first batch
of cumin seed was under the detection limit. But there were almost no
log reductions of both bacteria for the
second-batch sample. When analyzed,
the moisture contents of the two batch
samples were different due to variations
in sample characteristics. Spices are imported from various countries and there
is a huge variability in samples. 7 Thermal images of two batch samples clearly
show the differences in heating rates
(Figure 1). When conditioned to the
same MC, the heating rate was similar
and so also the microbial inactivation.
We monitored cold-spot temperatures
of both first- and second-batch samples
and found that cold-spot temperatures
were similar when achieving more than
5-log reduction of Salmonella. The cold
spot of the third-batch sample with
the same MC was heated to the same
cold-spot temperature of the previous
two batches, and a 5-log reduction was
achieved as expected. Therefore, cold-spot temperature and MC are two critical process control parameters in RF
processing. These two parameters must
be monitored in the food industry as
Critical Control Points when this novel
technology is deployed as a microbial
After the same RF heating time under the same treatment conditions,
E. faecium always showed a lower log reduction than that of Salmonella. Therefore, we can conclude that E. faecium is
a good surrogate for Salmonella cocktail
strains in cumin seeds for RF processing. 7 n
Long Chen is a doctoral student in the Department
of Biological Systems Engineering at the University of
Jeyamkondan Subbiah, Ph.D., P.E., is the Kenneth E. Morrison Distinguished Professor of Food
Engineering in Biological Systems Engineering and
Food Science & Technology at the University of Nebraska–Lincoln.
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Figure 1. Top Surface Temperature Distribution Comparison between the First- and Second-Batch Cumin Samples after the Same RF Heating Time