As part of defining the problem, we looked at data on pathogens found in lots of
seeds that had been traced to illness outbreaks. Only two seed lots were reported to
be contaminated. The levels of pathogens enumerated in these lots were Salmonella
muenchen at 16. 2 CFU/kg and Salmonella mbandaka at 13. 3 CFU/kg. That certainly
is a low level of contamination, probably not enough to make anyone ill. It works
out, on average, to having to eat thirty-six 10-g servings of sprouts to ingest just one
pathogenic bacterium, if there were no increase in pathogens from that level found in the
seeds. So why have so many people gotten ill from eating green sprouts?
Unfortunately, pathogens do increase in numbers in sprouts that have been
conventionally grown. Many studies have found a huge increase in pathogens during the process of growing the sprouts. Obviously, at least one pathogen must be
present when the seeds are planted. The standard process of growing green sprouts is
in a slowly rotating drum, at room temperature, with “irrigation” water added every
15–30 minutes. This is a great environment not only for growing sprouts but also for
growing pathogens such as Salmonella, E. coli and even Listeria monocytogenes.
Our hypothesis is that the real “problem” is the huge increase in pathogens that
can occur during the process of growing sprouts. Therefore, we set out to find a
method to prevent that increase. We explored several paths, including organic acids,
bacteriocins and competitive inhibition with lactic acid bacteria. All good ideas, but
none of them really worked very well. What did work, and what the rest of this article will describe, was growing the sprouts at approximately 40 °F.
Preventing Pathogen Growth in Sprouts: What Worked
The first thing we did was to move from the conventional drum system to growing sprouts in what could be a retail container. This not only made the experiments
much easier to run, but it also just seemed like a better way to grow and merchandise
sprouts. Once the seeds are planted, the sprouts are not touched until the end-user
(consumer or foodservice) “harvests” them for use on a sandwich, salad or other use.
The container was designed so that it would contain all the water the seeds would
need to grow into sprouts but also keep the seeds from “drowning.” Therefore, our
container held 35 g of hydrated seeds (hydrated seeds = ~1 g of air-dried seeds that
had imbibed about 1 g of water) on a nonwoven hydrophobic “platform” and 60 g
of water for the seeds to use as they grew. The container was also designed to provide adequate ventilation, as determined by measuring oxygen and carbon dioxide
levels in the container during seed germination and growth (Figure 1).
Results of Inoculated Pack Pathogen Studies
Seeds were disinfected ( 2,000 ppm NaOCl adjusted to pH 6.0 with acetic acid for
15 minutes), rinsed and hydrated as described above. Thirty-five grams of hydrated
seeds and 60 g of water were planted in a container. At this point, the samples
were inoculated with either a cocktail of five pathogenic strains of Salmonella or
four strains of E. coli O157:H7. The target inoculation level was 103 to 105 CFU/g.
Enough trays were prepared so that at each evaluation time, three individual trays
could be analyzed separately. The sprouts were then grown at 70 °F and 40 °F for 2
or 21 days, respectively. Following the appropriate growth period, trays were placed
at 40 °F for the shelf-life phase of the
study. All studies were run at Deibel
Labs in Madison, WI. The geometric
mean of the three samples was calculated, and the data are displayed below.
Figure 2 provides a good visualization of the results. With seeds germinated and grown at 70 °F, there was the
expected 4- to 6-log increase in the population of both Salmonella and E. coli.
However, when the sprouts were grown
at 40 °F, there was not only no increase
in the populations of either pathogen,
but there was a several-log decrease in
both Salmonella and E. coli.
As can be seen in Tables 1 and 2,
there was no increase during the subsequent 4-week shelf life of these sprouts.
Interestingly, growing sprouts at 40 °F
not only prevents the growth of
Salmonella and E. coli, but also significantly
increases product shelf life.
We conducted several additional
inoculated pack studies with crimson
clover where we looked at the addition
of potential inhibitors, such as organic
acids and bacteriocins, in the soak water.
The results were essentially the same
as reported above: Germination and
growth at 40 °F was the key variable in
preventing the multiplication of
Salmonella or E. coli. There was no significant
additional inhibition observed with the
use of these inhibiting compounds.
We also replicated this study with
alfalfa seeds. We got the same results as
shown with crimson clover: a several-log
Figure 1. Tray with Filtered Water Reservoir and Pad, after Planting of Hydrated Seeds and
Growth Period ( 21 Days at 40 °F)
70 °F 40 °F
Figure 2. Effects of Sprout Growth Temperature on Pathogens