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Effectiveness of Acid/Chlorine rinses at
Eliminating E. coli O157:H7 from Alfalfa Seeds Prior to Sprouting
Megan Lang,
Steve Ingham, Barbara Ingham; University of Wisconsin-Madison
Scientists at the University of Wisconsin-Madison have studied the effectiveness
of two-step acid/hypochlorite treatments as alternatives to 20,000 ppm active
chlorine for eliminating E. coli O157:H7 from alfalfa seeds prior to sprouting.
Commercially available alfalfa seeds were dipped in a mixture of E. coli O157:H7
bacteria and then dried to attain roughly 1,000,000 bacteria per gram of seeds.
These treated seeds were then soaked using one of several treatments in an
attempt to remove the pathogenic bacteria.
Introduction
Acetic acid and lactic acid rinses were tested at various strengths and at
various temperatures, either alone or followed by various concentrations of
hypochlorite. Acetic acid is commonly found in vinegar (5% acid) and lactic acid
is a common food ingredient, produced in the manufacture of cheese and other
fermented foods.
Initially, 33 combinations of acid/hypochlorite rinse were evaluated based on
seed viability and germination. The 5 treatments that allowed for greatest
viability/germination (>90%) were selected for further study. Those 5
treatments were:
1) 5% (v/v) lactic acid for 10 min at 42°C
2) 5% acetic acid (v/v) for 10 min at 42°C
3) 2.5% lactic acid for 10 min at 42°C followed by 2,000 ppm active chlorine
(from calcium hypochlorite) for 15 min at 25°C
4) 5% lactic acid for 10 min at 42°C followed by 2000 ppm active chlorine for
15 min at 25°C
5) 20,000 ppm active chlorine for 15 min at 25°C.
Results
Each rinse treatment reduced the number of pathogenic bacteria inoculated onto
the seeds by about 1,000,000 bacteria per gram. A second series of laboratory
tests indicated that at least half of the bacteria were injured (not killed) by
the rinse treatment. A combination of both lactic acid AND active chlorine was
more effective at removing bacteria than either acetic acid or lactic acid
alone. Treatment with 20,000 ppm active chlorine appeared to kill all the
contaminating bacteria on the seeds. But, regardless of the rinse treatment
given to the seeds, E. coli O157:H7 increased to at least 10,000,000 bacteria
per gram during sprouting - even after those treatments where all contaminating
bacteria appeared to be destroyed before sprouting. Results of this study show
that:
a)
bacteria may be injured by rinse treatments and these injured cells may repair
during sprouting. Testing the effectiveness of any rinse treatment must take
into account these injured cells. Failure to account for the injured cells may
lead to a false sense of security regarding the effectiveness of a seed rinse
treatments.
b) an acid wash can reduce bacteria by 100 to 10000 per gram; without using
20,000 ppm active chlorine
c) rinsing the seeds with both lactic acid and hypochlorite destroyed more
bacteria than using just acid
d) none of the rinse treatments tested can prevent re-growth of surviving E.
coli O157:H7 during sprouting. This is true for the acid rinses, the
acid/chlorine rinses, and the 20,000 ppm active chlorine rinse.
Summary
It is important to test the effectiveness of any rinse treatment on both the
seeds and the resulting sprouts. The conditions for sprouting are ideal for
bacterial growth, and may promote the repair of injured cells. Alternatives may
exist to hypochlorite, depending on the level of destruction required to ensure
safety. In other words, is it sufficient to reduce bacterial counts by 1000 per
gram, or is a kill of 100000 required? This study used seeds artificially
inoculated with high levels of pathogenic E. coli 0157:H7. No conclusions can be
drawn as to how effectively this methodology parallels a situation where seeds
are naturally contaminated with low levels of pathogens.
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