International Specialty Supply
Supplying Sprout Companies Throughout the World
|
International Specialty Supply Supplying Sprout Companies Throughout the World
|
|
820 East 20th Street Cookeville, TN 38501 USA 931 526 1106
|
Seed
Sampling of Blended Lot SproutNet International Specialty Supply June 5, 2002 Dear Bob, "How reliable is your seed sampling
program if the lot has been blended with other lots?" Dear Grower, The statistical reliability is
only as good as the probability of detection of the smallest lot
that is blended in. For example, suppose you have a full load of alfalfa
seed with 880 fifty-pound bags. Let's also say that the shipment is
blended from 3 lots of 1,000, 15,000, and 28,000 lbs. It does not matter
if the lots are blended because the probabilities average out. If there is
a pathogen in the seed at the rate of 4 cfu per kg or higher, your probability
of capture for each is: 1,000 lbs =
20 bags sampled x 20 grams per sample = 400g
sampled. According to our charts, the probability of capture is 79.81% 15,000 lbs = 300 bags
sampled x 20 grams per sample = 6.0 kg sampled.
The probability of capture is 99.99999999% 28,000 lbs = 560 bags
sampled x 20 grams per sample = 11.2 kg sampled. The
probability of capture is virtually 100%. In this case, even though you
sampled and tested a total of 17.6 kg (39 lbs) of seed, you would need to figure
the sample is only 79.81% reliable because of the small sublot within the
lot. You are mixing the 1,000 lbs of seed with the other 43,000
lbs of seed. This reduces a 4 cfu per kg contaminated lot to 0.091
cfu/kg over 44,000 pounds of seed. On the other hand, you are
increasing the amount of seed sampled from 440g to 17.6 kg. If you plug in
the numbers, you find out that it comes out to be exactly 79.81%. A
statistical probability chart for seed sampling is located on our website
under ISS
Seed Safety. I will be the first to admit
that this method is not perfect. An example often brought up is,
"What if a mouse gets into one corner of one bag?" Careful inspection of the shipment
is critical to the integrity of the sampling and testing program. If there
is a mouse in a bag, there is probably a hole in the bag where the mouse
entered. And if the seed was stored or shipped in a rodent infested area,
the bag inspection and bag swab tests should reveal the problem. If a mouse got into a bag or into
the seed before the seed was bagged, and was only in one bag, the probability of
detection is very low. But it is only one bag, not 880. If your seed
sanitation does not completely kill the pathogens in the seed, hopefully your
post testing will detect it. If the pathogens make it past all three of
these hurdles, there could be a small, localized, outbreak. Not a
national outbreak, involving many growers, that lasts for months before the
source is detected. No doubt this is an effective risk reduction step and
probably the most effective of the three. None of the three methods employed
to alleviate pathogen problems in sprouts is 100% reliable. Post
testing is not completely reliable, and when using chlorine, unless you kill
every pathogen, the re-growth is back to about what it would have been if you
had not sanitized. There are a lot of researchers and
regulators who read the SproutNet so I'm going to through out the following
hypothesis and see if it sticks: There is an inverse
relationship between seed sampling & testing, and sanitation. That
is, it stands to reason that the more contaminated the seed is, the
less likely you are to be able to kill all of the pathogens using seed
sanitation. Yet, the more contaminated the seed is, the easier it is to
detect a pathogen using seed sampling and testing. To rely just on chlorine is folly.
A truckload of seed, contaminated at the rate of 4 cfu/kg, has roughly 72,575
individual pathogen cells total. the FDA recommended chlorine soak gives
a 2.5 - 3 log reduction. A three log reduction would bring the count down
to 72.5 viable pathogen cells in the truckload, which will multiply during
sprouting. In other words, unless the pathogens are detected in post
testing, there will still be an outbreak. Not every batch of sprouts will
be contaminated, just the theoretical 72.5 batches. But if the lot is
contaminated at 4 cfu/kg, the probability of detection using proper seed
sampling and testing is well over 99.9999%. On the other hand, seed that is
very lightly contaminated, say at the rate of .01 cfu/kg (that's 200 cells per
truckload), has a probability of detection of less than 3% using our method
of seed sampling and testing. But the chlorine would theoretically
reduce the cells to 1/5 of one cell, for an 80% chance of eliminating every
single cell. The two methods, seed
sampling and testing along with the FDA recommended chlorine soak, compliment
each other well to virtually eliminate the risk of salmonella or
E.coli 0157:H7 making it from the seed to the consumer. |