Food micro (long-ish)

Gary G see.signature at
Wed Dec 15 20:44:45 EST 2004

On Wed, 15 Dec 2004 11:16:00 +0200, "The Sceptical Chymist"
<polyzoni at otenet.invalid> wrote:

>Hello everybody.
>I would appreciate any comments on the following:
>We are a company that makes frozen dough products, some of which include
>cured meat in the filling. We’re having a problem with one of our customers,
>a major food multinational, and I would like to hear the opinions of others.
>The situation is as follows:
>Two of our products contain boiled ham and bacon, which are placed in an
>open dough base (they’re exposed). The cooking instructions state clearly
>that the product should be baked at 180 deg C for15-20 min. However the
>customer believes that the end-user might let the product thaw rather than
>take it straight from the freezer to the oven and additionally might pick
>and eat the ham or bacon. For that reason they treat those two ingredients
>as ready-to-eat and ask for an "absence of Salmonella spp in 375g" analysis.
>We’re using a Biomerieux Minividas instrument, and the protocol asks for:
>1. Homogenising 25g of sample in 225 ml of buffered peptone water (BPP).
>2. Incubating the resulting 250ml of homogenate at 37 °C for 16-20 hrs.
>3. Subculturing 0.1 ml of homogenate in 10 ml of Rappaport Vassiliadis (RV)
>4. Incubating the RV broth at 41.5 °C for 6-8 hrs.
>5. Subculturing of 1 ml from the RV broth in 10 ml of M-broth.
>6. Incubating the M broth for 16-20 hrs at 41.5 °C.
>7. Heating 1 ml of M broth at 100 °C for 15 min.
>8.  Adding 0.5 ml of heated M broth to the Vidas SLM test strip and reading
>the results in 45 minutes.
>Positive results are followed by confirmation by a reference method that
>involves plating the selective broth and running biochemical (API strips)
>and/or serological (agglutination) tests.
>If we were to follow the protocol to the letter we would have to test 15 25g
>samples (the instruments can only run 12 samples at a time) with all the
>reagents, incubator space, staff and time requirements that would entail.
>What we do instead is pool the 15 samples of 25g to 4 samples of 100g (that
>makes 400 rather than 375g). We dilute those 1:10 with BPP and homogenise,
>ending up with four homogenates of 1000 ml. These we incubate at 37 °C for
>16-20 hrs and then draw 0.1ml and subculture it in RV broth. From that point
>onwards we follow the rest of the protocol to the letter.
>The probability of the 400g sample carrying Salmonella remains the same as
>long as the sample is representative and drawn from different points. What
>does change is the probability that 0.1 ml drawn from 1000 ml of incubated
>homogenate carry Salmonella. There’s a better chance of detecting it if
>0.1ml is drawn from 250 ml as the protocol requires. However we believe that
>after incubating the homogenate at 37 °C for 16-20 hrs in BPP, if Salmonella
>is present it will multiply to such levels that this difference is
>I’m interested in other people’s opinion on the matter, so I would like to
>hear from you.
>I will be posting this to more than one food list/newsgroup, so apologies to
>anyone who reads it more than once.
>Kostas Polyzonis
>QA Manager
>Michael Arabatzis AVEE - Hellenic Dough
>To reply by private e-mail type replace invalid with gr

The sample could be fixed and imaged directly in SEM to view and count
Salmonella organisms.

Gary Gaugler, Ph.D.
Microtechnics, Inc.
Granite Bay, CA 95746
gary at microtechnics dot com

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