Culture, Microscopic, and Sampling Methods
219
models of the instrument are available, but model 400 is most widely used in food microbiology
laboratories. It can handle samples (diluent and specimen) of 40–400 ml.
The Stomacher has been compared to a high-speed blender for food analysis by a large number
of investigators. Plate counts from Stomacher-treated samples are similar to those treated by blender.
The instrument is generally preferred over blending for the following reasons:
1. The need to clean and store blender containers is obviated.
2. Heat buildup does not occur during normal operational times (usually 2 minutes).
3. The homogenates can be stored in the Stomacher bags in a freezer for further use.
4. The noise level is not as unpleasant as that of mechanical blenders.
In a study by Sharpe and Harshman
113 the Stomacher was shown to be less lethal than a blender
to Staphylococcus aureus, Enterococcus faecalis, and Escherichia coli. One investigator reported that
counts using a Stomacher were significantly higher than when a blender was used
129 whereas other
investigators obtained higher overall counts by blender than by Stomacher.
5 The latter investigators
showed that the Stomacher is food specific; it is better than high-speed blending for some types of foods
but not for others. In another study, SPC determinations made by Stomacher, blender, and shaking
were not significantly different, although significantly higher counts of Gram-negative bacteria were
obtained by Stomacher than by either of the other two methods.
63 Another advantage of the Stomacher
over blending is the homogenization of meats for dye reduction tests. Holley et al.
54 showed that
the extraction of bacteria from meat by using a Stomacher does not cause extensive disruption of
meat tissue, and, consequently, fewer reductive compounds were present to interfere with resazurin
reduction; whereas with blending, the level of reductive compounds released made resazurin reduction
results meaningless.
Another device, the Pulsifier
r
, is somewhat similar to the Stomacher. It creates a high level of
turbulence on food samples resulting in the release of microorganisms from the sample.
The Spiral Plater
The spiral plater is a mechanical device that distributes the liquid inoculum on the surface of a rotating
plate containing a suitable poured and hardened agar medium. The dispensing arm moves from the
near center of the plate toward the outside, depositing the sample in an Archimedes spiral. The attached
special syringe dispenses a continuously decreasing volume of sample so that a concentration range of
up to 10,000:1 is effected on a single plate. Following incubation at an appropriate temperature, colony
development reveals a higher density of deposited cells near the center of the plate, with progressively
fewer toward the edge.
The enumeration of colonies on plates prepared with a spiral plater is achieved by use of a special
counting grid (Figure 10–1A). Depending on the relative density of colonies, colonies that appear in
one or more specific areas of the superimposed grid are counted. An agar plate prepared by a spiral
plater is shown in Figure 10–1B, and the corresponding grid area counted is shown in Figure 10–1C.
In this example, a total sample volume of 0.0018 ml was deposited, and the two grid areas counted
contained 44 and 63 colonies, respectively, resulting in a total count of 6.1 × 10
4 bacteria per milliliter.
The spiral plating device described here was devised by Gilchrist et al.
44 although some of its
principles were presented by earlier investigators, among whom were Reyniers
105 and Trotman.
128
The method has been studied by a rather large number of investigators and compared to other methods
of enumerating viable organisms. It was compared to the SPC method by using 201 samples of raw
and pasteurized milk; overall good agreement was obtained.
30 A collaborative study from six analysts
219
models of the instrument are available, but model 400 is most widely used in food microbiology
laboratories. It can handle samples (diluent and specimen) of 40–400 ml.
The Stomacher has been compared to a high-speed blender for food analysis by a large number
of investigators. Plate counts from Stomacher-treated samples are similar to those treated by blender.
The instrument is generally preferred over blending for the following reasons:
1. The need to clean and store blender containers is obviated.
2. Heat buildup does not occur during normal operational times (usually 2 minutes).
3. The homogenates can be stored in the Stomacher bags in a freezer for further use.
4. The noise level is not as unpleasant as that of mechanical blenders.
In a study by Sharpe and Harshman
113 the Stomacher was shown to be less lethal than a blender
to Staphylococcus aureus, Enterococcus faecalis, and Escherichia coli. One investigator reported that
counts using a Stomacher were significantly higher than when a blender was used
129 whereas other
investigators obtained higher overall counts by blender than by Stomacher.
5 The latter investigators
showed that the Stomacher is food specific; it is better than high-speed blending for some types of foods
but not for others. In another study, SPC determinations made by Stomacher, blender, and shaking
were not significantly different, although significantly higher counts of Gram-negative bacteria were
obtained by Stomacher than by either of the other two methods.
63 Another advantage of the Stomacher
over blending is the homogenization of meats for dye reduction tests. Holley et al.
54 showed that
the extraction of bacteria from meat by using a Stomacher does not cause extensive disruption of
meat tissue, and, consequently, fewer reductive compounds were present to interfere with resazurin
reduction; whereas with blending, the level of reductive compounds released made resazurin reduction
results meaningless.
Another device, the Pulsifier
r
, is somewhat similar to the Stomacher. It creates a high level of
turbulence on food samples resulting in the release of microorganisms from the sample.
The Spiral Plater
The spiral plater is a mechanical device that distributes the liquid inoculum on the surface of a rotating
plate containing a suitable poured and hardened agar medium. The dispensing arm moves from the
near center of the plate toward the outside, depositing the sample in an Archimedes spiral. The attached
special syringe dispenses a continuously decreasing volume of sample so that a concentration range of
up to 10,000:1 is effected on a single plate. Following incubation at an appropriate temperature, colony
development reveals a higher density of deposited cells near the center of the plate, with progressively
fewer toward the edge.
The enumeration of colonies on plates prepared with a spiral plater is achieved by use of a special
counting grid (Figure 10–1A). Depending on the relative density of colonies, colonies that appear in
one or more specific areas of the superimposed grid are counted. An agar plate prepared by a spiral
plater is shown in Figure 10–1B, and the corresponding grid area counted is shown in Figure 10–1C.
In this example, a total sample volume of 0.0018 ml was deposited, and the two grid areas counted
contained 44 and 63 colonies, respectively, resulting in a total count of 6.1 × 10
4 bacteria per milliliter.
The spiral plating device described here was devised by Gilchrist et al.
44 although some of its
principles were presented by earlier investigators, among whom were Reyniers
105 and Trotman.
128
The method has been studied by a rather large number of investigators and compared to other methods
of enumerating viable organisms. It was compared to the SPC method by using 201 samples of raw
and pasteurized milk; overall good agreement was obtained.
30 A collaborative study from six analysts