Hydrophobic Grid Membrane Filtration

The most widely adapted approach to membrane filtration of foods is the hydrophobic grid membrane filter (HGMF). The HGMF is a membrane filter on the surface of which a hydrophobic material is applied in a grid pattern. The hydrophobic material used to form the grid is a physical barrier to colony spread, largely restricting bacterial colonies to the confines of the individual grid squares in which they were first formed (see Fig. 1). Using this technique, the counting range of a single filter was extended to as high as 104 colony-forming units (23,24).

Initially, the HGMF was seen as a water microbiology and research tool. Beginning in 1978, its potential applicability to food microbiology was established when a series of studies on the filtering characteristics of a wide variety of food homogenates was published. It was reported that most food homogenates, if blended using a "Stomacher" and, where appropriate, if treated with a surfactant and/ or an enzyme digestion, could be filtered (25). In a later study (26), a finely woven stainless steel cloth prefilter was used to remove particulate material from food homogenates immediately prior to filtration. The apparatus used to carry out this two-stage filtration is illustrated in Figure 2. This study established that neither prefiltration, nor surfactant treatment, nor enzyme digestions altered the bacterial counts obtained from food samples.

Figure 1. Coliform colonies growing on a hydrophobic grid membrane filter. The hydrophobic grid lines serve to confine colonies within the boundaries of the individual squares.

Figure 2. Filtration apparatus incorporating 5p stainless steel prefilter used to inoculate hydrophobic grid membrane filters with food sample homogenates.

Since then numerous specific applications of the hydrophobic grid membrane filter to food microbiology have been introduced. These applications are listed in Table 1, and several of them are discussed in the following sections.

Table 1. Applications of Hydrophobic Grid Membrane Filter to Food Microbiology

Analysis

References

Aerobic plate count

27,41-43

Coliforms /Escherichia coli

28-31,54

E. coli 0157:H7

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