Improvements In Sampling And Sample Preparation

One of the most useful instruments developed for sample preparation is the Stomacher (Tekmar, Cincinnati, Ohio). This instrument is designed to massage food samples in a sterile bag. The food sample is first placed in the sterile disposable plastic bag, and appropriate sterile diluents are added. The bag with the food is placed in the open chamber. After the chamber is closed, the bag is then massaged by two paddles for a suitable time, usually from 1 to 5 min. No contact occurs between the instrument and the sample. During massaging, microorganisms are dislodged into the diluent for further microbiological manipulation. Massaged slurries are then used for microbiological analysis. More than 30,000 units of the Stomacher have been sold worldwide since its introduction in 1975. A similar system named Masticator Homogenizer is marketed by IUL Instruments, Germany. Recently Sharpe invented a new instrument called Pulsifier that can dislodge bacteria from food by purification in a bag. Fung et al. (16) evaluated the Pulsifier versus the Stomacher and found that both instruments provided similar microbial counts from food samples. The Pulsifier resulted in less food debris in the sample, which is advantageous for pathogen detection using technology such as enzyme-linked immunosorbent assay (ELISA), Gene Probe, and polymerase chain reaction (PCR).

An instrument for sample preparation is the Gravimetric Diluter (Spiral Biotech, Bethesda, Maryland). One of the routine procedures in food microbiological work is to aseptically measure a sample of food (eg, 5 g of meat) and then aseptically add an exact amount of sterile diluent (eg, 45 mL) to make a desired dilution (1:10). With the Gravimetric Diluter, the analyst needs only to aseptically place an amount of food (eg, 5.3 g), into a Stomacher bag, set a desired dilution (1:10), and set the instrument to deliver the appropriate amount of sterile diluent (eg, 47.7 mL). Thus, the dilution operation can be automatically done. The dilution factor can be chosen by the analyst to satisfy the need (1:10, 1:50, 1:100, etc) simply by programming the instrument. Manninen and Fung (17) evaluated the Gravimetric Diluter and found that depending on the volume tested the accuracy of delivery for most samples was found to be in the range of 90 to 100%. A new version of this instrument is called Diluflo and has been in use satisfactorily in the author's laboratory since 1992. Recently Pbi of Italy marketed a similar instrument named Dilu-macher for automatic microbiological dilution.

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