Plate Count Agar

LAB 149

Description

A medium designed for use with the spiral plating system and other surface inoculation techniques. The formula is equivalent to A.P.H.A. Plate Count Agar and is suitable for the determination of total viable counts in food products by surface count and pour plate methods.

Method for reconstitution

Weigh 20.5 grams of powder, disperse in 1 litre of deionised water. Allow to soak for 10 minutes, swirl to mix then sterilise by autoclaving at 121˚C for 15 minutes. Cool to 47˚C then pour into Petri dishes.

Appearance: Pale straw colour, clear.

pH: 7.0 ± 0.2

Minimum Q.C. organisms: S. epidermidis NCIMB 50082 E. coli NCIMB 50034

Storage of Prepared Medium: Plates - up to 7 days at 2-8˚C in the dark.

Inoculation: Surface, or pour plate.

Incubation: 30˚C aerobically for 48 hours for aerobic mesotroph count. 6˚C aerobically for 10 days for aerobic psychrotroph count. 55˚C aerobically for 48 hours for aerobic thermotroph count.

Interpretation: Count all colonies or use spiral plating colony count equipment.

Method for reconstitution

Weigh 23.5 grams of powder, disperse in 1 litre of deionised water. Bring to the boil with frequent stirring to dissolve. Dispense into tubes and sterilise by autoclaving at 121˚C for 15 minutes. Cool to 44-46˚C for not more than 3 hours prior to use.

ROLL-TUBES. Add an additional 10g/litre Agar No. 1 prior to reconstitution of the medium.

Appearance: Pale straw coloured, clear gel.

pH: 7.0 ± 0.2

Minimum Q.C. organisms: S. epidermidis NCIMB 50082 E. coli NCIMB 50034

Storage of Prepared Medium: Plates - up to 7 days at 2-8˚C in the dark. Capped containers - up to 3 months at 15-20˚C in the dark.

Inoculation method: Pour plate technique or surface inoculation.

Incubation: 30˚C aerobically for 48 hours for aerobic mesotroph count. 6˚C aerobically for 10 days for aerobic psychrotroph count. 55˚C aerobically for 48 hours for aerobic thermotroph count.

Interpretation: Count all colonies and calculate the number of organisms (or ‘colony forming units' c.f.u.) per ml of sample allowing for dilution factors.

References

American Public Health Association (1972). Standard Methods for the Examination of Dairy Products. 13th edn. (ed. Hausler, W.J.) A.P.H.A., Washington.

American Public Health Association (1966). Recommended Methods for the Microbiological Examination of Foods, 2nd edn. (ed. Sharf, J.M.) A.P.H.A., Washington.

American Public Health Association (1976). Standard Methods for the Examination of Water and Waste Water, 14th edn. (ed.. Franson, M.A) A.P.H.A., Washington.