Metodo AOAC

17.2.07

AOAC Of fi cial Method 990.12

Aero bic Plate Count in Foods

Dry Rehydratable Film

(Petrifilmä Aer o bic Count Plate) Method

First Ac tion 1990

Fi nal Ac tion 1994

Re sults of the interlaboratory study sup port ing ac cep tance of the

method:

Flour: sr = 0.225; sR = 0.246; RSDr = 5.3%; RSDR = 5.8%

Nuts: sr = 0.272; sR = 0.674; RSDr = 7.4%; RSDR = 18.4%

Shrimp: sr = 0.540; sR = 0.615; RSDr = 9.8%; RSDR = 11.1%

Spice: sr = 0.274; sR = 0.303; RSDr = 6.0%; RSDR = 6.6%

Tur key: sr = 0.278; sR = 0.348; RSDr = 5.3%; RSDR = 6.6%

Veg e ta bles: sr = 0.310; sR = 0.454; RSDr = 6.3%; RSDR = 9.2%

A. Prin ci ple

See 989.10A (see 17.3.03).

B. Apparatus

See 989.10B(a) and (c)–(e) (see 17.3.03).

C. Re agent

Di lu tion wa ter.—To pre pare stock so lu tion, dis solve 34 g

KH2PO4 in 500 mL H2O, ad just to pH 7.2 with 1M NaOH (ca

175 mL), and di lute to 1 L with wa ter. To pre pare buffered wa ter for

di lu tions, di lute 1.25 mL stock so lu tion to 1 L with boiled and cooled

wa ter. Au to clave 15 min at 121°C.

D. Prepa ration of Test Suspension

See 966.23B (see 17.2.01).

E. De ter mi na tion

Place dry-film aer o bic count plate on flat sur face. Lift top film

and in oc u late 1 mL test sus pen sion onto cen ter of film base.

Care fully place top film down on inoculum. Dis trib ute sus pen sion

over pre scribed growth area with down ward pres sure in cen ter of

plas tic spreader de vice (re cessed side down). Leave plate

un dis turbed 1 min to per mit gel to so lid ify. In cu bate plates 48 ± 3 h at

35° ± 1°C.

In in cu ba tor, place plates in hor i zon tal po si tion, clear side up, in

stacks not ex ceed ing 20 units. Count plates promptly af ter

in cu ba tion pe riod. Af ter in cu ba tion is com plete, plates may be

stored frozen (£–15°C) up to 7 days. Avoid this as a rou tine practice.

Use stan dard col ony coun ter for count ing pur poses.

Mag ni fier-illuminator may also be used to fa cil i tate count ing. Col onies

stain in var i ous shades of red. Count all col o nies in count able range

(30–300 colonies).

To compute bacterial count, multiply total number of colonies per

plate (or av er age num ber of col o nies per plate if count ing du pli cate plates

of same di lu tion) by re cip ro cal of di lu tion used. When count ing col o nies

on duplicate plates of consecutive dilutions, compute mean number of

colonies for each dilution before determining average bacterial count.

Es ti mated counts can be made on plates with >300 col o nies and should

be re ported as es ti mated counts. In mak ing such counts,

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