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

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