AOAC 2018 Methods

(DI) or reverse osmosis (RO) water and adjust pH to 7.2 with 1 N NaOH and bring final volume to 1 liter

with DI or RO water. Add 99 mL to dilution bottles and sterilize 15 min at 121°C. Store in refrigerator.

Or purchased, e.g. Weber Scientific, Hamilton, NJ item # 3127-14 or alternative manufacturer

equivalent.

( b ) Buffered Peptone Water (BPW) .—Peptone 10 g, Sodium chloride 5 g, Disodium phosphate 3.5 g, Mono-potassium Phosphate 1.5 g Distilled water 1 L. Add 99mL to dilution bottles and sterilize 15 min at

121°C. Store in refrigerator. Final pH 7.2 ± 0.2 .

( c ) Neutralizing Buffered Peptone Water ( n-BPW ).—buffered peptone 20.0 g, soy lecithin 7 g sodium thiosulfate 1 g, Microbiologically Suitable (MS) water 1 L, sodium bicarbonate 12.5g pH 7.7 ± 0.5 at 25°C.

C. General Preparation

( a ) Observe Good Laboratory Practices for microbial testing. Avoid specimen contamination. ( b ) Test on a level surface, in a clean area, and free of dust and blowing air. ( c ) Avoid hand contact with test samples and Peel Plate EB medium. ( d ) Log serially dilute sample into BPW, Butterfields or microbiologically suitable water to obtain the countable range 1–150 CFU/plate or test multiple dilutions to attain the countable range.

D. Sample Preparation

( a ) Foods .--( 1 ) Add 25 g (25 mL if already liquid) of food (infant formula, butter, milk, ice cream, milk powder) to 225 mL of dilution buffer (BPW following ISO method), Stomach/homogenize for 1–2 min, let

settle 1 min. Following homogenization, 1:10 serial dilutions are performed in dilution buffer to the

desired concentration.

( 2 ) For cereal, add 25 g to 1225 mL of dilution buffer, Stomach/homogenize for 1–2 min, let settle 1 min.

( 3 ) Continue to dilute 10 mL of prior dilution in 90 mL of dilution blank to reach countable range (1 to

150 CFU/plate). Other volume/volume dilution schemes are acceptable.

( b ) Surfaces .--The sampling protocol followed ISO 18593 (1). Stainless steel surfaces were sampled by

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