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This International Standard specifies a method for the measurement of the acidification activity of lactic acid bacteria by continuous measurement of pH. The method is applicable to dairy starter cultures where these characteristic microorganisms are present. Two types of standardized milk are specified in the procedure: boiled milk with 9,5 % mass fraction dry matter (B-milk 9,5); and autoclaved milk with 9,5 % mass fraction dry matter (A-milk 9,5). It is possible that heat treatment of B-milk 9,5 does not inactivate all enzymes that are present, which can affect the activity of some cultures. In that case, cultures are tested with A-milk 9,5 in which all enzymes have been inactivated.