An on-line method for the measurement of diacetyl and acetaldehyde in yoghurt, fermented milk, butter and margarine.

A new method for the measurement of diacetyl and acetaldehyde in fats, ferm ented dairy products and batch-cultures of Lactococcus lactis sap. lactis b y. diacetylactis is proposed. A gas membrane sensor connected to a gas chro matograph is used for direct analysis. At 35 degrees C, the lowest limits o f detection are 0.005 mg . L-1 for diacetyl and 0.01 mg . L-1 for acetaldeh yde. At this temperature, linearity is observed from 0.005 to 50 mg . L-1 f or diacetyl and from 0.01 to 80 mg . L-1 for acetaldehyde. As variations in response were noted when this method was applied to commercial samples con taining fats, it is recommended to perform calibration in the tested produc t or in the fermentation media. In this study, the gas membrane sensor was shown to be a valuable tool for diacetyl and acetaldehyde determination in commercial products and for on-line measurement of various volatile compoun ds during lactic acid bacteria cultures. (C) Inra/Elsevier, Paris.