The traditional method is Gas Chromatography, but here is an article that analyzes diacetyl in yogurt using a spectrophotometer or fluorimeter based on a distillation sample preparation, and compares to gas chromatography method. I can send article if you want just PM me your email address.
Analysis of diacetyl in yogurt by two new spectrophotometric and fluorimetric methods
Eduardo J. Guerra^Hernández, Rosa García Estepa, Isabel Rodriguez Rivas
doi:10.1016/0308-8146(95)93939-O
Abstract
Two new methods (spectrophotometric and fluorimetric) are reported for the determination of diacetyl in yogurt. The diacetyl was isolated by distillation and condensed with isoniazide. This solution was then treated with Zr(IV) to form a complex, which was measured with these two methods. Average recovery of diacetyl was 96.8% for spectrophotometry and 99.1% for fluorimetry, while precision was RSD = 2.5 and 1.0%, respectively. The concentration of diacetyl in yogurt ranged from 0.79 to 2.77 μg/g. The two procedures gave statistically similar results. The fluorimetric method was compared with a traditional method (gas chromatography), for which acetaldehyde, acetone, methanol, and ethanol were also determined.
Also:
Rapid gas-chromatographic method for the determination of diacetyl in milk, fermented milk and butter
V. Macciolaa, G. Candelab, A. De Leonardisa, ,
doi:10.1016/j.foodcont.2007.08.014
Abstract
In this work, a simple and fast method for the determination of diacetyl by gas-chromatographic technique coupled with flame ionisation detector (GLC-FID) was developed. Diacetyl is the typical butter flavour, but it is also commonly present in others fermented dairy products. Recently, diacetyl determination has also attracted interest because it is one of the parameters on which lactic acid bacteria (L.A.B.) are characterized and valued. Only acetone and 2,3-pentanedione were used as chemicals. After centrifugation of acetone–milk mixture, supernatant was filtered and directly injected into gas-chromatographic apparatus, without a further purification procedure step.
This method was accurate and precise; diacetyl recovery on milk was 97% and the detection limit was 1 mg L−1. Finally, by using this method, diacetyl was easily determined in fresh and high-temperature treated milk, commercial butter, yoghurt and also in a series of L.A.B. performance tests.