The utility of the Voges-Proskauer (17) reaction in detecting bacterial metabolites responsible for specific off-flavors in frozen concentrated Florida orange juice has been denionstrated by Hill, Wenzel, and Barreto ( 7 ) and Byer ( 3 ) . This report is concernecl with the application of their niethod to California Valencia orange juice, particularly as compositional differences may affect evaluation of data from different citrus prodnciiig areas.The Voges-Proskauer reaction was selected by the original citrus workers because of its sensitivity for the oxidation-reduction compounds diacetyl and acetoin (acetyliiiethylcarbinol) . Several methods are known for the detection of diacetyl. One of these, employing hydroxylamine, yields dimethylglyoxime, which subsequently condenses with urea to give a yellow coinpound (19). The method does not appear to be sensitive below 15 p.p.m.Other workers (12, 18,11, 13) have reacted the dimethylglyoxime with a nickel salt to produce soluble tetravalent nickel dimethylglyoxime having a red color. The method, used on beer, appears to be sensitive at 0.2 p.p.m.The hydroxylamiiie reaction, although highly specific for diacetyl, is time-consuming and requires meticulous technique. Other methods based on reaction with chromotropic acid or analine extraction (15) are either not specific or sufficiently sensitive.The Voges-Proskauer reaction as modified by Barritt (1) results froin the reaction of diacetyl and peptone with u napthol to produce a red color. The test also determines 2,3 butanediol (2,3 butylene glycol). The detectable limits appear to be 0.05 p.p.m. diacetyl, 0.5 p.p.m. acetylmethylcarbinol, and 40,000 p.p.ni. 2,3 butylene glycol.The Voges-Proskauer reaction, hereinafter referred to as the V-P reaction, although highly specific for the above three named substances, can be affected by other compounds, some of which are naturally occurring or degradation products in orange juice. The Byer modificatioii ( 3 ) for citrus juices affords concentration of the significant materials to an easily detectable level and removal of same from a substrate containing a heterogeneity of interfering compounds.
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