Effect of citric acid concentration on the formation of diacetyl by certain lactic acid bactria.

Diacetyl has been associated with the "buttermilk off-flavor" sometimes found in orange juice and in frozen concentrated orange juice. Bacteria of the Lactobacillus and Leuconostoc genera have been identified as causative agents of this type of spoilage (Hays, 1951; Hays and Riester, 1952; Murdock et al., 1952, 1953; and Hill et al., 1954). Growth rates of these spoilage organisms have been investigated by Barreto (1953) and Rushing et al. (1956). Witter (1956) reported a Lactobacillus species capable of diacetyl production at a rate of 2.5 ppm per hr per million organisms per ml in a continuous culture system. This means that it would take 10,000 organisms per ml 4 hr to produce 0.1 ppm. This amount is well below the flavor threshold in orange juice (Hill et al., 1954). In any case, generation times were found to be insufficient for spoilage to occur in the juice or concentrates in the evaporator unless relatively static films or pockets of juice existed (Rushing et al., 1956). Other factors favoring diacetyl accumulations could be high initial populations, especially of efficient diacetyl producers, or recirculation of the juice in the evaporators. Christensen and Pederson (1958) found citric acid to be essential for diacetyl production by some strains of lactic acid bacteria, and in all species studied, found that diacetyl production in low sugar medium was enhanced by citric acid. The present investigation was undertaken to clarify some of the factors influencing the accumulation of diacetyl in citrus juice products. Conditions were kept within normal limits of pH, concentration of nutrients, and temperatures encountered in the processing of orange juice concentrates.