A Less Hazardous Enrichment Broth for Listeria without Cycloheximide and Acriflavine.

The hazardous substances cycloheximide and acriflavine are used as selective agents in standard listeria enrichment broths. This study was undertaken to develop a less hazardous listeria enrichment broth in order to avoid occupational-safety and waste-management problems. The replacement of cycloheximide (50 mg/l) and acriflavine (15 mg/l) with natamycin (25 mg/l), moxalactam (10 mg/l) and bacitracin (20 mg/l) did not inhibit the growth of Listeria monocytogenes . Furthermore, most of the competing microbial strains were suppressed equally well or better in this less hazardous modification than in a standard broth.

The Effect of GDL-induced pH Decrease on the Formation of Biogenic Amines in Meat.

The effect of pH on the formation of biogenic amines has mainly been studied in broths in which pH has been fixed before incubation. However, in the fermentation of dry sausage, pH quite rapidly decreases from the initial value to a certain level. In this study glucono-delta-lactone (GDL) was used to decrease pH in meat, Six minced meat samples were each divided into three portions (A-C): 0% (A), 0.5% (B), or 1.0% (C) of GDL was added and the samples were incubated at 20-22°C for 7 d. The amounts of biogenic amines (histamine, tyramine, putrescine, cadaverine, phenylethylamine, tryptamine, spermine, and spermidine) as well as pH, water activity, and the bacterial counts of lactic acid bacteria, fecal streptococci, coliforms, and total plate count were measured. Addition of GDL resulted in a significant decrease in pH and in the levels of histamine and putrescine as well as in the levels of fecal streptococci, coliforms, and total plate counts. Of 87 fecal streptococci, seven Enterococcus faecalis strains produced tyramine. All the coliforms and related strains isolated from violet red bile agar produced tyramine, putrescine, and cadaverine on agar plates. However, the proportion of histamine-positive strains of these strains, especially Hafnia alvei , increased from 0 to 57% during the incubation. The rate and level of pH decrease clearly affected amine formation in meat, indicating that the levels of, e.g., histamine produced could be decreased by optimizing the pH decrease during fermentation. Addition of GDL facilitates study of the effect of pH decrease without interactions between the starter culture and contaminant flora.