Overcoming the Bacteriostatic Activity of Onion in Making Standard Plate Counts

SUMMARY— The bacteriostatic activity of onion homogenates in making standard plate counts as reported in the literature was confirmed. It was found that potassium sulfite counteracts this activity. The sulfite must be added to the blending solution instead of to the petri plate. Five-tenths percent sulfite is sufficient and the concentration may be as high as 1.6%. Variety of onion did not appear to be a factor. This treatment was of value with both laboratory and commercially dehydrated onions. Coliform organisms were not inhibited by sulfite. It is recommended that all standard plate counts of onion be made with 0.5% K2SO8, in the blending solution.