Agar medium detects bacteria that discolors cured meat.

Aerococcus viridans, Carnobacterium viridans and Weissella viridescens cause discoloration in cooked bologna. The discoloration is the result of hydrogen peroxide ([H.sub.2][O.sub.2]) produced by these organisms, which in turn creates a greenish color on the product when the package is opened. This, of course, hurts product value and creates a bad impression on consumers.

The meat-agar test systems currently available that detect organisms which cause discoloration do not consider the growth of C. viridans and give false positive results for pediococci and leuconostocs, which do not produce enough [H.sub.2][O.sub.2] to cause any discoloration. Canadian scientists wanted to develop a simple, reliable agar-based medium for use in quality assurance laboratories that would anticipate the presence of these spoilage organisms before they become a problem in meats.

As [H.sub.2][O.sub.2] is responsible for causing green discoloration, a test system that indicates sufficient levels of its production was the object of their research. The scientists modified PTM, CTSI and MRS agars by changing their pH, substituting different sugars, and altering the temperature and time of incubation. A modified PTM agar with pH 8.1 containing 20 g per L of glucose was best for detecting all three greening organisms.

The researchers tested the agar with a wide spectrum of organisms normally present in meat, including producers of [H.sub.2][O.sub.2]. All three greening organisms produced brown-colored colonies on m-PTM, indicating high levels of [H.sub.2][O.sub.2]. Non-greening organisms produced blue or white colonies, depending on the levels of [H.sub.2][O.sub.2], which were low or nonexistent.

Some non-greening [H.sub.2][O.sub.2]-producing organisms generated brown colonies at low levels of glucose (0.2 g per L). But at 20 g per L, glucose produced brown colonies. This indicates that the greening organisms may use one or more alternate pathways to produce [H.sub.2][O.sub.2] when higher levels of glucose are present. [H.sub.2][O.sub.2]-producing but non-greening organisms cannot produce sufficient [H.sub.2][O.sub.2] to cause greening in bologna. So it appears that PTM may be a useful agar medium for detecting greening organisms in bologna.

Further information. Rick Holley, Department of Food Science, University of Manitoba, 248 Ellis Building, Winnipeg, R3T 2N2 Canada; phone: [phone]; fax: [phone]; email: [e-mail].