Detection of propionic acid bacteria in cheese

Area of Science:

• Food Microbiology
• Dairy Science
• Analytical Chemistry

Background:

• Accurate enumeration of propionic acid bacteria (PAB) is crucial for cheese quality.
• Existing methods for PAB detection face challenges with interference from starter cultures.

Purpose of the Study:

• To develop and validate a selective sodium lactate agar (SLA) medium for quantifying propionic acid bacteria (PAB) in cheese.
• To improve the accuracy of PAB enumeration in the presence of high starter bacteria populations.

Main Methods:

• Utilized sodium lactate agar (SLA) supplemented with cloxacillin (4 µg/ml) to inhibit starter bacteria.
• Compared PAB counts on the selective SLA with propionic acid levels in cheese samples.
• Evaluated the selectivity of the medium against common cheese contaminants.

Main Results:

• The addition of cloxacillin to SLA successfully inhibited mesophilic lactic starters and thermophilic lactobacilli, but not PAB.
• Low numbers of PAB could be accurately counted in the presence of high starter bacteria populations.
• A strong correlation (r=0.9) was observed between propionic acid levels and PAB counts in cheese.
• While other bacteria like Clostridium tyrobutyricum grew on the medium, they were morphologically distinguishable from PAB.

Conclusions:

• The cloxacillin-supplemented SLA is a valuable tool for the selective enumeration of propionic acid bacteria (PAB) in cheese.
• This method enhances the reliability of PAB quantification, aiding in cheese quality assessment and process control.
• Further differentiation of PAB from other cheese bacteria on the modified SLA is feasible based on colony characteristics.