Rapid Coliform, Yeast and Mold Testing of Yogurt

Testing of Yogurt and Other Dairy Products

Yogurt is a dairy product, which is made by blending fermented milk with various ingredients that provide flavor and color.  Manufacturers have responded to the growth in the yogurt market by introducing many different types of yogurt including low fat and no-fat, creamy, bio-yogurt, organic, baby, and frozen.  The popularity of yogurt has increased due to its perceived health benefit resulting in significant increase in consumption (Chandan et. al 2006) as shown in Figure.

Recently Reuters reported that Dannon, the world’s largest yogurt maker, announced that it expects annual double digit percentage dairy sales growth in the United States over the long term.

Microbiological Testing of Yogurt

Undesirable microorganisms constitute the primary hazard to safety, quality, and wholesomeness of milk and dairy foods.   The primary assays performed in yogurt are yeast and mold (requiring up to 7 days for results) and the coliform assay that is used as an indication of appropriate processing. Traditional methods are slow, tedious, labor intensive, and often not suitable for assessing the quality and shelf-life of perishable dairy products. 

 The emphasis on the programs based on HACCP (Hazard Analysis and Critical Control Points) for total quality management in the dairy industry and increased demand for microbiological surveillance of products, process, and environment have led to increased interest in rapid methods and automation in microbiology.

BioLumix technology can make the microbiological testing simpler, faster, and automated.  Saving time, labor and money. 

The BioLumix test method for detection of coliform and yeast & molds involves a direct addition of the sample into ready to use vials and automated monitoring of the samples in the instrument.  The coliform assay is completed in 12 hours, while most contaminated samples are flagged within a typical shift.  The yeast & molds assay is completed in 48 hours as compared to the 5 days required for the standard assay.

The ready–to-use vials come with media that is pre-adjusted for pH, such that after the addition of the sample the appropriate pH for the growth of the microorganisms is attained.  BioLumix has developed such a media that is custom made with a higher pH to accommodate the low pH of yogurt while still maintaining the capability to test as much as 1.0 gram of product directly in the vials.    

Coliform Assay:

The curves obtained by the addition of 1.0 gram of yogurt to a high pH vial (pH 8.2) of coliform are shown:

The Dark Blue curve shows the 1.0 gram sample.  All product interference has been eliminated as depicted by the flat part of the curve.  The Green Curve illustrates a yogurt sample inoculated with a coliform.

In North America, BioLumix has tested yogurt samples manufactured by a number of companies.  The Table below includes a series of products from one such company Yoplait, Ontario, Canada.  These samples include low fat and regular varieties. 

Lemon Cream Pie Light Fat Free
Key Lime Pie Original 99% Fat Free
Strawberry Shortcake Light Fat Free
Raspberry Mousse Whips
Strawberry Thick & Creamy Low Fat
Key Lime Pie Original
Delights Parfait Chocolate Raspberry

All un-inoculated products resulted in flat curves as seen in the Figure.  Product interference was not found.

All products tested were shown to be clean (<1 coliform/ gram).  All inoculated products detected in the system in less than 8 hours.

Yeast and Mold Assay:
Similar results were obtained for yeast and molds. The curves show the results obtained with a variety of yogurt products.

Fieldberry Stirred Yogurt Dark blue
Raspberry Stirred Yogurt Green
Raspberry Stirred Yogurt-inoc CAD L- blue
Yoplait Mingo Red
Yoplait Mingo-inoc ASP Purple
Tubes- Raspberry Yellow
Tubes- Grapes Tan
Irresistible Creamy yogurt Vanilla Dark green

The inoculated samples (light blue- inoculated with Candida albicans; purple inoculated with Aspergillus niger) detected in less than 24 hours.  All the un-inoculated samples were clean and did not detect in the vials.  A total of 50 different yogurts were tested using this method and the data showed that all the samples that contained either yeast or mold detected in the system and none of the clean samples detected.  The plate count results agreed 100% with the BioLumix vial results.


The BioLumix system is designed to accelerate product release with a simplified, automated approach. This yields fast, accurate, real-time results while reducing costs. The BioLumix offers real-time results of contaminated samples saving hours, possibly days. Completed Coliform or Enterobacteriaceae results are obtained within 12 hours; Yeast and Mold results are obtained within 48 hours.


R.C. Chandan, C.H. White, A. Kilara, and Y.H. Hui. 2006. in Manufacturing Yogurt and Fermented Milks. Blackwell Publishing.

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