Accelerating the detection of yeast and mold in yogurt

By Jennifer Johnson

What was once an obscure foreign dairy product, Greek yogurt has exploded into a billion dollar business, and it is still growing!  Greek yogurt sales are 50% higher than at this time last year and represent more than one-third of all yogurt sold in the U.S. By comparison, in 2007 Greek yogurt accounted for only 1% of the total yogurt market.

As with regular yogurt, Greek yogurt is made from fermented milk that is soured and thickened by adding specific lactic acid-producing cultures. However, Greek yogurt is strained more times than traditional yogurt to remove more whey. As a result, Greek yogurt is thicker and has more protein than regular yogurt. The basic cultures or probiotics used to make yogurt are Lactobacillus bulgaricus and Streptococcus thermophilus and many additional probiotic organisms are often added which include Lactobacillus acidophilus, Lactobacillus casei, and Bifidobacterium species, all of which may help to maintain the balance of bacteria needed to boost the immune system and promote a healthy digestive tract. According to the Obesity Action Coalition, yogurt is the number 1 natural source of probiotics eaten by Americans. The live microorganisms in Greek yogurt can help improve your digestive system, help your body absorb nutrients, and improve your health. Greek yogurt may also help ease gastrointestinal conditions like constipation, inflammatory bowel disease, lactose intolerance, and diarrhea.

It is this overabundance of helpful bacteria that can interfere with the detection of yeast and molds in Greek yogurt.

Yeast and Mold Testing in Yogurt

Undesirable microorganisms constitute the primary hazard to safety, quality, and wholesomeness of milk and dairy foods. Yogurt is susceptible to microbial contamination, especially by fungi which grow and reproduce in acidic environments containing oxygen.  In a study by Montagna (1998), a variety of differently manufactured yogurts from a variety of manufactures tested positive for fungi, of which 75% of the 7.2% positives were yeasts and 25% were molds with counts between 200 and >106 cfu/ml. Yogurts contaminated by fungi can have modified organoleptic characteristics. The most common molds found were Aspergillus and Penicillium and the most common yeast belonged to Candida, Cryptococcus, Rhodotorula species.

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. In the past few years rapid automated methods such as BioLumix and Soleris have been adopted to speed up time to results and automate and simplify the microbiologist tasks.

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

The BioLumix test method for detection of yeast & molds involves a direct addition of the sample into ready to use vials and automated monitoring of the samples in the instrument. This simple method saves labor and disposables.  The yeast & mold assay is completed in 48 hours as compared to the 5 days required for the standard assay.

The ready–to-use vial comes 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. The addition of the supplement to the vial results in the elimination of background growth from lactic bacteria resulting in flat curves.

The initial work with yogurts containing high levels of probiotics showed that the probiotics caused false detection in the instrument due to the CO2 production by the Probiotic bacteria.  As shown in the picture below, the addition of a supplement containing inhibitors to prevented the growth of the probiotics in the vials and resulted in a clear detection of fungi, when present, and a clean flat curve when organisms are not present. 

This newly developed assay has a high sensitivity, being able to detect a few yeast or molds in a gram of sample, is easy to perform (less than one minute hands on time), and result in quick time to results. Many different flavors of various brands of yogurt were tested and none created a false positive result.  Store bought contaminated yogurt detected in the system as did all the samples inoculated with low levels of yeast or mold.

The BioLumix System is designed to accelerate product release with a simplified, automated approach. The system yields fast, accurate, real-time results while reducing costs and eliminates the time required for the assays to be completed.  The system offers real-time results of contaminated samples saving hours, possibly days.

BioLumix will streamline and simplify the microbiological procedures, save labor and create a paperless laboratory, while generating results that correlate well with plate count methodology.

If you are interested in participating a field trial with Neogen’s new test, please contact


M.T. Montagna, R. Erroi, S. Sanapo, G. Caggiano, F. Bagordo, A. De Donno, Food products and fungal contamination. Note I. Preliminary investigation in commercial yoghurt. Journal of Preventative Medicine and Hygiene 39: 68-70. 1998