The Microbial Symphony of Yogurt

A Deep Dive into Its Cultural and Microbiological Significance
January 18, 2024 by
Abdi Nasser

The Historical Tapestry of Yogurt:


From the ancient civilizations of Mesopotamia to the nomadic tribes of Central Asia, yogurt has been a staple in various cultures. The process of yogurt fermentation, a result of the ntricate dance between bacteria and milk, was discovered serendipitously by our ancestors. The preservation of milk through fermentation not only extended its shelf life but also introduced a new spectrum of flavors and textures.

Microbial Maestros: L. delbrueckii subsp. bulgaricus and S. thermophilus:

In the world of yogurt production, not all microorganisms are created equal. Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus stand out as the dynamic duo responsible for the transformation of milk into the delightful yogurt we savor today. These bacteria work in harmony, breaking down lactose into lactic acid and contributing to the unique taste and texture of yogurt.

Microbiological Control: Ensuring Yogurt Quality and Safety:

Yogurt, a time-honored culinary delight spanning over 10,000 years of history, owes its existence and distinctive taste to microorganisms. Among the key players in its production are L. delbrueckii subsp. bulgaricus and S. termophilus. The meticulous control of these microorganisms is crucial, and ISO 7889:2003 outlines a standardized method for their enumeration, ensuring the quality and consistency of yogurt.

 Procedure Overview

 1. Preparation of Test Portion

For non-fruit yogurts, weigh 10g of the mixed sample following general requirements (ISO 8261). For fruit yogurts, blend and weigh 10g after 1 minute of mixing.

Microscopic Examination

Stain the sample smear with Methylene Blue to estimate the density of cocci and rods. This microscopic examination helps determine appropriate dilution ranges.

3. Preparation of Primary Dilution

Following ISO 8261, create a 1:10 dilution by blending the sample with diluent. The maximum sample weight with diluent should not exceed 50g.

4. Inoculation and Incubation

Transfer 1mL of each dilution to petri dishes for duplicates. For L. delbrueckii subsp. bulgaricus, use Acidified MRS Medium; for S. termophilus, use M17 Agar. Incubate under specific conditions for each microorganism.

5. Colony Counting

Count colonies on plates with 15 - 300 colonies, considering only those with characteristic features. This step ensures accurate enumeration under subdued light.

6. Confirmation

Stain selected colonies using the Gram method. Non-spore-forming Gram-positive, catalase-negative rods indicate colonies from Acidified MRS Medium, while Gram-positive, catalase-negative chains or diplococci signify those from M17 Agar.


ISO 7889:2003 provides a comprehensive protocol for enumerating key microorganisms in yogurt, guaranteeing its microbiological quality. By adhering to this procedure, producers can maintain the distinctive flavors and consistency that consumers have cherished for millennia. The confirmation step adds an extra layer of assurance, ensuring that only the characteristic microorganisms contribute to the delightful properties of yogurt.

Condalab Recommended Products

Ensure the success of your yogurt microbiological control with Condalab's high-quality products:

- CAT. 5058 Methylene Blue

- CAT. 1318:  M17 Agar

- CAT. 4600:  Gram Staining Kit

These products are designed to meet the stringent requirements of ISO standards, supporting accurate and reliable microbiological testing in yogurt production.

REST Client (Huachao Mao)