What Bacteria is Under Curd? A Deep Dive into Lactic Acid Bacteria

November 20, 2025 •

4 min read

The creamy, tangy texture of curd, also known as dahi or yogurt, is the result of a fascinating biological process involving millions of microorganisms. A small amount of starter culture, containing millions of beneficial microbes, is added to milk to begin the process, but what bacteria is under curd? The primary answer lies in the group known as lactic acid bacteria (LAB).

Quick Summary

The specific bacteria in curd belong to the lactic acid bacteria group, primarily Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. These microbes ferment lactose into lactic acid, causing the milk proteins to coagulate, which results in the characteristic thick texture and tangy flavor while also boosting its nutritional value.

Key Points

Primary Bacteria: Curd is fermented by Lactic Acid Bacteria (LAB), primarily Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus.
Symbiotic Relationship: In curd production, S. thermophilus first lowers the pH, and L. bulgaricus then thrives in the acidic environment, breaking down proteins to create food for the streptococcus.
Fermentation Process: The bacteria convert lactose (milk sugar) into lactic acid, which causes the milk's casein proteins to coagulate and form the semi-solid curd.
Nutritional Boost: The fermentation process increases the nutritional value of curd, particularly by enhancing its vitamin B12 content.
Digestive Health: The LAB act as probiotics, which promote healthy digestion by balancing the gut's microbial flora.
Temperature Matters: Temperature control is crucial for successful curd formation; the milk must be heated and then cooled to an optimal temperature (40-45°C) before adding the starter culture for the bacteria to thrive.

The Lactic Acid Bacteria Behind Curd Formation

The transformation of milk into curd is a testament to the power of microbial fermentation. The process relies on a cooperative symbiotic relationship between specific bacterial strains known as Lactic Acid Bacteria (LAB). This relationship is what gives curd its unique taste, texture, and nutritional profile. While many different LAB species can be found in various fermented milk products, a handful are most commonly associated with traditional and commercial curd production.

The Dynamic Duo: Lactobacillus bulgaricus and Streptococcus thermophilus

The most prominent and well-studied bacteria involved in making curd are Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. These two species are typically added as a starter culture to pasteurized milk. Their interaction is a classic example of proto-cooperation, where each species's growth is stimulated by the other.

Streptococcus thermophilus initiates fermentation: S. thermophilus is a thermophilic (heat-loving) bacterium that grows optimally at higher temperatures, typically around 40-45°C. It rapidly begins to consume the lactose in the milk and produces folic acid and formic acid, which are essential growth factors for L. bulgaricus.
Lactobacillus bulgaricus continues acidification and develops flavor: As the acidity of the milk increases, conditions become more favorable for L. bulgaricus. This rod-shaped bacterium is highly proteolytic, meaning it breaks down milk proteins (casein) into smaller peptides and amino acids. These released compounds, in turn, provide the nutrients that allow S. thermophilus to continue its growth. The peptides released by L. bulgaricus are also vital for developing curd's characteristic flavor.

The combined metabolic activity of this pair leads to rapid acidification, causing the milk's casein proteins to coagulate and thicken, resulting in the characteristic texture of curd.

The Fermentation Process Explained

The journey from liquid milk to thick curd is a biological marvel that can be broken down into a few simple steps:

Preparation: Milk is heated to a high temperature, typically between 85-90°C, and held for several minutes. This process, known as pasteurization, kills any undesirable bacteria and denatures the whey proteins, which helps create a firmer curd. The milk is then cooled to the optimal incubation temperature for the starter cultures, around 42-45°C.
Inoculation: A small amount of existing curd, containing the live LAB cultures, is added to the warm milk. This 'starter' contains the millions of bacteria needed to ferment the entire batch.
Incubation: The inoculated milk is left undisturbed in a warm environment. The S. thermophilus begins to grow first, producing lactic acid, which lowers the pH. This environment becomes ideal for L. bulgaricus to flourish, and its proteolytic activity releases the peptides needed for both species to continue multiplying.
Coagulation: As the lactic acid accumulates, the milk's pH drops to a point (around 4.6) where the casein proteins lose their negative charge and aggregate together, forming a semi-solid gel.
Refrigeration: Once the desired consistency and tartness are achieved, the curd is refrigerated. This cooling process halts the fermentation, preventing the curd from becoming excessively sour and preserving its texture.

Other Notable Lactic Acid Bacteria in Curd

While the L. bulgaricus and S. thermophilus are the standard for yogurt, other LAB species can also play a role, particularly in regional variations or as probiotic additions. These can include:

Lactobacillus acidophilus: A common probiotic that is well-adapted to surviving the acidic conditions of the human gut.
Lactococcus lactis: A mesophilic (moderate-temperature loving) bacterium often found in traditional fermented milk products, particularly buttermilk.
Lactobacillus casei and Lactobacillus rhamnosus: Other probiotic strains frequently added to enhance the health benefits of fermented dairy.

A Comparison of Traditional Curd vs. Probiotic Yogurt


Feature	Traditional Curd (Dahi)	Probiotic Yogurt
Primary Bacteria	Native starter cultures, often a mix of Lactococcus and Lactobacillus species adapted to local conditions.	Standardized starter cultures (S. thermophilus and L. bulgaricus) plus specific added probiotic strains like L. acidophilus or Bifidobacterium.
Microbial Viability	May contain active, live cultures, but the exact strains and quantity can be inconsistent, especially in homemade versions.	Guarantees a certain count of specified, viable probiotic microorganisms at the time of consumption.
Flavor Profile	Often varies from mild to quite sour, depending on the specific mix of bacteria and fermentation conditions.	Flavor is often more standardized and consistent, with some variations depending on added strains or flavorings.
Texture	Can range widely in consistency and firmness depending on milk type, starter culture, and incubation time.	Often has stabilizers or specific strains (like EPS-producing ones) to ensure a consistent, thick, and smooth texture.
Nutritional Profile	Contains vitamins and minerals from the original milk, with enhanced Vitamin B12 content due to fermentation.	Enhanced with additional specific probiotic strains aimed at targeted gut health benefits.

Health Benefits of Lactic Acid Bacteria in Curd

Beyond simply thickening milk, the LAB in curd offer a range of health advantages:

Improved Digestion: The bacteria aid in breaking down lactose, making curd more digestible for individuals with lactose intolerance.
Enhanced Nutrient Absorption: Fermentation increases the bioavailability of certain nutrients, most notably Vitamin B12.
Boosted Gut Health: As probiotics, these bacteria help maintain a healthy balance of intestinal flora, which is essential for overall digestive wellness and immunity.
Immune System Support: A healthy gut microbiome, supported by regular consumption of probiotics, is linked to a stronger immune response.

Conclusion

The bacteria under curd are a complex and cooperative community of Lactic Acid Bacteria, most notably Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. Through their symbiotic fermentation of lactose, these microbes produce the characteristic lactic acid that coagulates milk into the thick, tangy dairy product we know and love. The result is a food that is not only delicious but also a powerhouse of nutritional benefits, supporting digestive health and enhancing the absorption of vitamins. Whether homemade or commercially prepared, the microbial magic behind curd is a fascinating intersection of food science and natural biology.

Visit the official International Dairy Federation website for more information on the standards and microbiology of fermented milk products.

Frequently Asked Questions

The primary bacteria responsible for making curd are Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, which work together in a synergistic relationship during fermentation.

Bacteria turn milk into curd through fermentation. They consume lactose, the milk's natural sugar, and convert it into lactic acid. This acid lowers the milk's pH, causing the milk proteins (casein) to denature and coagulate, forming the solid curd.

Yes, the bacteria in curd, particularly lactic acid bacteria, are beneficial for human health. They are probiotics that help improve digestion, boost the immune system, and support a healthy gut microbiome.

Yes, there can be differences. Homemade curd relies on a native starter culture, which can vary in bacterial composition. Store-bought yogurt uses specific, standardized strains (L. bulgaricus and S. thermophilus), and many include additional probiotic strains for specific health benefits.

If the milk is too hot when the starter culture is added, the high temperature will kill the beneficial bacteria, and the fermentation process will fail. The milk will not set and will likely spoil.

Yes, lactic acid bacteria have the ability to synthesize vitamin B12 during the fermentation process, which enriches the nutritional quality of the curd.

Curd (or dahi) is traditionally made with native, locally sourced bacteria, resulting in a variable taste and texture. Yogurt is produced under controlled conditions using standardized, specific bacterial cultures (S. thermophilus and L. bulgaricus), resulting in a more consistent and predictable product.