Recipes & guides · 4 min read
What is Lactobacillus bulgaricus?
Meet the bacterium that gave Bulgarian yogurt its name: who found it, what it does in milk, and why it needs a partner.
Lactobacillus delbrueckii subsp. bulgaricus — "bulgaricus" for short — is the lactic acid bacterium that makes yogurt yogurt. It carries the name of a country because that is where it was found.
A Bulgarian discovery
In 1905 Stamen Grigorov, a young Bulgarian doctor working in Geneva, looked at home-made yogurt from his village under the microscope and described the rod-shaped bacterium responsible for it. It was later named after Bulgaria.
What it does in milk
Bulgaricus lives on lactose, the sugar in milk, and turns it into lactic acid. As the milk gets more acidic, its proteins settle into a soft gel — the set of the yogurt — and the flavour becomes sharp and fresh. It also makes the small aroma compounds that give real yogurt its typical smell.
Why it never works alone
In yogurt, bulgaricus always works with a partner: Streptococcus thermophilus. The two help each other along — thermophilus gets the fermentation started quickly, bulgaricus carries it on and gives the tang. That is why every proper yogurt culture, ours included, contains both.
Why temperature matters
Both bacteria like it warm: around 42–45 °C. Above about 50 °C they die; below about 38 °C they slow down and the yogurt sets thin. Keep the milk in that warm window for 7–8 hours and the culture does the rest. The step-by-step guide has the details.
