⚛️ Physics, Chemistry & Biology in a Spoonful of Curd

Sailekshmi · October 4, 2025, 4:35pm

CUBE ChatShaala Summary (04.10.2025)

Today’s discussion explored how the transformation of milk into curd beautifully connects physics, chemistry, and biology.

Milk Composition: Milk contains lactose (sugar) and casein (protein).
Role of Bacteria: When lactic acid bacteria (LAB) are introduced (via a spoonful of starter curd), they act as prokaryotes that synthesize enzymes.
Enzymatic Action: These enzymes convert lactose into lactic acid.
Protein Change: Lactic acid alters the structure of casein protein, causing milk to thicken into curd.
Starter vs. Control:
- With starter curd: Millions of LAB multiply rapidly through protein synthesis, producing curd.
- Without starter curd: Only random air bacteria are present, so curd formation is uncertain and delayed.

This model system helps us connect protein synthesis, enzymatic reactions, microbial growth, and food science.

Provocative Questions for the Audience

Can milk ever turn into curd without adding starter curd? If yes, how long would it take and why?
Why do we call lactic acid bacteria “prokaryotes,” and how does this matter for curd formation?
Is curd formation only a chemical reaction or also a biological process? Where does physics
enter the picture?
If curd bacteria can multiply so fast, why don’t we get sick when we eat curd?

What I Learned

Curd making is not just “grandma’s recipe” but a scientific process involving enzymes, proteins, and bacterial genetics.
The presence of starter culture (with LAB) makes the process predictable, unlike random air bacteria.
The term “prokaryote” literally means “before nucleus,” reminding us of the evolutionary simplicity of LAB.
Protein structure can be chemically altered by acids, showing a direct link between food science and molecular biology.

TINKE Moments (This I Never Knew Earlier)

Many thought curd forms just because of “air exposure,” but in reality, it needs specific bacteria (LAB).
Protein synthesis inside bacteria directly links microbial life to a visible kitchen phenomenon.
The exact role of temperature in accelerating or slowing down curd formation wasn’t deeply discussed—this could be our next exploration.
Could different bacterial strains produce curd with different tastes, textures, or nutritional values?

Reference

@Arunan, @SN1261, @sakshiconsultant2002, @magpie, @dhanraj7, @Jeene, @ankitcube, and others

kvk · October 4, 2025, 6:50pm

My doubts are on the timing of the curd formation and how the timing is dependent on various parameters like temperature or the volume of the ‘starter’ curd (Where did I read about measuring curdling time with one teaspoon of starter curd and one tablespoon?).

Real-Time Monitoring of Yogurt Fermentation Process … by scientists at Biomeasurement Technology Laboratory, Graduate School of Agricultural Science, Kobe University shows that the fermentation process follows a S-curve with rapid change between 3.3 hours and 4.6 hours after start of curdling. As the Biomeasurements Technology Lab wanted to propose a non-invasive Near Infrared Spectroscopy to measure the curdling, they did not vary the temperature nor the quantity of starter yogurt nor its different types (only Bulgarian, no Japanese, nor Indian etc).

At CUBE we can try a Reflectometer to measure the timing of curd formation. Reflectometer costing around Rs 1000 are available in India. Then we can find the ideal temperature for fastest curd formation.

I feel that there is something like 50 % Growth time (like half-life for radio-activity) for the lactobacillus, say to grow from 8000 to 12000 in 1 cc. As it is easier to measure the number of bacillus after 2 hours, 4 hours, 6 hours etc most measurements are timed this way and the 50% growth time can be estimated. I think that the 50% Growth time for lactobacillus LAB is about 10 minutes. Then about 112 cc of milk will be curded in 70 minutes using 5 cc of starter curd. If 10 cc of starter curd is used, it will take 60 minutes to form 112 cc of curd.

The temperature of the milk is very important. The starter curd for non-sour curd is around 43 degree centigrade. But I have learned that it is more important to shake the milk after adding the starter curd. The Operating Procedure I apply is:

Add 15 cc of the starter curd to the 900 cc of milk at around 40 degree C in a 1000 cc vessel.
Shake the 1000 cc container with a spoon for 10 seconds in one direction.
Stop shaking the spoon for 10 secs.
Shake the container for 10 secs with the spoon in a direction +120 degrees to the first.
Stop shaking for 10 secs.
Shake the container for 10 secs with the spoon in a direction -120 degrees to the first.
Close the container.
After around 3 hours the container will be seen to contain well formed curd.

Maybe the churning, non-churning steps make the LAB to spread throughout the milk to form a solid looking curd.