DAIRY TECHNOLOGY: FERMENTED MILK
Diagram
Yogurt and curd (dahi) side by side
Yogurt
Milk set with a fixed pair of cultures at warm temperature: mild, smooth, and the same every time.
↓
Curd (dahi)
Milk set with a spoonful of yesterday’s curd: mixed wild cultures, tangier, and different in every home.
↓
The core difference
The starter. Yogurt uses defined cultures; dahi uses a back-slopped mixed culture.
Figure 1: Yogurt compared with curd (dahi). Drawn in our own style from FAO, The Technology of Traditional Milk Products in Developing Countries, 1990, and Milk Products of India, ICAR, 1964, chapter FERMENTED WHOLE MILK PRODUCT DAHI, p. 15.
Introduction
Yogurt and curd, called dahi, look alike in the bowl: both are white, set, tangy fermented milks. They differ in how they are started and in how much they vary.
The core difference is the culture. Yogurt is made with a defined pair of bacteria, Streptococcus thermophilus and Lactobacillus bulgaricus, at a controlled warm temperature. Dahi is made by adding a spoonful of the previous batch, which carries a mixed community of lactic bacteria, and setting it at room temperature. That is why yogurt tastes the same every time and dahi tastes different in every home.
Quick comparison
Table 1: Yogurt compared with Curd (Dahi)
| Aspect | Yogurt | Curd (Dahi) |
|---|---|---|
| Starter culture | Defined pair: S. thermophilus and L. bulgaricus | Back-slopping: a spoonful of previous dahi, mixed lactic cultures |
| Setting temperature | About 40 to 45 degrees C, held warm | Room temperature, about 25 to 35 degrees C in warm climates |
| Setting time | About 4 to 6 hours | About 6 to 12 hours, often overnight |
| Flavour | Mild, clean, consistent | Tangier and more varied, changes with the household culture |
| Texture | Smooth and firm, even set | Softer set, can be looser, sometimes with whey on top |
| Acidity | Mild to moderate, controlled | Often sharper, can turn very sour if left too long |
| Milk used | Usually cow milk, standardised | Cow milk or buffalo milk, often boiled and cooled at home |
| Production | Factory made with standard cultures | Home made daily, and also made in small dairies |
| Shelf life | About 1 to 2 weeks when cold | A day or two at room temperature, longer when cold |
| Main use | Eating plain, desserts, industrial products | Daily food with rice and meals, raita, lassi, kadhi |
Scientific explanation
Definition
Yogurt is milk fermented by the defined thermophilic cultures Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus until the acid sets it into a gel.
Curd, or dahi, is the traditional fermented milk of South Asia, made by inoculating boiled and cooled milk with a small amount of a previous batch of dahi and letting its mixed lactic cultures set the milk.
Working principle
Both products work the same way at the core: lactic bacteria eat lactose, make lactic acid, and the falling pH sets the casein into a gel.
The difference is the culture and the temperature. Yogurt cultures grow fast at 40 to 45 degrees C and give a clean mild acid. Dahi cultures are a mixed community that grows at room temperature and gives a broader, tangier flavour, and because the culture is carried over from batch to batch, it drifts over time.
Apparatus
For yogurt: an incubator or fermentation room held at 40 to 45 degrees C, and a defined starter culture.
For dahi: a clean earthen or steel pot, a warm corner of the kitchen, and a spoonful of the previous batch as starter. No incubator is needed in a warm climate.
Measurement procedure
For yogurt, heat the milk, cool it to about 42 degrees C, add the defined culture, hold it warm until it sets at about pH 4.5, then cool it at once.
For dahi, boil the milk, cool it to lukewarm, stir in a spoonful of previous dahi, cover the pot, and leave it undisturbed overnight at room temperature until it sets.
Factors affecting readings
Temperature decides the result: too cold and the milk does not set, too hot and the cultures die.
The starter decides the flavour: a fresh defined culture gives mild yogurt every time, while an old or contaminated dahi starter gives a sour or off flavour. Clean pots and fresh milk matter for both.
Formulas and calculations
The books give no formula for this pair. The setting of both products is followed by pH and by acidity, not by calculation.
Acidity is measured by titration and given as percent lactic acid: Titratable acidity (percent lactic acid) = (Volume of alkali used x Normality x 0.09 x 100) divided by the weight of the sample. Dahi left too long shows a rising acidity by this test.
Industrial applications
- Yogurt: eating plain, desserts, smoothies and factory-made products.
- Yogurt: the base for flavoured and Greek style yogurts in the dairy industry.
- Curd (dahi): the daily accompaniment to rice and meals across South Asia.
- Curd (dahi): raita, lassi, kadhi and marinades, where its tang suits the dish.
- Both: probiotic foods and a source of lactic bacteria in the diet.
Advantages and limitations
- Yogurt advantage: consistent mild flavour and firm set, batch after batch.
- Yogurt limitation: needs a defined culture and a warm incubator, so it is harder to make at home without them.
- Curd (dahi) advantage: easy to make at home with no special culture, just the previous batch.
- Curd (dahi) limitation: flavour and acidity vary, and a poor starter can give a sour or off product.
FAQ
Are yogurt and dahi the same thing?
No, though both are fermented milks. Yogurt uses a defined pair of cultures at warm temperature; dahi uses a mixed culture from the previous batch at room temperature.
Which is tangier?
Dahi is usually tangier and more variable, while yogurt is milder and more consistent.
Can I use dahi as a yogurt starter?
It will set milk, but the result is dahi, not yogurt, because the culture is the mixed dahi community, not the yogurt pair.
Can I use yogurt as a dahi starter?
Yes, it will set the milk, and the first batch will taste mild like yogurt, but carried over many batches it drifts towards a mixed culture.
Which keeps longer?
Yogurt, when kept cold in a sealed tub. Dahi at room temperature turns sour within a day, so it is made fresh daily.
References and further reading
- FAO, The Technology of Traditional Milk Products in Developing Countries, 1990 (Google Books page read)
- Srinivasan and Anantakrishnan, Milk Products of India, ICAR, 1964, chapter FERMENTED WHOLE MILK PRODUCT DAHI, p. 15 (Google Books page read)
- A. K. M. Humayun Kober, Milk and Dairy Foods: Nutrition, Processing and Healthy Aging, CRC Press, 2024, Ch. 3 and Ch. 11 (Google Books page read)
- FTN books on disk: Starter Cultures and Fermented Milk Products, Traditional Dairy Products (agrimoon dairy series).
