DAIRY TECHNOLOGY: FERMENTED MILK
Diagram
Yogurt and Greek yogurt side by side
Yogurt
Milk fermented with lactic cultures until it sets: thick, tangy, and full of whey.
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Greek yogurt
The same yogurt strained to drain the whey away: much thicker, higher in protein, lower in lactose.
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The core difference
Straining. Greek yogurt is yogurt with the whey removed, so everything solid is more concentrated.
Figure 1: Yogurt compared with Greek yogurt. Drawn in our own style from A. K. M. Humayun Kober, Milk and Dairy Foods: Nutrition, Processing and Healthy Aging, CRC Press, 2024, Ch. 3.
Introduction
Yogurt and Greek yogurt sit side by side in the same tubs, but Greek yogurt is not a different milk. It is yogurt that has been strained.
The core difference is the whey. Straining drains away the watery whey, which carries lactose and some minerals, and leaves the protein and fat behind in a smaller, thicker mass. Greek yogurt is therefore much thicker, higher in protein, and lower in lactose and sugar than regular yogurt.
Quick comparison
Table 1: Yogurt compared with Greek Yogurt
| Aspect | Yogurt | Greek Yogurt |
|---|---|---|
| How it is made | Milk fermented with lactic cultures until it sets | Yogurt strained through cloth or a separator to drain the whey |
| Texture | Soft, spoonable, pours slowly | Very thick, holds its shape on a spoon |
| Protein | About 3 to 5 percent protein | About 8 to 10 percent protein, roughly double |
| Lactose and sugar | Higher, whey keeps its lactose | Lower, much of the lactose leaves with the whey |
| Fat | Depends on the milk used | Higher per spoonful, because the solids are concentrated |
| Tanginess | Mildly tangy | More tangy and sharp |
| Whey | Stays in the tub | Drained away |
| Yield | One litre of milk gives about one litre of yogurt | One litre of milk gives much less Greek yogurt, because the whey is lost |
| Price | Lower | Higher, because more milk is needed per tub |
| Main use | Eating plain, smoothies, cooking | Eating plain, dips, desserts, and as a thick spread |
Scientific explanation
Definition
Yogurt is milk fermented by lactic acid bacteria, mainly Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, until the acid sets the protein into a gel.
Greek yogurt, also called strained yogurt, is yogurt from which part of the whey has been removed by straining or by centrifugal separation.
Working principle
Fermentation works because the cultures eat lactose and make lactic acid. As the acid builds, the pH falls to about 4.5, the casein loses its charge, and the milk sets into a gel that traps the whey.
Straining works because the gel holds the protein and fat while the watery whey drains through the cloth. Removing whey concentrates everything that stays behind, which is why the protein per spoonful roughly doubles.
Apparatus
A fermentation vat or incubator held at about 40 to 45 degrees C, and a filler for the tubs.
A straining cloth, a bag, or a centrifugal separator for Greek yogurt, plus a tank to collect the drained whey.
Measurement procedure
Heat the milk, cool it to about 42 degrees C, add the starter culture, and hold it warm until the pH reaches about 4.5 and the milk sets. Cool it at once to stop the acid building.
For Greek yogurt, pour the set yogurt into a cloth or a separator and let the whey drain until the texture is thick. Measure the protein and the total solids to check the straining.
Factors affecting readings
The milk used decides the result: richer milk gives thicker yogurt, and skim milk gives a thinner set.
The culture, the temperature and the time decide the tang: warmer and longer fermentation gives more acid and a sharper taste. Over-straining makes the yogurt dry, under-straining leaves it thin.
Formulas and calculations
The books give no formula for this pair. The protein gain from straining is found by measurement, not by calculation.
Worked example from measured values: regular yogurt at 4 percent protein strained to half its weight gives about 8 percent protein, because the same protein now sits in half the mass. The exact value depends on how much whey is drained, so it is measured on the batch.
Industrial applications
- Yogurt: eating plain with fruit, drinking as lassi, and cooking in curries.
- Yogurt: smoothies and dressings where a pourable texture is wanted.
- Greek yogurt: eating plain, where a thick creamy spoonful is wanted.
- Greek yogurt: dips such as tzatziki, desserts, and baking where thickness matters.
- Both: breakfast bowls, marinades and sauces, with the choice made on texture.
Advantages and limitations
- Yogurt advantage: mild, light and cheap, with more calcium per tub because the whey stays in.
- Yogurt limitation: thinner and lower in protein per spoonful.
- Greek yogurt advantage: very thick and high in protein, with less lactose and sugar.
- Greek yogurt limitation: more expensive, because more milk is needed per tub, and the drained whey must be used or thrown away.
FAQ
Is Greek yogurt just thicker yogurt?
Nearly. It is yogurt with the whey strained out, which makes it thicker and doubles the protein per spoonful.
Why does Greek yogurt have less sugar?
Much of the lactose, the milk sugar, leaves with the whey when the yogurt is strained.
Is Greek yogurt healthier?
It is higher in protein and lower in sugar, which suits many people, but regular yogurt keeps more calcium from its whey. Neither is better in every way.
Can I make Greek yogurt at home?
Yes. Line a sieve with a clean cloth, pour in plain yogurt, and let the whey drain in the fridge for a few hours until it is as thick as you like.
What happens to the whey?
It is the watery liquid that drains out. It holds lactose and minerals, and in factories it is used for drinks, animal feed or other products.
References and further reading
- A. K. M. Humayun Kober, Milk and Dairy Foods: Nutrition, Processing and Healthy Aging, CRC Press, 2024, Ch. 3 (Google Books page read)
- FTN books on disk: Starter Cultures and Fermented Milk Products, Yogurt Science and Technology (agrimoon dairy series).
