How Greek Yogurt Is Made: From Milk to Strained Yogurt, Village Cloth to Separator
Updated: 1 day ago
Every tub of Greek yogurt is the result of two operations. First the milk is fermented into yogurt by a pair of bacteria that have been working together for thousands of years. Then part of the whey is removed. The first step is shared with every yogurt on earth; the second is what makes it Greek.
Step 1: the milk
Industrial Greek yogurt is made almost entirely from cow's milk, standardised to the fat level printed on the tub: 0%, 2%, 5% or 10%. Sheep's and goat's milk are used for premium and traditional products; sheep's milk is naturally about 50% richer in protein and nearly twice as rich in fat as cow's milk, which is why sheep yogurt is denser even before straining.
Under the Greek Food and Beverages Code the only additions allowed are cream (to adjust fat) and milk proteins of the same animal species, within a limit of four percentage points of extra solids. Milk powder reconstitution is prohibited. Thickeners are not permitted in anything sold as yogurt.
Step 2: heat treatment
The milk is pasteurised at a higher temperature and for longer than drinking milk, typically 85–95 °C for several minutes. This kills competing microbes, but more importantly it denatures whey proteins so that they bind to casein during fermentation, producing a firmer, smoother gel that holds less free water. At home this is the "bring it almost to the boil" step.
Step 3: the cultures
The milk is cooled to about 42–45 °C and inoculated with the two mandatory yogurt bacteria:
*Streptococcus thermophilus* starts fast, consumes oxygen and produces formic acid and carbon dioxide that stimulate its partner.
**Lactobacillus delbrueckii subsp. bulgaricus** breaks milk proteins into peptides and amino acids that feed the streptococcus and produces acetaldehyde, the compound behind yogurt's characteristic "green apple" aroma.
Together they convert lactose into lactic acid. As the pH falls toward 4.6, casein micelles lose their charge and link into a gel. Greek law requires at least 10 million live cells per gram until the end of shelf life; the US standard requires live cultures at the time of manufacture unless the product is labelled as heat-treated. Some producers add further strains (acidophilus, bifidobacteria) and say so on the label; see Greek yogurt and probiotics.
Step 4: fermentation
Fermentation takes three to six hours at 42–45 °C, or longer at lower temperature for a milder flavour. Two styles exist:
Set yogurt ferments in its final container. Traditional Greek paradosiako yogurt is set in clay or glass pots and develops a skin (petsa) on top; Greek law requires the skin for anything labelled "traditional".
Stirred yogurt ferments in a tank, is cooled, then stirred and pumped. Strained yogurt starts as stirred yogurt because it has to be moved to the straining stage.
Step 5: straining
This is the defining step. The yogurt gel is separated from part of its whey.
Cloth bags
The village method, still used by small dairies and at home. Yogurt is poured into tightly woven cotton bags (tsantila) and hung or pressed for anything from four hours to a full day. Gravity does the work; the result depends on the weave, the ambient temperature and how long you wait. Yield is around a third of the starting weight.
Centrifugal separators
Large dairies use nozzle separators originally developed for quark. Warm yogurt (about 40 °C) is spun at high speed; the dense curd exits through nozzles while the whey is drawn off. Separation is continuous and takes seconds rather than hours, and the final solids content can be dialled in precisely.
Ultrafiltration
Membranes with pores small enough to hold back proteins and fat but let water, lactose and minerals through. Ultrafiltration can be applied before fermentation (concentrating the milk, so there is little whey to drain later) or after. It retains more of the whey proteins, which some manufacturers consider a quality advantage and purists consider a departure from the traditional profile.
Whatever the method, Greek law defines the result by protein content: a minimum of 5.6% for cow's and goat's milk strained yogurt and 8% for sheep's milk.
Yield and the whey problem
Straining is expensive because most of the milk leaves as whey. Measured production data from Greek dairies show that 100 kg of milk yields roughly 33 kg of strained yogurt and 67 kg of acid whey; manufacturers in the United States commonly quote three to four litres of milk per litre of Greek yogurt.
Acid whey is not the sweet whey of cheesemaking. It has a pH of 4.2–4.6, holds 3–5% lactose and only 0.2–0.7% protein, and its oxygen demand is so high (BOD around 46–50 g per litre) that discharging it untreated into waterways would strip them of oxygen. When US production exploded after 2007, a 2013 Modern Farmer investigation estimated the Northeast alone was producing more than 150 million gallons of acid whey a year. Producers responded with feed contracts, land application, anaerobic digestion and, more recently, membrane processes that strip the acid so the whey can be dried into usable powders. We look at the issue in Acid whey: Greek yogurt's byproduct problem.
Step 6: cooling, filling and storage
Strained yogurt is cooled rapidly to below 5 °C to slow the cultures, filled into tubs, and held cold through distribution. Properly chilled, unopened strained yogurt keeps three to four weeks; the bacteria continue slowly acidifying it, which is why a tub near its date tastes sharper than a fresh one.
What "Greek-style" producers do instead
A manufacturer who wants the texture without the yield loss can thicken regular yogurt with milk protein concentrate, whey protein concentrate, pectin, modified starch or gelatin. The product is legal in most markets if labelled "Greek-style", but it has not been strained and typically has half the protein. In Greece such a product cannot be sold as yogurt at all; it falls under the code's dairy-dessert article. How to tell the two apart is covered in Authentic Greek yogurt vs Greek-style.
Making it yourself
Everything above scales down to a saucepan, a thermometer, a spoonful of live yogurt and a cloth. The home method, with timings and troubleshooting, is in How to make Greek yogurt at home.
Sources
Dairy (MDPI) 2025, 6, 21 — Greek strained yogurt production, yields and acid whey characteristics: mdpi-res.com (PDF)
Greek Food and Beverages Code, Article 82 (Yogurt): aade.gr (PDF)
US FDA, amended standard of identity for yogurt (2021) and compliance guidance: fda.gov
Modern Farmer, "Whey Too Much: Greek Yogurt's Dark Side" (2013): modernfarmer.com
FoodNavigator, Greek yogurt producers respond on whey (2013): foodnavigator.com
Wikipedia, Yogurt (cultures, chemistry, history overview): en.wikipedia.org
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