Whey collecting on the surface of a yoghurt, known as syneresis or wheying off, is one of the first things a customer notices. A 2023 review of yoghurt syneresis in the Journal of Food Science and Technology lists weak gel firmness and syneresis among the texture defects that affect consumer acceptance.
The usual response is to add a stabiliser or increase it. That can work, but the review makes clear that syneresis is shaped just as much by how the milk base is built and processed.
Where the whey comes from
Yoghurt's texture comes from a protein gel that forms as lactic acid bacteria acidify the milk. Whey separates when that network cannot hold its serum, either because it formed weak or because it contracts during storage.
Timing during fermentation matters. In a Journal of Dairy Science study on acid milk gels, raising the temperature just after the gel had formed increased whey separation, while cooling at the same point reduced it. The authors concluded that yoghurt's final properties depend heavily on conditions during the early gel development phase, while the bonds in the network are still forming.
What the research identifies
The milk base
A review in the Asian-Australasian Journal of Animal Sciences names total solids content, heat treatment and incubation temperature among the processing variables that shape yoghurt texture.
The syneresis review describes enriching the milk base with skimmed milk powder, whey protein powders or casein-based powders as a way to reduce syneresis, and identifies casein-based powders as the most effective of the proteins reviewed.
Fat plays a part as well. In a 2025 study, yoghurt at 0.1% fat showed the highest syneresis, at 51.28%, against 14.75% to 22.48% for the yoghurts made at higher fat levels.
Heat treatment and homogenisation
The review reports that optimising the heat treatment of the milk, citing 85 °C for 30 minutes and 95 °C for 5 minutes, together with single or two-stage homogenisation, can decrease syneresis.
The culture
The starter culture's type, its proteolytic activity, whether it produces extracellular polysaccharides, and the inoculation rate can all affect water holding and syneresis.
Incubation and cooling
The review points to incubation at around 40 °C and a two-step cooling process as conditions that can decrease syneresis.
Stabilisers
Among the stabilisers reviewed, gelatin was the most effective. That makes it a useful benchmark, but gelatin is of animal origin and cannot be used in yoghurt sold as vegetarian, so plant-derived options need to be trialled against your own base.
Process settings from studies are starting points. Heat treatment, incubation temperature and cooling interact with your milk, culture and equipment. Change one at a time, against a control made the same day.
A diagnostic order
- Note when the whey appears. Whey visible at the end of fermentation points to how the gel formed. Whey that builds up in storage points to how it holds.
- Check the milk base. Total solids and fat for the affected batches, against specification.
- Check the heat treatment record. The time and temperature actually achieved, not only the set point.
- Check fermentation and cooling. Incubation temperature through the run, and how quickly, and in how many stages, the product was cooled.
- Check handling. Whether set yoghurt was moved or jolted before it had cooled and firmed.
- Then review the stabiliser, once the process is confirmed to be running as intended.