Hygienic production and high protein in Turkish strained yoghurt
Strained yoghurt is a product whose dry matter is raised after fermentation. Three production routes, three levers that set the protein content, and whey management.
Strained yoghurt — concentrated yoghurt or labneh in the international literature — is a product whose dry matter is raised by draining whey from the coagulum after fermentation. The manufacturing principles are identical to quark; the only difference is the culture. That matters in practice: a plant with a quark line can produce strained yoghurt and labneh on the same equipment.
Three production routes
| Route | How it works | Advantage | Drawback |
|---|---|---|---|
| A · Cloth bag (traditional) | Gravity draining, 8–16 h, in a cold room | Low investment | High labour; open system, microbiological inconsistency |
| B · Nozzle separator | Centrifugal separation after thermisation; product leaves through nozzles into a cyclone | Continuous flow, high capacity, closed system | Equipment investment |
| C · Ultrafiltration | Milk concentrated by UF before fermentation | Whey protein stays in the product → highest yield | Highest investment |
From traditional to modern: the thermisation difference
When the quark separator was first introduced, milk was pasteurised at about 73 °C before fermentation and separation — the traditional method.
The common method today combines two heat treatments: skimmilk receives high-temperature long-time (HTLT) pasteurisation at 85–95 °C for 5–15 minutes, and the acidified milk is then thermised at 56–60 °C for up to 3 minutes before separation. Together these improve yield appreciably: whey proteins denatured at high temperature precipitate with the casein network and stay in the product instead of being lost with the whey.
Process flow and parameters
| # | Step | Parameter |
|---|---|---|
| 1 | Skimmilk pasteurisation (HTLT) | 85–95 °C · 5–15 min |
| 2 | Cooling + culture addition | 25–28 °C |
| 3 | Rennet addition | ~2 ml liquid rennet / 100 kg milk |
| 4 | Fermentation | ~16 h · pH 4.5–4.7 |
| 5 | Coagulum stirring + thermisation | 56–60 °C · ≤3 min |
| 6 | Cooling | 37 °C |
| 7 | Centrifugal separation | Nozzle separator; whey from a separate outlet |
| 8 | Final cooling | DM 16–19% → 8–10 °C · DM 19–20% → 11–12 °C |
Why only a little rennet?
About 2 ml of liquid rennet per 100 kg of milk is added with the culture — roughly one tenth of ordinary cheesemaking. The aim is not to make cheese but to obtain a firmer coagulum, which means fewer fines lost during separation and a cleaner whey.
Three levers for high protein
| Method | Typical rate | Advantage | Watch for |
|---|---|---|---|
| UF retentate (from skimmilk) | CF 1.5–2.0 | Whey protein stays in the product; highest nutritional value | Membrane investment; retentate viscosity can slow fermentation |
| Skimmilk powder | ≤ 3% | Low investment, quick to apply | Above 3%: powdery mouthfeel and cooked flavour risk |
| Evaporation | 10–20% of volume | Natural concentration, no additions | Energy cost; added thermal load |
| Milk concentrate | variable | Flexible dosing | Supply and cold-chain dependency |
Under FAO/WHO principles the minimum milk solids-non-fat in yoghurt is 8.2%. Raising total dry matter — particularly the casein and whey protein fraction — gives a firmer coagulum and reduces the tendency to whey separation. This is why high-protein strained yoghurt needs less stabiliser.
Is a stabiliser needed?
Correctly produced natural yoghurt requires no stabiliser; a firm, high-viscosity gel forms on its own. Stabilisers (gelatin, pectin, starch, agar-agar) are typically used at 0.1–0.5% and only in fruit yoghurts or pasteurised yoghurt. The wrong type or an excess gives the product a rubbery, hard consistency.
Formulation suggestions
| Parameter | Classic strained | High protein | Labneh / spreadable |
|---|---|---|---|
| Dry matter | 16–18% | 19–21% | 22–24% |
| Protein | 7–8% | 9–11% | 9–10% |
| Fat | 3–5% | 0–2% | 8–12% |
| Target pH (filling) | 4.4–4.6 | 4.4–4.6 | 4.2–4.4 |
| DM route | Separator | UF retentate + separator | Separator + cream addition |
| Cooling target | 8–10 °C | 11–12 °C | 11–12 °C |
| Stabiliser | Not needed | Not needed | Optional, ≤0.3% |
Why cooling temperature depends on dry matter
At 16–19% dry matter the product can be cooled to 8–10 °C. At 19–20% it should only be cooled to 11–12 °C. Cooling further raises viscosity beyond what can be pumped and the line blocks. When developing a high-protein product this limit directly shapes the filling line design.
Critical hygiene points
- The cloth bag route is the riskiest step. Draining takes 8–16 hours with the product exposed to room air throughout. Bags must be washed and disinfected after every use and the cold room held below 4 °C. Plants that scale up abandon this method not mainly for labour reasons but for microbiological inconsistency.
- Separator outlet and cyclone form a closed system and must be connected to CIP. The nozzle area needs periodic inspection for blockage.
- The cream addition line is a separate circuit to the dynamic mixer and needs its own CIP programme.
- The cooling exchanger counts as a heated surface; use a programme with an acid step.
- For long-life product, stabilisers must be fully dispersed in the buffer tank before heat treatment, or the protein system collapses during final heating.
Whey: cost or revenue
The largest operational issue with strained yoghurt is the side stream. Doubling dry matter means separating roughly half the input milk as whey. This stream carries a high organic load; direct discharge is both costly and, in most places, prohibited.
This is where the economics of the UF route become clear: whey protein stays in the product, so yield rises and the load to be disposed of falls. On the separator route the whey must be treated as a product in its own right — whey protein concentrate (WPC) production or sale to the feed industry are the two most common paths.
Summary
- Strained yoghurt runs on the same line as quark; the difference is the culture.
- HTLT (85–95 °C / 5–15 min) plus thermisation (56–60 °C / ≤3 min) improves yield.
- 2 ml rennet per 100 kg with the culture firms the coagulum.
- The most effective route to high protein is UF retentate; skimmilk powder should not exceed 3%.
- Above 19% DM, do not cool below 11–12 °C.
- Whey is not waste but a side stream that must be planned for.
Technical data (temperatures, times, concentration ranges and concentration factors) are taken from the relevant chapters of the Dairy Processing Handbook (Tetra Pak). Formulation and roadmap suggestions are application recommendations built on the process limits in that source; they should be validated against each plant's milk composition and equipment.