Milk cooling in a yogurt production line has three main jobs: protect raw milk before processing, bring heat-treated yogurt milk to the correct fermentation temperature, and reduce the temperature after fermentation to control further acidification. The equipment and target temperatures change with the product. Stirred yogurt is normally cooled before filling, while set yogurt is fermented and cooled in its package.
A small plant may use jacketed batch tanks for several steps. A larger line may combine regenerative plate heat exchangers, fermentation tanks, low-shear yogurt coolers, chilled-water systems and a packaged-product cooling room. Getting the connections and capacities right matters as much as choosing the individual machines.
Where Does Cooling Take Place in a Yogurt Production Line?
The first cooling decision is made before the yogurt line starts. Raw milk received for later processing is chilled and held under controlled conditions. Once production begins, the milk is standardized, homogenized and heat-treated. It is then brought to the temperature required by the starter culture. After fermentation, another cooling step protects the yogurt's acidity and texture.
| Cooling point | Typical temperature target | Purpose | Common equipment |
|---|---|---|---|
| Raw milk receiving and holding | Approximately 4°C when cold storage is needed | Maintain raw milk quality while waiting for processing | Bulk cooling tank, insulated storage tank, plate pre-cooler |
| After yogurt-milk heat treatment | Commonly 40–45°C for thermophilic cultures | Prepare the milk for starter addition and fermentation | Multi-section plate heat exchanger, temperature-control loop |
| After fermentation — stirred yogurt | Often first reduced to about 15–22°C | Slow further acidification while preserving a workable texture | Jacketed fermentation tank, low-shear pump, yogurt plate or tubular cooler |
| After fermentation — set yogurt | Intermediate cooling commonly to about 18–20°C, then cold storage around 5°C | Cool the packaged gel with minimal disturbance | Incubation/cooling room, cooling tunnel, refrigerated storage |
These are useful process reference points, not one fixed temperature schedule for every recipe. The appropriate setpoints depend on culture, product viscosity, packaging and the production method. The Tetra Pak Dairy Processing Handbook's fermented milk chapter describes the different set and stirred yogurt routes in detail.
Raw milk cooling happens before yogurt processing
When milk arrives well before the production shift, it may need to be cooled and stored close to 4°C. A tank is selected around the incoming volume, collection schedule, cooling time and the capacity needed for the next production run. A site processing milk immediately may use a different reception arrangement from one that holds milk overnight.
NexAgri supplies milk cooling and storage tanks, sanitary pumps and rapid milk pre-cooling systems for the receiving area. The raw milk tank should be sized separately from the fermentation tanks and finished-yogurt buffers.
How Is Yogurt Milk Cooled After Heat Treatment?
Yogurt milk often receives a more intensive heat treatment than ordinary pasteurized drinking milk. A commonly used treatment is 90–95°C for around five minutes, which helps develop the protein structure needed for a stable fermented gel. The heat exchanger and holding section must be sized to achieve the selected temperature and holding time at the operating flow rate.
Once the milk leaves the holding section, a continuous line commonly cools it in two steps:
- Regenerative cooling. Hot, heat-treated yogurt milk transfers part of its heat to the incoming milk through separate channels of a sanitary heat exchanger. This reduces both heating demand and the load on the final cooling section.
- Temperature trimming. A controlled water circuit brings the product to the required inoculation temperature, often around 40–45°C. An outlet temperature sensor and control valve keep the product close to its setpoint before it enters the fermentation system.
A typical equipment arrangement is:
Balance tank → regenerative heating → homogenization (as specified) → final heating and holding → regenerative cooling → water-cooled temperature adjustment → culture addition → fermentation tank
The hygienic flow path, measuring points and diversion arrangements are defined in the process and instrumentation diagram. If the product temperature goes outside the permitted range, the controls should prevent it from reaching the culture-dosing or fermentation stage until the batch has been handled under the plant's approved procedure.
For set yogurt, an alternative arrangement involves cooling the treated milk to approximately 5°C for temporary holding, then heating it to incubation temperature before culture dosing and filling. This can make preparation and filling schedules easier to manage.
Our plate processing units can be configured with regeneration, holding, cooling sections, temperature recording and automatic flow or valve controls. The actual arrangement depends on whether the same line also produces drinking milk or other dairy products.
How Do Set and Stirred Yogurt Cooling Systems Differ?
The most important difference comes after fermentation. A set yogurt cup is already filled before the protein gel forms. Stirred yogurt develops in a tank and must be moved through cooling and filling equipment without excessively breaking down its texture.
| Process step | Set yogurt | Stirred yogurt |
|---|---|---|
| Culture addition | Before filling | Before or during fermentation-tank filling |
| Fermentation | Inside the final cup or container | In an insulated fermentation tank |
| First cooling step after fermentation | Cool filled cups using circulated air | Cool the yogurt in the tank and/or through a hygienic product cooler |
| Main equipment | Incubation room, cooling tunnel or combined chamber, cooling racks | Jacketed tank, low-shear pump, sanitary plate or tubular cooler |
| Packaging stage | Before fermentation | Usually after initial yogurt cooling |
| Main engineering concern | Uniform cup-center temperature and gentle handling | Viscosity, pump shear, backpressure and consistent filling temperature |
Cooling stirred yogurt
Stirred yogurt is commonly fermented at around 42–43°C until the selected pH endpoint is reached, often in the region of 4.2–4.5. Cooling then reduces the activity of the culture and slows further acidification. The gel receives controlled mechanical treatment to produce a smooth texture before or during transfer through the cooler.
For relatively low-viscosity products, a hygienic plate cooler designed for yogurt can be used. A tubular or double-pipe cooler may be a better fit for more viscous products or a line with strict pressure-drop and shear limits. The final choice depends on the yogurt's viscosity, the allowed pressure drop, desired outlet temperature and filling rate.
NexAgri can configure a jacketed fermentation tank, variable-speed low-shear pump, plate or tubular cooler, temperature and pressure measurement, and a buffer tank ahead of the filler. A fruit-yogurt line may also need a separate metering and mixing section. For that application, see our guide to stirred yogurt separation after fruit addition.
Cooling set yogurt
Set yogurt requires a different layout. After filling and incubation, cups are moved into a cooling room or tunnel. Air must pass around the individual packages and through the crates or racks. A cold room thermometer alone does not tell the operator how quickly the middle of each yogurt cup is cooling.
Package size, cup material, number of cups, crate spacing, air distribution and door openings all affect the result. A cooling tunnel that handles small cups at one speed may require a different layout or dwell time when a customer introduces larger containers. Final cooling normally continues in refrigerated storage.
For set yogurt projects, we can supply the incubation room, cooling room, air circulation equipment, racks and refrigeration components, with cooling-load calculations based on the intended packaging and production schedule.
Which Cooling Equipment Should a Small Yogurt Plant Choose?
The first question is whether the plant needs to cool milk before fermentation, viscous yogurt after fermentation, or sealed yogurt cups. These are different equipment duties.
| Equipment | Suitable duty | What determines the selection |
|---|---|---|
| Jacketed batch tank | Small-batch heating, fermentation and cooling when the vessel is equipped for the intended duty | Batch size, dimple/coil/double-wall jacket design, heat-transfer area, coolant connections, agitation and cycle time |
| Multi-section plate heat exchanger | Continuous yogurt-milk heating, regeneration and cooling to inoculation temperature | Product flow, holding time, temperature profile, heat recovery, pressure drop |
| Hygienic yogurt plate cooler | Cooling pumpable stirred yogurt | Viscosity, gentle channel design, pump type, pressure drop, cleaning access |
| Tubular or double-pipe yogurt cooler | More viscous stirred yogurt, or products requiring wider flow passages | Viscosity at outlet temperature, pump pressure, heat-transfer area, cleaning arrangement |
| Chilled-water system | Supplies the cooling medium to tanks and exchangers | Concurrent equipment loads, supply/return temperature, summer ambient conditions |
| Cooling room or tunnel | Cooling sealed set-yogurt cups | Cup-center cooling curve, package size, air distribution, batch loading pattern |
Choosing the cooling medium
For raw milk cooled to approximately 4°C, a final plate-cooling section may use ice water at around 1–2°C. Some installations use a separate food-grade propylene glycol circuit operating below 0°C (for example, around −2 to −4°C) where the equipment and required product temperature justify it. The glycol remains on the utility side of a sanitary heat-transfer surface; coolant selection also has to address freeze protection, pressure separation and leak detection.
For finished stirred yogurt, the coolant is selected together with the product outlet temperature, viscosity and exchanger geometry. Very cold coolant combined with slow product-side movement can create strong temperature gradients and local thickening near a tank wall or heat-transfer surface. A controlled-temperature jacket circuit, or a properly sized wide-channel plate or tubular cooler with stable coolant flow, can achieve the cooling target while keeping the yogurt's mechanical treatment under control. The coldest available water circuit is not necessarily the best setting for a viscous yogurt batch.
For 200 L batches, jacket cooling may provide a compact, economical solution when the cooling schedule is relatively relaxed. At 500 L, a jacketed tank combined with an external cooler can shorten turnaround time. For 1,000 L batches or repeated production shifts, an external product cooler may offer a more efficient arrangement. These are starting points for discussing a layout; actual equipment selection comes from the required batch cycle and product characteristics.
For a small batch pasteurizing or fermentation tank, check the jacket construction (dimple jacket, external coil or double-wall water jacket), effective heat-transfer area, coolant inlet/outlet size and agitation arrangement. These details determine whether the tank can complete the required cooling cycle or needs an external product cooler.
In a continuous system, the pasteurizer, fermentation tank, cooler and filling machine should be sized as a connected line. A 2,000 L/h upstream section does not give a plant 2,000 L/h of finished yogurt if the downstream cooling or filling stage can only process 1,000 L/h.
How Do You Calculate Yogurt Cooling Capacity?
Start with the heat that must leave the product. The basic sensible-heat equation is:
Cooling duty (kW) = Mass flow (kg/h) × Specific heat (kJ/kg·K) × Temperature drop (°C) ÷ 3,600
For batch cooling, replace mass flow with the total mass and divide by the cooling time in seconds.
Example 1: A 1,000 L/h yogurt-milk preparation line
Assume a line cools heat-treated yogurt milk from 95°C to 43°C, with milk density approximated as 1.03 kg/L and specific heat as 3.9 kJ/(kg·K).
- Product flow: 1,000 L/h × 1.03 kg/L = 1,030 kg/h
- Temperature drop: 95 − 43 = 52°C
- Product-side cooling duty: 1,030 × 3.9 × 52 ÷ 3,600 = 58.0 kW
This is the heat removed from the product across the specified temperature change. In a regenerative plate system, much of that heat can be transferred to incoming milk. The final water-cooling section and mechanical refrigeration system are sized using the remaining duty, the actual cooling-water temperatures and any loads operating at the same time.
Example 2: Cooling a 1,000 kg stirred-yogurt batch
Suppose the desired first cooling stage is 43°C to 20°C in 30 minutes. Using a specific heat of 3.9 kJ/(kg·K):
1,000 × 3.9 × (43 − 20) ÷ 1,800 = 49.8 kW
That is the average product-side cooling load during the stated 30-minute stage. The cooling system must also account for tank heat transfer, pumping, available temperature difference and the required cooling curve. For a highly viscous yogurt, the exchanger and pump must be checked together at the cold-end viscosity.
Cooling packaged set yogurt
For a cooling room, the product heat load is only one part of the calculation. The refrigeration design must also include heat through walls and ceilings, warm air entering when doors open, fans, lighting and packaging materials. Cooling time must be checked against the temperature inside the cup, not just the temperature of the cooling-room air.
At NexAgri, we use the intended process schedule to size heat exchangers, chilled-water circulation, pumps, fermentation tanks and refrigeration equipment. This is particularly useful when a plant plans to run fresh milk and yogurt on a shared preparation line or add more batches later.
Why Is a Yogurt Cooling System Taking Longer Than Before?
A longer cooling cycle is easier to diagnose when the product-side and cooling-water-side measurements are recorded together.
| What you observe | What to check first | Typical engineering response |
|---|---|---|
| Cooling-water supply is warmer than the design condition | Chiller capacity, water temperature, condenser performance, seasonal load | Recalculate available refrigeration duty and circulation capacity |
| Product outlet temperature rises while throughput stays the same | Water flow, valve position, exchanger temperature difference | Inspect controls and verify operating flow |
| Pressure drop across the product side is increasing | Fouling, blocked channels, filter condition | Review CIP performance and inspect the exchanger if necessary |
| Flow fluctuates during stirred-yogurt transfer | Viscosity, pump speed, suction pressure and backpressure | Match the pump and cooler to the actual yogurt viscosity |
| Set-yogurt cups cool unevenly | Rack spacing, airflow paths, package size and loading pattern | Adjust air distribution and test cup-center temperatures |
| Cooling performance falls after a product change | New recipe viscosity, flow rate, solids content or temperature requirements | Recheck the duty and hydraulic design for the new product |
Routine CIP helps maintain heat transfer and hygiene. When a cooler gradually loses capacity, compare product and coolant temperatures, operating flow and pressure drop with a clean-equipment baseline. An increase in pressure drop can indicate fouling or restricted passages; unchanged pressure drop with warmer coolant may point instead to the refrigeration circuit.
For heated milk-processing circuits, a reference CIP programme may circulate 0.5–2.0% alkaline cleaner at around 75°C, followed where needed by 0.5–1.5% acid cleaner at around 70°C, with appropriate rinsing between steps. Sanitary pipe circuits commonly use a CIP solution velocity of about 1.5–3.0 m/s to provide cleaning action. These figures refer to cleaning conditions, not to the transfer speed of shear-sensitive yogurt. The final programme is set for the plate/gasket materials, product deposits and chemical supplier's recommendations, then checked for effective cleaning. Oklahoma State University's CIP guidance explains the interaction of time, temperature, chemistry and flow. Thorough rinsing and draining must be completed before returning to production.
We can supply automatic CIP connections, temperature and flow monitoring, control interlocks, replacement or upgraded cooling equipment, installation support and operator training. During commissioning, a product trial can be used to record the actual temperature curve, flow, pressure and system response.
What Information Should You Send Before Ordering a Yogurt Cooling System?
A useful equipment proposal starts with the production schedule, not just the nameplate capacity of a tank. For a new installation or retrofit, share the following details:
- Yogurt type: set, stirred, drinking or a combination, including whether fruit is added.
- Output: liters per hour, batch volume, batches per day and filling-machine speed.
- Temperature programme: heat-treatment temperature/holding time, culture inoculation temperature, fermentation endpoint, first-cooling target and final storage temperature.
- Product characteristics: viscosity range, fat and solids content, and any ingredients that change pressure drop or heat transfer.
- Cooling utilities: available chilled-water supply/return temperatures, existing chiller capacity and the site's summer design conditions.
- Production layout: available floor area, tank positions, pipe lengths, cleaning connections and future expansion plans.
- Acceptance checks: required product outlet temperature, cooling time, flow, pressure drop and data-recording requirements.
NexAgri can combine dairy processing equipment—including fermentation tanks, sanitary pumps and CIP—with the relevant dairy production line systems. We can prepare a proposed process flow, equipment list, layout, utility requirements and control connections, followed by installation guidance, commissioning and staff training.
For a cooling-system review, send us your process temperatures, product type and target capacity. We can identify the appropriate cooling stages and prepare an equipment configuration around your actual production requirements.
Frequently Asked Questions
Does yogurt milk always need to be cooled to 4°C before fermentation?
No. When the heat-treated milk is going directly into fermentation, it is commonly cooled to the culture's inoculation temperature, often around 40–45°C. Some set-yogurt preparation lines use chilled temporary storage before reheating to the required incubation temperature.
Is a plate heat exchanger suitable for cooling finished stirred yogurt?
It can be, provided the plate design, pump and pressure drop suit the yogurt's viscosity and shear sensitivity. Tubular or double-pipe cooling is another option for thicker products. A heat exchanger used for milk before fermentation is not automatically the right choice for finished yogurt.
Can a 1,000 L batch of yogurt be cooled with the tank jacket alone?
That depends on jacket area, cooling-water temperature, agitation and the cooling-time target. A jacket may suit a slower batch schedule, while an external sanitary cooler can shorten the cycle and release the tank sooner for the next batch.
What is the difference between cooling capacity and the chiller's rated capacity?
Cooling capacity at the product side describes how much heat must be removed from milk or yogurt. The external chiller handles the portion assigned to the chilled-water circuit after regeneration and other heat-recovery stages, alongside any simultaneous loads. Its selection also depends on the required water temperature and site conditions.
Conclusion
A yogurt line needs different cooling equipment at different points. Raw milk storage, cooling after heat treatment, stirred-yogurt transfer and set-yogurt cup cooling each place different demands on temperature control and heat transfer.
The most reliable way to plan the system is to map the actual production route, calculate the heat load at each stage, and size the pumps, coolers, tanks and refrigeration equipment to the filling and batch schedule. NexAgri can supply and integrate these components into a small-batch system or a continuous yogurt processing line.
