Fundamentals of hygienic design in a dairy plant
Hygiene is a design decision, not a cleaning programme. Which detergent you use on a line that cannot be washed makes no difference. Where soil adheres, the four measurable CIP variables, and material selection.
Cleaning discussions in dairy plants usually revolve around detergent brand and dosage. Yet three things determine the outcome, and two of them are decided at the purchasing stage: how the surface gets soiled, whether the equipment was designed to be washed, and only then the parameters of the cleaning programme.
Why soil forms: two surfaces, two problems
When milk is heated above 60 °C, milk stone begins to form — a sticky layer of coagulated protein, calcium phosphate and fat. After production runs longer than eight hours this deposit adheres tightly and cannot be removed with alkali alone; an acid step becomes mandatory to dissolve the calcium salts.
Cold surfaces (tanks, transfer lines, pre-filling pipework) behave differently. Here the deposit is an undried milk film. Cleaning must start as soon as the system is emptied; a dried film turns into a layer that is far harder to remove.
Practical consequence
Heated-surface circuits and cold circuits cannot share one CIP programme. The acid step is mandatory on heated circuits and usually unnecessary on cold ones. A plant running a single programme either under-cleans the heat exchanger or wastes chemicals on tanks.
The four variables of CIP
The result of mechanised cleaning is the resultant of four variables. When one is reduced, another can compensate — which is also the key to cost optimisation.
| Variable | Range | Note |
|---|---|---|
| Detergent concentration | Alkali 0.5–1.5% · Acid 0.5–1.0% | Diluted during cleaning; must be checked in-circuit |
| Temperature | Alkali ≥70 °C · Acid 68–70 °C | Alkali should match the temperature the product saw |
| Flow velocity (mechanical effect) | 1.5–3.0 m/s | Turbulent in this range; CIP pumps sized above product pumps |
| Duration | 10–30 min | Tank wall 10 min alkali; plate heat exchanger 20 min nitric acid |
Raising the concentration seems intuitive but often backfires: foaming increases, rinsing takes longer and chemical cost rises. The mechanical effect is the mechanised equivalent of the scrubbing brush — the goal is not to move product but to create turbulent flow.
CIP programme for a heated circuit
| # | Step | Duration | Temperature / Concentration |
|---|---|---|---|
| 1 | Pre-rinse (warm water) | ~10 min | max. 55 °C |
| 2 | Alkaline detergent circulation | ~30 min | 75 °C · NaOH 0.5–1.5% |
| 3 | Intermediate rinse (warm water) | ~5 min | — |
| 4 | Nitric acid circulation | ~20 min | 70 °C · 0.5–1.0% |
| 5 | Post-rinse (cold water) | — | — |
| 6 | Gradual cooling | ~8 min | cold water |
The pasteuriser is usually disinfected in the morning before production: hot water at 90–95 °C for 10–15 minutes. The clock starts once the return temperature is at least 85 °C — not the temperature at the heater outlet.
On cold circuits the acid step is normally omitted: water rinse → alkali → rinse.
Rinse water: two details that get missed
The final rinse should use softened water. Hard water leaves lime scale on the cleaned surface and seeds the next run's soil layer. Target hardness is 2–4 °dH.
The second is overnight bacterial growth. After strong alkali and acid the system is practically sterile, but residual rinse water supports growth until morning. Acidifying the final rinse to below pH 5 with phosphoric or citric acid largely prevents this.
Material choice decides hygiene
| Material | Risk | Consequence |
|---|---|---|
| Copper, brass, tin | Attacked by strong acids and alkalis | Even traces of copper cause an oxidised flavour |
| Stainless steel | Chlorine solutions | Corrosion risk |
| Mixed steel grades | Cleaning with cation-active agents | Electrolytic corrosion |
| Elastomers (gaskets) | Chlorine and oxidising agents | Blackening, cracking, particles into the milk |
| Plastics | Milk fat and detergents dissolve some components | Migration; food-contact certification required |
Sequence error: the most common cause of corrosion
If the programme starts with alkali and ends with acid and a chlorinated disinfectant follows, acid residues cause rapid corrosion. Neutralise with a weak alkaline flush before moving to a chlorinated chemical.
Positions that cannot be cleaned
CIP flow cleans only the surface it reaches. A line is only as cleanable as its worst point.
- Dead legs — blind branches, unused connections, capped old lines
- Weld seams — a rough internal weld creates a shelter no mechanical effect reaches
- Gasket seats — a wrongly sized gasket protrudes into the bore and forms a step
- Sampling cocks and gauge connections — classic dead legs
- Drainage slope — any section that does not drain is a growth site
On lines where product mixing is a risk, mix-proof (double-seat) valves are used: when the lines are isolated, any leakage goes to drain rather than into the product.
Summary
- Write separate programmes for heated and cold circuits.
- Check flow velocity and time before raising concentration.
- Use softened water (2–4 °dH) and acidify the final rinse to pH < 5.
- Never let acid residues meet a chlorinated disinfectant.
- Write dead legs, weld quality and drainage slope into the equipment specification.
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.