A dairy in India adds a 100 mm transfer line between its pasteuriser and a new yoghurt tank and connects it to the existing clean in place (CIP) circuit. Every night the CIP screen shows green ticks: correct temperature, correct caustic strength, correct step times. Six weeks later, routine tests start finding coliforms in yoghurt, and ATP swabs on a valve cluster fail. Nothing is wrong with the chemistry. The CIP supply pump, sized for 50 mm pipes, pushes solution through the new line at a quarter of the velocity it needs, and the pipe walls are barely scrubbed.
In short
- Clean in place (CIP) cleans the inside of pipes, tanks, heat exchangers and fillers by circulating rinse water and cleaning solutions through them, without dismantling.
- Cleaning depends on four linked factors, known as Sinner’s circle: time, temperature, chemical action and mechanical action (flow). If one falls short, another must rise, within limits.
- A typical dairy sequence is pre-rinse, caustic wash (about 0.5 to 2 % sodium hydroxide at about 70 to 85 °C), intermediate rinse, acid wash, final rinse and sanitising.
- Pipes need turbulent flow with enough wall shear; a mean velocity of about 1.5 m/s or more is the usual design target.
- Prove each clean with conductivity, return temperature, flow and time records, and verify the programme with inspection, ATP swabs and microbiological swabs.
What is clean in place CIP?
Clean in place, usually shortened to CIP, is the cleaning of internal product-contact surfaces of closed equipment by circulating water and cleaning solutions through it under automatic control, without taking it apart. It is standard wherever pumpable foods are processed.
Cleaning and disinfection are separate jobs. Cleaning removes soil, meaning fat, protein, sugar, starch and minerals, and the microbes in it. Disinfection, often called sanitising, then reduces the remaining microorganisms to a safe level. A sanitiser applied to a dirty surface performs poorly, because soil consumes the chemical and shields cells. The soil decides the chemistry:
| Soil | Typical source | Removed mainly by |
|---|---|---|
| Fat | Milk, cream, oils | Hot alkali (saponification) and surfactants, above the fat’s melting point |
| Protein | Milk, egg, meat juices; baked on in heaters | Alkali, sometimes with chlorine or enzymes |
| Sugars | Juices, syrups | Warm water; alkali if caramelised |
| Starch | Sauces, desserts | Alkali or enzymes |
| Minerals (milkstone, beerstone, scale) | Heated milk, beer, hard water | Acid, such as nitric or phosphoric |
Saponification is the reaction of alkali with fats that turns them into water-soluble soaps. Plants run two kinds of system. Single-use CIP makes fresh solution for each clean and suits heavily soiled circuits such as UHT plants. Recovery CIP returns caustic and acid to storage tanks and tops up their strength, saving chemicals, water and energy, but the solutions gradually accumulate soil.
What is Sinner’s circle?
Sinner’s circle is the principle, described by the chemist Herbert Sinner in the 1950s, that cleaning results from four interacting factors: time, temperature, chemical action and mechanical action. If one factor is reduced, one or more of the others must increase to achieve the same result.
- Time: contact time of the solution with the soil, counted once the circuit has reached temperature and strength.
- Temperature: heat speeds chemical reactions, melts fat and lowers viscosity.
- Chemical action: the type and concentration of detergent, chosen for the soil.
- Mechanical action: in CIP, flow velocity in pipes and spray impact in tanks, because nobody scrubs.
Manual cleaning leans on brushing at moderate temperatures and concentrations, for operator safety. CIP leans on heat, stronger chemicals and turbulence. The trade-offs have limits: too hot a pre-rinse bakes raw protein onto steel; no caustic, however strong, cleans a dead leg, a pipe branch that sees no flow; and no amount of alkali dissolves milkstone.
What is a typical CIP sequence?
A typical CIP cycle for a dairy or similar liquid food circuit runs pre-rinse, caustic wash, intermediate rinse, acid wash, final rinse and sanitise, with each step’s time, temperature, concentration and flow validated for that circuit.
- Pre-rinse with water, often recovered final-rinse water, to remove loose soil. Use cool or warm water on raw protein soils.
- Caustic wash with sodium hydroxide (NaOH) at about 0.5 to 2 % and about 70 to 85 °C to remove fat and protein. Heavily fouled heat exchangers need the upper end of these ranges or more.
- Intermediate rinse to flush out caustic and loosened soil.
- Acid wash, commonly nitric or phosphoric acid at about 0.5 to 1.5 % and about 50 to 70 °C, to dissolve mineral deposits. Many plants run acid daily on heated surfaces and less often on cold ones.
- Final rinse with potable water until conductivity or pH shows the chemicals have gone.
- Sanitise with hot water (often 85 to 95 °C) or a chemical sanitiser such as peracetic acid, at the end of cleaning or just before production.
Conductivity is a measure of how well a solution carries electric current. It rises with caustic or acid concentration, so the CIP controller uses it to dose chemical and to detect where water ends and detergent begins. The relationship shifts with temperature and soil load, so check sensors by titration. Make-up is simple arithmetic: 2000 L of 1.5 % w/v caustic needs 30 kg of NaOH, which is 60 kg, or about 39.5 L, of 50 % w/w stock with a density of 1.52 kg/L.
CIP chemicals are hazardous. Concentrated caustic and nitric acid cause severe burns, and chlorinated products mixed with acid release chlorine gas. Fail-safe separation, such as double-seat valves with leakage detection or a physical break, must keep cleaning solutions out of product lines.
What flow velocity does CIP need in pipes?
Most CIP designs target a mean velocity of at least about 1.5 m/s in every pipe of the circuit, checked in the largest-diameter pipe, because that gives fully turbulent flow with enough wall shear stress to remove soil.
Turbulent flow is chaotic flow with eddies that reach the pipe wall, unlike laminar flow in smooth layers. It is predicted by the Reynolds number, Re = ρvD/μ, where ρ is density, v mean velocity, D internal diameter and μ viscosity; pipe flow is turbulent above roughly 4000. Turbulence alone is not the target, because hot solutions are turbulent even at low velocity. What removes soil is wall shear stress, the drag of the moving liquid on the wall, which rises roughly with the square of velocity.
Q = v × π × D² / 4
Worked example
What flow must a CIP supply pump deliver for 1.5 m/s? For 50 mm internal diameter, Q = 1.5 × π × 0.050² / 4 = 0.00295 m³/s, or 10.6 m³/h. For 100 mm, Q = 1.5 × π × 0.100² / 4 = 0.0118 m³/s, or 42.4 m³/h. Doubling the diameter quadruples the flow needed.
Now run the 10.6 m³/h pump through the Indian dairy’s new 100 mm line. Velocity = 0.00295 / (π × 0.100² / 4) = 0.375 m/s. Taking water properties at 75 °C (density about 975 kg/m³, viscosity about 0.38 mPa·s), Re = 975 × 0.375 × 0.100 / 0.00038 ≈ 96,000, so the flow is still turbulent. Yet wall shear stress is less than a tenth of its value at 1.5 m/s.
The fix is a larger pump or a separate circuit for the 100 mm line, then verification swabbing.
Pump sizing for CIP means matching the pump curve to the flow and pressure drop of the most demanding circuit, the kind of calculation taught in Intermediate Food Process Engineering. Tanks are cleaned by spray devices: static spray balls rely on a falling film, rotating jet heads add impact. Every internal surface must be wetted, including under the manway and around fittings, and the return pump must keep up so solution does not pool. Riboflavin tests, in which a fluorescent riboflavin solution is applied to inner surfaces and checked under UV light after rinsing, are a common way to prove spray coverage.
How do you verify that a CIP clean worked?
Verify CIP at two levels: check every cycle’s recorded parameters against their limits, and periodically test the cleaned surfaces with inspection, ATP swabs and microbiological swabs. Measure at the return line, which proves the whole circuit reached its targets.
| Parameter | How it is measured | A failure often means |
|---|---|---|
| Step time | Controller log, timed from return conditions | Route short-circuit, manual override |
| Return temperature | Calibrated sensor in the return line | Heater or steam fault, heat loss |
| Concentration | Conductivity, checked by titration | Dosing fault or dilution |
| Flow | Flow meter on the supply | Worn pump, blocked spray device, partly closed valve |
| Route | Valve position feedback | Wrong circuit or swing bend position |
| Final rinse | Conductivity or pH back to water baseline | Chemical residue risk in product |
ATP (adenosine triphosphate) is a molecule found in all living cells and in most food residues. ATP swabs use a firefly enzyme reaction to produce light in proportion to the ATP on a surface, giving a result in relative light units within minutes. A high reading shows organic residue, not specifically microbes, and pass limits depend on the instrument, so set them from the site’s own baseline. Microbiological swabs and rinse samples, such as total viable count or Enterobacteriaceae, show whether microbes survived. Periodic strip-downs of valves, plate packs, pump casings and gaskets find what sensors and swabs miss.
Trend the data. Return temperatures creeping down or step times lengthening often warn of a failing heat exchanger, blocked spray device or worn pump weeks before a hygiene failure. The Food Technology for Industry Professionals course builds this kind of CIP review into its hygienic design and cleaning module.
Why does CIP fail, and how can you save water and energy safely?
Most CIP failures come from design or circuit changes rather than chemistry. The usual causes are:
- low velocity after a line is added, a pipe is enlarged or a pump wears;
- dead legs: hygienic design guidance, such as that from EHEDG, keeps any unavoidable branch very short, about one pipe diameter deep or less;
- air trapped at high points, and poor drainage in dead-level pipes;
- blocked or damaged spray devices, and return pumps that cannot keep up;
- valve seats, gaskets and seal cavities not reached by the main flow;
- recovered detergent overloaded with soil.
Savings are legitimate goals. Reusing the final rinse as the next pre-rinse, conductivity-based phase separation and optimised step times can cut water and chemical use. Each change alters a hygiene control, though. Apply Sinner’s circle: if caustic temperature drops from 80 °C to 65 °C, something must compensate, such as longer time, higher concentration or a better detergent, and the new programme must be verified with inspection, ATP and microbiological results over several cycles before it becomes standard.
Frequently asked questions
How long should a CIP caustic wash last?
There is no universal time. The caustic step lasts as long as validation shows is needed for that circuit and soil, counted from when the return line reaches target temperature and concentration. Lightly soiled cold pipework needs far less than a heavily fouled UHT heater. Confirm the time with inspection, ATP and microbiological swabs, and revalidate after any change.
What concentration of caustic is used in CIP?
Sodium hydroxide at about 0.5 to 2 % is typical for dairy and beverage CIP, run at about 70 to 85 °C. Lightly soiled cold surfaces sit at the lower end, and heat exchangers with baked-on protein at the higher end, sometimes with additives such as surfactants or enzymes. The right value is the one your validation data support, held by conductivity control and checked by titration.
Why is 1.5 m/s used as the CIP velocity?
At about 1.5 m/s, flow in typical food pipework is fully turbulent and produces enough wall shear stress to remove soil, so it is widely used as a design target. It is a rule of thumb, not a law. What matters is that every section of the circuit, especially the largest pipe, reaches adequate velocity and that cleaning results confirm it.
What does an ATP swab tell you after CIP?
An ATP swab measures adenosine triphosphate on a surface, a marker of organic residue from food and microorganisms, as light output in relative light units. A low result, available within minutes, shows little organic residue remains. It does not identify or count microbes, so pair it with periodic microbiological swabs, and set pass limits from your own baseline because readings vary between instrument brands.
Can conductivity records alone prove that CIP worked?
No. Conductivity records show the solution was at the intended strength, and with temperature, flow and time records they show the cycle ran as designed. They cannot show that the design cleans every surface. That evidence comes from validation and periodic verification: inspection of opened equipment, ATP and microbiological swabs, and product results. Records prove delivery; verification proves effectiveness.
Next step. The Food Technology for Industry Professionals course covers soil chemistry, Sinner’s circle, CIP sequence design, pipe velocity and detergent make-up calculations, and how to verify and trend CIP data, alongside hygienic design and environmental monitoring. It finishes with a proctored final assessment and an ASC certificate, and you can see all eleven food science and technology courses.
Sources. Codex Alimentarius Commission, General Principles of Food Hygiene (CXC 1-1969, revised 2020), available from the Codex Alimentarius website. ISO 22000:2018, Food safety management systems. A. Y. Tamime (ed.), Cleaning-in-Place: Dairy, Food and Beverage Operations, 3rd edn (Blackwell Publishing, 2008). P. J. Fellows, Food Processing Technology: Principles and Practice, 5th edn (Woodhead Publishing, 2022). European Hygienic Engineering and Design Group (EHEDG) guidelines on hygienic equipment design.
This article is general guidance and is not a substitute for the applicable standard, legislation, your chemical supplier’s safety data or the advice of a qualified hygiene professional.