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In an indirect ultra-high-temperature skim-milk line operating at 137°C for 3–4 s, the fouling layer formed on plate heat exchanger surfaces is not homogeneous. It consists of a porous protein network and a mineral phase that changes from brushite on the hot-water side to hydroxyapatite-like material at the product–metal interface. The calcium phosphate fraction determines the mechanical response of the deposit to alkali. Where the atomic calcium-to-phosphorus ratio rises above 1.3, the basal mineral network is sufficiently continuous to survive caustic swelling of the overlying β-lactoglobulin gel. In such locations, the deposit does not detach by bulk sloughing but dissolves slowly from the outer surface inward. This produces a residual calcium phosphate film that remains visible under scanning electron microscopy as a compact layer of submicron crystals after 30 min of exposure to 1.0% w/w sodium hydroxide at 75°C. The film is not detected by routine ATP bioluminescence if the protein has been hydrolysed, but it provides a nucleation substrate for subsequent fouling and reduces heat-transfer coefficients. The phenomenon is well documented in dairy heat-exchanger cleaning trials, although published numerical removal kinetics vary because deposit composition depends on milk preheating, calcium chloride fortification, carbon dioxide content, and serum phosphate equilibrium. The relevant cleaning prerequisite is expressed in 21 CFR 117.35, which requires that food-contact surfaces be cleaned as frequently as necessary to protect against allergen cross-contact and microbial contamination, and in ISO/TS 22002-1:2009 clause 11.3, which addresses cleaning of equipment and facilities.
Alkaline cleaning of β-lactoglobulin-rich deposits proceeds through four overlapping events: hydration of the peptide matrix; ionization of acidic side chains causing electrostatic repulsion and swelling; nucleophilic attack of hydroxide on peptide bonds; and oxidative cleavage of disulfide bonds when hypochlorite is present. In a protein-only deposit, the initial swelling ratio at pH 12.5 and 80°C can exceed 2.0, producing fissures that allow convective transport of fresh sodium hydroxide to the metal surface. Calcium phosphate alters this sequence by introducing ionic crosslinks between carboxylate groups and divalent calcium, by occupying the void volume with submicron prismatic crystals, and by buffering local pH through phosphate deprotonation. The resulting composite has a lower water uptake and a higher elastic modulus than the protein gel. The deposit cannot disintegrate by shear when the calcium phosphate volume fraction exceeds roughly 0.15–0.20, as inferred from model deposit studies using heat-denatured whey protein isolate and colloidal calcium phosphate. In such deposits, hydroxide uptake is diffusion-limited and the apparent cleaning front velocity falls from protein-dissolution control to mineral-reaction control. The required caustic contact time therefore scales with the square of the deposit thickness. Industrial validation of this effect is typically performed by measuring the decrease in electrical conductivity during cleaning and by comparing the ratio of dissolved protein to dissolved calcium in spent clean-in-place solutions. A decrease in the protein-to-calcium ratio over successive wash cycles indicates accumulation of mineral-dominated residue. Operators should not interpret a low turbidity rinse as complete cleaning because calcium phosphate films may be visually transparent and only 5–10 µm thick.
Within a falling-film evaporator concentrating skim milk to 35–45% total solids, the deposit on calandria tubes is subjected to thermal gradients that promote type A fouling in the first effect and type B fouling in the later effects. The transition from protein-dominated to mineral-dominated fouling is not abrupt; it follows the increase in soluble calcium and phosphate as water is removed and as pH decreases due to carbon dioxide evaporation. A deposit formed at 68°C may contain only 8–12% ash, while a deposit formed at 95°C in the same evaporator may contain 35–50% ash on a dry-weight basis. The calcium phosphate content influences the choice between a single-stage caustic wash and a two-stage caustic–acid sequence. When the ash content exceeds 25–30%, caustic alone cannot reliably remove the deposit to the bare metal surface within the maximum permissible clean-in-place time of 45–60 min on a production line. The acid step typically uses 0.5–1.5% w/w nitric acid at 60–70°C or 0.5–1.0% w/w phosphoric acid at 60–70°C, with nitrate or phosphate sequestering action. Published data for this specific configuration is limited because the deposit composition varies with milk protein genetic variant, seasonal serum phosphate, and calcium chloride added in cheese-milk standardisation. Nevertheless, the general result is that calcium phosphate acts as an alkali-insoluble cement, and no caustic soak time alone can compensate for its continuous phase.
The standard alkaline cleaning formula for dairy plate heat exchangers is 0.5–2.0% w/w sodium hydroxide with 50–200 mg/kg available chlorine from sodium hypochlorite, heated to 70–85°C and circulated for 20–45 min at a flow rate sufficient to produce a wall shear stress of 2–10 Pa. Sodium hydroxide hydrolyses peptide bonds and saponifies triglycerides, while hypochlorite oxidises residual protein and improves wetting. However, neither reagent dissolves the hydroxyapatite basal layer. To remove calcium phosphate in the same caustic cycle, the detergent must contain a chelating agent. Ethylenediaminetetraacetic acid tetrasodium salt is effective because the calcium-EDTA complex has a conditional stability constant log K′CaEDTA of approximately 10.7 at pH 10 and 25°C. Nitrilotriacetic acid has a lower log K′ of approximately 6.4 for calcium under similar conditions but is used less frequently due to environmental and occupational restrictions. Gluconates and glucoheptonates have lower calcium stability constants but provide additional iron and copper control in hard water. The chelator concentration must exceed the stoichiometric equivalent of calcium in the foulant plus the calcium in the water make-up and the calcium dissolved from the outer deposit layers. For a deposit of 1 kg dry mass containing 30% calcium phosphate, the calcium content is approximately 0.2 mol, requiring 0.2 mol of tetrasodium EDTA per cycle if the entire layer is to be dissolved. In practice, the chelating agent is dosed at 0.2–0.5% w/w in the caustic stage, but this addition may be insufficient because part of the chelator is consumed by magnesium, iron, and copper from the deposit and the water supply. The pH of the caustic-EDTA blend is maintained above 11.5 to keep the EDTA in the fully deprotonated form, but the formation of calcium-EDTA reduces free hydroxide. The spent solution’s calcium concentration is monitored by complexometric titration or inductively coupled plasma optical emission spectrometry according to ISO 11885:2007. If the free EDTA is exhausted before the calcium phosphate dissolves, the remaining mineral phase recrystallises to a lower-surface-area hydroxyapatite that is more difficult to remove. Therefore sodium hydroxide concentration is not the sole cleaning variable; free chelator capacity determines the endpoint for calcium phosphate dissolution.
| Deposit class | Protein content (% w/w dry) | Ash content (% w/w dry) | Ca/P atomic ratio | Response to 1.0% w/w NaOH at 80°C |
|---|---|---|---|---|
| Type A proteinaceous | 70–80 | 10–20 | 0.8–1.2 | Removed within 20–30 min |
| Mixed type A/B | 50–70 | 20–35 | 1.2–1.5 | Partial removal; mineral residual remains |
| Type B mineral | 30–50 | 35–55 | 1.5–2.0 | Not removed; separate acid stage required |
| Dense UHT mineral | 15–30 | 50–70 | 1.5–2.0 | Not removed; acid plus chelator necessary |
At pH values above 10.5, the surface charge of precipitated hydroxyapatite becomes increasingly negative, which reduces the adsorption of anionic surfactants and may limit the wetting of the mineral surface. This is one of the reasons that high-caustic cleaning without chelating agents leaves the metal interface enriched in calcium phosphate. The negative surface potential at high pH favours the adsorption of cationic surfactants, but these are not commonly used in rotary spray cleaning of milk tanks due to foam generation and incompatibility with anionic soil emulsifiers. Nonionic ethoxylated alcohols with cloud points above 80°C are selected for high-temperature caustic cleaning, but they do not bind calcium. In waters with magnesium hardness greater than 50 mg/L as CaCO₃, the high pH also causes precipitation of magnesium hydroxide, which can deposit on top of the calcium phosphate film and mask it from acid detection. The resulting mixed magnesium phosphate–calcium phosphate scale is harder to remove than either phase alone because it combines the acid resistance of magnesium hydroxide with the acid solubility of calcium phosphate. This interaction has been observed in reconstituted whey processing lines where the wash water hardness is uncontrolled. For this reason, clean-in-place make-up water for dairy lines is softened to less than 10 mg/L as CaCO₃ or treated by reverse osmosis before caustic detergent preparation.
When the calcium-to-phosphorus atomic ratio of the basal layer rises above 1.5, the deposit acquires the chemical signature of calcium-deficient hydroxyapatite rather than brushite. The transition is detected by X-ray diffraction broadening of the 002 and 211 reflections and by Fourier-transform infrared spectra showing a shift in the phosphate ν₃ antisymmetric stretching band from 1035 cm⁻¹ toward 1020 cm⁻¹. This phase is less soluble in citric acid than brushite, and it is resistant to caustic even in the presence of EDTA at concentrations below 0.3% w/w. The effect is most severe in indirect ultra-high-temperature plants where the product-side wall temperature exceeds 105°C for less than 60 s; there the fouling layer is often only 20–40 µm thick but has a calcium-to-phosphorus ratio above 1.6. Because the deposit is thin, it does not alter pressure drop enough to trigger a clean-in-place alarm, but it reduces the overall heat-transfer coefficient by 25–40%. After a caustic cycle of 30 min at 1.5% w/w sodium hydroxide and 80°C, the residual film remains at the metal surface. The only reliable removal method is a subsequent acid cycle using 0.5–1.0% w/w nitric acid at 65°C for 15–20 min, sometimes followed by an alkaline rinse to recondition the stainless steel. This caustic–acid–caustic sequence is effective because the acid dissolves calcium phosphate while the final caustic removes acidic residues and restores passivation.
Hypochlorite is added to caustic detergents for dairy protein removal because it oxidizes β-lactoglobulin disulfide bonds and generates N-chloro derivatives that are more water soluble. In the presence of calcium phosphate, hypochlorite consumption increases disproportionately because the mineral surface catalyses oxygen evolution and the entrapped protein requires longer oxidation. The measured available chlorine can drop from 150 mg/L to less than 20 mg/L within 10 min at 75°C when a calcium phosphate-rich deposit is present. This leaves the outer protein layer partially hydrolysed but the basal mineral layer untouched. In addition, chlorination of organic matter produces trichloromethane and other disinfection by-products; for this reason, many European dairy operations limit hypochlorite in caustic clean-in-place stages and use enzymatic pre-treatment or acid cleaning. The replacement of hypochlorite by hydrogen peroxide in alkaline media is not straightforward because hydrogen peroxide decomposes rapidly at pH 11 and at 75°C, particularly in contact with manganese and iron oxides in the water or the stainless steel. Therefore, caustic cleaning without chlorine may require longer contact times or a two-stage caustic/acid approach when calcium phosphate is the limiting fouling phase.
Electropolished stainless steel surfaces with surface roughness Ra below 0.4 µm retain less calcium phosphate after caustic cleaning than mechanically polished surfaces with Ra 0.8–1.5 µm. The reduction is attributed to lower surface density of crevices and to a more homogeneous passive layer. But surface finish alone cannot eliminate calcium phosphate residues when the deposit contains hydroxyapatite. In electrochemical terms, the passive chromium oxide film on 316L stainless steel has a slightly negative surface charge at pH 11, which repels anionic phosphate; however, calcium ions can bridge phosphate to anionic sites on the metal oxide, forming a tenacious calcium phosphate monolayer. The monolayer does not reduce the corrosion resistance of 316L in hot caustic solutions, but it provides sites for further mineral growth. 3-A Sanitary Standards 603-05 for plate heat exchangers specify crevice-free design and surface finish criteria, yet cleaning efficiency on production surfaces remains a function of deposit age, thermal history, and water hardness. Equipment with dead legs, gaskets, and probe ports may retain a calcium phosphate film even after validated clean-in-place cycles. Therefore swab sampling for calcium or phosphorus is recommended before the surface is returned to production.
In plate heat exchangers with nitrile gaskets, prolonged exposure to 1.5% w/w sodium hydroxide at 80°C can cause gasket hardening and loss of compression. EPDM gaskets have better alkaline resistance but can adsorb calcium phosphate particles, which remain in the gasket crevice and re-contaminate the cleaned surface. Spray-ball systems in milk silos have comparatively low wall shear stress below 1 Pa, so calcium phosphate residues in the shadow zone behind the spray ball are not removed without acid descaling. The calandria tubes of falling-film evaporators are particularly prone because the product film is distributed unevenly and the deposit alternates between protein-rich and mineral-rich bands. In these geometries, caustic cleaning in turbulent flow may erode the protein-rich bands but leave behind hard mineral bands, causing a corrugated fouling profile that reduces the heat-transfer coefficient more than a uniform layer of equivalent mass. The phenomenon is observed during inspection of evaporator tube bundles after service. Published data for this specific configuration is limited, but the observed behaviour is consistent with the mineral-bridging model described earlier.
Validation of a caustic cleaning cycle on a dairy protein fouling layer containing calcium phosphate requires direct measurement of residual calcium and phosphate on the surface, not only visual inspection. The typical analytical sequence includes a pre-rinse to collect suspended solids, a caustic wash sample at 5, 15, and 30 min for protein concentration by ultraviolet absorbance at 280 nm, and a final acid rinse sample for calcium and phosphorus by inductively coupled plasma optical emission spectrometry according to ISO 11885:2007. The total residual mineral load on the surface is then compared with the amount removed during the acid stage. If the ratio of calcium removed in the acid stage to calcium removed in the caustic stage exceeds 3:1, the caustic stage is not dissolving the mineral and the cleaning programme requires a chelating agent or separate acid step. X-ray diffraction of residual deposits on test coupons placed in bypass lines is used to identify brushite or hydroxyapatite and to monitor phase transitions after repeated cleaning cycles. Scanning electron microscopy with energy-dispersive X-ray spectroscopy on the coupons provides visual confirmation of mineral continuity at the metal interface. Swab tests for calcium based on arsenazo III or ion-selective electrode are used for field verification with a detection threshold of approximately 5 µg/cm² for calcium. When residual calcium exceeds 10 µg/cm² on a cleaned surface, a subsequent fouling cycle is more rapid because the calcium phosphate nuclei remain. The validation report should include the exact sodium hydroxide concentration, temperature, circulating time, flow velocity, system volume, and water hardness. Without these parameters, the cleaning result is not reproducible.
In operations where the water hardness is above 15°dH and the milk is fortified with calcium chloride, the calcium phosphate content of the deposit increases rapidly. The incompatibility of caustic-only cleaning with such deposits is compounded by the use of anionic surfactants that precipitate with calcium and leave soap scum. Nonionic surfactants do not precipitate but may be oxidised by hypochlorite. The combination of sodium hydroxide with ammonium bifluoride in acidic cleaners is prohibited on stainless steel because fluoride attacks the passive layer. In a combined caustic–acid clean-in-place cycle, the acid stage should not use hydrochloric acid on 316L stainless steel because chloride pitting can occur above 50°C at pH below 2. Nitric acid is preferred at 0.5–1.0% w/w because it dissolves calcium phosphate and passivates the surface. The acid stage should be followed by a potable water rinse until the rinse-water pH is between 6.5 and 7.5. If the caustic stage contains tetrasodium EDTA, the spent solution should be pH-adjusted and treated for heavy metals before discharge according to local wastewater permits. The process boundary for alkaline cleaning of dairy protein fouling is therefore defined by the deposit’s calcium phosphate volume fraction, the available chelator capacity, the temperature window of 70–85°C, and the material compatibility of elastomers and stainless steel.
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