Phosphatase Test in Milk: Principles, Procedure, and Applications
By Dr. Zubair Khalid, DVM, MS, PhD ·
Introduction to the Phosphatase Test in Milk
The phosphatase test in milk is a biochemical assay used to verify the adequacy of pasteurization by detecting residual activity of the enzyme alkaline phosphatase (ALP). Pasteurization, the process of heating milk to destroy pathogenic microorganisms, also inactivates native milk enzymes. Among these, alkaline phosphatase is the most heat-sensitive enzyme that is naturally present in raw milk at measurable concentrations. Its thermal inactivation profile closely mirrors the destruction of significant milk-borne pathogens, including Mycobacterium tuberculosis and Coxiella burnetii, making it an ideal biological marker for process verification.
The test was developed in the 1930s and rapidly became a regulatory cornerstone in dairy science. The fundamental logic is straightforward: if alkaline phosphatase activity is absent, the milk has been heated sufficiently to kill pathogens; if activity remains, the milk is either raw, inadequately pasteurized, or contaminated with raw milk after pasteurization. This single assay thus serves as a sentinel for both food safety and quality assurance. Over decades, the test has evolved from simple colorimetric procedures to highly sensitive fluorometric methods, yet the underlying principle remains unchanged: detect the presence of an enzyme that should not survive proper heat treatment.
For the undergraduate studying molecular biology or biotechnology, the phosphatase test offers a compelling case study in applied enzymology—demonstrating how enzyme kinetics, protein denaturation, and analytical chemistry converge in a routine industrial assay.
Biological Basis: Alkaline Phosphatase in Milk
Enzyme Structure and Function
Alkaline phosphatase (EC 3.1.3.1) is a ubiquitous enzyme found across taxa, from bacteria to mammals. In bovine milk, the enzyme is a membrane-bound glycoprotein associated with the milk fat globule membrane. It is a dimeric metalloenzyme, with each subunit containing two zinc ions (Zn²⁺) and one magnesium ion (Mg²⁺) at the active site. These metal ions are essential for catalytic activity: the zinc ions participate in substrate binding and catalysis, while the magnesium ion stabilizes the enzyme's active conformation.
The enzyme catalyzes the hydrolysis of phosphomonoesters, removing a phosphate group from a variety of substrates:
R-O-PO₃²⁻ + H₂O → R-OH + HPO₄²⁻
In milk, the natural substrates include phosphoproteins, nucleotides, and sugar phosphates. The enzyme exhibits maximal activity at alkaline pH (8.0–10.0), which is why it is termed "alkaline" phosphatase. This pH optimum is exploited in the assay design, where buffers maintain the reaction environment at the enzyme's preferred pH.
The gene encoding bovine alkaline phosphatase is the ALPL gene, which produces the tissue-nonspecific isozyme (TNAP). In milk, the enzyme is identical to the bone/liver/kidney isoform found in other bovine tissues. Its presence in milk is not a deliberate secretion but rather a consequence of mammary gland cell turnover and the shedding of milk fat globule membranes during milk synthesis.
Thermal Inactivation Kinetics
The utility of alkaline phosphatase as a pasteurization marker derives from its thermal inactivation kinetics. The enzyme is inactivated by heat through protein denaturation—unfolding of the tertiary structure leads to loss of catalytic activity. The kinetics of this inactivation follow first-order behavior:
d[ALP]/dt = -k[ALP]
where k is the rate constant that increases exponentially with temperature according to the Arrhenius equation. For bovine milk alkaline phosphatase, the D-value (time required for a 90% reduction in activity at a given temperature) is approximately 2.5 minutes at 62.8°C (the low-temperature long-time pasteurization temperature). At 71.7°C (high-temperature short-time pasteurization), the D-value drops to approximately 0.25 seconds.
Critically, the thermal resistance of alkaline phosphatase is slightly greater than that of the most heat-resistant non-spore-forming pathogen of concern in milk, Coxiella burnetii. This means that when alkaline phosphatase is fully inactivated, the milk has received a heat treatment sufficient to destroy these pathogens with a margin of safety. Conversely, the enzyme is more heat-sensitive than some bacterial spores, so the test cannot verify sterility—only the adequacy of pasteurization against vegetative pathogens.
The inactivation is not always irreversible. Under certain conditions, particularly with ultra-high temperature (UHT) processing followed by prolonged storage, some reactivation of alkaline phosphatase can occur. This phenomenon, discussed in detail in a later section, complicates interpretation and requires careful consideration in quality control.
Principle of the Phosphatase Test
Substrate Selection
The phosphatase test relies on the enzymatic hydrolysis of a synthetic phosphomonoester substrate that, upon dephosphorylation, yields a detectable product. The ideal substrate must be:
- Highly specific for alkaline phosphatase
- Soluble in aqueous buffer systems
- Capable of producing a measurable signal (color or fluorescence)
- Stable under assay conditions
The most common substrates are:
Phenyl phosphate: Hydrolyzed to phenol and inorganic phosphate. Phenol can be measured colorimetrically after reaction with a chromogenic reagent.
p-Nitrophenyl phosphate (pNPP): Hydrolyzed to p-nitrophenol, which is yellow in alkaline solution (λmax = 405 nm). This is the substrate of choice for quantitative spectrophotometric assays.
Fluorophos® (4-methylumbelliferyl phosphate): Hydrolyzed to 4-methylumbelliferone, which fluoresces at 450 nm when excited at 360 nm. This substrate enables highly sensitive fluorometric detection.
The choice of substrate determines the sensitivity, speed, and complexity of the assay. For regulatory purposes, the fluorometric method using Fluorophos® has become the international reference method due to its superior sensitivity and speed.
Colorimetric Detection
In colorimetric methods, the product of the enzymatic reaction is detected by its inherent color or after a secondary chemical reaction. For example:
- Phenyl phosphate method: The phenol released by enzymatic action reacts with 2,6-dibromoquinone-4-chloroimide (CQC) in alkaline conditions to produce a blue indophenol dye. The intensity of the blue color is proportional to the amount of phenol released, which in turn reflects the enzyme activity.
- pNPP method: The p-nitrophenol product is directly yellow at alkaline pH. The absorbance at 405 nm is measured spectrophotometrically. This method is simpler because no secondary reaction is needed.
The color intensity is compared either visually against standards or measured precisely with a spectrophotometer. In quantitative formats, the enzyme activity is expressed in international units (U/L) or in micrograms of phenol released per milliliter of milk.
Standard Methods for Phosphatase Testing
Scharer Rapid Test
The Scharer rapid test is a classical colorimetric method that uses phenyl phosphate as the substrate. It is designed for speed and simplicity, making it suitable for field testing and small-scale operations.
Principle: Milk samples are buffered to pH 10 and incubated with phenyl phosphate. If alkaline phosphatase is present, phenol is released. After incubation, a color-developing reagent (2,6-dibromoquinone-4-chloroimide) is added. The phenol reacts with this reagent to form a blue indophenol dye.
Procedure outline:
- Add 1 mL of milk to 10 mL of buffered substrate solution
- Incubate at 37°C for 10 minutes
- Add 0.1 mL of CQC reagent
- Observe color development
Interpretation: A blue color indicates phenol production and thus the presence of active alkaline phosphatase. The intensity of blue correlates with enzyme activity. The test is qualitative or semi-quantitative, with results reported as negative (no blue), trace, or positive.
The Scharer test has a detection limit of approximately 1 μg of phenol per mL of milk, corresponding to roughly 0.1% raw milk contamination in pasteurized milk. This sensitivity is adequate for detecting gross failures in pasteurization but may miss low-level contamination.
Fluorometric Method
The fluorometric method, standardized as the ISO 11816 and AOAC 991.24 methods, uses Fluorophos® as the substrate. This is the current gold standard for regulatory testing.
Principle: Fluorophos® (4-methylumbelliferyl phosphate) is hydrolyzed by alkaline phosphatase to yield 4-methylumbelliferone, a highly fluorescent compound. The fluorescence intensity is measured at 450 nm (emission) after excitation at 360 nm.
Advantages:
- High sensitivity: detection limit of 0.02 U/L (approximately 0.003% raw milk contamination)
- Rapid: results in 3 minutes
- Quantitative: fluorescence intensity is directly proportional to enzyme activity
- Objective: results are read by a fluorometer, eliminating subjective color interpretation
Procedure outline:
- Warm substrate solution to 38°C
- Add 0.1 mL of milk to 2.0 mL of substrate
- Incubate at 38°C in the fluorometer
- Measure fluorescence at defined intervals (typically 1, 2, and 3 minutes)
- Calculate enzyme activity from the rate of fluorescence increase
The fluorometric method has largely replaced colorimetric methods in commercial dairies and regulatory laboratories due to its speed, sensitivity, and objectivity.
Colorimetric Method
The colorimetric method using p-nitrophenyl phosphate is a quantitative alternative that does not require a fluorometer. It is often used in teaching laboratories and in settings where fluorometric equipment is unavailable.
Principle: pNPP is hydrolyzed to p-nitrophenol, which absorbs light at 405 nm in alkaline conditions. The absorbance increase is measured spectrophotometrically.
Procedure outline:
- Mix 1 mL of milk with 4 mL of pNPP substrate in glycine buffer (0.1 M, pH 10.5)
- Incubate at 37°C for 30 minutes
- Stop the reaction by adding 1 mL of 0.5 M sodium hydroxide
- Measure absorbance at 405 nm against a blank
Interpretation: The absorbance is converted to enzyme activity using a standard curve prepared with known concentrations of p-nitrophenol. Results are expressed as U/L of alkaline phosphatase.
This method is less sensitive than the fluorometric approach (detection limit approximately 0.5 U/L) but is more accessible and provides quantitative data.
Step-by-Step Procedure for the Phosphatase Test
Sample Preparation
Proper sample handling is critical for accurate results. The following steps apply to the fluorometric method, the most widely used in industry:
- Collection: Collect milk samples in sterile containers. Samples must be representative of the batch being tested. For packaged milk, select containers from different points in the production run.
- Storage: If not tested immediately, store samples at 2–8°C. Testing should occur within 24 hours of collection. Do not freeze samples, as freezing can disrupt the milk fat globule membrane and potentially affect enzyme accessibility.
- Warming: Before testing, warm the sample to room temperature (20–25°C) and mix thoroughly by gentle inversion. Avoid vigorous shaking, which can create foam and introduce air bubbles that interfere with fluorescence readings.
- Substrate preparation: Reconstitute or warm the Fluorophos® substrate to 38°C in a water bath. The substrate must be at the correct temperature before adding the milk sample to ensure consistent reaction kinetics.
Incubation and Reagent Addition
- Blank measurement: Place 2.0 mL of substrate in a clean cuvette and measure the baseline fluorescence. This accounts for any spontaneous substrate hydrolysis.
- Sample addition: Add 0.1 mL of milk to the substrate and mix immediately by gentle pipetting or inversion. Begin timing immediately.
- Kinetic measurement: Place the cuvette in the fluorometer maintained at 38°C. Record fluorescence readings at 1, 2, and 3 minutes after sample addition. The instrument calculates the rate of fluorescence increase (ΔF/min).
- Reaction termination: For colorimetric methods, add the stopping reagent (e.g., sodium hydroxide) after the incubation period to halt the enzymatic reaction before measuring absorbance.
Reading and Interpreting Results
- Fluorometric method: The fluorometer automatically converts the rate of fluorescence increase to enzyme activity in U/L using a calibration factor. The instrument should be calibrated daily using a standard solution of 4-methylumbelliferone.
- Colorimetric method: Measure absorbance at 405 nm. Convert to enzyme activity using a standard curve:
- Prepare standards of 0, 1, 2, 5, and 10 μg/mL p-nitrophenol
- Plot absorbance versus concentration
- Interpolate sample values from the standard curve
- Scharer test: Visually compare the color of the test sample against a set of standards or a color chart. Report as negative, trace, or positive.
Interpreting Results and Quality Control Limits
Qualitative vs Quantitative Results
The phosphatase test can be performed as either a qualitative or quantitative assay, and the interpretation differs accordingly.
Qualitative tests (e.g., Scharer rapid test) provide a simple positive/negative answer. A negative result (no color development) indicates that alkaline phosphatase activity is below the detection threshold, which is interpreted as adequate pasteurization. A positive result (color development) indicates the presence of active enzyme and thus inadequate pasteurization or post-pasteurization contamination with raw milk.
Quantitative tests (fluorometric or colorimetric) provide a numerical value for enzyme activity. This allows for:
- Grading the severity of the failure
- Distinguishing between gross under-pasteurization and minor contamination
- Monitoring trends in processing efficiency over time
- Setting precise regulatory thresholds
Regulatory Standards
Regulatory limits for alkaline phosphatase activity in pasteurized milk vary by jurisdiction but are generally harmonized:
| Method | Regulatory Limit | Equivalent Raw Milk Contamination |
|---|---|---|
| Fluorometric (ISO 11816) | ≤ 350 mU/L | ~0.1% |
| Colorimetric (pNPP) | ≤ 0.5 U/L | ~0.1% |
| Scharer (visual) | No blue color | < 0.1% |
The European Union and Codex Alimentarius specify a maximum of 350 mU/L for pasteurized milk when measured by the fluorometric method. The United States FDA uses a similar threshold. These limits are set well above the detection limit of the assay to allow for the small residual activity that may occur naturally in some milk samples, while still being sensitive enough to detect meaningful contamination.
It is important to note that the regulatory limit is not zero. Trace amounts of alkaline phosphatase activity can arise from:
- Reactivation of the enzyme after pasteurization
- Naturally occurring enzyme variants with slightly higher heat resistance
- Analytical noise at the detection limit
Factors Affecting Test Accuracy
Microbial Phosphatase
Some microorganisms, particularly certain strains of Bacillus and Pseudomonas, produce their own alkaline phosphatase enzymes. If these bacteria are present in milk in high numbers, they can contribute to the measured phosphatase activity, leading to false-positive results.
This is particularly problematic in milk that has been stored improperly, allowing bacterial growth. The microbial enzyme may survive pasteurization if the bacteria form heat-resistant spores or if the enzyme itself is more heat-stable than the milk enzyme. To mitigate this issue, samples should be tested promptly after collection, and results should be interpreted with knowledge of the sample's microbial quality.
Enzyme Reactivation
Alkaline phosphatase can undergo reactivation after pasteurization—a phenomenon where the heat-denatured enzyme refolds into an active conformation. This is most commonly observed in:
- Cream and high-fat products
- Milk subjected to ultra-high temperature (UHT) treatment
- Products stored for extended periods
The reactivation process is time- and temperature-dependent. It occurs most readily when milk is heated to temperatures above 80°C, then cooled and stored at 20–30°C. The reactivated enzyme is often more heat-labile than the native enzyme, meaning it can be inactivated by a second heat treatment.
For quality control purposes, the International Dairy Federation recommends that samples be tested within 24 hours of collection and that results be interpreted with caution for UHT-treated products. Some regulatory frameworks allow higher thresholds for products where reactivation is known to occur.
Interfering Substances
Several substances can interfere with the phosphatase test:
Chemical inhibitors: Heavy metals (e.g., mercury, lead) and some detergents can inhibit alkaline phosphatase activity, causing false-negative results. Residual sanitizers on equipment can contaminate milk samples.
Color interference: Highly colored milk products (e.g., chocolate milk) can interfere with colorimetric detection. The fluorometric method is less susceptible to this issue.
pH effects: The assay relies on maintaining the reaction at alkaline pH. If the milk sample is acidic (e.g., from microbial fermentation), it can overwhelm the buffer capacity and reduce enzyme activity.
Fat content: Alkaline phosphatase is associated with the milk fat globule membrane. In skim milk, the enzyme may be present at lower concentrations, potentially leading to false-negative results if the assay is not optimized for low-fat products.
Applications and Limitations in Dairy Industry
Quality Assurance
The primary application of the phosphatase test is in verifying the adequacy of pasteurization. It is used:
- At the processing plant: Every batch of pasteurized milk is tested before release. This is a legal requirement in most jurisdictions.
- In regulatory inspection: Government agencies test retail milk samples to ensure compliance with pasteurization standards.
- In HACCP programs: The test serves as a critical control point verification for the pasteurization step in Hazard Analysis and Critical Control Point (HACCP) systems.
- In equipment hygiene monitoring: The test can detect post-pasteurization contamination when pasteurized milk comes into contact with raw milk residue on processing equipment.
- In shelf-life studies: Monitoring alkaline phosphatase activity over time can detect enzyme reactivation and help establish product shelf-life.
The test is also used for other dairy products, including cream, cheese, and flavored milk, with appropriate modifications for the product matrix.
Limitations
The phosphatase test has several inherent limitations that must be acknowledged:
Not a sterility test: The test only verifies the destruction of vegetative pathogens. It does not indicate the absence of bacterial spores, which can survive pasteurization and cause spoilage.
Limited to alkaline phosphatase: The test specifically detects one enzyme. It does not provide information about other quality parameters, such as protein content, fat content, or the presence of antibiotics.
Reactivation issues: As discussed, enzyme reactivation can produce false-positive results in certain products, particularly UHT milk.
Sensitivity to contamination: The test cannot distinguish between under-pasteurization and post-pasteurization contamination with raw milk. Both scenarios produce positive results.
Matrix effects: The test requires validation for each product type. Factors such as fat content, viscosity, and the presence of additives can affect performance.
Not applicable to all heat treatments: The test is calibrated for conventional pasteurization. It is not appropriate for verifying the adequacy of UHT or sterilization processes, where the enzyme is completely destroyed and its absence provides no useful information.
Common Pitfalls and Troubleshooting
Contamination
Problem: False-positive results due to contamination of the sample or reagents with raw milk.
Prevention:
- Use dedicated pipettes and cuvettes for the assay
- Never handle raw milk and pasteurized milk samples in the same area
- Clean all glassware with acid-wash followed by thorough rinsing with distilled water
- Verify that the substrate solution is not contaminated by running a reagent blank
Troubleshooting: If a positive result is obtained, repeat the test with a fresh sample and fresh reagents. If the repeat test is negative, the initial result was likely due to contamination.
Timing Errors
Problem: Inconsistent incubation times lead to variable results.
Prevention:
- Use a timer for all incubation steps
- Start timing immediately upon sample addition
- Ensure that the incubator or fluorometer is at the correct temperature before starting
Troubleshooting: If results are borderline, repeat the test with careful attention to timing. The kinetic fluorometric method is less susceptible to timing errors because it measures the rate of reaction rather than an endpoint.
Reagent Handling
Problem: Degraded or improperly stored reagents produce unreliable results.
Prevention:
- Store substrates and reagents according to manufacturer instructions
- Protect light-sensitive reagents (e.g., Fluorophos®) from light
- Check reagent expiration dates before each use
- Reconstitute lyophilized reagents with the correct volume of diluent
Troubleshooting: If the substrate solution appears discolored or turbid, discard and prepare fresh. Run a positive control (raw milk) and a negative control (boiled milk) with each batch of tests to verify reagent performance.
Temperature Control
Problem: Incorrect reaction temperature affects enzyme activity and results.
Prevention:
- Calibrate water baths and fluorometers regularly
- Allow samples and reagents to equilibrate to the correct temperature before mixing
- Monitor temperature during the assay
Troubleshooting: If the fluorometer temperature is incorrect, readings will be systematically biased. Recalibrate the instrument and repeat the test.
Sample Quality
Problem: Poor-quality samples (e.g., curdled, contaminated, or improperly stored) give unreliable results.
Prevention:
- Test samples within 24 hours of collection
- Store samples at 2–8°C before testing
- Mix samples thoroughly before testing
- Do not test samples that show signs of spoilage
Troubleshooting: If a sample is visibly spoiled, note this on the report and request a fresh sample.
Frequently Asked Questions
What is the principle of the phosphatase test in milk?
The phosphatase test detects the presence of active alkaline phosphatase enzyme in milk. Raw milk naturally contains this enzyme, which is inactivated by the heat of pasteurization. The test provides a substrate (such as phenyl phosphate, p-nitrophenyl phosphate, or Fluorophos®) that the enzyme hydrolyzes to produce a detectable product—either a colored compound or a fluorescent molecule. If the enzyme is present, the product forms and is detected; if the enzyme is absent (because pasteurization was adequate), no product forms. The presence of enzyme activity indicates that the milk was not adequately pasteurized or was contaminated with raw milk after pasteurization.
Why is the phosphatase test used for milk?
The phosphatase test is used because alkaline phosphatase has thermal inactivation kinetics that closely match the destruction of pathogenic microorganisms in milk. When the enzyme is fully inactivated, the milk has received sufficient heat treatment to kill significant pathogens, including Mycobacterium tuberculosis and Coxiella burnetii. The test thus serves as a biological marker for pasteurization adequacy. It is rapid, relatively inexpensive, and can detect very small amounts of raw milk contamination (as little as 0.1%) in pasteurized milk.
What is the procedure for the phosphatase test in milk?
The procedure varies by method. In the fluorometric method (the current standard): warm the Fluorophos® substrate to 38°C, add 0.1 mL of milk to 2.0 mL of substrate, incubate at 38°C while measuring fluorescence at 1, 2, and 3 minutes, and calculate enzyme activity from the rate of fluorescence increase. In the colorimetric method: mix milk with p-nitrophenyl phosphate in glycine buffer (pH 10.5), incubate at 37°C for 30 minutes, stop the reaction with sodium hydroxide, and measure absorbance at 405 nm. In the Scharer rapid test: incubate milk with phenyl phosphate at 37°C for 10 minutes, add the color-developing reagent, and observe for blue color development.
What is the Scharer rapid test for milk phosphatase?
The Scharer rapid test is a colorimetric method that uses phenyl phosphate as the substrate. Milk is incubated with the substrate at 37°C for 10 minutes. If alkaline phosphatase is present, it hydrolyzes phenyl phosphate to release phenol. A reagent called 2,6-dibromoquinone-4-chloroimide (CQC) is then added, which reacts with phenol to produce a blue indophenol dye. The development of blue color indicates the presence of active alkaline phosphatase and thus inadequate pasteurization. The test is rapid (approximately 15 minutes total) and simple, making it suitable for field use, but it is less sensitive than fluorometric methods.
What does a positive phosphatase test indicate?
A positive phosphatase test indicates the presence of active alkaline phosphatase enzyme in the milk sample. This means one of the following: (1) the milk was not pasteurized at all (raw milk), (2) the milk was inadequately pasteurized (insufficient temperature or time), or (3) pasteurized milk was contaminated with raw milk after pasteurization (post-pasteurization contamination). In all cases, a positive result indicates a food safety risk and requires investigation and corrective action.
Can the phosphatase test give false positives?
Yes, the phosphatase test can give false positives. Common causes include: (1) microbial phosphatases produced by bacteria contaminating the milk, (2) reactivation of the enzyme after pasteurization (particularly in UHT-treated products), (3) contamination of samples or reagents with raw milk during testing, and (4) chemical interference from sanitizers or other substances. To minimize false positives, samples should be tested promptly, reagents must be handled carefully, and results should be interpreted in the context of the product type and processing history.
What is the difference between qualitative and quantitative phosphatase tests?
Qualitative tests (such as the Scharer rapid test) provide a simple positive or negative result based on the presence or absence of color development. They are rapid and simple but have limited sensitivity and do not provide information about the degree of contamination. Quantitative tests (such as the fluorometric method) measure the actual enzyme activity in units per liter (U/L) or millunits per liter (mU/L). They are more sensitive, provide objective numerical results, and allow for the detection of low-level contamination that qualitative tests might miss. Quantitative results also enable trend analysis and more precise regulatory compliance assessment.
Key Takeaways
- The phosphatase test detects residual alkaline phosphatase activity in milk as a marker for pasteurization adequacy; the enzyme is naturally present in raw milk and inactivated by proper heat treatment.
- Alkaline phosphatase is a dimeric metalloenzyme with zinc and magnesium ions at its active site; its thermal inactivation kinetics closely parallel the destruction of milk-borne pathogens.
- The test principle involves enzymatic hydrolysis of a phosphomonoester substrate (phenyl phosphate, p-nitrophenyl phosphate, or Fluorophos®) to produce a detectable colored or fluorescent product.
- The fluorometric method using Fluorophos® is the current international standard, offering high sensitivity (detection of ~0.1% raw milk contamination) and rapid results (3 minutes).
- Regulatory limits for pasteurized milk are typically ≤350 mU/L by the fluorometric method; results above this threshold indicate inadequate pasteurization or post-pasteurization contamination.
- False positives can arise from microbial phosphatases, enzyme reactivation (especially in UHT products), and sample contamination; proper technique and prompt testing minimize these risks.
- The test verifies pasteurization against vegetative pathogens but does not confirm sterility; it is not applicable to UHT or sterilized products where the enzyme is completely destroyed.
Further Reading
- Murthy GK et al. Rapid Colorimetric Test for Alkaline Phosphatase in Dairy Products. Journal of food protection. 1979. PubMed 30812132
- Murthy GK, Peeler JT. Collaborative Study of Rapid Methods for Alkaline Phosphatase in Milk and Cream. Journal of food protection. 1979. PubMed 30812125
- Scintu MF, Daga E, Ledda A. Evaluation of spectrophotometric and fluorometric methods for alkaline phosphatase activity determination in ewe's milk. Journal of food protection. 2000. PubMed 10983802
- Murthy GK, Peeler JT. Rapid methods for differentiating reactivated from residual phosphatase in milk and cream: collaborative study. Journal - Association of Official Analytical Chemists. 1979. PubMed 227830
- Yu L et al. Disposable lateral flow-through strip for smartphone-camera to quantitatively detect alkaline phosphatase activity in milk. Biosensors & bioelectronics. 2015. PubMed 25771303
- Claeys WL, Van Loey AM, Hendrickx ME. Kinetics of alkaline phosphatase and lactoperoxidase inactivation, and of beta-lactoglobulin denaturation in milk with different fat content. The Journal of dairy research. 2002. PubMed 12463692
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