β-Carotene serves as an essential nutrient in infant formula, functioning both as a provitamin A source and as a protective antioxidant for the delicate lipid matrix found in these specialized nutritional products. As regulatory standards have evolved to ensure the safety and nutritional adequacy of infant formula, robust analytical methods have become indispensable for quality control laboratories. The internationally recognized AOAC 2016.13 / ISO 23443:2020 method provides a validated approach for determining β-carotene and lycopene in infant formula and adult nutritionals using reversed-phase ultra-high-performance liquid chromatography with diode array detection (RP-UHPLC-DAD).
The Importance of Carotenoids in Infant Nutrition
Carotenoids represent a family of over 1,100 naturally occurring pigments that play vital roles in human health and development. Among the most biologically important is β-carotene, which exhibits provitamin A activity—the ability to convert into retinol within the body. This conversion capability makes β-carotene particularly crucial for infant nutrition, as vitamin A is essential for normal growth and development, immune system function, and vision.
Human breast milk naturally contains β-carotene at concentrations ranging from 380 to 490 µg/L, with levels increasing substantially in colostrum during the first days of lactation. Formula-fed infants who receive products without carotenoid fortification show significantly lower plasma β-carotene levels compared to breastfed infants, underscoring the importance of appropriate supplementation. Beyond its provitamin A function, β-carotene serves as an antioxidant that protects the polyunsaturated fatty acids (PUFAs) in infant formula from oxidative degradation, helping maintain product stability and nutritional quality.
Understanding the Analytical Challenge
Analyzing carotenoids in complex matrices like infant formula presents several unique challenges. These highly unsaturated molecules are prone to oxidative degradation, geometric isomerization (cis-trans conversion), and photodegradation when exposed to light, heat, oxygen, and acidic conditions. The all-trans form of β-carotene predominates in most natural sources and commercial products, but improper handling during sample preparation can induce unwanted isomerization, producing cis isomers that complicate quantification and may exhibit different biological activities.
The high lipid content of infant formula further complicates extraction procedures, as fats must be efficiently saponified to allow complete recovery of carotenoids without co-extracting large quantities of triglycerides that would interfere with chromatographic analysis. These analytical challenges necessitate carefully controlled sample preparation protocols that balance extraction efficiency with protection against degradation.
The AOAC 2016.13 / ISO 23443:2020 Method
This internationally harmonized method specifies procedures for the quantitative determination of β-carotene and lycopene in infant formula and adult nutritionals across solid (powders) and liquid (ready-to-feed and concentrates) formats. Following a rigorous collaborative study involving ten laboratories from seven countries, the method achieved Final Action status for β-carotene and lycopene, demonstrating excellent precision and reproducibility.
Sample Preparation Overview
The method employs a multi-step extraction process designed to efficiently isolate carotenoids from the complex infant formula matrix:
Saponification: Test samples are spiked with an internal standard (apocarotenal) and treated with potassium hydroxide (KOH) to hydrolyze lipids and xanthophyll esters. This alkaline treatment simplifies the matrix by converting triglycerides and phospholipids into water-soluble soaps that partition away from the target carotenoids.
Liquid-Liquid Extraction: Following saponification, samples undergo sequential extractions with a combination of solvents specifically selected for their ability to efficiently extract carotenoids. The method specifies extraction with methyl tert-butyl ether (MTBE) and tetrahydrofuran (THF), followed by hexane. This solvent combination provides comprehensive recovery of both polar and non-polar carotenoids while maintaining compatibility with subsequent chromatographic separation.
MTBE offers excellent solvating properties for carotenoids and has become increasingly popular in carotenoid analysis due to its lower toxicity compared to traditional solvents like chloroform. THF complements MTBE by enhancing the extraction of more polar carotenoid species and facilitating penetration into the sample matrix. Hexane provides a final non-polar extraction phase that ensures complete recovery of hydrophobic carotenoids like β-carotene.
Critical Nitrogen Blowdown Step: After extraction, the combined organic supernatants are dried under a stream of nitrogen gas. This evaporation step concentrates the carotenoids and removes the extraction solvents, allowing reconstitution in a mobile phase-compatible solvent. The nitrogen blowdown process represents a critical juncture where improper technique can result in analyte loss, oxidative degradation, or isomerization.
Reconstitution and Analysis: The dried residue is reconstituted in 2-propanol (isopropanol), which provides excellent compatibility with the reversed-phase chromatographic mobile phase. Following reconstitution, samples are filtered through 0.2 µm PTFE filters and analyzed by UHPLC using a C30 column specifically designed for carotenoid separations.
Chromatographic Separation Using C30 Columns
The method specifies a C30 reversed-phase column, which has become the gold standard for carotenoid analysis. Unlike conventional C18 columns, C30 stationary phases feature polymeric triacontyl (30-carbon) chains that provide markedly greater hydrophobicity and exceptional shape selectivity. This enhanced hydrophobicity allows C30 columns to effectively separate geometric and positional isomers of carotenoids, distinguishing all-trans-β-carotene from its cis isomers as well as resolving β-carotene from structurally related compounds like α-carotene, lycopene, lutein, and zeaxanthin.
The C30 phase's ability to discriminate subtle molecular differences makes it indispensable for accurate quantification, as different isomeric forms may exhibit varying detector responses and biological activities. The column provides sufficient phase thickness to enhance interactions with long-chain conjugated molecules, enabling baseline resolution of complex carotenoid mixtures within a single chromatographic run.
Diode Array Detection for Carotenoid Quantification
The AOAC/ISO method employs diode array detection (DAD), which monitors absorbance across a range of wavelengths simultaneously. Carotenoids exhibit characteristic UV-visible absorption spectra due to their extended conjugated double bond systems, with β-carotene showing maximum absorbance around 450-460 nm. DAD provides several advantages for carotenoid analysis:
High Sensitivity: Despite carotenoids' high molar extinction coefficients making them excellent chromophores, DAD delivers exceptional sensitivity that often surpasses other detection modes for these compounds. For β-carotene analysis in infant formula, the method achieves limits of detection around 0.08 µg/100 g and limits of quantification around 0.27 µg/100 g on a reconstituted basis.
Spectral Confirmation: DAD captures full UV-visible spectra for each chromatographic peak, enabling spectral matching for compound identification and purity assessment. The characteristic absorption maxima and spectral fine structure help distinguish β-carotene from potential interfering compounds and can identify the presence of cis isomers through their characteristic "cis peak" around 332 nm.
Method Robustness: Studies comparing DAD with electrochemical detection for carotenoid analysis have demonstrated that DAD often provides superior precision and accuracy, particularly for compounds with high molar extinction coefficients. The light-absorbing properties of carotenoids translate to more reliable quantification compared to methods relying on other detection principles.
The Critical Role of Nitrogen Blowdown Evaporation
Among all the sample preparation steps, nitrogen blowdown evaporation demands particular attention due to carotenoids' susceptibility to degradation. This technique uses a stream of dry nitrogen gas directed onto the sample surface to remove organic solvents, offering several advantages over alternative evaporation methods.
Why Nitrogen Blowdown?
Nitrogen blowdown provides gentle, controlled evaporation that operates at or near atmospheric pressure, avoiding the harsh conditions associated with high-vacuum techniques. The continuous flow of inert nitrogen gas creates a protective blanket that displaces oxygen from the sample headspace, minimizing oxidative degradation of the oxygen-sensitive carotenoid molecules. This oxygen exclusion proves particularly important for β-carotene, which readily undergoes oxidation that can lead to analyte loss and formation of degradation products.
The technique also offers excellent control over evaporation rate through adjustment of both nitrogen flow rate and bath temperature, allowing optimization for different solvents and sample types. This flexibility enables laboratories to balance the competing demands of rapid throughput and gentle treatment required for heat-sensitive carotenoids.
Best Practices for Nitrogen Blowdown of Carotenoid Samples
To achieve optimal recovery and prevent degradation during nitrogen evaporation of carotenoid extracts, several critical parameters must be carefully controlled:
Temperature Control: The evaporation bath temperature should be maintained below 35°C to prevent thermal degradation and isomerization of β-carotene. While higher temperatures accelerate evaporation, they also increase the risk of cis-trans isomerization and oxidative breakdown. The AOAC/ISO method and best practice guidelines recommend temperatures between 20-40°C, with many protocols specifying 30-35°C as optimal. This temperature range provides efficient solvent removal while staying well below the threshold where significant thermal isomerization occurs.
Research has demonstrated that β-carotene exhibits good stability at temperatures up to 4°C for extended periods, with minimal degradation observed at room temperature for short durations when protected from light. However, elevated temperatures dramatically accelerate degradation kinetics, making temperature control during evaporation a critical quality control point.
Use of Amber Glassware: All sample preparation should be conducted using amber (brown) glass vessels to protect carotenoids from photodegradation. Light exposure, particularly direct sunlight and ultraviolet radiation, catalyzes cis-trans photoisomerization and oxidative decomposition of carotenoids. Studies have shown that exposure to light can induce isomerization of all-trans-β-carotene to cis isomers, with the 9-cis form often predominating in photoisomerization reactions.
The use of amber glassware provides a simple yet effective safeguard, filtering out much of the harmful UV and short-wavelength visible light that drives these degradation reactions. For maximum protection, sample preparation should be conducted under subdued lighting conditions, and samples should be processed expeditiously to minimize the duration of light exposure.
Nitrogen Flow Rate Optimization: The nitrogen flow rate must be carefully optimized to create a visible dimple on the sample surface without causing excessive turbulence or splashing. Too low a flow rate results in prolonged evaporation times that increase the risk of oxidative and thermal degradation, while excessive flow generates turbulence that can cause sample loss through aerosolization and cross-contamination between adjacent samples.
The optimal flow rate depends on sample tube diameter and solvent volatility. Small samples (10-30 mm diameter tubes) can typically be efficiently concentrated using 19-gauge needles at moderate flow rates, while larger vessels may benefit from wider-bore needles that accommodate higher flow without generating excessive turbulence. Individual needle valves that allow independent adjustment of gas flow to each sample position prove particularly valuable when processing samples with varying volumes or evaporation endpoints simultaneously.
Use of Dry Nitrogen: The nitrogen gas must be dry to promote efficient evaporation and prevent moisture condensation that could dilute samples or introduce water into organic extracts. Most high-purity nitrogen from cylinders or generators contains minimal moisture, but compressed air used as an alternative must be properly dried before use.
Protective Additives: While the AOAC/ISO method incorporates antioxidants like pyrogallol into the extraction procedure, the addition of stabilizers warrants careful consideration. Pyrogallol and other phenolic antioxidants can protect carotenoids during saponification and extraction by scavenging free radicals and reactive oxygen species. However, for clinical and some food analysis applications, the consensus suggests that the protective effects of such additives in modern analytical workflows may be marginal, particularly when samples are processed rapidly under controlled conditions.
The decision to include antioxidants should be based on method validation data demonstrating improved recovery or reduced variability. For research applications where absolute carotenoid content must be determined without potential interference from added compounds, omission of antioxidants may be preferable provided that samples are handled with appropriate care.
Organomation's Nitrogen Evaporators: Purpose-Built for Demanding Applications
For laboratories conducting carotenoid analysis in infant formula and other challenging matrices, Organomation's N-EVAP nitrogen evaporators provide the precision, control, and reliability required for method compliance and reproducible results. With over 65 years of experience in nitrogen evaporator manufacturing, Organomation has continuously refined their designs to meet the evolving needs of analytical laboratories.
Precise Temperature Control: All N-EVAP models equipped with heated baths feature accurate temperature control systems that maintain bath temperatures within tight tolerances. This precision enables compliance with method specifications requiring temperatures below 35°C, preventing thermal degradation while ensuring efficient evaporation. The availability of both water bath and dry bath configurations allows laboratories to select the optimal heat transfer medium for their specific applications.
Individual Flow Control: Each sample position features a precision needle valve machined from solid brass and chrome-plated for durability and chemical resistance. These individually adjustable valves enable precise control of nitrogen flow to each sample, accommodating simultaneous processing of samples with different volumes or solvents without compromising recovery or introducing cross-contamination. This flexibility proves invaluable when processing real-world sample sets that often vary in matrix complexity and analyte concentration.
Flexibility for Multiple Sample Sizes: The N-EVAP's unique spring-tensioned sample holder accommodates test tubes ranging from 10-30 mm outside diameter without requiring instrument modifications or custom inserts. This adaptability streamlines workflow by eliminating the need to change hardware when processing different sample types. Optional large sample holders extend the range to accommodate vessels up to 70 mm in diameter, providing versatility for laboratories handling diverse sample preparation protocols.
Scalable Capacity: Organomation offers N-EVAP models with capacities ranging from 6 to 45 sample positions, enabling laboratories to match instrument capacity to their throughput requirements. This scalability ensures that both small research laboratories and high-throughput quality control facilities can find an appropriately sized solution. The circular rotating design common to many N-EVAP models provides convenient front-access to all sample positions, facilitating easy sample insertion and retrieval throughout the evaporation process.
Durability and Longevity: Case studies document N-EVAP instruments remaining in productive service for over 15 years, testament to their robust construction and reliable performance. The stainless steel bath construction and quality brass valve components resist corrosion and wear, even under demanding analytical laboratory conditions. This longevity translates to excellent return on investment and reduced instrument lifecycle costs.
Optional Features for Enhanced Performance: Organomation offers several optional configurations to optimize performance for specific applications. The Z-Purge system provides positive pressure purging capability for applications requiring enhanced protection against oxidation. Acid-resistant coatings protect instruments and needles when processing corrosive samples. These customization options allow laboratories to configure instruments precisely matched to their analytical requirements.
Practical Recommendations for β-Carotene Analysis in Infant Formula
Based on the AOAC/ISO method specifications and established best practices, laboratories implementing or optimizing β-carotene analysis should consider the following recommendations:
Method Implementation: Follow the AOAC 2016.13 / ISO 23443:2020 protocol closely, paying particular attention to the specified extraction solvents (MTBE, THF, hexane), saponification conditions, and nitrogen evaporation parameters. The method's validation across multiple laboratories and matrices provides confidence in its robustness when properly implemented.
Sample Protection Throughout Workflow: Protect samples from light exposure from the moment of collection through final analysis by using amber glassware and conducting operations under subdued lighting. Minimize the time samples spend at elevated temperatures, and process expeditiously to reduce opportunities for degradation.
Nitrogen Evaporation Parameters: Maintain evaporation bath temperatures at or below 35°C, adjust nitrogen flow rates to achieve visible surface agitation without excessive turbulence, and use high-purity dry nitrogen to ensure efficient solvent removal. Monitor individual samples during evaporation to prevent over-drying, which can make reconstitution difficult and may contribute to oxidative losses.
Quality Control: Include appropriate reference materials, spiked samples, and internal standards in each analytical batch to monitor recovery and detect systematic errors. The use of apocarotenal as an internal standard, as specified in the AOAC/ISO method, provides correction for variations in extraction efficiency and instrument response.
Instrument Maintenance: Regularly inspect and clean nitrogen delivery needles, maintain bath temperature calibration, and ensure nitrogen supply purity. Contaminated or clogged needles can lead to uneven gas flow and poor reproducibility, while temperature drift compromises method compliance and analytical performance.
Conclusion
The analysis of β-carotene in infant formula represents a critical quality control measure that ensures products meet nutritional specifications and regulatory requirements. The AOAC 2016.13 / ISO 23443:2020 method provides a validated, internationally recognized approach that delivers the accuracy and precision required for this demanding application. Success with this method hinges on meticulous attention to sample preparation details, particularly the nitrogen blowdown evaporation step where carotenoid degradation and isomerization risks peak.
By implementing best practices for nitrogen evaporation—maintaining temperatures below 35°C, using amber glassware for light protection, optimizing gas flow rates, and employing high-quality nitrogen evaporators with precise control capabilities—laboratories can achieve excellent recovery and reproducibility for β-carotene determinations. Organomation's N-EVAP nitrogen evaporators provide the robust, reliable performance needed to meet these demanding requirements, backed by decades of proven service in analytical laboratories worldwide.
As infant formula manufacturers continue to optimize nutritional profiles and regulatory oversight intensifies, the importance of robust analytical methods for carotenoid analysis will only increase. Laboratories equipped with appropriate instrumentation, validated methods, and a thorough understanding of the factors affecting carotenoid stability will be well-positioned to meet these evolving challenges and ensure that infant formula products deliver their intended nutritional benefits.