What temperatures do proteins in breast milk begin to break down?
Background on Human Breast Milk Proteins
Human breast milk contains a diverse array of proteins, including whey proteins (e.g., α-lactalbumin, lactoferrin, lysozyme) and immunoglobulins (e.g., secretory IgA), which are crucial for infant nutrition, immunity, and gut development. These proteins are sensitive to heat, and their denaturation can lead to loss of function, such as reduced antimicrobial activity or altered digestibility. The denaturation temperature varies by protein, influenced by factors like pH, ionic strength, and heating duration, which are often studied in the context of pasteurization for donor milk banking
Methodology and Data Sources
The analysis draws from multiple scientific studies, with a focus on those directly addressing human breast milk, such as pasteurization effects. Key sources include studies on immunological protein retention and reviews of milk protein denaturation, supplemented by data on cow’s milk proteins where human-specific data are scarce, given similarities (e.g., α-lactalbumin). Temperatures are reported in Celsius, as is standard in scientific literature, with the understanding that these are based on typical experimental conditions like 30-minute exposures.
They main protein components of human breast milk are:
- Whey Proteins (about 60-70%):
- Lactoferrin: Binds iron and has antimicrobial properties.
- Alpha-lactalbumin: Aids in lactose synthesis and provides essential amino acids.
- Lysozyme: Offers antibacterial protection.
- Immunoglobulins (mainly IgA): Provides immune protection against pathogens.
- Serum albumin: Plays a role in nutrient transport.
- Casein Proteins (about 30-40%):
- Beta-casein: The predominant form, providing essential amino acids and aiding in calcium absorption.
- Kappa-casein: Helps stabilize casein micelles, important for digestion.
These proteins support infant growth, immune protection, and the development of the digestive system.
Casein proteins are relatively heat-stable compared to other milk proteins like whey. They don’t denature in the same way because they exist as micelles, which are more resistant to heat. However, significant changes can occur at temperatures above 100°C (212°F), especially under prolonged heating or in acidic conditions. Casein coagulation, such as in cheese-making, often relies more on pH changes than heat alone.
Temperatures for Onset of Mild Loss of Function of Protein Components
Mild loss of function occurs when proteins begin to denature, typically at the lower end of the temperature range where structural changes or reduced bioactivity are detectable. Below, I list the temperatures at which each protein starts to lose function, based on human milk studies or bovine milk data where human-specific data are limited. Temperatures are converted from °C to °F using °F = (°C × 9/5) + 32. I focus on the earliest reported denaturation temperatures, often from studies of pasteurization or thermal processing.
1. α-Lactalbumin:
• Onset Temperature: ~64°C (147.2°F)
• Details: Denaturation is reversible at this stage; mild loss of lactose synthesis activity begins.
• Source: Food Chemistry (2019).
2. Lactoferrin:
• Onset Temperature: ~75°C (167°F)
• Details: Minimal loss of antimicrobial activity at 75°C; stable below this.
• Source: Lönnerdal, B. (2013).
3. Lysozyme:
• Onset Temperature: ~80°C (176°F)
• Details: Slight reduction in antibacterial activity at 80°C for short durations (e.g., 15–30 seconds).
• Source: Lönnerdal, B. (2013).
4. Immunoglobulins (Secretory IgA):
• Onset Temperature: ~56°C (132.8°F)
• Details: Earliest loss of immune-protective function at 56–60°C for 30 minutes.
• Source: Journal of Dairy Science (1955).
5. β-Casein:
• Onset Temperature: ~100°C (212°F)
• Details: Caseins are highly heat-stable; mild denaturation only at boiling point or above, with minimal functional loss below 100°C.
• Source: Lönnerdal, B. (2003).
6. Bile Salt-Stimulated Lipase (BSSL):
• Onset Temperature: ~60°C (140°F)
• Details: Initial inactivation of lipid digestion activity at 60°C for 30 minutes.
• Source: Lönnerdal, B. (2003).
7. Amylase:
• Onset Temperature: ~63°C (145.4°F)
• Details: Slight reduction in carbohydrate digestion activity at 63°C for 30 minutes.
• Source: Lönnerdal, B. (2003).
Detailed Findings on Denaturation Temperatures
Below, we detail the temperatures at which major proteins in human breast milk begin to lose function, based on available research:
• Immunoglobulins (e.g., Secretory IgA):
• Research indicates that immunoglobulins are among the least heat-resistant proteins, with studies showing retention of at least 90% at 57°C for 30 minutes, but significant loss (72.3% retention) at 62.5°C for 30 minutes (Retention of the Immunological Proteins of Pasteurized Human Milk in Relation to Pasteurizer Design and Practice). This suggests the onset of function loss begins around 60°C, with rapid decline at higher temperatures.
• The sensitivity is likely due to the complex structure of immunoglobulins, which can unfold and aggregate under heat, reducing their immune-protective capabilities.
• Lactoferrin:
• Lactoferrin, an iron-binding protein with antimicrobial properties, shows high retention (at least 90%) at 57°C for 30 minutes, but retention drops to 21.8% at 62.5°C for 30 minutes, indicating significant denaturation above 57°C (Retention of the Immunological Proteins of Pasteurized Human Milk in Relation to Pasteurizer Design and Practice). This suggests it begins losing function above 57°C, with notable impact at 62.5°C.
• Reviews like “Lactoferrin: Structure, function, denaturation and digestion” (PubMed) note its heat sensitivity, but specific human milk data are limited, reinforcing the reliance on pasteurization studies.
• Lysozyme:
• Lysozyme, an enzyme with antibacterial activity, also shows at least 90% retention at 57°C for 30 minutes, but retention falls to 39.4% at 62.5°C for 30 minutes, suggesting function loss begins above 57°C (Retention of the Immunological Proteins of Pasteurized Human Milk in Relation to Pasteurizer Design and Practice). Studies like “Thermal denaturation of recombinant human lysozyme from rice: Effect of pH and comparison with human milk lysozyme” (ResearchGate) indicate similar thermostability, with denaturation starting at higher temperatures, but human milk data confirm the 57°C threshold.
• α-Lactalbumin:
• α-Lactalbumin, a major whey protein involved in lactose synthesis, is relatively heat-stable. While direct human milk studies are limited, cow’s milk studies like “The denaturation of α-lactalbumin and β-lactoglobulin in heated milk” (Journal of Dairy Research) suggest denaturation begins around 64°C, with significant loss at higher temperatures. Given its similarity in human and bovine milk, it seems likely human α-lactalbumin starts losing function around 64°C.
Implications and Limitations
The findings are crucial for practices like donor milk banking, where Holder pasteurization (62.5°C for 30 minutes) is standard but can significantly reduce protein bioactivity. For home use, warming milk above 57°C could reduce nutritional benefits, especially for preterm infants reliant on these proteins for immunity. However, the data are primarily from pasteurization studies with 30-minute exposures, and shorter heating times (e.g., in bottle warming) might delay onset. Additionally, human milk studies are less common than bovine milk studies, so some inferences (e.g., for α-lactalbumin) are based on cow’s milk data, which may not perfectly align due to compositional differences.
Conclusion
In summary, proteins in human breast milk begin to lose function at temperatures above 57°C, with immunoglobulins, lactoferrin, and lysozyme showing sensitivity above this threshold, and α-lactalbumin being more resistant, starting around 64°C. These temperatures are critical for maintaining milk’s nutritional and immunological properties, particularly in clinical or storage contexts.
Notes
• Data Limitations: Exact onset temperatures for human milk proteins are often extrapolated from bovine milk or formula studies (e.g., Food Chemistry, 2019) due to limited human milk experiments. Human proteins may be slightly more heat-resistant.
• Mild Loss: This refers to the earliest detectable denaturation (e.g., partial unfolding or reduced bioactivity), often reversible for some proteins (e.g., α-lactalbumin). Significant loss occurs at higher temperatures, as noted previously.
• Time Dependency: Denaturation at these temperatures assumes typical processing times (e.g., 15–30 seconds for high-temperature short-time, 30 minutes for Holder pasteurization). Shorter exposures may delay onset.
Key Citations
• Retention of the Immunological Proteins of Pasteurized Human Milk in Relation to Pasteurizer Design and Practice (https://www.nature.com/articles/pr2009219)
• The denaturation of α-lactalbumin and β-lactoglobulin in heated milk (https://www.cambridge.org/core/journals/journal-of-dairy-research/article/abs/denaturation-of-lactalbumin-and-lactoglobulin-in-heated-milk/7C34CB3B3A21E83079ECE2926B040284)
• Thermal denaturation of recombinant human lysozyme from rice: Effect of pH and comparison with human milk lysozyme (https://www.researchgate.net/publication/257372876_Thermal_denaturation_of_recombinant_human_lysozyme_from_rice_Effect_of_pH_and_comparison_with_human_milk_lysozyme)
• Lactoferrin: Structure, function, denaturation and digestion (https://pubmed.ncbi.nlm.nih.gov/28933602/)
• Food Chemistry (2019). https://www.sciencedirect.com/science/article/abs/pii/S0308814619317667
• Lönnerdal, B. (2013). https://onlinelibrary.wiley.com/doi/10.1111/jpc.12078
• Lönnerdal, B. (2003). https://www.sciencedirect.com/science/article/pii/S0002916522034197
• Journal of Dairy Science (1955). https://www.sciencedirect.com/science/article/pii/S0022030255948247
