Some may wonder when warming or reheating frozen breast milk, what temperature may affect the proteins and nutrition in the milk. This article will discuss the protein components and the heat resistance of them.
Human breast milk contains several key protein components, which can be categorized into two main types: whey and casein proteins. While there are minimal studies specifically for human breast milk protein denaturing and protein breakdown, each of the components that make up breast milk has been studied. Other components of human milk do have varying levels of heat resistance that we will explore in other articles.
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, we 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 can be converted from °C to °F using °F = (°C × 9/5) + 32. We will  focus on the earliest reported denaturation temperatures, often from studies of pasteurization or thermal processing.
1. α-Lactalbumin:
• Denaturation Temperature: Begins at ~64–70°C (reversible); significant denaturation at 75–80°C for 15–30 seconds; complete at ~90°C for 15 seconds.
• Source: Food Chemistry (2019). https://www.sciencedirect.com/science/article/abs/pii/S0308814619317667
2. Lactoferrin:
• Denaturation Temperature: Partial denaturation at 80–90°C for 15–30 seconds; complete at ~90°C for 15 minutes.
• Source: Food Chemistry (2019); Lönnerdal, B. (2013). https://onlinelibrary.wiley.com/doi/10.1111/jpc.12078
3. Lysozyme:
• Denaturation Temperature: Begins at ~80–85°C for 15–30 seconds; complete at ~100°C for 10 minutes.
• Source: Lönnerdal, B. (2013). https://onlinelibrary.wiley.com/doi/10.1111/jpc.12078
4. Immunoglobulins (e.g., Secretory IgA):
• Denaturation Temperature: Begins at ~56–60°C for 30 minutes; significant loss at 70–75°C for 15–30 seconds.
• Source: Journal of Dairy Science (1955). https://www.sciencedirect.com/science/article/pii/S0022030255948247
5. β-Casein:
• Denaturation Temperature: Minimal denaturation below 100°C; significant at ultra-high temperatures (e.g., 145°C for 3 seconds).
• Source: ScienceDirect (2003). https://www.sciencedirect.com/science/article/pii/S0002916522034197
6. Bile Salt-Stimulated Lipase (BSSL):
• Denaturation Temperature: Inactivated at ~60–65°C for 30 minutes; significant loss at 75°C for 15 seconds.
• Source: Lönnerdal, B. (2003). https://www.sciencedirect.com/science/article/pii/S0002916522034197
7. Amylase:
• Denaturation Temperature: Partial denaturation at 63°C for 30 minutes; significant loss at 75–80°C for 15–30 seconds.
• Source: Lönnerdal, B. (2003). https://www.sciencedirect.com/science/article/pii/S0002916522034197
Notes and Limitations
• Data Sources: Most precise denaturation temperatures come from bovine milk or model infant formula studies (e.g., Food Chemistry, 2019) due to limited human milk experiments. Human milk proteins (e.g., α-lactalbumin, lactoferrin) are often more heat-resistant than bovine counterparts.
• Variability: Denaturation depends on heating duration, pH, and protein interactions. Short-time high-temperature treatments (e.g., 80°C/30 s) cause less damage than prolonged heating (e.g., 63°C/30 min) for some proteins.
• Minor Proteins: Growth factors, cytokines, and peptides (e.g., EGF, TGF-β) lack specific denaturation data due to low concentrations and analytical challenges.
• Pasteurization Context: Holder pasteurization (63°C/30 min) inactivates BSSL and immunoglobulins but preserves lactoferrin and lysozyme. Higher-temperature treatments (e.g., 80°C/30 s) denature most proteins significantly.
Source Links
• 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
