The stability of L-Arginine in dairy products

L-Arginine (abbreviated as L-Arg), an essential basic amino acid in the human body, is commonly used as a nutritional fortifier in dairy products to enhance the protein nutritional value of products and regulate physiological functions (such as promoting nitric oxide synthesis). However, its stability in dairy products is not constant; it is affected by multiple factors including dairy processing techniques, storage conditions, and chemical components in the system. L-Arginine may undergo degradation, oxidation, or structural changes, thereby affecting its activity and nutritional value. The following analysis focuses on three aspects: the core factors influencing stability, performance in typical dairy products, and stabilization strategies.

I. Core Factors Influencing the Stability of L-Arginine in Dairy Products

The system characteristics of dairy products (e.g., pH, water activity, fat content) and the physical and chemical conditions during processing and storage are key determinants of L-Arginine stability. These factors trigger chemical transformations (e.g., deamination, cyclization, oxidation) or physical migration (e.g., binding to other components) of L-Arginine, leading to a decrease in its content or loss of activity.

1. Thermal Stability During Processing: High Temperature as the Primary Destructive Factor

Heat treatment commonly used in dairy processing (e.g., pasteurization, ultra-high temperature instantaneous sterilization (UHT), spray drying, cooking) is the primary factor affecting L-Arginine stability. The molecular structure of L-Arginine contains a guanidino group (-C(=NH)NH₂) and an amino group (-NH₂), both of which are sensitive to high temperatures and prone to the following reactions under heating:

Deamination reaction: High temperatures promote the detachment of the amino group from L-Arginine, producing L-Citrulline and ammonia (NH₃)—the main pathway of heat-induced degradation. For example, during UHT treatment (135–150℃, several seconds), if the dairy product is neutral or alkaline, the loss rate of L-Arginine can reach 5%–15%; prolonged heating (e.g., traditional boiling sterilization) or repeated heating can further increase the loss rate to over 20%.

Cyclization reaction: At high temperatures (especially above 120℃) and in acidic conditions, the guanidino group of L-Arginine may undergo endogenous cyclization with the adjacent carboxyl group, generating pyrroline-5-carboxylic acid. This product lacks the physiological activity of L-Arginine and reduces the amino acid nutritional value of the product.

Maillard reaction: The amino group of L-Arginine easily undergoes the Maillard reaction (non-enzymatic browning) with lactose (a reducing sugar) in dairy products. The reaction rate accelerates significantly under high temperatures (above 100℃) and low water activity (e.g., the spray drying stage in milk powder production). The reaction consumes L-Arginine and produces melanoidins, aldehydes, and other substances, which not only inactivate L-Arginine but also affect the color (browning) and flavor (burnt taste) of dairy products.

In contrast, low-temperature processing techniques (e.g., pulsed electric field, ultraviolet sterilization in cold sterilization technologies) cause minimal damage to L-Arginine, maintaining a retention rate of over 95%. These techniques are more suitable for dairy products requiring high thermosensitive nutritional components (e.g., low-temperature yogurt, fresh milk).

2. System pH: Alkaline Environment Accelerates Degradation, Acidic Environment Enhances Stability

The pH of dairy products (typically 4.0–7.5, varying by category) directly affects the chemical stability of L-Arginine. As a basic amino acid (isoelectric point pI≈10.76), L-Arginine exists in different forms (protonated or free) under different pH conditions, resulting in significant differences in reaction activity:

Alkaline environment (pH＞7.0): Unfermented fresh milk (pH≈6.5–6.8, near neutral to slightly alkaline), cream (pH≈6.0–7.0), or modified milk added with alkaline additives (e.g., sodium bicarbonate) provide conditions where the guanidino and amino groups of L-Arginine tend to be in a free state, with enhanced chemical activity and increased susceptibility to deamination and oxidation. For example, in a simulated milk system with pH=8.0, the loss rate of L-Arginine after pasteurization (65℃, 30 minutes) is 8%–12% higher than that at pH=6.0; long-term storage (1 month at 4℃) further accelerates the degradation rate in an alkaline environment.

Acidic environment (pH=4.0–5.5): In fermented dairy products (yogurt pH≈4.0–4.5, fermented milk beverage pH≈4.2–4.8), the amino and guanidino groups of L-Arginine are protonated (forming -NH₃⁺), increasing molecular polarity and chemical stability, and reducing susceptibility to degradation. Studies show that in a yogurt system (pH=4.3), the retention rate of L-Arginine remains over 90% after 30 days of storage at 4℃, significantly higher than in neutral milk systems. However, if the pH is too low (＜4.0, e.g., high-acidity fermented milk), a small amount of cyclization may be triggered to generate pyrroline-5-carboxylic acid, but the overall degradation degree is much lower than in alkaline environments.

3. Oxidation: Synergistic Destruction by Metal Ions and Oxygen

Naturally occurring metal ions (e.g., Fe²⁺, Cu²⁺, mainly from raw milk, processing equipment, or packaging materials) and residual oxygen in dairy products damage the structure of L-Arginine through oxidation, a process particularly prominent during storage:

Metal ion-catalyzed oxidation: Transition metal ions such as Fe²⁺ and Cu²⁺ act as catalysts to activate oxygen or peroxides (e.g., hydroperoxides from milk fat oxidation) in dairy products, generating reactive oxygen species (e.g., hydroxyl radicals ・OH, superoxide anions O₂⁻). These radicals attack the C-N bonds in the guanidino group of L-Arginine, causing guanidino group cleavage and producing inactive products such as L-Ornithine and urea. They may also break peptide bonds (if L-Arginine exists in peptide form), further reducing its bioavailability. For example, in whole milk powder without added antioxidants, the loss rate of L-Arginine can reach 10%–15% after 3 months of room-temperature storage due to Fe²⁺ released from milk fat oxidation, while the loss rate in low-fat or skim milk powder can be reduced by 30%–40%.

Direct oxidation by oxygen: Under aerobic storage conditions (e.g., dairy products with poor packaging tightness or repeated exposure to air after opening), the amino group of L-Arginine is easily directly oxidized by oxygen, generating imino acids or ketonic acids, accompanied by slight yellowing. This oxidation is accelerated at high temperatures (above 25℃) and under light (especially ultraviolet radiation). For example, the retention rate of L-Arginine in transparent bottled modified milk containing L-Arginine can drop to below 75% after 1 week of storage under direct sunlight.

4. Interaction with Proteins and Other Components: Binding Limits Activity

Dairy products are rich in macromolecular proteins such as casein and whey protein (e.g., α-lactalbumin, β-lactoglobulin). L-Arginine may form non-covalent bonds (e.g., electrostatic interactions, hydrogen bonds) with these proteins. Although this does not directly destroy its chemical structure, it causes "inactivation" (inability to be directly absorbed and utilized by the human body):

Electrostatic binding: L-Arginine carries a positive charge (especially after protonation in acidic environments), while casein (isoelectric point pI≈4.6) carries a negative charge in acidic systems such as fermented milk. The two easily form "L-Arginine-casein complexes" through electrostatic interactions. These complexes reduce the free concentration of L-Arginine; although L-Arginine can be released through protease decomposition during human digestion, absorption is delayed. During processing (e.g., homogenization), shear forces may cause complex aggregation, indirectly increasing the risk of L-Arginine oxidation.

Interaction with other additives: If dairy products contain organic acids (e.g., citric acid, lactic acid, used to adjust flavor or pH) or vitamin C (ascorbic acid), reactions with L-Arginine may occur. For example, vitamin C generates dehydroascorbic acid during oxidation, which may undergo condensation with the amino group of L-Arginine to form Schiff bases, reducing the activity of both; high-concentration organic acids may exacerbate the cyclization of L-Arginine, especially during heating.

II. Stability Performance in Typical Dairy Products

Significant differences in processing techniques and system characteristics (e.g., pH, fat content, water activity) among different dairy products lead to distinct stability performances of L-Arginine. The following are specific conditions for several typical products:

1. Liquid Dairy Products: UHT Milk Has Lower Stability Than Pasteurized Milk; Fermented Milk Is More Stable

Fresh milk (pasteurized): Pasteurization uses low temperatures (60–85℃, several minutes), causing minimal damage to L-Arginine, with a retention rate typically reaching 85%–95%. However, if the raw milk has high metal ion content (e.g., from contaminated milking equipment) and is stored at high temperatures (＞10℃), L-Arginine may lose 5%–10% due to oxidation during the shelf life (usually 7–15 days).

UHT pure milk/modified milk: Ultra-high temperature instantaneous treatment (135–150℃, 2–6 seconds) results in short sterilization time, but high temperatures still cause deamination and slight Maillard reactions of L-Arginine, reducing the retention rate to 75%–85%. For modified milk (which may contain lactose, stabilizers, etc.) with alkaline pH (e.g., added with sodium bicarbonate to adjust taste), the loss rate of L-Arginine increases by a further 5%–10%; products using aseptic cold filling have no secondary contamination afterward, controlling L-Arginine loss within 5% during storage (6 months at room temperature).

Fermented milk (yogurt, kefir): Lactic acid bacteria produce acid during fermentation, lowering the system pH to 4.0–4.5, which significantly enhances the stability of L-Arginine. Even if a small amount of L-Arginine is lost during pasteurization before fermentation, the retention rate of L-Arginine in the fermented system still reaches 80%–90%. In addition, the metabolic activity of lactic acid bacteria may slightly degrade L-Arginine (e.g., converting it to citrulline), but the degradation amount is usually ＜5%, and some studies suggest this metabolite (citrulline) still has certain physiological value; meanwhile, the binding between whey protein and L-Arginine in fermented milk is weak, with a higher proportion of free L-Arginine, making it more easily absorbed by the human body.

2. Milk Powder Products: Significant Impact of Spray Drying; Oxidation and Moisture Absorption Prevention Required During Storage

The core step in processing milk powder (whole-fat, skim, modified milk powder) is spray drying (usually inlet air temperature 150–200℃, outlet air temperature 70–90℃). High-temperature airflow causes Maillard reactions (binding with lactose) and deamination of L-Arginine, resulting in a retention rate of 70%–85%. Among these, whole-fat milk powder has lower retention rate (5%–10% lower than skim milk powder) due to free radicals from fat oxidation exacerbating L-Arginine oxidation; milk powder added with antioxidants (e.g., vitamin E, rosemary extract) can control the oxidative loss of L-Arginine within 3%.

During storage, the water activity (Aw) of milk powder is a key influencing factor: if packaging is poorly sealed and milk powder absorbs moisture (Aw＞0.3), L-Arginine degradation and oxidation accelerate. Under room temperature (25℃) and high humidity (relative humidity＞60%), the loss rate of L-Arginine can reach 15%–20% after 3 months of storage; milk powder stored in vacuum packaging or nitrogen-filled packaging under low temperature (＜20℃) and dry conditions maintains an L-Arginine retention rate of over 80%.

3. Milk Fat Products (Cream, Butter): Stability Affected by Fat Oxidation and pH

Cream (whipping cream, light cream) and butter mainly consist of milk fat (content 80%–99%). As a water-soluble additive, L-Arginine is mainly dispersed in the aqueous phase (small amount of water) of the product. These products typically have a pH of 6.0–7.0 (near neutral), where L-Arginine tends to be in a free state with high chemical activity; Fe²⁺ and Cu²⁺ from milk fat oxidation catalyze its oxidative degradation. For example, cream without added antioxidants has an L-Arginine loss rate of 10%–15% after 1 month of refrigerated storage (4℃); when heated at high temperatures (e.g., in baking) above 100℃, L-Arginine undergoes rapid deamination, with the loss rate instantly increasing to 20%–30%.

III. Core Strategies to Enhance the Stability of L-Arginine in Dairy Products

To address the aforementioned influencing factors, L-Arginine degradation and loss can be reduced, and its activity and nutritional value maintained, through optimizing processing techniques, regulating system environment, and adding protectants:

1. Optimize Processing Techniques: Reduce Thermal Damage and Oxidation Triggers

Adopt low-temperature sterilization technology: For dairy products requiring high thermosensitivity (e.g., low-temperature yogurt, fresh milk), prioritize pasteurization, pulsed electric field (PEF), high-pressure homogenization, and other non-thermal or low-temperature sterilization techniques instead of UHT treatment, which can increase the L-Arginine retention rate by 10%–20%.

Control spray drying parameters: During milk powder production, appropriately lower the inlet air temperature (e.g., from 180℃ to 160℃) and increase the outlet air temperature (e.g., from 70℃ to 80℃) to shorten the residence time of L-Arginine in the high-temperature zone; simultaneously reduce milk powder moisture content (Aw＜0.25) to lower the risk of Maillard reactions and oxidation.

Avoid repeated processing and heating: Secondary heating of dairy products (e.g., reheating modified milk, repeated melting of cream) exacerbates L-Arginine degradation and should be minimized during production and use.

2. Regulate System Environment: Stabilize pH and Inhibit Oxidation/Binding Reactions

Adjust system pH to weakly acidic: For dairy products with adjustable pH (e.g., modified milk, milk beverages), add a small amount of organic acids (e.g., lactic acid, citric acid) to control pH at 5.0–6.0, keeping L-Arginine in a protonated state to enhance chemical stability; for fermented milk, control fermentation time (avoid over-fermentation leading to pH＜4.0) to balance the stabilizing effect of acidic environment and the risk of cyclization.

Reduce contact with metal ions and oxygen: During processing, use stainless steel (304 or 316) equipment, avoid copper or iron pipes and containers to reduce metal ion contamination; use barrier materials (e.g., aluminum foil composite film, light-proof glass bottles) for product packaging, and adopt vacuum or nitrogen-filled packaging to isolate oxygen and light—especially suitable for milk powder and liquid milk products.

3. Add Protectants: Build "Barriers" to Reduce Degradation

Add antioxidants: Incorporate food-grade antioxidants (e.g., vitamin E, sodium ascorbate, rosemary extract) into dairy products to scavenge reactive oxygen species in the system and inhibit oxidative degradation of L-Arginine. For example, adding 0.02%–0.05% vitamin E to whole milk powder can reduce the loss rate of L-Arginine to below 8% after 3 months of room-temperature storage.

Use encapsulation technology: Microencapsulate L-Arginine with materials such as cyclodextrin (e.g., β-cyclodextrin) and whey protein concentrate (WPC) to form a "protective layer", reducing direct contact with lactose, metal ions, and oxygen, while minimizing high-temperature damage. Studies show that the retention rate of β-cyclodextrin-encapsulated L-Arginine after UHT treatment is 15%–20% higher than that of the unencapsulated group.

The stability of L-Arginine in dairy products is the result of interactions between processing techniques, system environment, and components. Core challenges include high-temperature-induced degradation (deamination, cyclization, Maillard reaction), oxidation triggered by alkaline environments and metal ions, and binding reactions with proteins. Among different dairy products, fermented milk has the best stability due to its acidic environment, while UHT milk and whole-fat milk powder have poor stability due to high temperature, oxidation, and other factors. By optimizing processing techniques (low-temperature sterilization, controlling drying parameters), regulating system pH and packaging (acidic environment, oxygen/light isolation), and adding antioxidants and encapsulation protection, the retention rate of L-Arginine can be effectively improved, ensuring its nutritional and functional value in dairy products.