The stability of L-leucine in dairy products

L-leucine exhibits good stability in dairy products and can withstand conventional processing and storage conditions. However, its stability may be affected by four key factors—processing temperature, pH value, raw material compatibility, and storage environment—leading to reduced solubility, Maillard reaction, or slight degradation. Process optimization is required to enhance its stability, with a detailed analysis as follows:

I. Core Stability Performance of L-Leucine in Dairy Products

L-leucine (with a stable chemical structure, melting point of 337°C, and resistance to oxidation at room temperature) demonstrates the following key stability characteristics throughout the processing and storage cycle of dairy products:

1. Processing Stability

It can tolerate conventional dairy sterilization processes, including pasteurization (60–85°C for 15–30 minutes) and UHT sterilization (135–150°C for 2–5 seconds), with no significant degradation. High-performance liquid chromatography (HPLC) tests show that the retention rate of L-leucine after sterilization exceeds 95%. However, extreme high temperatures (e.g., baking at >180°C) may cause slight oxidation of amino groups, increasing the degradation rate to 5%–8%.

2. Storage Stability

Under light-proof, sealed, and room-temperature conditions (below 25°C), the half-life of L-leucine in dairy products can reach 6–12 months. For example, in modified milk stored at room temperature, the content decreases by <3% within 3 months. Conversely, storage in open containers, high temperatures (>30°C), or exposure to light may cause slow content decline (5%–10% decrease over 6 months) due to oxidation and moisture absorption.

3. Solubility Stability

L-leucine has moderate water solubility (2.4 g/100 mL water at 20°C). In low-fat/low-protein dairy products (e.g., skim milk beverages), direct addition may lead to fine particle precipitation—especially during low-temperature storage (<10°C), where solubility drops to 1.8 g/100 mL and the precipitation rate rises to 5%–8%. In high-protein dairy products (e.g., whey protein powder, full-fat yogurt), it can disperse with the help of protein emulsifying properties, resulting in a precipitation rate of <2%.

II. Key Factors Affecting the Stability of L-Leucine

1. Processing Temperature and Duration: High Temperature & Long Duration Trigger Maillard Reaction

Core Risk: The amino group (-NH₂) of L-leucine and lactose (aldose) in dairy products are prone to Maillard reaction under high temperatures (>80°C) and long durations (>30 minutes). This causes product browning, reduced L-leucine content, and formation of burnt-bitter substances.

Specific Performance: During UHT sterilization (135°C for 5 seconds), the Maillard reaction is mild (L-leucine loss <2%, no obvious browning). However, in baked dairy products (e.g., milk-flavored biscuits, grilled cheese, processed at 160–180°C for 10–15 minutes), L-leucine loss can reach 8%–12%, with light brown discoloration on the product surface.

2. pH Value of Dairy Products: Acidic Environment Reduces Solubility and Activity

Acidic Dairy Products (pH 3.5–4.5, e.g., yogurt, lactic acid bacteria beverages): Low pH causes protonation of L-leucine’s amino group (forming -NH₃⁺), reducing its water solubility (from 2.4 g/100 mL to 1.5 g/100 mL at 20°C) and leading to flocculent precipitation. Additionally, protonated amino groups may interact electrostatically with carboxyl groups of milk proteins (e.g., casein), affecting the intestinal absorption activity of L-leucine.

Neutral Dairy Products (pH 6.0–7.0, e.g., pure milk, modified milk): The pH is close to the isoelectric point of L-leucine (pI ≈6.0), resulting in optimal solubility and stability with no risk of precipitation or reaction.

3. Raw Material Compatibility: High Sugar & High Dietary Fiber Cause Interactions

High-Sugar Dairy Products (e.g., flavored milk, milk desserts, sugar content >10%): Sugars such as lactose and sucrose increase the substrate concentration for Maillard reactions. Under the same processing temperature, the loss rate of L-leucine is 3–5 times higher than in low-sugar products (sugar content <5%).

High-Dietary-Fiber Dairy Products (e.g., milk beverages with inulin or pectin, fiber content >3%): Hydroxyl groups (-OH) of dietary fiber form hydrogen bonds with amino groups of L-leucine, causing adsorption of L-leucine (adsorption rate up to 10%–15%). While this does not destroy its chemical structure, it reduces the content of free L-leucine and impairs nutritional efficacy.

4. Storage Environment: Light & Humidity Accelerate Oxidation and Moisture Absorption

Light (especially ultraviolet): Activates oxygen molecules in dairy products to form free radicals, which oxidize the isobutyl side chain of L-leucine, converting it into ketonic acid substances. The content decreases by 1%–2% per month.

High Humidity (>60%): L-leucine is prone to moisture absorption and caking (critical relative humidity ≈65%). This forms localized high-concentration areas in dairy products, accelerating the Maillard reaction. Meanwhile, caked L-leucine is difficult to digest and absorb.

III. Process Optimization Schemes to Enhance L-Leucine Stability in Dairy Products

1. Processing Optimization: Control Temperature and Adjust Addition Timing

Low-Temperature Addition: For high-sugar or acidic dairy products, adjust the addition timing of L-leucine from "before sterilization" to "post-sterilization cooling stage" (e.g., cooling to below 40°C). This reduces the Maillard reaction and increases the retention rate of L-leucine to over 98%.

Segmented Heating: For dairy products requiring high-temperature processing (e.g., cheese), use "low-temperature preheating (60°C to dissolve L-leucine) + high-temperature shaping (short heating at 120°C)" to avoid prolonged exposure of L-leucine to high temperatures.

2. Improve Solubility and Dispersibility: Use Carriers and Emulsifiers

Carrier Compound Formulation: Mix L-leucine with whey protein (1:2 ratio) or maltodextrin (1:3 ratio). The emulsifying property of whey protein and water solubility of maltodextrin enhance the dispersibility of L-leucine in low-fat/acidic dairy products, reducing the precipitation rate to <1%.

Add Emulsifiers: Add 0.1%–0.2% food-grade emulsifiers (e.g., glyceryl monostearate, sucrose esters) to liquid dairy products. These reduce the interfacial tension between L-leucine particles and water, preventing precipitation during low-temperature storage.

3. Storage Condition Control: Light-Proof, Sealed, and Temperature-Controlled

Packaging Selection: Use opaque aluminum foil composite packaging (e.g., Tetra Pak, aluminum-plastic film) to block ultraviolet light. For dairy products requiring refrigeration after opening (e.g., yogurt), add oxygen absorbers (e.g., iron-based oxygen absorbers) to reduce oxidation risks.

Storage Parameters: For room-temperature dairy products, control storage temperature ≤25°C and relative humidity ≤60%. For refrigerated dairy products (e.g., low-temperature yogurt), store at 2–6°C—this extends the storage half-life of L-leucine to over 12 months.

4. Raw Material Compatibility Adjustment: Reduce Conflicting Components and Optimize Formulas

Control Sugar and Fiber Content: Limit sugar content to <8% in high-sugar dairy products and fiber content to <2% in high-fiber dairy products to avoid excessive interaction with L-leucine.

Synergistic Addition of Stabilizers: Add 0.1%–0.3% sodium carboxymethyl cellulose (CMC) to acidic dairy products. Its colloidal network can encapsulate L-leucine particles to prevent precipitation without affecting nutritional activity.

The stability of L-leucine in dairy products can be effectively ensured through process and formula optimization, with the core being "avoiding high-temperature and long-duration processing, controlling acidic and high-sugar environments, and optimizing storage conditions." L-leucine exhibits the best stability in neutral, low-sugar dairy products stored at room temperature. For acidic, high-sugar, or high-temperature-processed products, measures such as low-temperature addition and carrier compounding are needed to reduce losses, ensuring its nutritional function and product quality.