Technical Scheme for Membrane Concentration in Dairy Processing

1.0 Design Basis and Objectives

• Processing Target: Dairy feedstocks such as raw milk, skim milk, and whey.

• Core Objectives:

  1. Energy Saving and Consumption Reduction: Utilize low-energy membrane separation technology to replace or partially substitute high-energy thermal evaporation, thereby reducing the energy cost per unit of solids produced.

  2. Quality Preservation: Operate at low temperatures (typically <50°C) to maximally preserve the native conformation, biological activity, and flavor compounds of proteins (especially whey proteins and immunoglobulins), avoiding thermal denaturation.

  3. Component Separation and Standardization: Achieve precise separation and recombination of milk components such as milk protein, lactose, and minerals to produce high-value-added products (e.g., Milk Protein Concentrate, Demineralized Whey Powder).

  4. Improved Yield and Flexibility: Enhance the recovery rate of dry matter and enable flexible production adjustments through modular design.

2.0 Core Membrane Technology Selection

Based on the molecular weight and separation precision of the target components, the following four primary membrane technologies are employed:

3.0 Typical Process Flow Design

The system design follows the logic of "Pretreatment → Membrane Separation → Post-treatment". Process combinations vary depending on different product targets. The core process framework is illustrated in the diagram below:

Step-by-Step Process Explanation:

Stage 1: Raw Material Pretreatment

1. Standardization: Adjust the fat/protein ratio of the raw milk to meet product specifications.

2. Pasteurization: Apply heat treatment (e.g., 72-85°C for 15 seconds) to destroy pathogens, ensuring food safety and the biological stability of the membrane system.

3. Skimming/Clarification: Obtain skim milk and cream via centrifugal separation, or remove fine particles from whey.

Stage 2: Membrane Separation Core Section (Combined based on product orientation)

• Scheme A: Milk Protein Concentrate/Isolate Production

  1. MF for Clarification/Sterilization: Subject skim milk to MF (0.1-1.4 μm) to retain bacteria and spores, achieving cold sterilization.

  2. UF/Diafiltration Concentration: Use UF membranes (10-30 kDa) to concentrate proteins. Employ diafiltration (DF) by continuously adding deionized water during concentration to continually wash out lactose and ash into the permeate, significantly increasing the final product protein purity (can exceed 85%).

• Scheme B: Whey Utilization

  1. MF Clarification: Remove residual casein fines and fat from sweet or acid whey.

  2. UF Concentration: Concentrate whey proteins to produce Whey Protein Concentrate (WPC) with varying protein contents.

  3. NF Demineralization: Apply NF to the UF permeate or products requiring partial demineralization to selectively remove monovalent salts (Na⁺, K⁺, Cl⁻), improving product flavor and functional properties.

• Scheme C: Liquid Milk/Condensed Milk Pre-concentration

  1. RO Pre-concentration: Subject standardized whole milk or skim milk to RO treatment, increasing total solids from approximately 12% to 22-25% at ambient temperature, significantly reducing energy consumption and thermal load in subsequent evaporation steps.

Stage 3: Post-treatment and Finishing

1. Standardization: Make final adjustments to the fat, protein, etc., content of the membrane concentrate according to the final product specifications.

2. Sterilization: Apply Ultra-High Temperature (UHT) processing or secondary pasteurization to ensure commercial sterility.

3. Homogenization: Apply homogenization to fat-containing products to prevent cream separation and enhance product stability.

4. Drying: For powdered products, the high solids content of the membrane concentrate significantly reduces the energy consumption and cost of subsequent spray drying.

4.0 Key Design Parameters and Operational Control

1. Membrane Material Selection: Prioritize hygienic-grade, cleanable membrane materials such as Polyethersulfone (PES) or hydrophilic Polyvinylidene Fluoride (PVDF), whose surface properties help mitigate protein adsorption and fouling.

2. Optimization of Operational Parameters:

   • Temperature: Typically controlled within the range of 10-50°C. Lower temperatures favor quality preservation, while moderately higher temperatures reduce viscosity and increase flux.

   • Transmembrane Pressure (TMP): Optimize operating pressure to balance flux and selectivity, avoiding excessive compaction of the fouling layer.

   • Cross-flow Velocity: Maintain a sufficiently high cross-flow velocity (typically >4 m/s) to generate turbulence and mitigate concentration polarization and fouling on the membrane surface.

3. Fouling Control and Cleaning:

   • Establish a regular Clean-In-Place (CIP) protocol. A typical cleaning sequence includes: Water rinse → Alkaline wash (e.g., 0.5-1.0% NaOH, 50-55°C) to remove proteins and fats → Acid wash (e.g., 0.5-1.0% HNO₃ or citric acid, 40-45°C) to remove inorganic scale → Final water rinse to neutrality.

   • Initiate the cleaning procedure based on an increase in TMP or a decline in flux.

5.0 Scheme Advantages and Techno-economic Summary

1. Quality Advantage: Low-temperature physical process maximally preserves nutritional components and functional activities, resulting in superior product flavor.

2. Energy Efficiency Benefit: The energy consumption for RO pre-concentration is only 1/5 to 1/10 that of thermal evaporation, leading to a significant reduction in overall process energy consumption.

3. Product Value Addition: Enables precise component separation for producing high-value-added ingredients like high-purity milk proteins and demineralized whey, expanding the product portfolio.

4. Environmental & Yield Benefits: No phase change reduces carbon emissions; the closed system minimizes loss of aromatic compounds, improving product yield; the permeate (primarily water and lactose) can be utilized as a resource.

Summary: The membrane concentration process outlined in this scheme represents a clean, efficient, and precise separation technology suitable for the modernization of the dairy industry. Its successful implementation relies on meticulous process design tailored to specific raw materials and product targets, strict control of operational parameters, and a scientific membrane maintenance strategy. Comprehensive pilot-scale validation is recommended prior to large-scale investment to determine the optimal process route and economic benefits.