L8: Homogenization

Milk Processing Overview

• Preheating Process

  • Function: Utilize outgoing pasteurized milk to preheat incoming cold raw milk.

  • Location in Process: Occurs before homogenization.

  • Typical Temperature: Post-preheating temperature is approximately 58 to 60 degrees Celsius.

• Homogenization Unit

  • Purpose: Reduces size of fat globules in milk to create a stable emulsion.

  • Benefits of Elevated Temperature:

  • Milk fat softens and is in a liquid state.

  • Easier to force liquid fat through small gaps in the homogenization unit, saving energy compared to processing solid fat.

• Pasteurization Process

  • Post-Homogenization: Milk is pasteurized to a final temperature of 161 or 72 degrees Celsius.

  • Objective: Deactivates lipase enzymes that might attack milk fat.

Homogenization Details

• Two-Stage Process:

  • High Pressure First Stage: Breaks down fat globules to smaller sizes.

  • Low Pressure Second Stage: Ensures complete separation of smaller fat globules.

• Parameters:

  • High Pressure: 2,500 PSI (first stage).

  • Low Pressure: 500 PSI (second stage).

  • Elevation of Temperature: Usually set around 60 degrees Celsius post-preheating.

Stokes' Law in Emulsion Stability

• Stokes' Law: Describes the rate at which a particle rises in a fluid.

  • Formula Components:

  • Rate of rise is dependent on:

    • Gravity ($g$)

    • Density of oil ($d_{f}$)

    • Density of water ($d_{w}$)

    • Viscosity of water ($
      u$)

    • Diameter of the spherical particle ($d$)

  • Implication: Larger $d$ results in a faster rate of rise.

  • If fat globule size decreases, the rate of rise also decreases, causing slower separation.

Conceptual Understanding of Forces in Homogenization

• Conceptual Exercise:

  • Consider fat globules of various sizes (large, medium, small). If released simultaneously, larger globules rise faster due to higher diameter, illustrating the dependency on particle size.

  • If fat globules are too small, they will rise much slower, effectively making their rise negligible within practical time limits (e.g., shelf life).

Homogenization Valve and Mechanics

• Function: Acts similarly to a nozzle, controlling the gap size through which milk is forced under high pressure.

• Process Stages Summary:

  • Before Homogenization: Fat particles are of various sizes.

  • Post First Stage: High-pressure treatment reduces large globules to smaller sizes; some may combine again.

  • Post Second Stage: Guarantees complete separation of fat particles and stabilizes the emulsion.

USPH Index and Quality Control

• USPH Index Overview:

  • Developed by the U.S. Department of Public Health to assess the effectiveness of homogenization.

  • Measured by comparing fat content in the top 10% of a milk sample with the remaining 90%: A homogeneous product should show no significant separation of fat content.

• Case Study: A manufacturer faced issues with cream layer formation despite normal USPH readings due to the omission of the second low-pressure homogenization stage, leading to fat globule clumping over time.

Changes in Fat Globule Sizes

• Before Homogenization: Fat globule sizes range from 1 to 20 microns, average about 3-4 microns.

• After Homogenization: Sizes drop to between 0.1 to 4 microns with an average below 2 microns.

• Impact on Surface Area:

  • Breaking a large fat globule into smaller ones increases total surface area, reducing the protective effect of membrane materials.

  • Additional proteins can adhere to the surface of smaller globules to form a protective layer, ensuring stability despite reduced surface protection due to increased surface area.

Flow of Milk in the System

• Process Schematic:

  1. Cold raw milk is held in massless tank (constant level).

  2. Milk is pumped to plate heat exchanger (regenerative heat sections), where it is preheated.

  3. Optional vacuum deaerator can remove odors.

  4. Milk is sent to a cream separator to yield cream and skim milk.

  5. Cream undergoes homogenization while skim milk is mixed back for standardization.

  6. Mixed milk goes to plate heat exchanger again and is pasteurized at 161 degrees Celsius.

  7. Flow diversion valve ensures proper temperature is maintained; returns milk to the beginning if necessary.

  8. Milk is cooled to 2-4 degrees Celsius before packaging.

Conclusion of the Lecture

• Final Notes: Understanding the entire homogenization and pasteurization process is crucial for ensuring product quality, preventing separation, and maximizing shelf life.