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Milk Processing: Clarification, Pasteurisation & Homogenisation

This presentation explains the key steps of milk processing, including clarification, pasteurisation and homogenisation. This topic is taken from the course FN-201: Food Science andd microbiology.Useful for Food Science, Nutrition and Public Health students.

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Food Science Overview•
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Milk processing Aktarafun Zannat Assistant Professor (Food and Nutrition) Govt. College of Applied Human Science, Dhaka
Milk Processing Operations  Once the milk has been received at the milk processing centre, or dairy, it should be immediately strained, cooled & processed so as to prevent any bacterial growth.  There are 3 operations of milk processing- Clarification Pasteurisation Homogenisation
Clarification  Milk is passed through a centrifugal clarifier.  The speed of the clarifier is adjusted so that cream is not separated.  Dirt, filth, and cells from the udder are removed.  Some bacteria are also eliminated.  The resulting clarified milk is ready for pasteurisation.
Pasteurisation  Named after Louis Pasteur.  Involves heating milk to a high temperature that destroys pathogenic microorganisms (e.g., tuberculosis, fever-causing organisms).  Does not affect composition or properties of milk significantly.  Must be followed by immediate cooling to prevent microbial regrowth.  Considered essential in producing butter, ice cream, cheese.  Also inactivates natural enzymes like lipase.
Methods of Pasteurisation 1. Holding or Batch System  Milk/cream heated to 65°C for at least 30 minutes.  Sometimes 68.3°C for 20 minutes.  Requires careful control of time and temperature.  Effective against tuberculosis organisms, but Q fever organism (Coxiella burnetii) requires at least 63°C for 30 minutes.
2. High Temperature Short Time (HTST) / Continuous System  Milk rapidly heated to 72°C for 15 seconds.  Followed by quick cooling.  Advantage: no cooked flavour, cream line remains intact.
3. Ultra High Temperature (UHT) System  Milk heated to 93.4°C for 3 seconds OR 149.5°C for 1 second.  Provides complete pasteurisation.  Widely used in dairy industry.  Shelf life longer than other methods.  Milk rapidly cooled to 7°C or lower.  Disadvantage: milk has a more pronounced cooked flavour.
Role of Phosphatase in Pasteurisation  Raw milk contains alkaline phosphatase enzyme, used as an indicator of pasteurisation adequacy.  This enzyme is heat-sensitive and is destroyed at temperatures similar to those required for proper pasteurisation.  Presence of phosphatase in pasteurised milk = incomplete pasteurisation.  Test principle: Phosphatase liberates phenol from phenyl phosphate → reacts with indicator produces deep blue colour. →  No blue colour after incubation = successful pasteurisation.  Test is highly sensitive (detects even 0.1% raw milk contamination).
Effects of Pasteurisation 1. Nutritive Value  Minimal effect overall.  Slight loss of heat-labile vitamins (e.g., thiamine, ascorbic acid).  Whey proteins denature slightly; minerals unaffected.  Pasteurised milk forms finer curd when digested. 2. Flavour  No objectionable “cooked” flavour.  Natural flavouring compounds remain unchanged.
3. Micro-organisms  Pasteurised milk is not sterile.  Pathogens destroyed: Mycobacterium tuberculosis, Salmonella typhi, Corynebacterium diphtheriae, Brucella.  Non-pathogens and spores may remain.  About 99% bacteria, yeasts, and moulds destroyed.  Heat-resistant spores may survive. 4. Enzymes  Phosphatase and lipase inactivated, preventing quality deterioration.
 Pasteurised milk is not sterile must be quickly cooled to → prevent multiplication of surviving bacteria.  After pasteurisation, milk is tested for temperature, fat, specific gravity, solids-not-fat, acidity, and bacterial count.  Shelf life: about one week or more if kept cool; at room temperature, it spoils in a day.
Homogenisation  Homogenisation = process of making stable emulsion of milk fat and serum.  Achieved by passing warm milk/cream under high pressure through a small orifice.  This breaks fat globules into very small globules (0.1–20 µm diameter).  The number and surface area of fat globules increase.  Newly formed fat globules are coated with absorbed protein + phospholipids, preventing reuniting.  Prevents cream separation (no rising of cream layer).  Results in stable milk emulsion.
Effects on Milk  Homogenised milk has creamier texture, bland flavour, and whiter appearance.  Provides greater whitening power when added to coffee/tea.  Produces soft curd on coagulation easily digested. →  In evaporated milk & ice-cream: prevents fat separation → smoother texture.  Accelerates action of lipase risk of rancidity (so done before → pasteurisation to inactivate lipase).
Freezing of Milk/Cream  When frozen at a slow rate, the protein film around fat globules (which acts as an emulsifier) is weakened and ruptured.  This causes fat globules to coalesce (merge together).  Freezing also disturbs the dispersion of protein and calcium phosphate.  On thawing and standing, both protein and calcium phosphate settle out.  This leads to a reduction in the whiteness of milk.
References  Srilakshmi, B. (2019). Food Science (8th Edition). New Age International Publishers.
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