Dairy processing – an overview
Practical Action
Animals should be inspected each day for disease (especially for mastitis) to prevent
bacteria from an infected animal being passed by hands to healthy animals or into the
milk.
Personal hygiene rules are observed (see Technical Brief: Hygiene and safety rules in
food processing).
Milk is kept in containers that are covered and it is cooled as quickly as possible.
Milk is transported to collection centres in shortest time possible.
Incoming milk in the dairy should be cooled to below 4oC (see below). It should be tested to
ensure that it is fresh, safe, has been properly handled, and has not been adulterated with added
water.
The level of quality assurance (QA) in processing depends on the risk associated with the
particular product and is assessed by risk (or hazard) analysis using the HACCP (Hazard Analysis
Critical Control Point) system (details in How to HACCP). Dairy manufacturers should carry out a
hazard analysis for each of their products (see Technical Briefs for pasteurised milk, soured
milks and yoghurt, butter and ghee, cheese making, ice cream and dairy confectionery).
Appearance
Fresh milk should be slightly viscous, white with a yellowish tinge depending on the fat content
(the fat content varies with the species of animal, the breed and the time of year). There should
not be discoloration, lumps in the milk or a high viscosity. It should have a bland, slightly sweet
taste and a pleasant smell. If a spoon is dipped into a sample of milk and slowly withdrawn,
there should be no strings or threads of milk.
Density
The density of milk can be measured using an instrument known as a ‘lacto-densimeter’ (or
‘lactometer’) (Figure 2). If a low reading is obtained, it indicates that the milk may have been
diluted with water.
Bacteria
A microbiological test that is appropriate for small-scale
dairy processors is the ‘methylene blue’ test. Other tests
are either too expensive, or require the specialist equipment
and skills at a university Food Science department or Bureau of
Standards.
The method is:
1. Mix a 20 ml sample of milk to ensure that the cream is evenly
mixed in.
2. Place in a test tube and add 0.5 ml of dye
solution (the dye is a 0.0075% solution of
methylene blue1). The liquid dye should be
stored in a foil-covered bottle in a cool, dark
Figure 2: Lacto-densimeter with thermometer
Photo: Courtesy of Ningbo Goodwill Foreign Trade
Co.Ltd,
place away from sunlight, and can be kept for 2 months.
3. Seal the tube with a stopper and mix the contents by inverting the tube.
4. Hold in a covered water bath at 36-38oC, so that the sample remains in the dark.
5. Record how long it takes for the dye to fade (as an indication, if the colour remains for 30
minutes2 the milk has a satisfactory quality.
1 Methylene blue dye is a powder that may be available from pharmacies or specialist suppliers in the capital city.
The dye is made by dissolving the powder in distilled water.
2 Different times may be used in local dairy regulations and these should be consulted at the Bureau of Standards
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