Determination of fat in milks and milk products: Gerber Method.

The quality of drinking milk as well as its derivatives (cheese, creams, yoghurts, etc.) depends firstly on the quality of the raw material, but also on the conditions of transport, handling and conservation.

The determination of the fat content in milk is essential in quality control for:

• Detecting fraudulent adulterations that can modify the organoleptic properties and even alter the health of consumers.
• Determining the nutritional content.
• Obtaining products derived from milk.

The spectrum covered by this technique goes beyond cow’s milk, as it is also used for the quantification of fat in sheep, goat, and alpaca milk.

It is also carried out with other types of plant materials such as almonds, soybeans, oats, however it requires an experimental phase to adapt the method to the raw material.

About the Gerber Method.

The Gerber method consists of separating and measuring the fat contained in milk or its derivatives. It was developed in 1892 and is still used for its speed and accuracy.

The fat percentage scale of the products to which this method is applied is between 0 and 16% fat.

The products to which fat determination is applied are milk, yogurt, cheese, ice cream.

All types of milk, whole, skimmed, or semi-skimmed, have a percentage of fat, so we can thus assess whether the milk has been adulterated with other types of fats (usually fats of vegetable origin).

The measurement process is carried out in a device called a butyrometer in which a reaction is generated to obtain a direct reading of the percentage of fat.

For each type of milk and its derivatives, there are different types of butyrometers on the market, depending on the established fat ranges for that type of food. Thus, there are butyrometers for goat cheese, sheep’s milk, cow’s yogurt, etc.

Equipment and reagents needed:

Equipment:

• To favour the deposit of the fat in the upper part of the butyrometer, a specific centrifuge with heating is needed, like the Lacter 21.
• A Bain-Marie with adjustable temperature of at least 100ºC

Reagents for the determination of fat by the Gerber Method:

• Sulfuric acid (90%)
• Amyl alcohol

Given the aggressiveness of the samples, their handling in extraction hoods or laminar flow cabinets is recommended.

Materials:

• Glass pipettes and absorption bulbs.
• Butyrometers for the type of dairy, according to its origin.

Process:

• Partial digestion of the sample:

10 ml of 90% sulfuric acid must be placed at the bottom of a butyrometer, keeping it in a vertical position so that it does not stir.

Using a pipette, 11 ml of milk needs to be added through the walls of the tube in order to form a layer of milk on top of the previously added sulfuric acid.

Next, 1 ml of amyl alcohol must be added.

To start the digestion, a stopper must be placed in the mouth of the butyrometer, then it must be wrapped in a towel and shaken vigorously to facilitate the mixing of the different components of the sample.

It should be able to be perceived that the sample produces heat as a result of the reaction that occurs in the tube, hence the recommendation to use a towel to wrap the tube.

• Separation:

The butyrometer must be put in a bain-marie (water bath) for 10 minutes at 65 ºC.

Subsequently, the tube needs to be placed in a centrifuge at 65ºC for 2 minutes at 1,100 RPM.

The next step is to put the butyrometer back in a rack inside the bath in a vertical position for 5 more minutes at 65ºC.

• Reading:

The reading of the percentage of fat is made directly on the scale of the upper part of the tube, after extracting it from the centrifuge.

To do the measurement on the tube, the inaccuracy of the reading must be considered, the following image should be taken as a reference; the lower value of the line on which the meniscus rests must always be taken.

This value gives us the measurement based on the size of the fat layer.

REF Norm ISO 2446:2008 Milk determination of fat content.