In this opportunity I would like to continue sharing information about the types of chemical analysis performed on whole milk to control the quality of the product, addressing on this occasion the determination of fat content by the Rose-Gottlieb method.
Source: Image designed by @yusvelasquez, contains a public domain image.
For the milk industry, as well as for our food, the quality of milk is very important, being the composition of milk one of its main control factors, and in this, milk fat plays an important role, since it determines not only the nutritional, physical and chemical properties of this food, but also of the products obtained from it, influencing also the purity and finally the price of the product.
And with respect to purity, it is a parameter of special value, since benefiting the fat content in milk can be an attractive practice to increase its price, so it is essential to be able to detect any adulteration that may cause a change in the nutritional value of the product and put people's health at risk. An excellent quality milk contains at least 3.5% fat and 3.2% protein, a fraudulent method to increase the yield of milk is to add water, and to mask the wateriness, urea, starches and glucose are incorporated, but this practice alters the solids and fat content of the milk. For this reason, standards such as Venezuelan COVENIN 903-1993 and 798-1994 require a minimum fat content of 3.2% for raw and pasteurized milk, respectively, which must be free of foreign substances.
It is for this reason that the determination of fat content, along with other parameters are of vital importance, in order to ensure that a sample complies with the established legal values. So let's review one of the methods used to determine this value and continue to share with the community the methods of chemical analysis of whole milk.
It is the lipid component of milk, which supplies half of the calories of this important food, and its content serves to typify and standardize dairy products, making the distinction between whole, semi-skimmed or skimmed.
Milk fat is found forming globules surrounded by a membrane of lipoprotein nature composed mainly of phospholipids and glycoproteins and whose nucleus consists mainly (95%) of triglycerides (TAG), and has a hydrophobic character. But in its composition we can also find other simple lipids such as monoglycerides, diglycerides and cholesterol esters, and some more complex ones such as tocopherols.
Milk fat has a high content of short and medium chain fatty acids, easily absorbable, which makes it different from other edible fats, constituting a source of immediate energy with a low tendency to be stored in adipose tissue.
Lipid composition of cow's milk. Source: image edited by @yusvelasquez, background image is public domain.
Basis of analysis
Fat in milk, as in any other food, can be determined by various methods, the most commonly used being direct extraction by means of a suitable solvent or indirect extraction by reacting it with an acid or a base.
The Roese-Gottlieb method is based on the extraction of fat with a mixture of ethyl ether and petroleum ether in the presence of a concentrated solution of ammonia and ethyl alcohol. The ammonia solution dissolves the proteins and milk fat, ethyl alcohol breaks the emulsion and combination of fats and proteins, avoiding the formation of a gelatinous mixture, the ethyl ether acts as a solvent for the fat, but to prevent it from also dissolving other components present in the aqueous phase, it is combined with petroleum ether to reduce the solubility of the aqueous phase in the ethyl ether. Finally, the fat is extracted, dried and subsequently determined by weighing.
The method described here is based on the gravimetric method of Roese Gottlieb.
Materials and reagents
Ammonium hydroxide (NH4OH)
95% ethyl alcohol
Special extraction tubes
Common laboratory volumetric equipment
- Adjust the temperature of the sample to 20 °C.
- Weigh 10 g of the sample to a dry and previously weighed Erlenmeyer.
Weighing of the sample. Source: @yusvelasquez.
- Add 1.5 mL of ammonium hydroxide (2 mL if the milk is very acidic).
- Add 10 mL of ethyl alcohol, mix well and shake for 30 seconds.
- Add 25 mL of ethyl ether and shake vigorously for one minute. Then add 25 mL of petroleum ether and shake again. A few drops of phenolthalein solution can be added if desired to better visualize the separation of the two phases.
Add the solvents to the sample. Source: @yusvelasquez.
- Transfer to the extraction tube. Depending on the type of extraction tube, it can be centrifuged or left to stand for 30 minutes.
The mixture is added to a decanting tube and allowed to settle. Source: @yusvelasquez.
- Decant the solvent phase (transparent layer) to a previously tared Erlenmeyer.
- To repeat the extraction of the remaining aqueous phase in the tube two consecutive times, using only 5 mL of alcohol and 15 mL of ethyl ether in the second one, and 15 mL of each solvent in the third one.
- Evaporate the contents of the Erlenmeyer over a heating plate to complete dryness, then dry in the oven at 102 °C for one hour.
Dry the sample in an oven. Source: @yusvelasquez.
- Let the Erlenmeyer cool in a desiccator and repeat the drying until a constant weight is observed. This residue represents the amount of fat in the sample.
- Calculate the percentage of fat contained in the sample by difference of weight between the empty Erlenmeyer and with the fatty residue.
Expression of results
The fat content is expressed in % w/w and is determined by the following expression:
P = mass of the empty Erlenmeyer flask in grams.
P1 = mass of the Erlenmeyer plus the fatty residue, in grams.
M = mass of the sample, in grams.
The following image shows some average values in measurements made on samples of pasteurized milk sold in markets.
Percentage of fat in whole milk samples. Source: @yusvelasquez.
Fat content is a very important parameter to determine both the quality of milk and its nutritional value, being also very important its determination in the dairy industry to quantify the quality of the product and detect any adulteration.
Although there are other methods to determine fat content, the Roese-Gottlieb method is one of the most accurate and routine methods used in food chemical analysis laboratories, and can be applied to fluid milk (whole, condensed or evaporated) and its derivatives (butter, cheese, yogurt). Therefore, the described method is essential to ensure the quality of this product.
Well friends, this is the end of this post, I hope you liked the information presented. Remember the importance of analytical methods in food quality control. See you next time!
- Calderón-Rangel, A., Rodriguez, V., Martinez, N. (2013). Determinación de adulteraciones en leches crudas acopiadas en procesadoras de quesos en Montería (Córdoba). Disponible en ORINOQUIA - Universidad de los Llanos - Villavicencio, Meta, Colombia. Vol. 17 - No 2.
- COVENIN 903-1993: Leche cruda.
- COVENIN 798-1994: Leche completa y sus derivados.
- Calvo, M.V., Castro-Gomez, M., García-Serrano, A., Rodríguez, L.M., Juárez Iglesias, M., Fontecha, A. (2014). Grasa láctea: una fuente natural de compuestos bioactivos. Disponible en Alimentacion, Nutricion y Salud. Vol. 21, N.º 3, pp. 57-63
- Central lechera asturiana (2019). Grasa láctea.
- COVENIN 931-1997. Leche y sus derivados. Determinación de grasa por el método Roesse Gottlieb (2da. Revisión).