Determination of N/Protein in Milk by Kjeldahl Method (CAT#: STEM-EA-0043-ZJF)

Introduction

Milk and dairy products play a role of primary importance in the human food and are essential to the balance and the adequacy of the food ration. The milk composition (water, fat, minerals, lactose and proteins) is an important parameter for its quality evaluation. The protein fraction includes caseins (80%) and whey proteins (20% such as lactalbumin, lactoglobulin) and it is of interest, not only from the nutritional point of view, but also from a technological one. In fact, milk proteins have particular chemical properties which contribute to create typical sensory features of dairy products (i.e. cheese and yogurt), influencing the whole food processing.
Kjeldahl is nowadays the most used method for determining nitrogen and protein contents in foods and feeds , thanks to the high level of precision and reproducibility and to its simple application. The modern Kjeldahl method consists in a procedure of catalytically supported mineralization of organic material in a boiling mixture of sulfuric acid and sulfate salt at digestion temperatures higher than 400 °C. During the process the organically bonded nitrogen is converted into ammonium sulfate. Alkalizing the digested solution liberates ammonia which is quantitatively steam distilled and determined by titration.