Parallel evaporation after extraction of liquid dairy samples following the Mojonnier method (ISO 1211:2010) – gravimetric fat determination

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Courtesy of BÜCHI Labortechnik AG

Parallel evaporation after extraction of milk samples with the Mojonnier

The fat in liquid dairy samples is usually extracted with an organic solvent that is then evaporated to complete dryness prior to gravimetric analysis. In this short note, initial conditions and evaporation settings for the parallel evporation of milk fat dissolved in diethyl ether / light petroleum are presented. These settings have been optimized for fast and accurate results with simultaneously high solvent recoveries. 

Introduction

The quantitative fat content determination of food is usually done via extraction using a lipophilic solvent. For milk and dairy products there are specific regulations for the quantitative determination since a protein shell generally encloses the fat in such products. This means that fat needs to be released, i.e., by denaturing and removing the protein shell, prior to the actual analysis. In the Röse-Gottlieb and the Mojonnier methods, this denaturation is done applying alkaline conditions. The sample protein is digested with ammonia (25 % solution) and the realeased fat is subsequently extracted with a mixture of diethyl ether (bp 34-35 °C, peroxide-free) and light petroleum (bp 30-60 °C). The fat is extracted twice (three times for high fat contents) with the diethyl ether / light petroleum mixture. Ethanol (96 vol%) acts as antigelling compound and is added to the denaturated sample prior the first two extraction steps. The organic solvents are distilled off after complete phase separation and the isolated fat is dryed and weighted. After distillation of the organic solvents (including ethanol), the drying (1 h at 102 ± 2 °C) is repeated until the mass decrease is less than 1 mg.

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