For the determination of the total fat content in challenging samples, the most commonly used method is solid/liquid extraction (picture 2) with prior hydrolysis. There are various standard methods like Röse-Gottlieb (RG) and Schmid-Bondzynski-Ratzlaff (SBR), but the most relevant and reliable (regarding accuracy and repeatability) reference method for fat determination is Weibull-Stoldt. In case of uncertain result or doubts using RG or SBR, Weibull-Stoldt is the definitive reference. For many samples Weibull-Stoldt is recommended to determine total fat content and for some dairy samples it is even compulsory:

• Infant foods - ISO 8262-1 / IDF 124-1
• Edible ices and ice mixes - ISO 8262-2 / IDF 124-2
• Special cases - ISO 8262-3 / IDF 124-3

Competitors with Microwave Digestion Systems claim that these instruments are capable of performing a determination acc. to Weibull-Stoldt. But according to the principle, the digestion and the extraction have to be done in two separate work steps and two separate containers to avoid an interaction of digestion reagent and extraction solvent. The microwaves although actually perform a liquid/liquid extraction (picture 1) which means that digestion reagent and extraction solvent are in the same container. In certain samples a major error source of this method is the emulsifier in the sample. The emulsifier binds the two liquids and causes an inaccurate extraction phase. This makes a proper analysis acc. to Weibull-Stoldt impossible. Samples with emulsifier are:

• Egg yolk
• etc.

Additionally microwaves have general disadvantages:

• only low sample weights possible - not suitable for samples with low fat content + bad repeatability
• safety issues: digestion with closed tubes can cause high pressure in the tubees with the threat of an explosion
• samples in one batch should have similar properties
• waiting time for cooling down to avoid opening vessel pressurized
• fat analysis not compliant with official methods