The Role of Extraction Thimbles in Fat Analysis

Regulatory food standards require food products and supplements to have nutrition labels to enable consumers to make informed decisions and take control of their overall health. Commonly located on the back of your favorite snacks and food products, the nutritional label provides nutritional content, including calories per serving, and breaking it down to total protein, total cholesterol, total fat, sodium and total carbohydrates.

While basic labeling requirements began in 1906 and were expanded in 1938, modern nutrition labeling as we know it—including the inclusion of trans and saturated fatty acids under total fat, commonly referred to as crude fat—became legally required in 1994 following the passage of the Nutrition Labeling and Education Act (NLEA) in 1990¹

Differentiating between trans fats and saturated fats on nutrition labels became a critical step in helping consumers make healthier dietary choices. Trans fats—especially artificial trans fats created through partial hydrogenation of vegetable oils—are widely recognized as the most harmful type of dietary fat. They increase levels of LDL (bad) cholesterol, lower HDL (good) cholesterol, and are strongly linked to a higher risk of heart disease, stroke, and type 2 diabetes. Due to these significant health risks, the U.S. Food and Drug Administration (FDA) has effectively banned artificial trans fats from the U.S. food supply². 

Saturated fats, on the other hand, are not universally harmful. They occur naturally in many foods, including dairy products and meats. While high intake of saturated fat is still associated with an increased risk of cardiovascular disease, small amounts can fit into a balanced diet, especially when consumed in the context of whole foods rather than processed products. 
 
The process of quantifying Total Fat is highlighted in different food standards:

• AOAC Official Methods For crude fat, AOAC Method 920.39 (Ether Extract)3 is one of the most used. This method involves Soxhlet extraction using petroleum ether or diethyl ether to extract fat from the food sample.
• ISO Standard, ISO 1443⁴ for meat and meat products and ISO 6492 for animal feeding stuffs.
• AOCS Methods⁵ The American Oil Chemists' Society (AOCS) has methods like AOCS Am 5-04 for crude fat in oilseeds and meals. 

Even though these standards have different applicability, it follows one specific procedure which is gravimetric analysis of fat post Soxhlet extraction. 

Soxhlet Extraction

Soxhlet extraction is a classical technique used to solubilize fat from solid food samples using an appropriate non-polar solvent, typically hexane or petroleum ether. The food sample is placed inside an extraction thimble—a microporous tube, usually made of cellulose or glass fiber, that holds the sample while allowing fats to pass through.

During the process, the solvent is heated using a Digital Hotplate, causing it to evaporate. The solvent vapors rise into the condenser, where they cool and return to liquid form. The condensed solvent then drips into the Soxhlet extractor, submerging the sample. As the solvent fills the chamber and reaches the siphon, it flows back into the round-bottom flask to be evaporated again. This continuous cycle—repeating over a period of 4–6 hours—washes the food sample with fresh solvent. The hot solvent melts the fats, which dissolve and pass through the thimble’s pores, while solid food particles are retained. The result is a purified fat extract, free from solid contaminants. 

After thorough washing, the solvent will flush all the fat extracts to the tared round bottom flask. This tared round bottom flask is then placed in an air oven, set to the ideal evaporation temperature of the solvent used. After repetitive weighing, the average mass of the flask that contains the fat sample can be determined using the equation: 

mf = mass of the round bottom flask together with the extracted fat sample

m1 = mass of the round bottom flask before extraction

Selection criteria for thimbles

Because Soxhlet extraction involves prolonged and repeated solvent circulation at elevated temperatures, it is important to select a solvent that can withstand high heat without degradation. Likewise, the choice of extraction thimble plays a critical role in ensuring efficient and reliable performance. Sterlitech Extraction Thimbles, made from porous cellulose or glass fiber media, are designed to provide excellent temperature resistance while maintaining optimal solvent flow from the sample to the Soxhlet extractor. To achieve the best fit and performance, it is essential to match the extraction thimble to your equipment by carefully considering the internal diameter and height of your Soxhlet extractor.

As long as the extraction thimble is slightly smaller than the internal diameter and height of the Soxhlet extractor, it will fit properly and function effectively within the system.

In addition to size, material selection is another important consideration. The choice of thimble material should be guided by the solvent being used and the temperatures involved in the extraction process. Different materials—such as cellulose or glass fiber—offer varying levels of chemical resistance and temperature stability, making it essential to match the thimble to your specific application.

The Sterlitech Extraction Thimbles portfolio offers a comprehensive collection of different thimble height, diameter, and material that ensures compatibility with most Soxhlet extractors in the market. Need assistance with product selection? Ask an Expert!

References:

1.      Janssen, WF (1981), The Story of the Laws Behind the Labels, FDA Consumer Magazine, retrieved from: The Story of the Laws Behind the Labels, Date Retrieved: June 06, 2024

2.      21 CFR 101.9 Small Entity Compliance Guide: Trans Fatty Acids in Nutrition Labeling, Nutrient Content Claims, and Health Claims, US FDA 2003-D-0035

3.      4.5.01 AOAC Official Method 920.39 Fat (Crude) or Ether Extract in Animal Feed

4.      ISO 1443:2025 Meat and meat products — Determination of total fat content

5.      AOCS Approved Procedure Am 5-04 Rapid Determination of Oil/Fat Utilizing High Temperature Solvent Extraction