Determination of Fat Content in Cookies Using Soxhlet Extraction
Mission:
To determine the fat content, expressed as a mass percent, in cookies.
Discussion:
The simplest form of solid - liquid extraction is the treatment of a solid with a solvent. An example of this
would be in separating a sand/salt mixture by using water. Swirl for a few moments and decant, or pour
off, the salt (now in the water layer). The sand remains since it is INSOLUBLE. In all solvent extractions
an attempt is made to take advantage of differences in solubility between the species being separated.
Classical extraction methods for most solid samples involve a substantial amount of wet chemistry. If one
were not versed in the chemical arts, it is hard to be confident and proactive in heading off experiment
failure due to lack of education, experience, or understanding. A viable alternative to the traditional wet
methods of solvent extraction is a Soxhlet type of apparatus which allows for near 100% active material
recovery and user friendliness.
An Overview of the Soxhlet Apparatus
The apparatus, first described in 1879, is a versatile tool that can be used to separate a single gram to
hundreds of grams with near 100% recovery. The basic procedure calls for a solid sample to be placed in
a porous container and allowing condensed solvent to extract continuously. The basic components of a
Soxhlet apparatus include:
•
Condenser: Function is to cool the solvent vapor and cause it to condense (turn back into a
liquid). Figure 1.1 shows a ring type of condenser which functions by increasing the surface
area that can be used to transfer heat. This is very similar to a heat exchanger or automotive
radiator that is used to dump excess heat.
•
Porous Container: Function is to hold the solid sample and allow for the condensed solvent to
saturate and pass through thereby extracting active material. This can be likened to a filter in
that it retains the insoluble components and allows whatever is dissolved to pass.
•
Distilling Pot: Function is to hold the solvent pool and serve as a reservoir for the
concentrated material.
Safety Protocols:
Students are to wear eye protection and gloves during this experiment. Reactions are to be done in a
fume hood under supervision by either myself or a qualified laboratory assistant. Your setup must be
approved before you begin. Dispose of all materials according to directions given by the instructor.
Figure 1.1. Experimental Setup for Soxhlet Extraction
Procedure:
Using a mortar and pestle, grind your cookie into pieces small enough to fit inside the extraction thimble.
Weigh the extraction thimble with a piece of glass wool inside first; then reweigh the thimble, cookie
sample and glass wool again. Calculate the mass of cookie sample by difference. Be sure to gently pack
the thimble and its contents with a piece of glass wool to prevent the contents from escaping the thimble.
Measure between 90 and 100 mL of methylene chloride solvent and place into the distilling pot. Add 1 or
2 boiling stones to the pot. Assemble the Soxhlet apparatus in an operating fume hood making sure to
clamp everything together. Have either myself or the lab assistant verify your setup before beginning the
extraction.
Turn on the heat source and set the power source on the heating mantle to a setting of 5 or 6. One
should observe the distilling pot contents boiling within 10-15 minutes; if this does not happen, increase
the setting on the power source.
Allow the Soxhlet apparatus to operate for a minimum of 6-8 cycles; a cycle is defined as the solvent
filling up the area that houses the extraction thimble and then flushing out returning the solvent back into
the distilling pot. An ideal “pace” is to have an extraction cycle occurring every 5-6 minutes. Record the
time for each cycle, and adjust the heat setting so that the recommended interval is reached.
Once the extraction is concluded, turn off the power source and disassemble your apparatus. Remember
that the distilling pot is warm, so protect your hands.
Transfer the solvent to a pre-weighed evaporating dish. Place the dish and contents into a fume hood and
allow the solvent to concentrate by evaporation for 24 hours. Label your dish so that it can be identified.
Determine the mass of the dish and residue.
Calculate the %fat content in the cookie sample.