STUDY UNIT 4: LIPID EXTRACTION AND SOLUBILITY

Learning Objectives

Upon completion of the following exercises, the student should be able to:

• Use a weighing balance.

• Distinguish the aqueous phase from the organic phase.
• Differentiate between solubilities of different solvents.

4.1. Introduction

Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins,
monoglycerides, diglycerides, phospholipids and functions in storing energy, signaling, and acting as
structural components of cell membranes. Nevertheless, over-consumption of certain lipid components
can be detrimental to our health, e.g. cholesterol and saturated fats. In many foods the lipid component
plays a major role in determining the overall physical characteristics, such as flavor, texture, mouthfeel
and appearance. For this reason, it is difficult to develop low-fat alternatives of many foods, because
once the fat is removed some of the most important physical characteristics are lost. Finally, many fats
are prone to lipid oxidation, which leads to the formation of off-flavors and potentially harmful
products. Some of the most important properties of concern to the food analyst are:

 Total lipid concentration

 Type of lipids present
 Physicochemical properties of lipids, e.g., crystallization, melting point, smoke point, rheology,
density and color
 Structural organization of lipids within a food

4.2. Properties of Lipids in Foods

Lipids are usually defined as those components that are soluble in organic solvents (such as ether,
hexane or chloroform), but are insoluble in water. This group of substances includes triacylglycercols,
diacylglycercols, monoacylglycercols, free fatty acids, phospholipids, sterols, caretonoids and vitamins
A and D. The lipid fraction of a fatty food therefore contains a complex mixture of different types of
molecule. Even so, triacylglycercols are the major component of most foods, typically making up more
than 95 to 99% of the total lipids present. Triacylglycerols are esters of three fatty acids and a glycerol
molecule. The fatty acids normally found in foods vary in chain length, degree of unsaturation and
position on the glycerol molecule. Consequently, the triacylglycerol fraction itself consists of a complex
mixture of different types of molecules. Each type of fat has a different profile of lipids present which
determines the precise nature of its nutritional and physiochemical properties. The terms fat, oil and
lipid are often used interchangeably by food scientists. Although sometimes the term fat is used to
describe those lipids that are solid at the specified temperature, whereas the term oil is used to describe
those lipids that are liquid at the specified temperature.

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4.3 Solubility of lipids

Solubility of a chemical is determined by its structure and solution conditions. Aqueous solubility is
important because, except for gases, chemical agents in the environment are transported and made
available to living systems as aqueous solutions. Chemicals are either hydrophilic (water-soluble) or
hydrophobic (water-insoluble).

It is important to be able to accurately determine the total fat content of foods for several reasons:

 Economic (not to give away expensive ingredients)

 Legal (to conform to standards of identity and nutritional labeling laws)
 Health (development of low-fat foods)
 Quality (food properties depend on the total lipid content)
 Processing (processing conditions depend on the total lipid content)

The principle physicochemical characteristics of lipids (the "analyte") used to distinguish them from the
other components in foods (the "matrix") are their solubility in organic solvents, immiscibility with
water, physical characteristics (e.g., relatively low density) and spectroscopic properties. The analytical
techniques based on these principles can be conveniently categorized into three different types: (i)
solvent extraction; (ii) non-solvent extraction and (iii) instrumental methods.

The fact that lipids are soluble in organic solvents, but insoluble in water, provides the food analyst
with a convenient method of separating the lipid components in foods from water soluble components,
such as proteins, carbohydrates and minerals. In fact, solvent extraction techniques are one of the most
used methods of isolating lipids from foods and of determining the total lipid content of foods.

Experiment Guide:

Part 1: Extraction of lipids from foods

Material:

 Test tubes
 Test tube rack
 Spatula
 Pestle & mortar
 Water bath/ hot plate
 Ethanol and chloroform

Method:

1. Clean and dry 2 test tubes

2. Grind and crush food sample into small pieces
3. Accurately weigh small amount of your food sample (5g) place it in each test tube
4. Add ethanol or chloroform to about 5 mL above the level of the sample and shake thoroughly
5. Allow the solid to settle (about 5 min) for lipids to be extracted
6. While you are waiting for the lipids dissolve, accurately weigh two clean small beakers (record the
mass)
7. Carefully transfer solvents from test tubes containing your food sample to weighed beakers
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8. Place the beaker on a hot plate or water bath to evaporate solvents
9. When you think it has all evaporated, weigh beakers again (record the mass)
10. Calculate the mass of lipid extracted from the food sample determine the weight percentage of fat in
that food

Analysis:

From what food item are you extracting lipids?

Mass of food sample ……………………………………g
Mass of empty beaker…………………………………..g
Mass of beaker and lipid……………………………….g
Mass of lipid extracted………………………………….g
Weight percentage of lipid in food………………%

Part 2: Solubility of lipids

Material:

Test tubes
Test tube rack
Olive oil
Distilled water
Ethanol, acetone and chloroform
Water bath/ hot plate

Method:

1. Place 0.5ml of oil in 4 test tubes containing 4ml of different solvents (acetone, chloroform, ethanol
and water).
2. Shake the tubes thoroughly, then leave the solution for about 1 minute.
3. Make observations and record in a table.

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