FOOD CHEMISTRY

Module 3. Food lipids

Lesson 8
EDIBLE FATS AND OILS - CLASSIFICATION AND CHEMICAL COMPOSITION

8.1 Introduction

Lipids are a broad group of naturally occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (A, D, E and K), triglycerides, diglycerides, monoglycerides, phospholipids, and others. Lipids are formed from structural units with a pronounced hydrophobicity. This solubility characteristic, rather than a common structural feature, is unique for this class of compounds. Lipids are soluble in organic solvents but not in water.

Fats and oils may be obtained from vegetables (various vegetable oils, cocoa butter, etc.), animal source (milk fat, lard, tallow, etc.) and marine (whale oil, cod liver oil, etc.). They play an important role in nutrition as well as physiological functions as they are rich energy source (9 kcal/g) and as a source of essential fatty acids and fat soluble vitamins. Some lipids are amphiphilic in nature (contain both hydrophilic & hydrophobic groups) with surface-active properties. As a whole, fats enrich the nutritional quality and impart the desired body & texture, rich mouth feel to the food. It also contributes characteristic flavour to food and produces a feeling of satiety or loss of hunger.

8.2 Classification of Lipids

Lipids are classified on several basis, i.e. based on its complexity, ability to react with alkali (saponification process) to form soap and polarity (charge on its components).

A. Based on Structure/Complexity: Based on structure, lipids can be classified into three groups, i.e. simple, complex and derived lipids.

I. Simple lipids: These lipids are composed of fatty acids and alcohol components, and include fats, oils and wax esters. They can be hydrolyzed to two different components, usually an alcohol and an acid.

II. Compound lipids: These lipids include glycerophospholipids (phospholipids), glyceroglycolipids (glycolipids), and sphingolipids. On hydrolysis, it yields three or more different compounds.

III. Derived lipids: They meet the definition of a lipid but are not simple or compound lipids and include fatty acids and alcohols; which are the building blocks for the simple and complex lipids. It also includes sterols, vitamins, pigments, hydrocarbons, etc.

B. Based on Polarity

Based on polarity lipids are classified into two groups, i.e. polar lipids and non-polar lipids.

I. Polar lipids

1. They are charged molecules

2. Soluble in polar solvents like alcohol, acetone, etc. e.g. phospholipids, glyceroglycolipids, fatty acids, etc.

II. Non-polar lipids

1. They are uncharged molecules

2. Soluble in non-polar solvents like ether, benzene, hexane, etc. e.g. Glycerides, sterols, sterol esters, Carotenoids, waxes, vitamins, etc.

C. Based on Saponifiction

It is classified based on the ability of lipids to react with alkali (saponification process) to form soap. Based on this reaction, lipids are grouped as saponifiable lipids and unsaponifiable lipids.

I. Saponifiable lipids

• React with alkali and form soap

• Present in large amount. e.g. Glycerides, phospholipids, fatty acids, cholesterol ester, etc.

II. Unsaponifiable lipids

• Do not react with alkali to form soap

• Present in relatively small amount. e.g. Fat soluble vitamins, sterols, hydrocarbons, carbonyls, etc.

8.3 Traditional Classification of Edible Fats/Oils

It is classified based on the source of fat/oil and the constituent fatty acids present.

1. Milk fat

a) They are derived from milk of mammals, particularly from buffalo, cow, goat and sheep.

b) Major fatty acids of milk fat are palmitic (C_16:0), stearic (C_18:0) & oleic (C_18:1) acids.

c) Contains appreciable amounts of short chain fatty acids (C_4:0, C_6:0, C_8:0, C_10:0).

d) Butyric acid (C_4:0) is a characteristic fatty acid to milk fat.

2. Lauryl or Lauric acid fat

a) Characteristic fatty acid is Lauric acid (Almost 40 - 50 % of total fatty acids).

b) Low amount of unsaturated fatty acids and thus having low melting point.

c) Contain moderate amount of C_6:0, C_8:0, C_10:0 fatty acids.

d) Obtained from certain species of palm, ex. Coconut.

3. Vegetable butters

a) Obtained from the seeds of various tropical trees, e.g. cocoa.

b) Characterized by their narrow melting range, i.e. due to arrangement of fatty acids in the triglyceraldehyde molecules.

c) Widely used in the manufacture of confectionary products, ex. Chocolates, etc.

4. Oleic –linoleic acid fats

a) Fat present in this group are the most abundant and of vegetable origin.

b) Contain large amounts of oleic and linoleic acids

c) Contain less amount of saturated fatty acids (i.e. less than 20%)

d) Cottonseed, corn, peanut, sunflower, palm olive and sesame oils are important examples.

5. Linolenic acid fats

a) Contain large amount of linolenic acid.

b) Soybean, rapeseed, wheat germ, hempseed, etc. with soybean being the most important.

c) Linolenic acid in soybean oil is responsible for off-flavour, i.e. flavour reversion problem.

6. Animal body fats

a) It is known as depot fats from domestic land animals e.g. lard and tallow.

b) Contain large amounts of C₁₆>C₁₈ fatty acids

c) Contain medium amounts of unsaturated fatty acids (mostly C_18:1> C_18:2)

d) Contain appreciable amounts of saturated triacylglycerols and shows high melting points.

e) Egg lipids are important due to their emulsifying properties and high content of cholesterol.

7. Marine oils

a) Contain large amounts of omega-3-polyunsaturated fatty acids, with up to six double bonds

b) Usually rich in vitamins A & D.

8.4 Chemical Composition

Table 8.1 Gross chemical composition of fats of various species

Sr. No.	Class of lipids	Cow milk fat	Buffalo milk fat	Human milk fat
		(% weight basis)
1.	Triacylglycerols	97.5	98.6	98.2
2.	Diacylglycerols	0.36	0.4	0.7
3.	Monoacylglycerols	0.02	0.03	traces
4.	Cholesterol	0.31	0.3	0.25
5.	Cholesterol Esters	traces	0.1	traces
6.	Phospholipids	0.6	0.5	0.26
7.	Free fatty acids	0.027	0.5	0.4

8.5 Unsaponifiable Matter of Various Fats and Oils

All the fats and oils have a tendency to form soap when they are allowed to react with alkali. This reaction is called saponification reaction. It is defined as “The number of milligrams of KOH required to saponify one gram of fat”. The saponification value (SV) is related to molecular weight of the constituent fatty acids in a particular fat. Fats and oils contain an average of 0.2-1.5% unsaponifiable compounds. The reaction involved in saponification process is shown below.

Fig. 8.1 Saponification reaction

The unsaponifiable fraction of fats consists of sterols, terpenic alcohols, aliphatic alcohols, squalene, and hydrocarbons. The composition of various components of unsaponifiable fraction in some fats and oils is given in Table 8.2. In most fats the major components of the unsaponifiable fraction are sterols. Animal fats contain cholesterol whereas plant fats and oils contain phytosterols with no or only trace amounts of cholesterol. The predominant phytosterol is 3-sitosterol; the others are campesterol and stigmasterol. Sterols are compounds containing the perhydrocyclopenteno- phenanthrene nucleus, which they have in common with many other natural compounds, including bile acids, hormones, and vitamin D.

The sterols provide a method of distinguishing between animal and vegetable fats by means of their acetates. Cholesterol acetate has a melting point of 114°C, whereas phytosterol acetates melt in the range of 126 to 137°C. This provides a way to detect adulteration of animal fats with vegetable fats. The various constituents present in unsaponifiable matter of lipids are discussed below:

a) Hydrocarbons: All edible oils contain hydrocarbons with an even/odd carbon number (C₁₁-C₃₅). Olive, rice and fish oils are particularly rich in this class of compounds. The main hydrocarbon constituents of olive oil (1-7g/kg) and rice oil (~3.3g/kg) is linear tri-terpene known as squalene (C₃₀). This compound is used as analytical indicator for olive oil. It is also present in substantially high amount in fish liver oil.

b) Sterols: Sterols are compounds containing perhydro cyclopenteno–phenanthrene tree nucleus. The steroid skeleton contains 4 condensed rings A, B, C and D. A characteristic in steroids is the presence of an alcoholic –OH group in position 3. In most fats, the major component of unsaponifiable fraction is that of the sterols. In animal fats, it is mainly cholesterol while in plant fats/oils it is phytosterol. The prominent phytosterol is β-sitosterol. Cholesterols are obtained biosynthetically from squalene. In animals, cholesterol is the precursor for the biosynthesis of steroids and bile acids. Cholecalciferol (vitamin D₃) is formed by the photolysis of 7-dehydro cholesterol.
The main steroid of yeast is ergosterol (pro-vitamin D₂). This is converted by irradiation (UV) into ergocalciferol (Vitamin D₂)

c) Tocopherols and Tocotrienols: The methyl derivatives of tocol are denoted as tocopherols. Some methyl derivatives of Tocotrienols are also found in foods. Because α-tocopherol is the most abundant tocopherol and it appears to have the greatest biological activity, so, α -tocopherol content of foods is usually considered to be the most important. These redox type lipids are important as antioxidants in foods containing fats and oils.

d) Carotenoids: They are polyene hydrocarbons biosynthesized from 8 isoprene units and have 40 carbons. They provide the intensive yellow, orange or red colour to a great number of foods of plant origin. They are synthesized only by plants. However, they reach animal tissues via the feed and can be modified and deposited there.
Carotenoids are divided into two classes:

• Carotenes: carotenes are pure polyene hydrocarbons.
• Xanthophylls: Xanthophylls have oxygen in the form of hydroxy, epoxy or oxo groups and are present in corn, green leaves, egg yolk, etc.

Table 8.2 Composition of the unsaponifiable matter of some fats and oils

Fats/Oils	Hydrocarbons	Squalene	Aliphatic Alcohols	Terpenic Alcohols	Sterols
Olive	2.8-3.5	32-50	0.5	20-26	20-30
Linseed	3.7-14.0	1.0-3.9	2.5-5.9	29-30	34.5-52
Teaseed	3.4	2.6	-	-	22.7
Soybean	3.8	2.5	4.9	23.2	58.4
Rapeseed	8.7	4.3	7.2	9.2	63.6
Corn	1.4	2.2	5.0	6.7	81.3
Lard	23.8	4.6	2.1	7.1	47.0
Tallow	11.8	1.2	2.4	5.5	64.0