Fats

Overview of Fats as Natural Products

Fats are a major class of natural lipids that serve primarily as energy storage materials in living organisms. In chemistry, “fats” usually refers to triacylglycerols (also called triglycerides): esters of glycerol with three fatty acid molecules. They are predominantly found in animals (e.g. adipose tissue) but also in plants (e.g. seeds, nuts, fruits).

Fats are:

• Hydrophobic and insoluble in water, but soluble in nonpolar solvents.
• Energy-dense: their complete oxidation releases much more energy per gram than carbohydrates or proteins.
• Structurally simple compared with many other natural products, yet highly variable due to the many possible fatty acids.

In this chapter, the focus is on the specific structure, properties, and key reactions of fats as natural products, and on their biological and technical relevance.

Structural Features of Fats

Triacylglycerols: Glycerol Triesters of Fatty Acids

Chemically, a fat is a triacylglycerol:

• The alcohol component is glycerol (systematic name: propane-1,2,3-triol).
• Each of the three hydroxyl groups of glycerol is esterified with a fatty acid.

The general structure can be represented as:

$$
\text{Glycerol backbone: } \ce{HO–CH2–CH(OH)–CH2–OH}
$$

After esterification with three fatty acids $\ce{R1COOH}$, $\ce{R2COOH}$, $\ce{R3COOH}$:

$$
\ce{HO–CH2–CH(OH)–CH2–OH + 3 RCOOH ->[\text{condensation}]
CH2–O–CO–R1 \\
\quad\quad | \\
CH–O–CO–R2 \\
\quad\quad | \\
CH2–O–CO–R3 + 3 H2O}
$$

In a more compact form:

$$
\ce{(HOCH2–CHOH–CH2OH) + 3 RCOOH -> (RCOO–CH2–CHO–CH2–OOC–R) + 3 H2O}
$$

where $\ce{R1}$, $\ce{R2}$, and $\ce{R3}$ are hydrocarbon chains (alkyl groups) of the fatty acids. If all three are the same, the triacylglycerol is simple; if they differ, it is mixed.

Fatty Acid Residues in Fats

The properties of a fat depend largely on the fatty acids it contains. In natural fats, fatty acids are typically:

• Straight-chain, unbranched.
• With an even number of carbon atoms (often C16 or C18).
• Saturated (no C=C double bonds) or unsaturated (one or more C=C).

Common saturated fatty acids (as residues in fats):

• $\ce{C15H31COOH}$: palmitic acid (hexadecanoic acid, C16)
• $\ce{C17H35COOH}$: stearic acid (octadecanoic acid, C18)

Common unsaturated fatty acids:

• $\ce{C17H31COOH}$: oleic acid (cis-9-octadecenoic acid, C18:1)
• $\ce{C17H29COOH}$: linoleic acid (C18:2, essential)
• $\ce{C17H27COOH}$: linolenic acid (C18:3, essential)

In natural plant and animal fats, the double bonds in unsaturated fatty acids are usually:

• In the cis configuration.
• Isolated (separated by at least one $\ce{CH2}$ group), not conjugated.

This geometric feature strongly influences the physical properties of fats.

Physical Properties of Fats

Fats vs Oils

In everyday language, there is a distinction:

• Fats: triacylglycerols that are solid or semi-solid at room temperature (e.g. butter, lard).
• Oils: triacylglycerols that are liquid at room temperature (e.g. olive oil, sunflower oil).

Chemically they are the same class of compounds; the difference is mainly in melting point.

The melting point of a triacylglycerol is influenced by:

1. Chain length of the fatty acids:
• Longer chains $\Rightarrow$ stronger van der Waals interactions $\Rightarrow$ higher melting point.
2. Degree of saturation:
• More saturated $\Rightarrow$ chains can pack closely $\Rightarrow$ higher melting point.
• More unsaturated (especially cis double bonds) $\Rightarrow$ “kinks” in the chain $\Rightarrow$ poorer packing $\Rightarrow$ lower melting point.

Thus:

• Animal fats with many saturated fatty acids tend to be solid.
• Plant oils rich in unsaturated fatty acids tend to be liquid.

Solubility and Density

• Fats are nonpolar (apart from the ester groups, the molecule is dominated by hydrocarbon chains).
• They are insoluble in water but soluble in nonpolar or weakly polar solvents such as hexane or chloroform.
• Fats are less dense than water and float on top of aqueous phases.

Odor, Taste, and Rancidity

Pure triacylglycerols are almost odorless and tasteless. The characteristic odors and flavors of natural fats and oils come from:

• Minor components (e.g. free fatty acids, aldehydes, ketones).
• Products of oxidation or hydrolysis.

Rancidity is the deterioration of fats, often accompanied by unpleasant smell and taste. Two main forms:

• Hydrolytic rancidity: partial hydrolysis of triacylglycerols to free fatty acids by enzymes (lipases) or moisture.
• Oxidative rancidity: reaction of oxygen with unsaturated fatty acid residues, forming peroxides and breakdown products (aldehydes, ketones, short-chain acids).

Conditions promoting rancidity:

• Light, heat, oxygen, and traces of metal ions (catalyze oxidation).

Chemical Reactions of Fats

Ester Hydrolysis: Saponification

Fats are esters and undergo hydrolysis. Under strongly acidic or enzymatic conditions, hydrolysis is reversible; under basic conditions, it becomes practically irreversible.

Acidic Hydrolysis (Conceptual)

In the presence of water and a strong acid, triacylglycerols can be hydrolyzed to glycerol and free fatty acids:

$$
\ce{Triacylglycerol + 3 H2O <=> Glycerol + 3 RCOOH}
$$

In living organisms, hydrolysis is catalyzed by lipases (enzymes), rather than by strong mineral acids.

Basic Hydrolysis (Saponification) and Soap Formation

In aqueous base (e.g. $\ce{NaOH}$, $\ce{KOH}$), fats undergo saponification:

• The ester bonds are cleaved.
• Fatty acid anions form salts with the metal cations.
• Glycerol is released.

For a triacylglycerol with three identical fatty acids $\ce{RCOOH}$:

$$
\ce{(RCOO)3C3H5 + 3 NaOH -> C3H5(OH)3 + 3 RCOO^- Na^+}
$$

Products:

• Glycerol (propane-1,2,3-triol).
• Sodium or potassium salts of fatty acids: these are soaps.

The long nonpolar hydrocarbon chain $\ce{R}$ and the polar carboxylate group $\ce{COO^-}$ make soap molecules amphiphilic, an important property for cleaning and emulsifying. The detailed behavior of soaps belongs to surfactant chemistry and is considered elsewhere.

Hydrogenation of Unsaturated Fats

Unsaturated fats contain C=C double bonds that can be hydrogenated:

• Addition of hydrogen across the double bond.
• Carried out with a catalyst (typically nickel, palladium, or platinum).

Simplified reaction (for a single double bond):

$$
\ce{R–CH=CH–R' + H2 ->[Ni] R–CH2–CH2–R'}
$$

Applied to triacylglycerols:

• Partial hydrogenation converts some unsaturated fatty acids into more saturated ones.
• This raises the melting point and changes texture (e.g. from liquid oil to semi-solid fat).

Industrial applications:

• Production of margarine and shortening from plant oils.
• Adjusting spreadability and stability of fats.

A side effect of certain hydrogenation conditions is the formation of trans fatty acids, where cis double bonds are converted to trans double bonds rather than being completely hydrogenated. Trans fatty acids have different physical and biological properties than the natural cis forms, with notable nutritional implications.

Oxidation of Unsaturated Fats

Unsaturated fatty acid residues are sensitive to autoxidation in the presence of oxygen:

• Initiated by radicals, heat, or light.
• Leads to formation of hydroperoxides, which can decompose to aldehydes, ketones, and carboxylic acids.

Simplified scheme:

$$
\ce{RH + O2 -> ROOH -> (aldehydes, ketones, acids, etc.)}
$$

where $\ce{RH}$ represents a lipid (fatty acid residue). This process contributes to oxidative rancidity and loss of nutritional quality.

To slow oxidation, fats and oils are often:

• Protected from light and oxygen.
• Stabilized with antioxidants (e.g. vitamin E, butylated hydroxytoluene, BHT).

Enzymatic Reactions: Digestion and Metabolism (Overview)

In biological systems, fats undergo specific enzymatic reactions:

1. Digestion:
• In animals, dietary triacylglycerols are emulsified by bile salts and hydrolyzed by pancreatic lipase.
• Products: free fatty acids and monoacylglycerols, which can be absorbed.
2. Storage and mobilization:
• In adipose tissue, triacylglycerols are stored in fat droplets.
• When energy is needed, lipases break them down, releasing fatty acids for further metabolism.

The detailed pathways of lipid metabolism and energy generation are covered in biochemical chapters.

Classification of Fats

Simple vs Mixed Fats

• Simple triacylglycerols: all three fatty acid residues are identical.
• Mixed triacylglycerols: contain two or three different fatty acids.

Natural fats are mostly mixed, resulting in a broad melting range rather than a sharp melting point.

Animal vs Plant Fats

• Animal fats (e.g. lard, tallow, butter):
• Often higher in saturated fatty acids.
• Tend to be solid or semi-solid at room temperature.
• Plant fats and oils (e.g. olive oil, sunflower oil, palm oil):
• Often richer in unsaturated fatty acids.
• Usually liquid at room temperature (exceptions: palm oil, coconut fat, cocoa butter, which are relatively rich in saturated or particular solidifying fatty acids).

Because of these differences, plant oils are often chosen for hydrogenation and texturization to produce spreads and shortenings.

Dietary “Visible” and “Hidden” Fats

From a nutritional perspective, it is useful to distinguish:

• Visible fats: obvious fats and oils used directly (butter, oil, margarine, lard).
• Hidden fats: triacylglycerols present within other foods (meat, cheese, nuts, pastries, chocolate).

Chemically they are the same type of compounds, but their source and fatty acid composition vary, influencing nutritional impact.

Biological Roles of Fats

Fats play several key roles in living organisms:

1. Energy storage:
• Triacylglycerols represent a highly efficient energy reserve.
• Per gram, they yield more energy upon oxidation than carbohydrates or proteins.
2. Thermal insulation and protection:
• Subcutaneous fat layers in animals insulate against cold.
• Fat deposits can mechanically protect organs.
3. Source of essential fatty acids:
• Some unsaturated fatty acids (e.g. linoleic acid, linolenic acid) cannot be synthesized by many animals and must be obtained from the diet.
• These are precursors for important signaling molecules.
4. Transport and storage of fat-soluble substances:
• Fats can dissolve and store fat-soluble vitamins (A, D, E, K) and other lipophilic natural products.

The detailed biochemical consequences and health aspects of dietary fats, including the roles of saturated, unsaturated, and trans fats, are part of nutritional and biochemical discussions.

Industrial and Technical Uses of Fats

Food Technology

Fats are central ingredients in food products, providing:

• Texture (creaminess, flakiness in pastry).
• Flavor and aroma carriers.
• Satiety and high caloric content.

In food technology, fats may be:

• Refined (removal of free fatty acids, pigments, odors).
• Hydrogenated or fractionated to adjust melting behavior.
• Interesterified (exchange of fatty acid residues between triacylglycerols) to tailor physical properties.

Soaps and Detergents

Traditional soap manufacture is based on saponification of natural fats and oils:

• Animal fats (tallow) and plant oils (coconut oil, palm oil) are treated with base.
• The resulting fatty acid salts are formulated into solid or liquid soaps.

The detailed chemistry of surfactants and detergents is covered in the corresponding chapters, but the connection to fats is the fatty acid origin of many surfactants.

Nonfood Applications

Natural fats and their derivatives are used as:

• Raw materials for fatty alcohols, fatty acid esters, and surfactants.
• Components of cosmetics and pharmaceutical ointments (as emollients and carriers).
• Lubricants and processing aids in various industries.

Hydrolysis, hydrogenation, and subsequent chemical modification of fatty acid derivatives derived from fats enable a broad range of industrial products.

Summary

Fats are triacylglycerols: glycerol triesters of fatty acids. Their structural variability arises from the different chain lengths and degrees of unsaturation of the fatty acids they contain. This variability governs their physical properties (solid vs liquid, melting range), behavior upon hydrogenation and oxidation, and biological effects.

Key reactions include:

• Hydrolysis (acidic, enzymatic, or basic) leading to glycerol and fatty acids or their salts (soaps).
• Hydrogenation of unsaturated fatty acid residues to adjust texture and stability.
• Oxidation of unsaturated residues, responsible for rancidity and quality loss.

Biologically, fats serve as dense energy stores, thermal insulation, and reservoirs of essential fatty acids and fat-soluble substances. Technically, they are important feedstocks in food technology, soap manufacture, surfactant production, and cosmetics, linking the chemistry of natural products to numerous everyday applications.