Difference Between Saturated Fat and Unsaturated Fat

FOOD CHEMISTRY: LIPIDS

Diagram

Figure 1: A saturated fatty acid has no double bonds and a straight chain; an unsaturated fatty acid has double bonds that kink the chain. Drawn in our own style from the fatty acid discussion in the FTN Food Chemistry notes, Unit 5.

Introduction

A saturated fat and an unsaturated fat differ in one thing: double bonds. A saturated fatty acid has no double bonds, so its chain is straight and it packs tightly. An unsaturated fatty acid has one double bond (monounsaturated) or more than one (polyunsaturated), and each double bond puts a kink in the chain, so it packs loosely. That difference decides whether the fat is solid or liquid at room temperature, and how fast it goes rancid.

Quick comparison

Table 1: Saturated fat compared with unsaturated fat

Scientific explanation

Definition

A fatty acid is a chain of carbon atoms with a carboxyl group at one end and a methyl group at the other. A saturated fatty acid has no double bonds, so its chain is straight and it packs tightly. An unsaturated fatty acid has one double bond (monounsaturated) or more than one (polyunsaturated), and each double bond puts a kink in the chain, so it packs loosely. A fat is a triacylglycerol, glycerol joined to three fatty acids, and a cooking oil is more than 95 percent triacylglycerol.

Working principle

Consistency follows from saturation: saturated fats pack tightly and melt high, so they are solid at room temperature; unsaturated fats pack loosely and melt low, so they are liquid. A fat is solid when its crystals form a network that traps liquid oil. Unsaturated fats also oxidise: autoxidation is the reaction of unsaturated fatty acids with oxygen from the air, in a chain reaction with three stages, initiation, propagation, and termination. The peroxides break down into aldehydes and ketones that taste painty, cardboard-like, and tallowy. Light, heat, and metals such as iron and copper speed the reaction, while antioxidants such as tocopherols slow it by catching radicals.

Apparatus

The laboratory tests use titration glassware: a flask for the fat and the reagent, and a burette for the standard thiosulphate or acid. The iodine value test adds iodine reagent to the fat; the peroxide value test adds potassium iodide so the peroxides free iodine, which is titrated with standard thiosulphate.

Measurement procedure

The iodine value is found by adding a known excess of iodine reagent to the fat, letting it add across the double bonds, and titrating the iodine left. It is reported as grams of iodine absorbed by 100 g of fat, and a high iodine value means a more unsaturated fat. The peroxide value is found by adding potassium iodide to the fat, so the peroxides free iodine, and titrating the iodine with standard thiosulphate. The acid value is found by titrating the fat dissolved in alcohol with standard alkali.

Factors affecting readings

The iodine value fails if the reagent is old or the reaction time is short, because not all double bonds react. The peroxide value is the most fragile: peroxides break down as oxidation advances, so a badly rancid oil can show a low peroxide value while its anisidine value, which measures the later breakdown products, is high. A professional reads peroxide value together with anisidine value and with smell, never peroxide value alone. Light, heat, and metals speed oxidation, so storage changes the readings.

Formulas and calculations

Iodine value (IV). IV = (B minus S) x N x 12.69 / W, where B is the blank titre, S is the sample titre in millilitres, N is the normality, and W is the sample weight in grams. The 12.69 is 0.1269 x 100, the 100 converting grams of fat to 100 g of fat.

Worked example: W = 0.25 g, B = 40.0 mL, S = 15.0 mL, N = 0.1. IV = (40.0 minus 15.0) x 0.1 x 12.69 / 0.25 = 25 x 0.1 x 12.69 / 0.25 = 126.9. That is a highly unsaturated oil, near sunflower oil. A butter at IV near 30 would give a much smaller difference between blank and sample.

Peroxide value (PV). PV = (S minus B) x N x 1000 / W, in milliequivalents per kg. Worked example: W = 5.0 g, S = 8.5 mL, B = 0.5 mL, N = 0.01. PV = (8.5 minus 0.5) x 0.01 x 1000 / 5.0 = 16 milliequivalents per kg. Fresh refined oil should sit below 10, so 16 says the oil has started to oxidise.

Industrial applications

• Saturated fat: solid fats for spreads and pastry, where a plastic texture gives a short, crumbly texture, and butter, which is about 80 percent fat and browns well.
• Saturated fat: hard fats for frying in the past, such as tallow, the traditional frying fat.
• Unsaturated fat: cooking and salad oils that must stay liquid and clear, including oils chosen for frying because fats high in oleic acid and low in polyunsaturated acids last longer at 170 to 180 C.
• Unsaturated fat: the source of the essential fatty acids linoleic and alpha-linolenic acid in seed oils.
• Both: hydrogenation turns a liquid unsaturated oil into a solid fat for spreads and bakery, and interesterification rearranges fatty acids to make a spread without making trans fatty acids.

Advantages and limitations

• Saturated fat, advantage: stable to oxidation and solid at room temperature, so it gives structure to pastry and spreads.
• Saturated fat, limitation: solid and high melting, so it cannot serve as a pourable oil, and partial hydrogenation of unsaturated oils can make trans fatty acids, which are linked to higher blood cholesterol.
• Unsaturated fat, advantage: liquid and pourable, and it supplies the essential fatty acids the body cannot make.
• Unsaturated fat, limitation: it oxidises fast by autoxidation, so it goes rancid in air, light, and heat, and it needs cool, dark, sealed storage and antioxidants.

FAQ

What makes a fatty acid unsaturated?

It has one or more double bonds in its carbon chain, which puts a kink in the chain.

Why is a saturated fat solid and an unsaturated fat liquid?

Saturated chains are straight and pack tightly, so they melt high; unsaturated chains are kinked and pack loosely, so they melt low.

What does a high iodine value tell you?

The fat holds many double bonds, so it is more unsaturated and oxidises faster.

Which fatty acids are essential?

Linoleic acid and alpha-linolenic acid: the body cannot make them, so they must come from food.

Why does oil left open in a warm kitchen turn bitter?

Its unsaturated fatty acids react with oxygen by autoxidation, making peroxides that break down into aldehydes and ketones that taste painty and cardboard-like.

References and further reading

• FTN subject: Food Chemistry, Unit 5: Lipids.
• Zeece, Michael. Introduction to the Chemistry of Food. Academic Press, 2020.
• Coultate, Tom. Food: The Chemistry of Its Components. 7th ed., Royal Society of Chemistry, 2023.
• Damodaran, Srinivasan, and Kirk L. Parkin. Fennema’s Food Chemistry. 5th ed., CRC Press, 2017.
• Wang, Dongfeng, Hong Lin, Jianqian Kan et al. Food Chemistry. De Gruyter, 2015.
• Google Books pages read for this article:
• Gunstone, Frank. Vegetable Oils in Food Technology. Wiley, 2011 (Google Books page read)
• O’Brien, Richard D. Fats and Oils: Formulating and Processing for Applications, 3rd ed., CRC Press, 2008 (Google Books page read)
• Provost, Joseph J. et al. The Science of Cooking. John Wiley and Sons, 2016 (Google Books page read)