Fats and Oils¶
As with other lectures in this unit, these notes are going to continue on from Food Science - Carbohydrates
Lipids are a heterogeneous class of naturally occurring compounds, classified together based on their common property of solubility, that is; they are soluble in organic solvents with low polarity and are insoluble in high polar solvents like water.
In the body we find lipids in cell membranes, energy stores and steroid hormones.
As a Storage¶
As a method of storage, lipids hold much more energy than carbohydrates for a few reasons:
- They are in their most reduced form
- They pack together really well because of their shape
- They can exclude water, which would otherwise take up space
They are stored in large organelles in cells and can be released when the energy intake of the animal is less than the required energy expenditure. This storage also has added benefit as a cushioning and insulating medium, as well as acting as a location to store fat soluble nutrients, such as vitamins A, D, E and K.
Types of Lipids¶
Fatty Acids¶
Fatty acids are made of a carboxylic acid head and an unbranched hydrocarbon tail. Naturally occurring fatty acids will have a chain length of between 10 and 20. The fatty acids can be; saturated, monounsaturated or polyunsaturated and their unsaturated bonds can be in the cis or trans stereochemistry.
Common saturated naming conventions:
- Lauric acid (dodecanoic acid) - \(\ce{C12}\)
- Myristic acid (tetradecanoic acid) - \(\ce{C14}\)
- Palmitic acid (hexadecanoic acid) - \(\ce{C16}\)
- Stearic acid (octadecanoic acid) - \(\ce{C18}\)
Unsaturated naming conventions:
- Palmitoleic acid (hexadecenoic acid) - \(\ce{C16}\)
- Oleic acid (octadeceneoic acid) - \(\ce{C18}\)
Trans fatty acids are generally considered bad for human health as we don’t have the enzymatic machinery to be able to digest them properly
Polyunsaturated fatty acids have more than one double bond and if they’re cis double bonds, they can aid in digestibility. Cis polyunsaturated fatty acids are more readily metabolised
Properties of Fatty Acids¶
| Fatty acid | Length | DOU | Melting Point |
|---|---|---|---|
| Palmitic actid | 16 | 0 | 62 |
| Stearic acid | 18 | 0 | 69 |
| Palmitoleic acid | 16 | 1 | 0 |
| Cis-Oleic acid | 18 | 1 | 13 |
| Trans-Oleic acid | 18 | 1 | 45 |
| Linoleic acid | 18 | 2 | -9 |
From the above table we can see a few particular trends:
- The MP increases sa the chain length increases as there will be more VdW interaction area
- The MP of trans unsaturated fatty acids is greater than of unsaturated fatty acids due to their inability to pack as effectively
- The MP decreases as the nuber of double bonds increases, as the sp2 carbons won’t back quite as well as the sp3 ones
Essential Fatty Acids¶
The human body can synthesise mote fatty acids from carbohydrates or other fatty materials, but humans don’t produce enough poyunsaturated fatty acids and require them from their diet. Examples include linoleic acid and α-linoleic. In infants, deficiencies can cause dermatitis.
These are the ω3 and ω6 fatty acids, described as such, because they have cis double bonds at carbons 3 and carbon 6 from the ω end. They prevent blood platelets from sticking together prematurely, which would cause blood clots.
In our ancestral hunter gather, the ration of ω6:ω3 FA was 1:1, however we consume closer to 10:1 The recommended is <5:1
The recommendations are to increase the consumption of oils with a low ω6:ω3 ratio
Commercial Fatty Acids¶
When a product is labelled as being a particular kind of fatty acid, such as a polyunsaturatd fatty acid, it will contain a combination of poly/monounsaturated and saturated acids, but the advertised one will be in the majority.
Prostoglandins¶
Fatty acid derivatives of \(\ce{C20}\) with a cyclopentane in the middle
Waxes¶
Waxes are the ester product of a fatty acid and a long chain alcohol. They have an ester group in the middle and hydrocarbon tails on either side of it. These are often used by animals for waterproofing (e.g. in hair, fur, etc.)
Triglycerides¶
Are the esterified product of three fatty acids and a glycerol molecule.
They can be classified into simple triglycerides, which rarely occur in nature and have three identical fatty acids and mixed triglycerides which will have three different triglycerides.
Fatty acids aren’t typically used as an energy store due to their reactive acid, however they are stored as the fatty acid and can be easily hydrolysed when not needed. They are considered long term storage and will stick around seasonally
Glycerophospholipids¶
Steroids¶
Are a class of lipid based on the sterone ring core. they all have a polar head group coming off the terminal cyclohexane ring that will either be a hydroxyl, or a ketone. They are comprised of a variety of substitutions to the outside of the ring that gives them their specific specificity as a substrate
Sterols are a way in which the the cell can modulate the membrane fluidity. They enter the cell membrane and interfere with the stacking of the phospholipids making them less strongly bonded together. They’re also used to increase the blood viscosity.
Cholesterol is the main sterol in animal cells and is transported in the blood. excessive amounts can cause to an over-increase in the viscosity of the blood. Mamals can either obtain it from food or synthesise it in the liver.
Chemical Properties of Triglycerides¶
The main property is that triglycerides can be hydrogenated with a Ni catalyst to reduce any double bonds to single bonds. this has the effect of increasing the viscosity for uses such as margarine and copha. Hydrogenation can also be partially accomplished, resulting in some remaming double bonds and/or some conversion of cis → trans
Triglycerides can also be “saponified” to salt forms of the fatty acids. This is done in the soap making process and has two main salt forms. Na salts form solid soaps and K salts form liquid soaps.
Stoichiometry plays a role here though and the ration of base to triglyceride will be 3:1 and the ration of base to FA will be 1:1
Profiling Fats¶
Saponification number (SN)¶
Saponification can also be used to quantify the average molecular weight of the fatty acid
Definition
The \(mg\) of \(\ce{KOH}\) required to saponify \(1\:g\) of fat or oil
The higher the SN, the lower the average MW of the FA
Calculating the SN of a known FA/TG¶
- Calculate the MW of the FA/TG
- Calculate the moles in \(1\:g\) of FA/TG
- Calculate the moles of salt required to saponify (watch stoichiometry)
- Calculate the mass of the salt in mg to get the SN
Calculating the SN of an unknown FA/TG¶
- Find the mass of the salt in \(g\) required to saponify \(1\:g\) of FA/TG
- Convert that to moles of salt
- Convert that to moles of FA/TG (watching stoichiometry)
- Divide \(1\:g\) by that many moles to get the SN
Iodine Number (IN)¶
Is a measure of the degree of unsaturation of a FA/TG
Definition
The \(g\) of \(\ce{I2}\) consumed by \(100\:g\) of fat or oil
One mole of \(\ce{I2}\) will react with one mole of double bonds. The In will increase as the # double bonds increases and may help in predicting the shelf life of a particular fat/oil. The double bonds are a weak point for oxidation and so the more double bonds, the more prone to oxidation the fat is
It’s also a predicter of the Titre point, which is the temperature at which the fat will solidify. the titre point decreases as the IN increases, so the lower the IN, the more likely the fat will be solid.
Example IN of common products¶
| Fats and Oils | SN | IN |
|---|---|---|
| Coconut oil | 251 | 7-12 |
| Palm kernel oil | 238 | 16-19 |
| Palm oil | 203 | 44-51 |
| Olive oil | 190 | 80-88 |
| Peanut oil | 190 | 84-105 |
| Canola oil | 192 | 105-120 |
| Sunflower oil | 191 | 125-144 |
| Soybean oil | 190 | 120-136 |
| Cocoa butter | 191 | 35-40 |
| Stearic acid | 206 | 3 |
Calculating the IN of a known FA/TG¶
- Calculate the MW of the FA/TG
- Calculate the moles in \(100\:g\) of FA/TG
- Calculate the number of moles of \(\ce{I2}\) required to add across the double bonds (watch stoichiometry)
- Convert the moles of \(\ce{I2}\) to mass of \(\ce{I2}\) in \(g\)
Calculating the # double bonds of an unknown FA/TG¶
- Find the mass of the \(\ce{I2}\) in \(g\) required to break \(100\:g\) of FA/TG double bonds
- Convert that to moles of \(\ce{I2}\)
- Find the moles of FA/TG (from provided MW)
- Divide the moles of fat by the moles of iodine to get the average # double bonds per molecule
Biodiesel¶
These are a non-petrol based alternative to diesel fuel that’s produced from renewable biological sources, such as waste vegetable oils. It’s non toxic and biodegradable.
It’s produced by reacting triglycerides with ethanol or methanol in the presence of \(\ce{NaOH}\) or \(\ce{KOH}\), resulting in methyl or ethyl esters. The remaining glycerol can be extracted off and used in the production of soaps and other products.