A beginners guide to fat

Our bodies need fat to function. For example, after allowing for water (70-80% by weight), our brains are mostly fat (health-conscious zombies beware). Fats in the brain provide the electrical insulation for our myriad nerve fibres, and have a role in nerve signaling and neurotransmission. Just as for vitamins, some fats are ‘essential’ – that is, our bodies cannot manufacture them and they must be obtained from our diet. The omega-3 and -6 fatty acids are examples.

This post is about getting to know fat a little better – in 10 questions.

1. What is the difference between fat and oil?

Nil.

Oil is a culinary term for a fat that is liquid at room temperature. At the molecular level, oil is fat – they are both triglycerides (mostly).

2. What is the difference between fat and fatty acid?

The terms are often used interchangeably, which is incorrect and confusing.

Fatty acids are chains of carbon atoms, with two (or one) hydrogen atoms attached to each carbon (the ends of the chains have other atomic arrangements that can be ignored for simplicity). No carbon can have more than 2 hydrogen atoms attached without breaking the chain.

Fats (triglycerides) are made up of 3 fatty acids attached to a short glycerol molecule, like flags to a flagpole.

3. What is the difference between saturated and unsaturated fat?

The difference of consequence is the structural deformity of their fatty acids.

FAT-TriglycerideIf every carbon atom in the fatty acid bonds to 2 hydrogen atoms, then the fatty acid is saturated (the carbon chain is saturated with hydrogen, no more can be added).

With this configuration, the carbon chains take on a linear zig-zag-zig etc configuration.

In some fatty acids, adjacent carbon atoms can both break one of their hydrogen bonds and form a second bond with each other instead (called a double bond). It is now an unsaturated fatty acid because it does not contain the maximum possible number of hydrogen atoms.

With an unsaturated fatty acid, a zag in the zigzagging carbon chain is skipped. This deforms the configuration and introduces a sharp bend to the fatty acid.

If just one carbon pair does this, the fatty acid is mono-unsaturated. If more than one pair does, it is poly-unsaturated (and more deformed).

The picture illustrates a basic triglyceride. The blue represents the glycerol molecule. The zigzag chains are the fatty acids (with a carbon atom at each vertex, omitted for clarity). This glycerol has 3 fatty acids attached, two of which (top) are saturated (straight chains), while the third is mono-unsaturated (a single double bond shown in red that kinks the chain).

4. What effect do bends in fatty-acids have on the physical properties of fats?

They determine whether the fat is an oil or a fat in culinary terms.

Because saturated fatty acids are a ‘straight’ zigzag, they can nestle in close to each other. This alignment increases the short-range attractive force between the molecules (van der Waals force) and holds them together. As a result, a fat made up primarily of saturated fatty acids is usually a solid at room temperature.

Because unsaturated fatty acids are misshapen, they are unable to get close enough for the weak molecular forces of attraction to bind them together. Fats made up of unsaturated fatty acids usually don’t solidify and remain liquid at room temperature.

5. What do saturated or unsaturated weights for fats on food labels measure?

Fatty acid composition.

nutritionIndexA fat can contain a maximum of 3 fatty acids that can be all of one type (e.g. saturated) or a mix of types (e.g. 1 saturated and 2 mono-unsaturated). A culinary fat will contain a mix of fats that in turn hold a mix of fatty acids. The saturated vs. unsaturated weights that are on food labels measure the totality of the fatty acids of each type.

By the way, if you add those food-label weights up, it never comes to the ‘total fat’ weight. This is because total fat weight also includes the weight of the glycerol molecule and because the FDA allow any weight less then 0.5g to be listed as 0g.

6. What is margarine? Is it a fat?

Yes, margarine is unsaturated fat that is made more saturated.

margarineThe aim is to make oil solid at room temperature, and hence a fat. To make margarine, unsaturated fats are processed in the presence of water (hydrogenated). This can break some double carbon bonds and the carbon atoms then re-bond with hydrogen from the water. The fat becomes more saturated and therefore straighter. This results in the oil becoming more solid. The degree to which this is done determines spreadability.

A byproduct of the margarine manufacturing process used to be the generation of trans-fats.

7. What are trans-fats?

An unsaturated fat that behaves like a saturated fat.

Remember that adjacent carbon atoms in an unsaturated fatty acid are missing a hydrogen atom and have double-bonded with each other instead. The missing hydrogen atoms are usually from the same side of the carbon atoms. Chemists use the prefix cis– to describe this arrangement (cis is Latin for ‘on the same side’). The kink in the linear shape of an unsaturated fatty acid occurs only with the cis-form. Virtually all unsaturated fatty acids take the cis-form.

In margarine manufacture some double carbon bonds can be left with hydrogen atoms that are missing from opposite sides of the carbon atoms. Chemists use the prefix trans– to describe this form (trans meaning ‘opposite side’). This is a more balanced configuration, meaning that even though the fatty acid contains a double bond (and is therefore unsaturated), there is no longer a significant kink in the chain.

This ‘straightened’ unsaturated fatty acid then behaves as though it were a saturated fatty acid.

FAT-CIS_TRANSThe picture shows a section of a saturated fatty acid (top). The red dots represent carbon atoms each with two hydrogen atoms attached. The zigzag has been removed for simplicity. The middle chain contains a double bond in which the hydrogen has been removed from the same side (cis). This introduces a bend in the same direction on each carbon atom and therefore a kink. In the bottom, the hydrogens have been removed from opposite sides (trans) which reverses the kink and straightens the chain.

Trans-fatty acids do occur in nature at trace levels. Our main dietary sources are the ruminants (cows and sheep for example, but not pigs). Microbes in the rumen of these animals produce trans-forms of fatty acids that are incorporated into the animal’s meat and dairy products. However, artificial and naturally occurring trans-fatty acids are chemically different, and their health effects are still under investigation. Trans-fats can also develop in repeatedly-heated deep frying oil.

The FDA considers there is enough preliminary evidence to take a cautionary approach to artificial trans-fatty acids. Although the FDA does not regulate levels (there is no scientific data to set these), it does now require levels to be reported on food labels.

Margarine is presently manufactured in such a way that its trans-fat content is reduced to trace levels. It has become a competitive marketing issue. The simple act of compulsory labelling created a marketing pressure.

8. What gives meat from different animals different flavours?

Fat.

Meat tastes ‘meaty’ beause of the proteins and many other molecules in the muscle cells. However, muscle fibres differ little between species (they all have the same job to perform) and it is not the proteins that set chicken apart from beef for example, it is their fats and fat-soluble molecules.

Microbes in the rumens in beef and lamb can generate fatty acids from carbohydrates (grain and grass for example). This influences flavour. Omega-3 fatty acid can only be produced by grass fed beef, not grain fed. Grain fed tends to have a milder flavour. Salt-marsh fed lamb is a Normandy specialty because of the effect of the lambs’ diet on fat flavours.

It doesn’t follow that a well marbled meat will be more flavourful. The visible fat (triglyceride) deposits don’t carry the most flavour, although they might add succulence. Muscle cell walls are constructed from fats (strictly lipids) with one fatty acid missing (the di-glycerides). These are not visible to the naked eye, but carry the unique flavours of the meat variety and reflect the upbringing of the animal.

9. How do we taste fat?

Taste, aroma, mouthfeel.

Cooking does little to fat and doesn’t break down fat molecules. We can tell this because oil heated to deep-frying temperatures is still oil. If heated excessively, fats do break down but the fragments have unpleasant taste and aroma.

The body breaks down fats into fatty acid fragments through the action of enzymes and gut bacteria.

In the mouth, it seems human saliva contains an enzyme (lipase) that breaks down some of the fat into fatty acids as we chew. Recently, taste receptors for long chain fatty acids have been identified on the tongue. In rodent studies, activation of this receptor up-regulates other taste receptors, particularly umami (savoury). Human studies are ongoing and of much interest, especially for obesity research.

A completely separate system (trigeminal) detects fat by mouthfeel and sends another pleasurable signal to the brain.

Smaller lighter fats will be released by chewing and travel retro-nasally to be detected by the olfactory system.

Not surprisingly, the system is working in concert to reward us for eating the food our body needs.

10. What is warmed-over flavour?

Breakdown of fatty acids by oxidation.

Unsaturated fatty acids, particularly those composing the walls of muscle cells, are vulnerable to oxidative break-down (we’re all told to eat more antioxidents). It is this oxidation that causes warmed-over flavours to develop in cooked meat.

Oxidation is accelerated by the presence of iron. Meat proteins contain abundant iron locked away in the protein molecule. With cooking, the denaturation (unraveling and breakdown of the protein) releases this iron which then catalyzes the oxidative breakdown of the fatty acids.

Lamb seems to be particularly prone to this, and a reason that it is often cooked with Mediterranean herbs such as rosemary and thyme is that these are high in antioxidents that retard development of warmed-over flavours.

 

Summary

If you got this far, you’re a legend.