In this new instalment of scientific food blogging we are going to
be looking into the taboo world of fat. Whether you believe fats are in your
health favour or not we can all agree they do make stuff taste better! A
perfectly scientifically valid argument to this point lies with the cocoa bean;
aka the chocolate bean. Cocoa in its ground up natural form is rancid (that’s a
fat joke, we will come to that) which no amount of healthy
antioxidants can make up for. However add in delicious butter and cream to
the harsh tasting cocoa, with a dash of sugar and the result makes the world a
much better place. Skipping the introductory quips, in this post we are going to explore the differences
between oil and fat, and whether these terms can be used interchangeably. We will also look into the origins of fat,
examine the battle royal that is butter vs oil vs margarine, and finally delve
into the end of a fats life – its expiration date.
Biological
and Culinary Uses
We are told that fats have serious health implications for us so
why then are fatty foods so desirable to us? This is a side effect of our
evolution; in nature fats are used as an effective energy store. Traditionally
fats offered lots of energy that could be stored for times of limited food or
for doing intensive work; like running away from dinosaurs. We as the human
species have then evolved to like the taste and texture of fats ensuring we eat
them; granting us enough energy to get away from any roaming T-rex we may bump
into. Yes you could argue that athletes today are more likely to eat a balanced
carbohydrate rich diet now, but carbs are not as effective as energy stores as
fat /oil per unit weight. For example if we stored our excess energy supply as
carbohydrate rather than fat we would be considerably heavier and unable to out
run and escape the snapping jaws of death. More interestingly there could be a
strong argument made for athletes to return to fat based diets over carbs due
to our species evolving using fats for energy. Bread and pasta are rather
modern inventions in comparison to eating nuts etc.
Fats have wide applications in food that are not always obvious to
us and here we will quickly examine where their culinary genius. Initially we think of oil as a cooking medium
where fats are used to prevent food sticking to pans, while also providing good
energy transfer to cook food. Alternatively water could be used to cook food
but it would not yield the same results. Water cannot achieve the same high temperatures
as fat (due to water evaporating ~100 oC at normal earth pressures),
prohibiting the formation of intense meaty sweet flavours and the crisp
textures associated with oil frying. More subtlety fats trick our brains into
thinking food is moister than it is. When chewing food we use saliva to help us
swallow it therefore the drier the food the more saliva will be used. Oil
lubricates food allowing it to be easily swallowed, making us think a piece of
meat is juicer than it actually is. Using this information we can now
understand why low fat meats have a small “cooked” window before we consider
them overcooked and dry. It also offers an explanation as to why when we eat
younger animals and cuts of meat with less fat they initially are considered
moist, but the more we chew the drier they become in our mouths. Room
temperature solid fats have also been widely used as a preservative, for
example in duck confit. When used as a preservative the fat acts as an oxygen
barrier, protecting the intended food from becoming oxidised and spoiling. Fats
also offer the cook the ability to form tender cakes, flaky pastry and even a
method of cleaning their hands.
Fats and
oils what’s the difference
So far I have used the terms fats and cooking oils interchangeably but
are cooking oils and fats the same? Yes, they are both from the same family of
molecules called triglycerides.
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| The structure of an example triglyceride molecule. |
A triglyceride molecule. Tri- means the three chains/fatty acids and
the glycerol is group that links the three chains. This raises an interesting
question; if all fats are triglycerides then “why are some fats solid and
others liquid?” The key to solving this conundrum is saturation. The words
saturated and unsaturated have been used to describe the world of fat for a
while now with little in the way of explanation for what the terms mean. Here I
shall put to rest the organic chemistry jargon. Simply a saturated molecule is
one where all the carbon atoms are connected to other carbon atoms by one bond.
Subsequently an unsaturated molecule is one where carbon is connected to
another carbon atom through multiple bonds. This is drawn below:
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| Top Left: An unsaturated molecule that has double carbon carbon bonds in its structure. Top Right: A saturated molecule that has single carbon carbon bonds. The letters C and H represent carbon and hydrogen atoms respectively and dashed lines represent the bonds between the atoms. Middle Left: A striped down overall version of the overall molecular structure of an unsaturated chain. Middle Right: A striped down overall version of the overall molecular structure of a saturated chain. Bottom Left: Poor packing of unsaturated fat molecules. Bottom Right: Tight molecular packing of saturated fat molecules. |
Saturated molecules adopt a straight line shape that packs together
very well which fits the definition of a solid; a tightly packed structure
where molecules have limited movement within their arrangement. From this
information saturated triglycerides must form solid fats. On the other hand
unsaturated triglycerides cause kinks in the carbon chains. These kinks
prohibit good packing of the molecules increasing the average distance between
the molecules; giving unsaturated molecules lower melting temperatures i.e.
they form liquid oils. As opposed to a solid, a liquid is made of molecules
that are more freely to move about next to one another enabling us to pour oil
at room temperature. Notice I said room temperature because I can buy olive oil
that becomes a solid at 4 oC (I said can, not does, due to the
diversity in oil purity and resulting melting temperature).
Where do
we get fats from?
As discussed, fats are fuels giving them an across species origin with
each source offering various pros and cons including flavour differences, heating
limits and health benefits. Here we will examine three unique ways in which we
harvest fat.
1 - Maybe unexpectedly, seeds have a high concentration of fat which is used as the energy source to start growing. Sunflower oil is prime example of a fat we harvest from seeds and is produced from crushing sunflower seeds. However cheaper sunflower oils may be made by heating up the process leading to a higher yield but often with detriment to the oil quality. This process is heavily seen in producing olive oil where heating the olives to get regular olive oil results in a loss of its delicate flavour. Olives that just get pressed (the good stuff) is denoted as extra virgin olive oil.
2 - Milk from animals is high in fat to provide energy to a mothers young. But milk is an emulsion with a high concentration of water. To yield a fat i.e. butter we churn milk, which enables fat droplets suspended in the milk to stick together forming a large solid (this is a huge generalisation but its fit for purpose here I think).
3 - The final fat source is again from animals but rather than collecting their milk, it’s a rather more invasive process where we render fat from their carcase. Animals use internal fat in much the same way we do; for heat insulation and as an energy store for when food is scarcer i.e. in the winter months. To render this heat is used to melt the solid fat in animals - typically this is seen as goose and duck fat around Christmas. Less favourable due to our current palates is beef dripping however similar processes are still used.
There are claims to which fats are better for you but I will not be so
bold to say which isn’t and which is. As with the nature of research with more
data comes better understanding and I know doubt that one day a definitive
answer will be found. I just hope its butter or duck fat.
Now I know some of you that have been paying attention will be saying
“hang on, then what’s margarine and where does that come from?” The answer is
margarine is fat that has been made to be like butter. Initially there was a
competition set out by Napoleon to make a synthetic edible fat due to a lack of
oil to meet modern industrial demands. Animal fats were initially used, namely
suet, which could give a buttery texture and a small amount of milk to give it
palatable taste. However it wasn’t until 1905 we made the swap from processed
animals to processed liquid vegetable oils to vet our cheaper buttery fix. As
discussed a liquid oil is more unsaturated and therefore to make oil into a
semisolid like butter we need to turn those unsaturated double carbon carbon
bonds into saturated ones. To do this we heat up the oil and add in high temperature
high pressure hydrogen gas, breaking the double bond whilst adding in 2 new
hydrogen atoms.
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| Figure to show the reaction of a unsaturated fat chain to a saturated one. |
Now that the oil has the texture of butter we need to make it taste like
butter through the addition of diacetyl (the molecule responsible for butter’s
flavour) not forgetting some vitamins, salt, preservatives and colour
enhancers.
When to
Use Which Oils
As discussed oils and fats have high boiling points that allow us
to cook food quickly. However when heated in the presence of oxygen the
triglycerides begin to break down into smaller molecules, the smaller bits of
triglyceride can now leave the bulk oil in the form of smoke. The temperature
at which oil begins to smoke is therefore referred to as its smoke point. Oils
should not be allowed to smoke due to formation of free fatty acids (a bit of
broken down triglyceride) leading to an rancid taste in the oil; which will be
transferred to the food. Repeated heating and cooling of the oil will lead to a
build-up of free fatty acids that is detrimental to the oil quality.
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| Figure to show the structure of an example of a saturated free fatty acid. |
It is clear then that when deep frying or cooking at very high
temperatures, we need an oil/fat that has a high smoke point. However fats themselves
are contaminated with other molecules which is often beneficial to their
flavour. These contaminants lower the smoke point of the oil, and this can be
seen readily in butter. Butter by its self will burn in moderate temperatures
but if you melt it first and skim off the solid precipitate we make ghee/clarified
butter, allowing a ~75 degrees C temperature increase before it smokes. As a
rough guide then lower smoking point fats have more flavour and should be
restricted to dressings and low temperature cooking. I have added a smoke point
chart below but this is not my work (there is no point re-inventing the wheel)
and I have referenced it somewhere in this article…
 |
| Table to show common kitchen fat smoke points. |
Keeping
Fat Fresh
We have already briefly talked about how high temperature heating
cycles deteriorate fats; but heating only speeds up this process of oxidation
leading to the forming of free fatty acids and a resulting rancid taste. We
should then try minimise the fats contact with air to preserve our fats. While
being at odds with water through non polar/polar interactions, water and fat
still shouldn’t be mixed. Water leads to hydrolysis of the triglycerides again
forming free fatty acids and rancidity which again increases with heating. This
can be a problem when deep frying. Water/steam from the cooked food is exposed
to the oil thereby reducing the life span of the oil in the deep fryer which
can lead to increased expenditure. To overcome regular oil changes in your
fryer you may wish to swap to a pressurised deep fryer, which increases waters
boiling point in the food resulting in more moisture staying in the food (not
escaping through steam) and exposing less water to the oil. Never the less oil
can still be exposed to light, iron and salt which have all shown to increase
the formation of free fatty acids. In other words just change the oil in your
fryer regularly! And do not be tempted to mix oils - this has been shown to
cause cross polymerisation reactions, especially at high frying temperatures. The
products of this are suggested to be carcinogenic.
It’s here that I will call it a day on fats for now. It’s a shame
that we ended on a carcinogenic bomb shell but I guess for many of us it’s as
true here as it in life. Next week’s topic “The Abattoir” will be much cheerier
featuring animations, real footage and first person accounts of its day to day
running. And for chemistry aficionados,
yes I did largely throw stereochemistry out of the window for the ease of
drawing figures in this article.
References
H. Mcgee, On Food and Cooking, Scribners, New York, 1st ed. 1984
http://jonbarron.org/diet-and-nutrition/healthiest-cooking-oil-chart-smoke-points#.VobPZvmLSM8
http://thetastee1.rssing.com/chan-8502785/all_p1.html
https://en.wikipedia.org/wiki/Smoke_point
