Curious Cook: A fat lot of good – Part 4


Read the other parts of this story:

Part 1

Part 2

Part 3


Earlier on, it was stated that unsuitable, rancid or overused cooking oils are really bad for health – and now we provide the justifications behind that accusation. It isn’t difficult as the science is quite clear on the subject – though we will first need to explain about how fats in cooking oil react when used and how they ultimately deteriorate.

It is important because the deterioration of fats causes fatty acids to break loose and become free fatty acids (FFAs) mingling in the oil – but once free in oil, they tend to behave like unruly children let loose in a fun fair with no supervision. It is quite unlike the FFAs during digestion, which are shunted along pre-determined pathways.

In cooking oils, FFAs combine readily with oxygen all over the place and get up to all kinds of chemical mischief.

In normal use and storage, cooking oils eventually deteriorate – the glycerol bonds in triglycerides break down and diffuse FFAs, sometimes very rapidly, due to oxidation.

The rate of oxidation is influenced by one or several factors operating in tandem. Oxygen is eight times more soluble in fats than in water so oxidation of FFAs happens faster than you might imagine, even though you generally can’t see it happening.

The main factors behind the oxidation processes are temperature, fatty acid composition, light (specifically UV light), certain metals, time, residual particles from earlier oxidation processes, and of course, access to air (oxygen).

Cooking oils can also deteriorate even without oxygen if water is present – this is a process called enzymatic peroxidation, where enzymes in plant oils and animal fats catalyse reactions between oil and water.

This is how butter usually becomes rancid in the refrigerator, as butter contains a significant amount of water as well as animal fats.

Used cooking oil that is collected from business and residential premises.
Used cooking oil that is collected from business and residential premises. Photos: The Star

Yet another way cooking oils deteriorate is via microbial action – bacteria, moulds and yeast can generate enzymes which break down lipids, although water usually also needs to be present as a partner-in-crime.

However, we will mostly focus on oxidation and what happens to cooking oils when they get oxidised.

At this point, you might be disappointed to learn that the documented benefits of polyunsaturated fats (PUFA) disappear when you use them for cooking – the oxidation processes (especially heating to cooking temperatures) cause the release of FFAs which react with oxygen (oxidise) rather rapidly and turn into peroxides, alkenals and aldehydes.

The same thing also happen with saturated and monounsaturated fats too but at a rather lower rate – oxygen molecules simply have a preference for bonding with the multiple double-carbon links in polyunsaturated fats.

Peroxides, alkenals and aldehydes are also known as oxidants or free radicals and are highly active compounds which are extremely prone to reacting with cells and generally messing up the normal chemical mechanisms of the body.

Free radicals are therefore commonly associated with cell damage, inflammation and general disruption of normal body functions.

Probably the most problematic free radicals are malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) – MDA promotes the mutation of cells while 4-HNE is simply toxic, especially to the kidneys and liver.

Critically, free radicals can also severely damage LDLs, causing them to be hunted and attacked by macrophages – and as we know, macrophages which pick up cholesterol quickly turn into foam cells and die, releasing cholesterol remnants which get stuck in damaged or inflamed cells. The LDLs damaged by free radicals are sometimes called “oxidised LDLs”.

Now we return to the subject of oils we must be wary of, especially when eating outside the home.

One thing to remember is that the FFA content in cooking oils should never be more than 2% when cooking begins, else it would be asking for trouble, health-wise.

It is hard to check for this – but nevertheless, if you see vats of smoking oils or the same deep-fat fryer used countless times, then it is a reasonable assumption that the FFA content would be more than 2% in these situations.

Frying ramen
Frying ramen

Unsuitable cooking oils

There are several unsuitable cooking oils which are commonly (and heavily) used in the food industry. Although PTFs (plant trans-fats) are banned in some countries for domestic use, they are not always banned for commercial use, especially in undeveloped countries.

In fact, bulk-use cooking oils in tropical countries probably need to be partially hydrogenated to preserve them in warm climates – else they will simply decompose very quickly in the heat.

However, the fact remains that cooking oils with trans-fats should be avoided wherever possible for the reasons stated earlier.

They are unnatural geometric isomers of unsaturated fats which profoundly mess with the lipoproteins in the body, among other things – and are really unfit for consumption in any form.

Other unsuitable cooking oils are those which have low smoking points – the smoking point is an indication of FFAs oxidising in air and the lower the temperature at which an oil begins to smoke, the stronger the indication of the formation of free radicals.

Not to be left out are the oils with high concentrations of PUFAs, especially when used for frying or cooking in high heat – as mentioned, they oxidise very quickly, producing damaging free radicals.

As such, it is a good idea to avoid cooking with sunflower, corn, soybean, canola, grapeseed and walnut oils – though other oils such as sesame seed oil, generic vegetable oils or margarines should also be spurned where possible.

The best oils for cooking would appear to be palm oil, extra virgin olive oil, coconut oil, lard and butter.

Rancid cooking oils

A couple of years ago, I was horrified to watch a BBC documentary about how oil merchants in China were recycling used cooking oils from drains and sewers.

It was so sickening I could hardly breathe due to fear and anger at the poisons they were inflicting on innocent people eating food cooked with their recycled oils – which were happily bought by restaurants and food stalls.

Not only were the oils rancid, they were clearly laced with detergents, bleaches, cleaning agents, sewage and unthinkable volumes of serious toxins.

This is not to say the same appalling situation exists in other countries but the commercial pressures of running a food outlet can mean that sometimes safety and storage standards can drop, especially in a slow month. And this can happen anywhere, except in the most rigorously monitored countries.

As rancidity is an indication of oxidation, this means that rancid oils already contain free radicals and these free radicals will catalyse a more rapid formation than usual of new free radicals when the oil is heated.

Frying tempura
Frying tempura

Please don’t assume that you can taste the rancidity of any cooking oil used, especially for fried foods – the batters and coatings of food very often hides the rancidity fully, so you can never know.

So if the food comes from a very cheap fried food stall, perhaps it is worth checking that they have fresh opened cans of oil hanging about.

Overused cooking oils

Overused oils suffer from the same deficiencies as rancid oils – and may actually be worse than rancid oils. This is because even more of the FFAs in such oils would have been converted to free radicals, so the food is actually fried in a chemical bath of peroxides, alkenals, aldehydes plus other oxidation compounds such as ketones, alcohols, aliphatic carbonyls, hydrocarbons, etc.

If you have a keen sense of smell, it is sometimes possible to detect the acridity when an oil is overused due to the presence of butanal, hexanal and a raft of other complex compounds such as trans, cis-2,4-decadienal, trans, cis-2,4-heptadienal, 1-octen-3-ol, etc.

On top of that, old food particles lingering in the oil would keep churning out streams of Advanced Glycation End-products (AGE) from protein and sugar residues – all the above-mentioned free radicals, oxidation compounds and AGEs either contribute to inflammation or toxicity, or quite often, both.

In short, there is not a single item of good news about eating food cooked in overused oils, so please be wary about places that may use them.

An aside: The length of fatty acids and why they might matter

You might have heard of stuff like short-, medium- and long-chain triglycerides (or fatty acids). They are usually abbreviated into SCT (SCFA), MCT (MCFA) and LCT (LCFA) and they may have some significance in our diets.

The differences between them are explained by the length of their aliphatic tails, and these aliphatic tails, by the way, is also the reason why oils don’t mix with water.

A diagram of a SCT aliphatic tail is as below, where the central bone of the tail is a sequence of carbon atoms, with hydrogen atoms linked to the carbon atoms. This kind of structure is hydrophobic and will not dissolve in water.

Diagram of an SCT (short chain triglyceride) aliphatic tail
Diagram of an SCT (short chain triglyceride) aliphatic tail

As you may surmise, the names themselves express the number of carbon atoms in the triglycerides – SCTs have short aliphatic tails of less than six carbon atoms, MCTs have between 6-12 carbon atoms and LCTs more than 12 carbon atoms.

SCTs are produced by the intestinal flora during the fermentation of dietary fibre and are readily metabolised in the gut. Although LCTs are much more common, both MCTs and LCTs come from the diet – but they are metabolised differently.

MCTs are commonly found in coconut and palm kernel oil. There have been some extravagant claims made about the health benefits of MCTs – but all we know is that MCTs are somewhat easier to digest (as they do not need bile), they have a different metabolic pathway (they are generally burned off in the liver thus expending food-derived calories more quickly) – and they are not stored in adipose (fat) tissue.

The main negative aspects are that MCTs are not essential fatty acids usable by the body, there are some documented side-effects (eg. digestive issues, minor cognitive problems, etc) – and they are also saturated fats.

None of the claimed benefits such as sustained weight loss, insulin regulation, stroke prevention or a cure for Alzheimer’s have been clinically proven in proper studies – and although there have been claims about low heart disease rates in countries where MCT consumption per capita is high, none of these countries (eg. Sri Lanka, Indonesia, Dominican Republic, etc) have particularly notable lifespan expectancies.

While deepfried foods are yummy, be wary of the oil used as rancid and overused cooking oils produce a nasty cocktail of chemical compounds in the food that are really bad for you.
While deepfried foods are yummy, be wary of the oil used as rancid and overused cooking oils produce a nasty cocktail of chemical compounds in the food that are really bad for you.

Japan usually has the longest living citizens and their diet is almost exclusively devoid of MCTs. As such, the biggest benefit of oils laden with MCTs in a modern kitchen may be that they generally have higher smoking points and therefore less prone to oxidise during frying or cooking.

I may return to the subject of MCTs later when larger peer-reviewed studies are available – all MCT clinical studies on humans to date seem to use small groups and therefore the results are not always significant.

Journey’s end

So, it has been quite a long thought journey since that lady bumped into me in that supermarket and I am honoured that you are still reading this (and not using the paper to wrap fish). I also have to get back to doing my proper day job.

In summary, I have concluded that perhaps it is not always too clever to ingest too much fatty meats after all, especially while eating out. It is not because of the fats (or cholesterol) in the diet (which cannot “stick” directly in the arteries as people think), but rather because of my preference for grilled, fried or roasted meats – and I am now a bit uncomfortable about possible issues with the preparation and ingredients used by strangers.

Also, dry-heat cooked meats often contain a lot of AGEs, especially around the crispy cooked fatty tissues. AGEs are inflammatory compounds and it does not make sense to increase any suspicion of arterial wall damage.

High blood pressure is also a leading cause of arterial wall damage so I am now using a blood pressure monitor and trying to chill out (even more). As I have always liked seafood, there are no plans to curb eating fish, which can help restore the Omega 6-Omega 3 balance.

The strategy is to avoid provoking arterial wall damage via inflammation and to reduce the number of LDLs flowing through the arteries.

So, in a roundabout way, my family is correct to warn me about fatty foods – they had the right motivation but not quite the right reasons.

The proper reasons, in my opinion, are the avoidance of excessive amounts of AGEs and by-products of frying in oil.

However, it does not stop me having a few tasty steaks at good restaurants – but now I am insisting on smaller portions of grass-fed beef in the hope that CLAs actually do have a beneficial effect. But mostly I cook at home whenever possible, and there I have made absolutely no changes to the fat content of the food.

But before the story ends, here is a list of ingredients of a typical pot of 1% low-fat cottage cheese (it is a list which I’ve just pinched from a shopping website) which we mentioned at the start of this series:

Cultured Skim Milk, Nonfat Milk, Whey Protein Concentrate, Cream, Salt, Guar Gum, Mono- and Di-glycerides, Locust Bean Gum, Xanthan Gum, Natural Flavours, Potassium Sorbate And Carbon Dioxide (Preservatives), Artificial Colour, Carrageenan, Polysorbate 80, Vitamin A Palmitate, Enzymes.

Although I am not a nerd, it is intriguing that the pot contained not only mono- and diglycerides (both are plant trans-fats) but also polysorbate 80. This is an emulsifier, also known as darbepoetin alfa, clinically linked with increased risks of blood clots, strokes, heart attacks, heart failures, and tumour growths or recurrences in patients with certain cancers. Yummy.

Woody Allen once said, “I am not as normal as I appear to be”. It’s just a silly, frivolous idea – but I think perhaps this quote should be on the labels of many processed foods and maybe also on the packaging from fast food outlets.

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