We cannot see, touch or feel these invisible molecular forces that dominate our day to day life.
Posted on July 24th, 2011

Dr Hector Perera     London

Ventilation

This is a gift we have; with or without your knowledge we understand certain things only by smell but not the common dangerous killer, carbon monoxide. When we cook, methane gets combusted in oxygen to form carbon dioxide. If less air is available for burning methane, then it is possible to form carbon monoxide. 

methane + oxygen —–> carbon monoxide + water.
2
CH4(g)  + 3O2(g)  ——>     2CO(g)   +   4H2O(l)

Carbon monoxide is a very poisonous gas. It cannot be seen or smelt. Faulty gas fires or boilers may produce carbon monoxide and poison the air in a room without anyone knowing. Every year, people die from carbon monoxide poisoning. Next time, open the kitchen window for fresh air, before you light the cooker.

Two types of food

There are two types of foods, vegetarian and non vegetarian. All these food have their own odour or smell at room temperature for example ripe bananas, mangos, pears or pineapples have the odour that is special to that particular fruit. A ripe mango never smells like a ripe pineapple because the smells have different chemicals. If you try to smell a ripe mango at 10 ft away or further then you cannot smell it but at very close distance from the nose, then you can smell the fruity odour. The smell is due to the volatile chemicals in the ripe mango but an unripe mango doesn’t smell as it hasn’t got volatile chemicals at room temperature. But some ripe fruits such as jack can be smelled more than 50 ft away that is while it is hanging up in the tree. Is it an invitation for natural distributors of seeds, crows and bats?

Mind the nose

 Here is another fruit called DURIYAN, with spiky or thorny peel which has a special strong odour that can be detected even at very far away. This is again due to highly volatile chemical given out from the ripe fruit, don’t try and smell it very closely, MIND THE NOSE.  Some says it is the peel that matters not the fruit but I am not sure. One would ask how you would eat it with that kind of odour. Some know the taste, so smell doesn’t matter, believe me, it’s really tasty.

Chemicals of ripe fruits

 Esters are quite common in our everyday life, as I mentioned. The sweet smell of fruits and flowers are nothing but esters, apart from that a number of things have esters.

The smells of fruits are due to the presence of complicated mixtures of chemicals, (not always just esters) rather than solely one ester, but individual esters are often important components.

 Many are responsible for the fragrance and flavour of flowers and fruits; for example, isopentyl acetate is present in bananas, methyl salicylate in wintergreen, and ethyl butyrate in pineapples. These and other volatile esters with characteristic odours are used in synthetic flavours, perfumes, and cosmetics. Certain volatile esters are used as solvents for lacquers and paints. Not forgetting the esters in nail varnish.

Though the size of the molecules varies, it seems that the presence of the O=C-O group is vital to the kind of smell. You might think this is beyond the knowledge of ordinary people but actually, esters are known even to GCSE students.

The chemical analysis of the esters such as using  Ethyl acetate, 2-methylbutyl acetate, butyl acetate (BA), and hexyl acetate or any other chemical can be detected by solid-phase micro-extraction and gasƒÆ’‚¢ƒ”¹- “‘liquid chromatography techniques.

Vegetables

Vegetables such as carrots, leeks, turnip, cabbages, chilies, tomatoes and spring onions, garlic have their particular odour but cannot be easily detected unless smelled at a close distance. The smell of garlic is due to a molecule containing some sulphur. When they are cooking then the smell can be detected from a fair distance because the volatile chemicals in them have escaped and mixed with the air. 

Fish Market experience

I am sure you know the smell of fresh prawns or fish; can you recollect the fish odour? I have been to Pettah and also to Negombo fish markets; it’s a different experience in spite of the smell of fish. The noise, the crowd, some people carry baskets, boxes or even fish on the back or head, oh what an experience! Again, mind the nose and the mouth, try not to throw out, you  soon get used to it, it’s true. Actually the fish doesn’t smell that bad but the rotting fish scales with bacterial action and dirty water mixed with blood lying in drains for the last few weeks gives this horrible smell. The fish itself actually has it’s natural smell but not rotten smell, you need to get this clear.

The chemistry of fish

The fish tissue contains an odourless chemical called trimethyl-amine oxide [TMAO]. Once the fish is killed and the fish tissue is exposed to air, the bacteria in the fish’s body breaks down this chemical into chemicals that are derivatives of ammonia and this gives a bad smell.

Did you ever think how would you eat fish with that odour? The others such as fresh chicken, beef, lamb and pork also have their own odour at room temperature but they do not have a high amount of TMAO as much as fish do.  This is because some chemicals in them are less volatile than in fish or prawns but once they are cooking then it spreads faster in the air. The raw fish odour is totally different to cooked fish or cooking fish. Would you not agree that the odour is due to some invisible chemicals in molecular form given out from these things mixed in the air?

Formulae of chemicals

Try and smell some vinegar or a tomato sauce, it has this vinegar smell due to a chemical of the simple formula CH3COOH. The others such as cooking gas methane has the formula CH4, smoke smell due carbon dioxide, has CO2 but carbon monoxide CO has no smell that is why it is too dangerous.

The present day students

 One must appreciate that the present day school children are very knowledgeable about science and technology hence when you teach cooking or talking about cooking to them it must be addressed with respect to their current high standard of knowledge.

They must be taught about germs, bacteria, virus and the chemicals in fruits, vegetables, edible roots, nuts, fish, meat and the spices we use for cooking.

If one tries to add this and that without any scientific and chemical explanation then it’s absolutely worthless, that’s not the scientific way to teach cooking. In that case, our two servants Piyasena and Roslyn would have done a better job than some of these British TV celebrity cooking demonstrations. Pardon me, if these so called celebrity cooks have no idea of science, how would they explain things scientifically or save energy?

Smell of cooking ingredients

Take some curry leaves or coriander leaves, rampae and sera, when they are very near the nose one can smell the chemicals in them. If they are crushed between the fingers then smell, the odour is very strong and this again due to a mixture of complicated invisible volatile organic molecules. What about other cooking ingredients such as chilly powder, nutmeg, cloves, cardamoms, cinnamon, pepper, cumin, coriander, mustard seeds, they all have odours that are special to each substance.


Reactions between chemicals

I always believe, we breathe, eat and drink nothing but chemicals. Would you not believe, that there are chemical reactions between different chemicals of the spices and the food we cook? As a result of these different spices actions, new chemical compounds would be formed due to intermolecular and intra-molecular reactions. The fruity smell is due to the formation of esters. How an ester is formed, this is the reaction between an acid and alcohol. In GCSE level as well they study simple esters such as ethyl-ethanoate formation between ethyl alcohol and ethanoic acid. Any unripe fruit has an acid that can be easily found by tasting unripe mangos or pineapples. The sharp sour taste is the acid in them. We know the taste of, “veralu achcharu or pineapple achcharu”, it has nothing but a mixture of fresh fruits but cannot easily explain the taste, one must eat to find out. Next time when you arrive Sri Lanka for a holiday, try and taste these long forgotten specials to Sri Lanka.


Gas for cooking

Majority of the people use gas for cooking but if they are not aware of the wastage, would you not agree that they waste gas, that is waste money on gas, pollute the atmosphere? Are you really worried about the gas bill going up, would you believe that I know the technique how to save some gas, may be 60%? I will be able to demonstrate how to save gas and cook scientifically, if I failed to save gas, I am prepared to pay Rs 500,000/00, half a million rupees,perera6@hotmail.co.uk

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