(Note: Visit post no. 2: Health, happiness, life and food, Part I, What are they? to understand what is meant by ‘life’ in this post and the next post to understand how life-giving oxygen can be life-threatening.)

Preamble: There are striking similarities between water and oxygen: (i) Both are chemically reactive and are found everywhere. (ii) Both make life itself possible in different ways. (iii) The properties and effects of both need to be controlled and modulated, failing which they can turn nasty. (iv)Though oxygen is strictly an element and water, a compound, both have simple molecular structures.

Oxygen in nature: Oxygen is a colourless, odourless gas, heavier than water vapour and much heavier than hydrogen. In free state, it is an intimate part of air around us along with nitrogen and traces of carbon dioxide, water vapour and a few other gases, not counting polluting particles. For our practical purposes, we can simplify air as a 1:4 mixture of oxygen and nitrogen, both in terms of number of molecules and volume. It is present in earth’s crust both as free oxygen and as chemically combined oxygen – by far most abundantly as silica. In vegetation, it exists in combined form as the most abundant natural organic polymer – cellulose. Many natural minerals of metals exist as their oxides i.e. combined with oxygen or as sulphides i.e. combined with sulfur – a ‘family member’ of oxygen.

Solubility of oxygen in water is poor, about 8 mg per liter water at 25⁰C, that too after saturation i.e. when the contact of air with water has been intimate and long. Hence, while oxygen is everywhere, it can’t find place within water. It is almost as if they have marked out their territories and won’t mess with each other! Not surprisingly, it is even less in the watery part of blood which already has a lot else dissolved in it. Most of the oxygen in blood is chemically bound to haemoglobin (found in red blood cells) because it has chemical affinity for oxygen. This oxygen is off-loaded at the cells in body tissues where it performs its designated role of oxidizing sugars and producing energy thru an extremely complex mechanism. This is the reason why physicians look for haemoglobin content of blood in a patient complaining of weakness; poor haemoglobin levels means poor oxygen-carrying capacity and poor availability of energy by combustion.  

How oxygen enters and exits our body and makes life happen: When we breath in the air, oxygen (along with nitrogen) reaches our lungs thru the wind-pipe (trachea) which branches profusely into tiny tubes called bronchi (bronchus in singular) as it enters the lungs. These become extremely thin ‘bronchioles’ when branched maximally at the end of which are tiny, thin-walled sacs called ‘alveoli’. Surrounding these alveoli are equally thin-walled blood vessels into which oxygen diffuses, to ‘oxygenate’ blood. These thin vessels then ‘regroup’ or ‘reverse branch’ into progressively larger vessels carrying progressively larger flow of blood till a single blood vessel (formed by merging of a branch each from either lung) returns the now ‘oxygenated blood’ to the heart.

 Heart pumps it out to all corners of the body, the first of which is the heart wall itself, thru the ‘coronary artery’!  The organs controlled by the Central Nervous System like the heart, lungs, liver, kidneys, pancreas etc. (i.e. those that work on their own unlike hands and feet) also use oxygen delivered to them thru blood. It reaches the cells of our body along with co-traveller, glucose – remember the ‘serum glucose level’ of, say, 100 mg/dL? How the oxygen-sugar duo travels across the wall of the cells and ultimately reacts (or the sugar ‘burns’) within the cells is the stuff of divinity. This reaction of oxygen with sugar releases energy that enables us to do everything we do (including the use of brain) and maintains body temperature. Unfortunately, the escorting of the sugars into cells requires one more player called ‘insulin’, but that’s another story.

Since oxygen is consumed in burning glucose, blood is now ‘de-oxygenated’ and hurries back to the heart to be pumped to the lung where the entering single blood vessel (a branch to each lung) branches profusely (a lot like the bronchi) to become the thin-walled blood vessels hugging the alveoli to pick up breathed-in oxygen. And on and on it goes making life happen. Thru oxygen from air and water in the blood replenished by the water you drink. Nitrogen has no haemoglobin looking for it so it is left to fend for itself in the blood at a negligible and inconsequential level. It cannot enter blood thru pure dissolution (unlike oxygen which chemically latches on to haemoglobin i.e. is pulled in) because blood is generally ‘saturated’ with nitrogen – though at a miniscule level because of poor solubility.

The route of oxygen in our body can be represented as: lungs to circulating blood within the lungs to heart to body parts to  back to heart to lungs…..When oxygen supply is limited for whatever reason, energy level drops. When working intensely in a poorly ventilated room, always go to the window and take deep breaths every now and then. Also, working out is best in an open ventilated area. Unused oxygen, along with carbon dioxide produced by sugar burning, traces the path exactly reverse to incoming air, till it leaves the body when we exhale. (Water is also co-produced during combustion but that is a part of the body!)

Note that the blood pathway is: deoxygenated blood from the body to heart to lungs (then oxygenated blood) to heart to the body, i.e. the lungs are the blood-oxygen meeting spot!

 

Working out, slimming and oxygen: The above segment shows why oxygen is called ‘praan-vayu’ in most Indian languages. In case of extreme or sustained muscular work, all our voluntary muscles use up the blood supply of glucose (when its level dips, it can come from glycogen – a water-insoluble glucose polymer – stored in the liver),  glucose released by breakdown of muscles’ own stored glycogen, fat stored in their own adipose tissue, then the subcutaneous (below the skin e.g. around the tummy) adipose tissue and ultimately proteins themselves. This is the basis of slimming thru aerobic exercise, ideally combined with smart dieting. But oxygen remains common as the reactant that ‘burns’ these chemical energy sources one by one for energy release. Note that you need to thank oxygen even for slimming!

When you work out, the body responds by increasing breathing rate i.e. oxygen intake rate and heart rate i.e. blood circulation rate. This ensures more and more blood picking up more and more oxygen for more and more energy release. Such increase happening smoothly in proportion to requirement and returning to normal quickly at the end of the requirement, is a sign of good heart-lung health. Roboticists can make robots dance to their tunes; will they ever be able to breathe ‘life’ into them?!

You can see life taking shape here in the interactions between the body and the environment and, within the body, among various organs. Obviously, oxygen can do nothing without the agency of lungs transferring it to blood and blood is helpless unless mobilized by the untiring exertions of the heart.

Oxygen in food processing: The above-mentioned ‘reactivity’ of oxygen  is usually a problem in food processing where it can cause undesirable reactions with food constituents, damaging the food and hence, the strategy is to keep oxygen (or air) out in most cases. But the same reactivity is sometimes exploited where the strategy is to ‘manage’ its access to the food being processed so that the reactions happen in the planned way.

Roasting of coffee beans: When hot coffee is raised to your lips and  sipped, the experience is described evocatively as its ‘bouquet’. Most of it comes from the coffee powder and no less than 500 distinct chemicals have been identified as responsible for this! This ‘coffeeness of coffee’ is created when its beans are roasted in specially designed roasters under tightly controlled conditions of heating, stirring, presence of air (i.e. oxygen) and roasting time. Each variety has its own roasting specifications, without significant variations across varieties, of course.

Importantly, roasted coffee is ideally ground closest to the consumption time because the extensive surface area exposed by grinding makes it vulnerable to the effect of the atmosphere. This is the reasons why Americans buy their coffee beans roasted (which is best left to experts) and maintain their own miniature grinders to get the powder right before brewing of their favourite cuppa. If you are a ‘control freak’ and want to roast at home, be careful; read the next post for more.

Roasted chicory: Chicory is a widely and, sometimes uncontrolledly growing shrub, whose off-white abundant roots are an extremely interesting agri-product. The roots are washed thoroughly after harvesting, cut into stubs and sun-dried. They are then roasted to brown ‘roasted chicory root stubs’ but with a lot less care than coffee beans, given their lowly status. These are powdered to get roasted chicory root powder, simply called ‘chicory powder’ in trade. It closely resembles roasted coffee powder in every way and, not surprisingly, finds its abundant way into commercial coffee products including the ‘instant’ ones. Unless your coffee expressly makes ‘100% pure coffee’ claim, it contains chicory. Chicory offers some serious benefits, though not in the small amounts that come with coffee.

Roasted cocoa beans: Everything about coffee beans applies to it with just one major difference: cocoa beans contain the valuable ‘cocoa butter’ which is a part of the bean exactly as groundnut oil is a part of its seed or kernel. It is called ‘butter’ just to denote its consistency at normal temperature ranges. Moulded bar chocolates owe their ‘snap’ and textural eating experience to cocoa butter.  This is a world in itself; suffice it to say that oxygen-mediated controlled roasting is crucial here also.

Caramel: One of world’s favorite both as a coloring and flavoring agent, it has many culinary versions. Being practically dry, sugar does not have the protection of evaporating water when heated and hence gets heated quickly. It soon melts and turns brownish because of a series of complex reactions with atmospheric oxygen. It is a skill to stop heating at the right stage and is called ‘caramelization’ of sugar. In the wet process, water is added to sugar before burning; water evaporates quickly but creates a time window for better control. When this process is stopped with just minor browning, the product is called ‘caramelized sugar’. When chefs expertly burn sugar crystals on a set cold dessert with a torch (‘flambe’), it forms a ‘brown glass candy’ on the top without heating the basic dessert much. Some nutritive properties have  been attributed to caramel.

In food science, the term caramelization is sometimes used to describe the heat- and oxygen-mediated browning of all carbohydrates.  Though serious alarm has not been raised about it, doubts persist about carcinogenicity of caramel and its products or dishes. Of particular concern is acrylamide – a known carcinogen, also formed when potatoes are fried as in wafers and French fries. Alarming?  Moderation is the name of the game.

(A stunning co-incidence: Coffee, chicory, cocoa, caramel – all processed silmilarly from similar starting materials. Have comparable color and flavor profile. All undergo varying degrees of aerial oxidation.  An adventurous food scientist should have a crack at ‘cheat (and cheap) coffee’ made from roasted chicory extract and caramel.

Miscellaneous:

1. Roasting eggplant or brinjal or aubergine on open flame (or in an oven) happens with the evaporating protection of water. However, this protection is not total which explains the development of mysterious flavor of the product. No need for alarm given the protection of water but don’t overdo it.
2. Whole spices contain delicate volatiles apart from water and usual plant tissue. It is common to carefully dry them initially so that they become crisp enough to break easily. During such drying with air some development of flavours is possible. The process of grinding generates heat and hence most industrial grinding is done with indirect cooling of the grinder. Chefs routinely toast (i.e. heat gently to primarily dry, with little flavor development) whole spices before grinding. The resulting fragile and dry spice develops subtle flavours which can be attributed to reactions with oxygen.
3. We use oxygen-mediated flavor (and color) development extensively in daily cooking. ‘Browning’ of onions, garlic, potatoes, bread slices, seeds like fenugreek, Bengal gram and Udad daal, etc. are examples. Note: (i) Incomplete combustion of food – a complex mix of largely organic molecules – produces mysterious flavours along with carcinogenic molecules. So the practice of ‘smoking’ food dishes at home (‘dhungar’ in most Indian languages) is dangerous. Note that carbon monoxide is the product of incomplete combustion of fuels. (ii) ‘Air friers’ are becoming popular for their limited use of cooking oils. Hot air brushing surface-oiled food articles can be dicey. This calls for a separate post but don’t use refined soybean oil for this. (iii)Usually, the things that can be browned easily have little free water; tomatoes cannot be browned easily.

Next post: Oxygen, food and life, Part II

The dark side of oxygen

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