Counterfeit brandy

Szalony kucharz via Wikipedia Commons

In the 15th and 16th centuries, working out the alcohol percentage of wine was no easy feat. For ease, the authorities taxed alcohol according to volume rather than percentage, making importing gin a better deal than wine or beer. And so, naturally, the merchants looked for a loophole, and they found one – or so they thought: distil down the wines, and add the water back in after passing customs. It seemed foolproof. But they had not accounted for one thing: warming wine changes its chemistry. Volatile chemicals are lost, other chemicals – esters, acids, aldehydes – decompose, or undergo reactions. When the merchants rediluted their wine, it tasted different. Wrong. “brandewijn”, or “burnt wine”, they called it, and nowadays, we call it brandy.

POPs

What are POPs?

There’s increasing concern about the growing mass of our discarded plastics – yet not because of their direct effects on wildlife (e.g. entanglement), but because plastics could be a crucial vector for the transport of key environmental contaminants: persistent organic pollutants, or POPs.

There are many thousands of POP chemicals, originating from agriculture, combustion processes, industrial syntheses, and products such as flame retardants, plasticisers and antimicrobials. In fact, our understanding and classification of them is ever evolving. They’re named not because of their chemical groups, but because of their behaviour. As the name “persistent organic pollutants” suggests, they’re long-lived and harmful.

Plastic debris on a beach. By epSos.de (Flickr).

What chemicals are we talking about?

Mosquito – By Alvesgaspar via Wikipedia Commons.

Typical culprits include DDT, polynuclear aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), halogenated flame retardants, and polybrominated diphenyl ethers (PBDEs). Although DDT is now banned in most countries, it’s still permitted in some places with a high incidence of malaria to fight off mosquitoes^[1].

POPs are polluting and persistent because of certain properties, such as their solubility, reactivity and volatility.

When is a Strawberry No Longer a Strawberry?

You take that first bite, tearing through the cells of the red berry and allowing its fragrance to erupt. Juices ooze, coating your mouth. The sweetness, modulated with a slight bitter hint, hits your tongue and lights up your senses, and the distinctive flavour of the strawberry overflows.

Why do we love strawberries so much?

Why do we love strawberries so much? There’s something alluring about them, something intense and complex in the flavour we experience that is not matched by any other berry – its chemistry.

When we recognise a flavour – the combined experience of taste, mouth feel and (crucially) smell – we are actually recognising the aromatic chemistry of a food, the volatile molecules given off when we bring it close to our nose or after we bite into it. When we understand the molecules that make up the distinct chemical profile of a flavour, we can replicate it by creating those chemicals in a lab and stirring them together. This helps us make artificial banana flavour, artificial vanilla flavour, and artificial fresh grass smell. But strawberries are awkward. Strawberry flavour is made up of a chemical medley of many, many different molecules (more than 350), variation exists strawberry species to strawberry species, and we just don’t know how important each chemical is.

This is in direct contrast to the raspberry, a humble berry that we recognise as 4-(4-hydroxyphenyl)butan-2-one, or “raspberry ketone”. Whilst cranberries and blackberries also contain this odour, they mix it with other chemicals, whilst in the raspberry it is exclusively responsible for the raspberry profile we instantly know.

Raspberry ketone, the molecule behind the smell we know so well. © Things We Don't Know.

Carbon-Based Hydrogen Bonding

Many chemists will be shocked to discover that carbon-based hydrogens can hydrogen bond. Weakly. But actually.

If you're not a chemist you're probably thinking that doesn't sound like much of a surprise... A hydrogen can hydrogen bond? Who've thunk it? And that is the first condition for hydrogen bonding – having a hydrogen.

However, until the discovery of the carbon-based hydrogen bond between acetone and halogenated hydrocarbons in 1937, it was believed that that the hydrogen could only make these bonds when attached to something very electronegative – electron-loving.

Why is this?

Electronegative atoms are electron-greedy and pull the electrons in a bond towards themselves. The electrons are shared unequally, leaving the electronegative atom a bit negative and whatever it’s bonded to a bit positive. We call this bond polar, because it has two differently charged ends, like a magnet. And, just like with two magnets, when the positive end of one comes near the negative end of the other, they bond. All polar molecules do this; we call it permanent dipole bonding. But hydrogens do it with an aggressive zeal that marks them out as unique.

Hydrogen bonds form between the slightly positive hydrogen atoms and slightly negative oxygen atoms in water molecules. Image via Wikipedia Commons.

There are three elements considered electronegative enough to polarise a hydrogen: fluorine, oxygen and nitrogen. Fluorine is the most electronegative element in the periodic table, given the Pauling electronegativity value (a made up scale we use to compare electronegativity values) 4. Oxygen is just behind it, with an electronegativity value 3.5, and nitrogen is 3. In contrast, hydrogen has an electronegativity value of 2.1, which means the difference between it and fluorine is 1.9 – considered enough to make a bond polar. But carbon is not very electronegative, and the difference between it and hydrogen is 0.4, surely too little to create polar bonds?

Apparently not.