Daily Science Factlet – Photonic crystal fibres

A friend and I are entering a video into the Optical Society’s Optics in a Minute video contest. We’re doing our video on photonic crystal fibres, which are allowing researchers to explore new ways of transmitting light for things like telecommunications and medical procedures.

Traditional optical fibres, that employ total internal reflection of light to carry information for things like the internet, can’t carry high powered laser light, because they would either melt, or distort the light – not great for accurately carrying data.

But unlike the solid glass traditional fibres, photonic crystal fibres are made up of a long hollow crystal of silica, so they carry the light in a different way (Fig 1). The sides of the crystal will allow certain wavelengths of light through but not others, due to an effect known as the photonic bandgap. And you can choose a crystal that reflects only the band of wavelengths you’re after (Fig 2).

This means that we can now guide powerful lasers within fibres that don’t melt and don’t distort the light.

Stay tuned for the video – we’ll put it up on youtube and I’ll post it here…

Daily Science Factlet – crustacean colouration

Whilst chowing down on some prawns recently, someone asked me why it was that they were a blue-grey colour when raw, and a pink colour once cooked. Here’s why…

If you look closely at a raw prawn, you can see that the colouration isn’t even, but is distributed in small spots on their shell, and on the flesh underneath. The spots are made up of pigment-containing cells called chromatophores, that you can find in all kinds of organisms from bacteria to birds and plants to parrotfish. The combination of overlapping cells containing different coloured pigments gives the final colour of the organism (and making the chromatophores expand or contract is how octopus and cuttlefish are able to change their colour).

The key pigments involved in the blue-grey to red change in prawns (and lobsters) are the red astaxanthin (that also makes salmon and flamingos pink), and the blue crustacyanin. Crustacyanin is actually made up of several astaxanthin molecules tied up together with a protein molecule. In raw prawns and lobsters you have a combination of the red astaxanthin and the blue crustacyanin in the cells, giving a greyish blue.

But when you heat prawns up, the protein holding the crustacyanin together unravels, freeing up the astaxanthins. These are very heat tolerant, so just stay red on heating, making the cooked prawns look that appetising pink colour…

Daily Science Factlet – the Perfect Cheese Toastie

Sometimes nothing will do but a crispy, savoury, oozy toastie. But which are the best cheeses to use? Well, science can help you out…

First of all, to get the gooey centre, you want a cheese (or combination of cheeses – let’s go all out here) that will give a ‘good melt’. There are some cheeses that would definitely not be good for this, particularly the acid-coagulated cheeses like paneer, ricotta and cottage cheese (see my post on why they don’t melt well).

The most important factor in melting is water content. High water content in the cheese dilutes the proteins that make it solid, meaning they are more weakly bonded together and so will flow past each other and melt at a lower temperature. Moist cheeses like fontina, gruyere and monterey jack are therefore good to provide the gooiness.

Watch out for using high fat cheeses – they are more likely to leak melted fat as the protein network breaks down with heat, and can make the toastie greasy. Though to be fair, that is often part of the enjoyment. It’s not a health food.

Now to stringiness. For some, another important aspect of a toastie. The reason a cheese goes stringy is that the little balls of milk protein (called casein micelles) in the cheese get linked up into long chains by calcium.

Cheeses tend to get less stringy with age. The first reason for this is that as a cheese ages, more and more lactose sugar is converted to lactic acid. Acid dissolves the calcium holding the micelles together, stopping them from forming stringy ropes and making them fall apart and flow more easily. The second reason is that as a cheese ages enzymes start to break down the micelles into small pieces that are too small to link up into chains.

So if you’re a string fan, then cheeses like emmental, mozzarella and young-ish cheddar are good. And if you prefer just a gooey centre, just use the moist cheeses above, and maybe an aged cheddar too.

Daily Science Factlet – watching yeast breathe

Big thanks to the Naked Scientists’ Kitchen Science website for providing me with the rough amounts to use.

Yeast are the tiny, single-celled relatives of mushrooms that we rely on to make bread and alcohol. When they gobble up sugars in flour or in fruit juice, in an environment with not much oxygen, they produce alcohol and carbon dioxide gas.

Glucose –> Ethanol + Carbon Dioxide

This gas is what makes dough rise and puffs up a loaf of bread, and is also what gets trapped in bottles of champagne, so that they fizz up when you pop the cork.

What you will need:

1 empty drinks bottle (500ml)

1/2 a sachet of dried yeast (about a teaspoon)

3 or 4 teaspoons of sugar

Warm water

1 balloon (blown up and let down a couple of times to stretch it)

What to do:

Pour the yeast and sugar into the bottle. Add warm water until the bottle is around half full (mine was a bit more than that). Screw on the lid and shake it around a bit to dissolve the sugar and disperse the yeast.  Take off the lid and stretch the balloon over the bottle opening. Leave it somewhere warm for about 20 minutes-half an hour.

You’ll see the balloon start to inflate and a thick layer of bubbly scum on the top of the water in the bottle. This is the yeast respiring! You can vary the experiment by comparing variables like the temperature of the water, the sugar source used etc.

And that’s it really. The video below is a speeded up version of the reason this whole experiment is worthwhile. I’ve removed the sound, so you can’t hear me giggling like a loon…

Daily Science Factlet – comfort food facts

Today’s factlet is more a little group of factlets really. Today I’ve been immersing myself in the world of food psychology; specifically the psychology of comfort food. Here’s some of the interesting little tid-bits I’ve found so far:

  • When people are sad they are more likely to indulge in ‘hedonic’ foods (e.g. M&Ms and popcorn) than when they are happy, in order to improve their mood.
  • But happy people are more likely to over-consume healthier ‘non-hedonic’ foods, in order to maintain their mood and to avoid later regretting eating something indulgent.
  • Men report preferring meal-type comfort foods like soup, pizza and pasta, but women report preferring snack-type comfort foods like crisps and chocolate. The researchers who carried out this study suggested that this could be because the men preferred foods that made them feel looked-after, and women found comfort in foods they didn’t have to spend a long time preparing (though I’m not really convinced by this as an argument in this age of higher gender equality in the kitchen)
  • Allowing ‘consumption norms’ like how much your friends are eating or only stopping eating once the plate is empty can lead to over eating.
  • Distractions like TV can increase consumption – so-called ‘mindless eating’, because they create patterns of eating behaviour that aren’t correlated with hunger

One thing I have noticed in doing this research is that the results of experiments done on participants who didn’t know what the experimenters were looking at don’t always agree with studies where people have volunteered information about their preferences and eating habits. It seems that people are often not aware of how their behaviour is changed by factors like mood and external cues.

This is a truly fascinating area – if you want to read more, I highly recommend Brian Wansink‘s website, and that of the Cornell University Food and Brand Lab. Both have great educational resources and tips for reducing your own ‘mindless eating’.

Daily Science Factlet – Acid cheese

So, in the run up to my shows at the British Science Festival in September, I’m going to be pinging out exciting little facts all about the science of bread, cheese, alcohol and microbes, plus probably a few on the science and psychology of comfort food too.

To kick off, a post on acid coagulated cheeses. These include ones like cream cheese, paneer, many soft goats cheeses, and ricotta, which differ from cheeses like Cheddar, Parmesan and Roquefort in that they do not use rennet in their production. To explain the difference, let’s take a quick step back to look at milk, and why it curdles, and how acid and rennet do it differently.

Cows’ milk is around 3.5% protein by weight (most of it is water), which can be split into two types; caseins and whey proteins. Caseins make up roughly 80% of the protein in milk, and are found bound up with calcium ions in little hairy-looking balls called micelles (check out my expertly drawn picture). It’s these micelles that rennet acts on (but more on that later). Whey proteins are only about 20% of the protein content, but include types of proteins called immunoglobulins, which can cause allergic reactions. They’re very stable once heated, and are what stabilises the bubbles in the milk foam you get on your cappuccino in the morning.

Curdling milk to make cheese means allowing the proteins to clump together. Rennet does this because it contains an enzyme that strips the casein micelles of their negatively charged coatings, stopping them from repelling each other and making them able to get closer together and form curds. If just using rennet to make cheese, the whey proteins are left by the ‘whey’-side (sorry…), and are drained off. But they do get in on the act if acid is used to curdle the milk. Many of us have experienced that horrible moment when that slightly-old-but-probably-still-ok-to-make-tea milk curdles when you add it to the cup. This is because the tannins in tea act like acids, neutralising the negative charges on the micelles, plus dissolving the Calcium bridges holding them together, and also denaturing the whey proteins, making a horribly lumpy cuppa.

Using acid to curdle or coagulate milk allows all of the proteins to bind together into a much finer, closely linked structure than if rennet had been used. This means that when acid-coagulated cheeses are heated, the first thing to be lost is water, actually making the cheese harder. This is why paneer can be cooked in whole pieces in curries without falling apart, why cottage cheese doesn’t melt like cheddar on a baked potato, and why ricotta stays in tasty little blobs on pizzas or in lasagna, rather than spreading out like mozarella.