MSci Project – Electric Fields in the Freezing Process.

The MSci project is, in some ways, a culmination of years’ worth of studying, in which you apply all the skills you’ve slowly acquired during the past few years of toil. With Imperial participating in such a vast range of research, it offers a similarly broad spectrum when it comes to choosing a topic for your final year project. As a sample of this variety, I thought I’d write a (short!) background of the area that tempted me. Enjoy!

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“If more of us valued food … above hoarded gold, it would be a merrier world.” – J.R.R. Tolkien

Food; it’s an essential (and very enjoyable!) component of all our daily lives. As such, any attempt made to improve its quality is, to my mind, a worthy cause of wholly altruistic intent. Paramount to this goal is the effective storage and preservation of foodstuffs, such that minimal damage is inflicted in its transit from place of production to our plates. Or hands, for the more feral among us.

The most common of preservation techniques is one with which we are all familiar: freezing. It is a wonderfully simple manner of keeping food for months on end. This works by slowing down the biological processes within the bacteria responsible for breaking down the food tissue- the colder it is, the slower the rate at which reactions proceed. Perfect, right?

Unfortunately, no. As straightforward as the freezing process appears, it is not without flaw. Suppose we have a piece of steak, being cooled to -20°C, as is typical of most domestic freezers. The steak has a very high water content, similar to that of human tissue. As the temperature falls below the melting point of the water, ice crystals start to form throughout the steak. Some crystals are found outside the cells, some inside. It is the latter of these two, so called intra-cellular crystals, that prove to be problematic.

As intra-cellular crystals form, the water content of the cell falls below that of its surroundings. This imbalance drives water into the cell, increasing the pressure on the membrane and forcing it to expand. This can burst cells, thereby degrading the texture and nutritional content of our – now ailing – steak. Not a brrr-illiant situation, I’m afraid.

Thus, we turn to physics for a potential solution! There is a wealth of experimental evidence to suggest that electric fields can affect the freezing process. Electric fields are regions in which charged particles, such as electrons, feel a force. Given that the water molecule has charge imbalances, it is reasonable to expect that this will influence the freezing in some way. The exact mechanism by which electric fields affect the transition from liquid to solid is very unclear, with experimental results yet to yield concordant results.

Perhaps the only reliable conclusion that can be drawn at this point is that the presence of a field increases the temperature at which freezing occurs in water. Recently, a negative correlation between the freezing temperature and the size of the resulting ice crystals has also been observed – the higher the freezing temperature, the smaller the resultant ice crystals’ size. When applied to a food sample, this could mean fewer ruptured cells and therefore tastier dinners for all.

What to make of all this, then? Enter: my project! We aim to investigate the very early stages of freezing, known as nucleation.

Nucleation occurs in systems close to freezing, with minute clusters of ice forming on the microscopic scale. Minute is no exaggeration; some of these clusters are only a few hundred atoms across – 10,000 times smaller than the width of a human hair! Most of these tiny clusters are unstable and hence short-lived, disappearing almost as soon as they come into existence. Some, however, are lucky enough to escape this fate. They continue growing, eventually becoming the large ice crystals we can measure in the lab. These clusters are termed supercritical.

We will use computer models to predict how adding an electric field could change the number of these supercritical clusters. Our idea is that more of these clusters would make freezing more likely, as well as meaning more, smaller crystals in total, thereby inflicting less damage on the food. A breakthrough in this field could see us all enjoying more satisfying dinners very soon!

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