Reading progress update: I've read 19 out of 219 pages.

The Science of Everyday Life - Marty Jopson

I've never been a fan of prawn crackers, but the section I've just read about how prawn crackers are made will ensure I'll never ever touch them (with or without the prawn powder). 


At first, I thought the fact that they are made from tapioca was off-putting:

The key ingredient in prawn crackers is tapioca starch, which itself contains some scientific surprises. It comes from the tuberous roots of the cassava plant.

These roots look a bit like sweet potatoes and are a staple crop for large populations of people living in the tropics. Given this, it’s a little alarming to discover that cassava is also a rich source of cyanide and can lead to both acute and chronic poisoning.

There are different varieties of cassava, and the ones known as bitter cassava contain dangerous amounts of a chemical called linamarin, which is basically glucose attached to cyanide. When the root is peeled, or chopped up, enzymes are released that break down the linamarin and release the cyanide. With this in mind, a vital part of the tapioca starch extraction process is to remove all of the cyanide.

Step 1 is to finely grate the cassava, which starts the breakdown of linamarin and the production of cyanide.

In step 2, you chuck it all in a vat of water and leave it to soak for a couple of days. The cyanide will dissolve into the water, and by changing the water a few times to give the grated cassava a rinse, all of the cyanide is flushed away.

You then take the resulting mush and squeeze all the liquid out. The mush you chuck away and the milky white liquid you leave to evaporate. What remains is a very fine, and very pure, starch powder.


But then Jopson described the industrial process of making prawn crackers:


For bulk manufacture they use tapioca starch, dried prawn powder and a tiny amount of water. The resulting powder is placed in a machine that compresses and heats the starch mixture. The pressure involved is huge, at up to 2 tonnes per square centimetre (about 35 pounds per square inch). At this point the starch melts, flows together and turns into a thermosetting plastic, which is just a fancy way of saying it’s a solid material that goes soft and runny when you heat it. This molten plastic starch is pooped out of the machine in little flat, translucent white discs. You can buy these uncooked prawn cracker discs in Chinese supermarkets and Asian food shops. In this form, as long as they are kept in a sealed plastic bag, they have an enormously long shelf life.

I said the water content at the start of the process was critical and when you drop your plastic starch disc into hot oil it is vital you have the right amount of water trapped inside the starch plastic. Two things happen: first, the thermosetting plastic starch heats up and turns soft and runny again. Secondly, the tiny amount of water it contains vaporizes, turning to steam and expanding in the process. As the specks of water turn into bubbles of water vapour surrounded by shells of plastic starch, the flat disc puffs up and turns into the familiar bubbly crisp. You then whip it out of the oil, before the starch has a chance to start browning and, as it cools, the starch plastic goes back to being a hard and brittle substance. But now, rather than a solid disc we have an aerated mass of crispy, crunchy loveliness. This may seem like a niche cooking method, but exactly the same process is used to make a number of everyday food products from puffed breakfast cereals to popcorn and even those, admittedly inedible, puffed packing worms you find crammed into boxes to protect the contents in transit. Not so unusual after all.

Yum....or rather not. But now we know why prawn crackers look like plastic fillers.