Answers for Dr Karl #1: Plastic Bags and Chicken Juice

Dr Karl is a smart guy. He has an extraordinary broad knowledge of science in general, as well as human and animal physiology.

The reason for this, of course, is that as a professional educator he gets to read a lot. I am, of course, intensely jealous – and one day hope to have the kind of job that he has.

I love listening to his regular segments on the ABC on Thursdays where people call in and ask him questions. Mostly he has an answer, but occasionally he gets asked a question relating to chemistry that he is unable to answer.

This should not surprise anyone, of course, – Dr Karl himself would be the first to admit that no one knows everything, and anyone who has a call-in radio show and is never stumped is having a lend of their audience. When I had my radio show on 6PR for example, it was quite common for me to say “I’ll have to look into that and get back to you next week.”

Anyhow, for the chemistry questions that Dr Karl can’t answer, luckily I am here.

Last week a caller asked why “chicken juice” leaks through plastic bags. What he was getting at was that whereas if you put water into a plastic bag it wouldn’t leak, if you had some chicken (or meat) in a plastic bag sitting on your kitchen bench top, when you pick it up later on invariably some of the juice has seeped through.

Why is it so?

The answer is simply that plastics are not completely impervious. That is, if you zoom down into the molecular microstructure of plastic bags you will see that it’s essentially a jumbled mess of strands with holes everywhere. The plastic bags that we get from the supermarket are PVC and because they are so thin there are enough faults in the microstructure to allow some things to seep through.

Imagine if you cooked up a kilogram of spaghetti and then tipped it out onto the floor and let it dry. When it had tried it would be a solid lump spread out on the floor. But although it would be a solid lump, between all the overlapping strands there would be gaps. Plastic is essentially the same. At a microscopic level it is made up of polymer strands all jumbled together with only weak interactions between them.

We know that there is weak interaction between the polymer chains, because the bag is flexible. That is, the polymer chains are able to move and flex in relation to each other.

And the reason that some liquids will leak through the bag, whereas water won’t, is probably related to the surface tension of water. If you put a drop of water on a hard surface and then put a drop of metho next to it you would observe a curious phenomenon: whereas the metho drop would flatten out on the surface, the drop of water would sit proud of the surface and would appear round on the edges.

The reason for this is the surface tension of water, which, because it is a polar compound, is very high. This surface tension would mean that the water would not be able to squeeze through the little faults in the plastic, but if there were other substances dissolved in the water they would tend to have the effect of decreasing the surface tension. This would mean that the liquid might be able to seep more readily through the pores.

Incidentally, anyone who works in the chemical industry knows this. Gloves ain’t gloves. If you are working with a toxic chemical that you need to protect your hands from, you need to know what type of glove to use, as some chemicals will seep right through certain gloves, and the most expensive chemical gloves are made up of multi layers of different plastics, to cover all their bases.

So that clears up one of life’s great mysteries – stay tuned for more “answers for Dr Karl.”

10171cookie-checkAnswers for Dr Karl #1: Plastic Bags and Chicken Juice