I was guessing this Yakult bottle was made of polystyrene. Its physical properties are quite similar to disposable Jollibee cups, both are light and break easily.
What are the ways to know if it is a polystyrene or not? The first is using this method, “Identifying a Plastic Container with No Recycling Symbol” and the second is the dissolution with oil or petroleum products such as diesel. The third way ? Please ask the manufacturer or the bottle molder. They may tell the accurate answer.
I poured diesel engine fuel into empty Yakult bottle. Let it stand overnight and observed.
The bottom of container was slightly deformed. The blurry transparency turned to opaque white. Container side also had patches of opaque white discolorations. It was obvious that liquid has been eating the container. Perhaps it is really polystyrene.
Is it wrong to use polystyrene as packaging of Yakult? Nope, it’s fine. Yakult contains live beneficial microorganisms. It needs a cold storage to maintain its potency. It is never subjected to any heat treatment. Contains no oil. Polystyrene polymer is not known to migrate to non-oily foods. It is perfectly safe as Yakult packaging.
According to wiki, yogurt containers are also made of polystyrene. This kind is cheap and easy to manufacture.
update as of June 16, 2016
Read updated post: Bad Smell From Yakult Bottle.