If you want the preprepared polenta, I can't help you. If you plan to cook it yourself, you can use cornmeal. The only difference is in the size of the pieces. It will be more like the Italian polenta is you can find course ground corn meal, but even the same cornmeal you use to make corn bread will make good polenta.
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Yes, polenta is made from crushed dry corn just like cornmeal. In fact you can use cornmeal to cook homemade polenta if you don't have official polenta. I use it all the time. The only difference I've ever noticed is that cornmeal is often ground finer than polenta. If you get course ground cornmeal it's the same thing.
Yes - ask for cornflour. DO NOT use cornflour - totally the wrong thing. eta: Cornflour in the UK is corn starch. This isn't the same as cornmeal! Still trying to find cornmeal in the UK, will add more when I know. BUT:- coarse cornmeal, polenta or maize meal or the more finely ground maize flour should be available in most big supermarkets often called - cornmeal, maize meal, maize flour, polenta, or polenta flour.
If it has been unopened and still looks good I would still use it
Yes, but you will need to spit in it first, as to get the right consistency
It has never stopped. Many people still use typewriters.
There should be an expiration date on the envelope or jar it comes in. If it past that date I would not recommend it for use.
Yes. As stated before. The use by date is a "best used by" date. You can eat this product after this date. Longer if the bag has not been opened yet.
Yes it is a best before date, not a use by date. It might be less papatable and have lost it's fizz.
Yes after arrived
It all depends whether it is a sell by date, use by date, or a best before date. If it is a sell by, then that would be an earlier date than an 'eat by' date so it would probably be OK. If it is a best before, then it probably won't taste as good but will still be safe; if it is after a use by date then it may not be safe so you shouldn't eat it.