Yoshon literally means "old". The Torah (Vayikra 23:14) forbids partaking from the new crop of grain (known as Chadash) before the second day of Pesach. This restriction applies to 5 types of grain, i.e. wheat, barley, oats, rye and spelt.
Although the origin of this prohibition is Biblical, there is an old dispute between the Halachic authorities whether this applies outside of Israel, and even in Israel whether it applies to the crops of a non-Jewish farmer. Many authorities rule that these restrictions do not apply outside of Israel, and consequently the Beth Din do not require our establishments and manufacturers to adhere to these stringencies. However, for those who wish to follow the more stringent opinion, we have compiled a list of products and sell-by dates which will assist in making observance of this mitzvah easier.
The only way to avoid using any new-season products containing wheat, barley, oats or rye is by stockpiling enough of these products to last until Pesach.
This sounds simple enough. But there's a problem. Often, before Pesach has arrived, something else arrives: infestation! Therefore, if you want to keep Yoshon, make sure to store flour etc in the freezer and other things in a cool, dry place. Also check carefully for infestation before use.
What products are affected? Initially it's mainly wheat flour, pasta, wheat grains, and cereals containing wheat. Barley and oats become affected only later in the season.
The Kashrut Department will issue regular bulletins updating information during the period when Chadash is a problem.