What is Yoshon?
In Judaism, Yashan (old [grain]) is a concept within kashrut (the Jewish dietary regulations), based on the Biblical requirement not to eat any Chadash— grain of the new year (or products made from it) prior to the annual Omer offering on 16th Nisan.
In classical Rabbinic Judaism, this requirement was considered restricted to the five classical grains of Judaism – wheat, barley, oats, spelt, and rye; any of these grains that are too young to pass the requirement (and products made from them) are referred to in Judaism as Chadash, meaning “new [grain].” Additionally, the Rabbinic interpretation requires grain to have taken root prior to the omer offering for it to become permitted; therefore, grains planted after Passover could only be consumed the following year.
Following the destruction of the Temple in Jerusalem in 70 C.E., the omer offering was no longer offered. Since this time, the new grain becomes permissible following the date on which the offering was brought in ancient times.
The applicability of the Chadash rules to grain grown outside the Land of Israel is a subject of debate among halachic authorities. The majority of medieval Jewish scholars (e.g. Maimonides, the Rif, and the Rosh) forbade its consumption. The later codifiers of Jewish law for Ashkenazic and Sephardic Jewry followed suit, both Rabbi Moses Isserles and Rabbi Joseph Caro declaring the stringent position. A radically novel lenient approach was presented by Rabbi Yoel Sirkis who felt it is permissible if the grain originally belonged to a non-Jew. Additionally, the manner in which various foods have historically been available has meant that Jewish populations would need to risk starvation to pursue stringent compliance with this aspect of Kashrut. These two factors led to a situation in which observation of the Yoshon regulation was relatively limited until very recently (at least in the Ashkenazic community).
In modern times, particularly in developed nations, food is much more readily available than it historically had been, and grain is in sufficient abundance that orthodox Jews have become more interested in following Yoshon requirements. Modern packaging practices, which in some nations involve the stamping of production dates on every package, often allow individuals to determine whether food is definitely Yoshon; packaging organizations sometimes add kashrut information to the packaging, and sometimes include in this information whether the product is known to be Yoshon.