If there is doubt whether grain or grain-containing products are prohibited as ḥadash, they may not be eaten, since one must be stringent in cases of doubt involving Torah prohibitions. This is the practice in Eretz Yisrael: any grain with kosher certification from the Chief Rabbinate is presumed to be free of any concern for ḥadash.
In principle, this should be the practice in the Diaspora as well. Since old grain is now readily available, and the situation is no longer one of pressing need, one should be stringent in cases of doubt concerning ḥadash (see Igrot Moshe, YD 4:46:4). In practice, however, most kashrut agencies in the Diaspora are lenient in such cases, relying on arguments developed in earlier times, when old grain was very difficult to obtain. One who wishes to be lenient outside Eretz Yisrael has authorities upon whom to rely. The more difficult it is to obtain food without concern for ḥadash, the more room there is to rely on the lenient views.
In Eretz Yisrael, however, one should not be lenient, even though most grain is imported. The overwhelming majority of authorities rule stringently, and this has long been the accepted practice in Eretz Yisrael, where cases of doubt can be avoided without difficulty. Therefore, any grain-containing product bearing overseas kashrut certification but lacking Rabbinate approval may not be eaten unless it states explicitly that it was made from old grain or was supervised by a rabbi who is careful about the prohibition of ḥadash, since most Diaspora kashrut agencies are known to be lenient when ḥadash is in doubt.
If ḥadash flour became mixed with other flour, it is not nullified, even if the other flour is a thousand times its quantity. Since ḥadash will become permitted after the day of the Omer, it is a davar she-yesh lo matirin – something that will eventually become permitted – and such an item is not nullified when mixed with its own kind (Nedarim 58a; MT, Laws of Forbidden Foods 15:10).
If, however, the mixture containing the ḥadash is ḥametz, it cannot be kept until the Omer, since the ḥametz must be destroyed before Pesaḥ. Therefore, if the mixture contains sixty times as much permitted food as ḥadash, the ḥadash is nullified and the mixture may be eaten (Ḥokhmat Adam 53; Binat Adam 28). The same applies if the mixture is a food that will spoil before the day of the Omer: the ḥadash is nullified in sixty (Ḥok Yaakov 489:24).
If ḥadash flour became mixed into a different kind of food, it is nullified in sixty, and one need not wait until the day of the Omer. Even a davar she-yesh lo matirin does not prohibit a food of a different kind unless it can impart flavor to it. Once there is sixty times as much permitted food, it can no longer impart flavor and is therefore nullified (SA YD 102:1).
