What does it mean that flour is yashan or chadash? Should Jews only use products made of yashan flour? If a Jew buys flour that they don't know whether it is yashan or chadash, can they use it?
Yashan and Chadash
Chadash and Yashan applies to things made only of the five grains: wheat, barley, rye, oats, and spelt. Chadash is grain that was planted less than two weeks before the 16th of Nisan and thus hadn't taken root before the Omer. Yashan is old grain that grew or took root before the 16th Nisan. There is a prohibition of eating grain that took root after the Omer started. That grain cannot be used until the next Omer starts.
The Shulchan Aruch (Siman 293 Se'if 2) says the prohibition of not using grain that is chadash applies both to inside and outside of Israel. This ruling is based on the three pillars: Rif, Rambam, and Rosh. All agree that we follow Rebbi Eliezer who says that the restriction of chadash applies outside of Israel. The reason for this is that even though this is not the majority of opinions, we find it a stam or unnamed and undisputed tractate in Orlah 3:9 of the same opinion
There are some authorities that claim there are some places that do not concern themselves with chadash or yashan. The Bach (siman 293) points out that the Rambam does not mention at all whether chadash applies to produce grown by gentiles. He also brings proof from tractate Rosh Hashanah that chadash produce that belongs to a gentile is not forbidden.
The Taz (the son-in-law of the Bach) disagrees with his father-in-law, but the Taz also brings a source to explain the custom in communities that are not concerned with chadash, which is that it could be that we should follow the majority of Poskim and not the minority. Since grian is something we rely upon, we can be lenient about only using yashan products.
The Shulchan Aruch says in Siman 239 Se'if 3. "Grain that did not take root before the 16th of Nissan is forbidden until the next Omer". Nevertheless, the Rama says all indeterminate grain is permitted after Passover because of a double doubt. Perhaps it comes from the previous year. And even if you’ll say that it came from this year, perhaps it took root before the Omer (Tur quoting the Rosh). And in types of grain that they certainly plant after Pesach, for example in some countries where they plant spelt and barley after Passover, then you should be stringent after the harvest. Unless most of the grain comes from another place where they plant before Passover.
And also, in a time when the winter season extends to after Passover, and in all of that region they plant the above mentioned crops after Passover, one should be stringent and suspect unless known otherwise. But one should not teach others in a place where most of their drinking and eating is from these species, because it is better that they be negligent and not to be pre-meditated.
Based on the above, whenever there is a double doubt, the Rama says that we can drive the benefit of the doubt. Since here we have double doubts that perhaps the grain is from last year and is not considered chadash anymore, even if you say that it is not from last year we can still say that perhaps it took root before the Omer.
However, when we are all sure that this grain was planted after Pesach, we only have one doubt, and we should be strict, unless we know that there is other grain that is coming from outside the community, so in that case we have another doubt that the grain is planted before Pesach.
There are a few questions that should be addressed regarding this double doubt.There is a well known concept in Yorei De'ah that anything that will be permitted later on, we do not use a double doubt. If we will wait until next Pesach, we can use this grain . Therefore we cannot use a double doubt. So how does the Rama use a double doubt?
The second question we have is that this double doubt is that it is considered a double doubt with one name. In other words, we can use ten doubts and say perhaps this grain is from 2, 3, 4, or 5 years ago, so the only doubt we have is whether it is chadash or yashan. The question is, why do we use a double doubt, when the doubt is simply whether the grain is chadash or yashan?
To answer the first question, the Shach says that the concept of things that will later on be permitted is only used when there is a mixture of something forbidden and something permitted, like on Pesach, so why nullify the forbidden part of the mixture now when it will later be permitted. However, here with regards to chadash, there is nothing to nullify, because the doubt is that perhaps it wasn't chadash in the first place. The second question is answered by
Or Letzion (vol 1 Yoreh De'ah 15) which says that it is two doubts because the first doubt is wether the grain was growing before or after Pesach, and this is something that is forbidden but will be permitted later on. The second doubt is about something that was never forbidden in the first place because when the grain grew before the omer, it didn't exist in the world, so the prohibition doesn't apply, and that makes it a double doubt with two characteristics.
There is a side problem for this double doubt, since in America it is possible to find out when the grain the flour was made of was harvested by the barcode, and therefore it is not considered unclear, so the leniency of the Rama does not apply. Nevertheless the Or Letzion says that for Shabbat and yom tov it is permitted if there is no yashan bread, to purchase regular kosher bread. Nevertheless, there are a few other doubts. The first is that maybe the wheat was planted at least 3 days before the omer and took root. The second doubt is that maybe the Halacha is to follow the Bach, who says that grain is not chadash in the diaspora. The third is that maybe we follow the Taz who says that if the grain belongs to a gentile, the prohibition of chadash doesn't apply.
Conclusion
If a person can easily find yashan flour, he should purchase this flour, even if it is more expensive than regular flour, and he can use money designated for ma'aser to buy the yashan flour if necessary. If one is in a place with no yashan flour available and he is outside of Israel, he can rely on the lenient opinion.