Why are trace amounts of chametz in milk or eggs bittul before Pesach, but not in spices or quinoa?

Question

(Note: It's entirely possible that one or more of the premises of this question is incorrect; if that's the case, I would very much appreciate any clarification of what the actual underlying halacha is.)

My understanding is that it is permissible to use eggs and (non-chocolate) milk on Passover, even without a KLP certification, provided they were purchased before the holiday, and that the reason for this permission is that any trace amounts of chametz that might be present become bittul before the chag begins. (Incidentally, I have also heard the same argument given with respect to olive oil, orange juice, and sugar, but there seems to be less consistency in this regard.)

But I have also been told that when purchasing quinoa, it is essential to make sure that it is sourced from a producer who ensures that the quinoa does not come into contact with other grains at any point in the harvesting or packaging process, and therefore quinoa can only be used if it is purchased with a KLP hechsher. The same, I am told, applies to ground spices.

Why doesn't the same principle that permits us to use eggs or milk without a KLP (provided it was purchased before Pesach) also apply to quinoa and spices — or, for that matter, tomato paste, apple juice, amaranth, or other non-kitniyot, "single-ingredient" products?

Answer 1

Plain milk contains added vitamins which pose a slight risk of chametz. Therefore, it is recommended to buy milk certified for passover, or if this is not possible, you can buy it before Pesach so the chametz will be battul.
Eggs and olive oil do not need to be certified for passover, and can even be bought on Pesach.

Quinoa, on the other hand, has a much higher risk of being contaminated. It is often grown with barley and stored in sacks that contained flour, see the Star-K's website for details.

Spices have an entirely different problem, where they might actually be adulterated with chametz or kitnios (think corn starch to prevent caking), which may be omitted from the ingredients list legally or illegally, and therefore must be certified for Passover. Additionally, some spices are grown near grain and might contain actual grains before it is ground.

A similar problem arises with sugar, where powdered sugar, which contains starch that may be chametz or kitniyos, can be re-worked into plain sugar, and should therefore be certified for Pesach, see this article from the Star-K. The OU disagrees, and says that plain white cane sugar can be used with no certification (and bought on Pesach).

Many of the other products you mentioned are not produced on dedicated equipment, and the equipment used to process them is very likely used for chametz or kitniyos as well. For example, tomato paste is cooked and then canned using canning equipment, any of which was likely used for chametz or kitniyos. Unless you know how the food was produced, you can't accurately asses the risk of containing chametz.

In short, it is recommended to buy kosher-for-passover milk and other similar products because of the risk of containing chametz, but in cases where this is not possible, because of the very low risk, it is permissible to buy the products before Pesach and rely on the fact that the possible chametz will be battul. However, other products are more likely to contain chametz or kitniyos, and in larger proportions, and therefore must be certified.

For more information, the OU has a list of common products, whether they must be certified, and why, as well as a list of products that are kosher for passover and may be bought on Pesach even without certification.