The Nodah Be Yehuda (YD Mah. Tin. 56) was asked about the kashrut status of a certain alcoholic drink that used non-kosher animal meat as part of the production process. He paskened that as the non-kosher ingredient was totally insignificant in the final product and did not add to the taste of the same – the ingredient was considered nullified (batel) and the final product kosher. As to the prohibition of purposefully nullifying a prohibited product- Noda Be Yehuda ruled that as the drink was manufactured by non-Jews for non-Jews – there could not be placed on a non-Jew any prohibition and one only had to look at the final product. If however the product was being manufactured by Jews then the product would be considered not kosher. The Nodah Be Yehuda rules that anything that is permissible be diavad in a Jewish home is permissible le chatchila if produced by a non-Jew for non-Jews.
Many centuries earlier the Rashba dealt with a similar problem. It involved a particular food that used miniscule amounts of non-kosher vinegar in the course of production. The Rashba ruled that the product remained forbidden. He argued that the laws of nullification apply only when an ingredient falls into a product accidentally. If however it was put in on purpose then regardless of the amount and regardless of whether done by Jew or non-Jew the product remains not kosher. Indeed the Nodah Be Yehuda acknowledges the Rashba – but dismisses him. He argues that the Halakha is not like the Rashba but rather like the Rambam(whom the Nodah Be Yehuda posits disagrees with the Rashba.)
Rav Henkin, IIRC, said that this Rashba is the basis for much of the kashrut business nowadays.
In America, though, he said that when you have one brand of an iten under hashgaha and one not under hashgaha, you must buy the one with the hashgaha. The Ktav Sofer said that one may rely on this leniency of the Rambam/Noda beYehuda only when there is no equivalent product available at the same price – but if there is an equivalent product available then one should be machmir and purchase a product that has not relied on this leniency.
If a company is producing a product for the general non-Jewish market, and the company does not request certification nor does it pay for it – but rather allows the kashrut agency to investigate the product and list it as kosher – then Halakhically one may rely on nullification and other leniencies that would normally be allowed only post facto in the home of a Jew, according to Kashrut Authority in Australia.
In other countries, they rely on these kashrut lists. R’ Abadi does exactly that- he investigates items to see if they are really kosher. Furthermore, R’ Shimon Efrati, a top Rabbanut kashrut official, authored an article in Pardes 40: 3 (pp. 12-13) endorsing the same viewpoint as R’ Abadi. In other countries, R’ Abadi’s approach seems to be accepted more or less, as evidenced in the fact that there are companies that consider the kosher market as totally insignificant in places like the UK, Australia, etc. – and they give permission to have their products checked. They expect no significant increase in turnover whatsoever. In such cases nullification may be employed, or at least a less thorough sub ingredient check, as well as some other leniencies mentioned in the poskim, according to an article by Kashrut Authority Australia.
I had a discussion regarding a “K” appearing on items with someone the other day, and it turns out that in a few cases, the K actually indicates some form of rabbinical supervision, but that depends on each company involved. Kellogg’s cereals with a K/KD are really KVH, ShopRite’s kashrut guide lists R’ Dov Hazdan, R’ Sheldon Goldsmith, R’ Benjamin Weinblatt (Plaza Torah Center/Bais Yechezkel in Queens), and even R’ Nota Greenblatt as those who supervise specific items produced by ShopRite. A few gelatin brands even post such letters on their websites (R’ Goldsmith supervises Royal Bbrand gelatin, which relies on the heter of the Ahiezer, which is also how the Rabbanut holds).