No pork; no shellfish; no milk with meat. Simple enough. But what about this yoghurt? Or that soy sauce?
And which brands of ice-cream can you serve at Friday-night dinner?
Now there's an app to answer all of those. In fact, there are two. They're both free, and they both ask the same question: Is It Kosher? That is the name chosen by British and Estonian developers who saw an opportunity to end the confusion. The UK version was created by north London software engineer Jonathan Levy, 28, and his friend, chemistry student Yoni Weiner.
Test your knowledge: Is it kosher?
Users' queries are checked in the London Beth Din list and answered with a quick "YES" or "NO". The United Synagogue has embedded it in their own app.
It has been downloaded 900 times since the pair, who both attend Alei Tzion Synagogue in Hendon, released it in June – ahead of a similar invention 1,500 miles away by developer Misha Beshkin.
His app searches databases from 70 countries to find people kosher food wherever they travel. He developed the Android app himself, before raising £5,000 on crowdfunding websites to make it available on Apple phones.
Both versions have been downloaded around 10,000 times.
Mr Beshkin, 38, said the idea came after repeated calls from a friend asking whether products were acceptable.
He said: "I was like a person in a call-centre, looking through different online databases to answer him, and then I thought: 'Hey, I'm a developer. I can do this in a different way.'"
Mr Levy stresses there is no rivalry between the two apps. He said: "It was a coincidence. We've been in touch with Misha and we have good relations, but we have slightly different ways of doing things.
"It started off as a hobby and it's remained that. We don't have any plans to make big money off it. We'll keep improving it as much as we can, though it's supposed to be simple. If you add too many features it ends up being complicated and tedious.
"We haven't really promoted it, but people discover it and like it. The app speaks for itself."
