There is no English word for Jooth, perhaps because this concept is not present in the Western culture. The idea is that one’s Karma and nature gets transferred to deeds one does, including cooking food. The Avastha of a person gets affected by the food one eats. Jooth in essence is some sort of contamination or pollution that is infected by a person who is a non-devotee (non-Gursikh). Somehow, the Karmic dirt of one's sins affects the food one prepares and this is called Jooth. This Jooth is avoided by Gursikhs, by eating food prepared by Gursikhs alone. Food prepared by non-devotees, who don't worship Vaheguru, commit blatant sins, is avoided by Gursikhs and this avoidance of food prepared by non-devotees and partaking food prepared by Gursikhs is called Dietary Bibek in Sikhi.
When we analyse the Saakhi of Malik Bhago and Bhai Laalo, we clearly see that the affect of one’s good or bad Karma passes on to the food one cooks or even buys.
In Malik Bhago’s case, the food may not have been touched directly by him but still his bad Karma got transferred to his food and Guru Sahib demonstrated to the world by squeezing blood out of his Roti. This proves that even if certain food is not directly touched by a person, it can still be Jootha i.e. if dishonest earnings are used to buy ingredients for food, such food too is Jootha, as far as Gursikhs are concerned.
There is certain protocol that is followed when preparing Guru ka Langar and this includes one doing full Ishnaan, wearing clean Gurmukhi Baana and keeping hands clean. How Jooth travels or affects is not too much of our concern. All we know is that food for Guru ka Langar should be cooked by Tyaar bar Tyaar Gursikhs and in a way that is in accordance to Gurmat Maryada.
The concept of Jooth is not explained in detail in any Gurmat literature but the details of doing Sewa are learned in the Sangat of Gursikhs. When we do Sewa with senior Sewadaars, we learn the details of how to prepare Langar, how to keep clean, what to do and what not to do etc. In Sikhi is not important or imperative to first understand everything about a certain principle, before adopting it. We are all supposed to wear Kakaars all the time but Guru Sahib jee has give us no clear reason as to why we must wear the Kakaars all the time. Same holds true for many other Rehits. At initial stages we just need to do what we are told to do by Guru Sahib and Sangat and with time, concepts become clear.