פירוש על עבודה זרה 76:15

Shetita is some sort of porridge that can be made with various liquids and grains/beans. If it is made with wheat or barley there is no concern that vinegar may have been added, which would be a problem because non-Jewish wine/vinegar is prohibited. If it was made with lentils and vinegar then obviously it is prohibited. The amoraim dispute when it has been made with lentils and water. Shmuel’s father and Levi are concerned lest by permitting this type of shetita people come to eat shetita with non-Jewish vinegar in it. Rav has no such concern.