Avodah Zarah 39B

אסורין בחותם אחד. חילתית, מורייס, פת, גבינה ⁠— מותרין בחותם אחד.

are all prohibited when they are found with only one seal; ḥiltit, fish stew [morayes], bread [pat], and cheese [gevina] are all permitted when they are found with one seal.

פת, למאי ניחוש לה? אי משום איחלופי קרירא בחמימא ⁠— מידע ידיע, דחיטי בדשערי נמי מידע ידיע, אי כי הדדי ⁠— כיון דאיכא חותם אחד לא טרח ומזייף.

The Talmud explains why only one seal is necessary for bread. With regard to what need we be concerned in the case of bread, that one might have thought it requires two seals? If it is due to the concern for the non-Jew exchanging the fresh bread of the Jew with his own bread that is less fresh, the difference between cold bread and warm bread is known, and the Jew will realize that there has been an exchange. Likewise, if there is a concern that a non-Jew might exchange the more valuable wheat bread of the Jew with his own less valuable barley bread, the Jew will also know about it in this case. And if it is due to the concern that a non-Jew might exchange similar kinds of bread with each other, it can be assumed that since there is one seal the non-Jew will not trouble himself and forge another seal just to exchange bread of equal value.

ורב, מאי שנא גבינה? דלא טרח ומזייף? חלב נמי לא טרח ומזייף! אמר רב כהנא: אפיק חלב, ועייל חתיכת דג שאין בה סימן.

The Talmud asks: And according to the opinion of Rav, what is different about cheese that it requires only one seal whereas milk requires two? If the reason is that a non-Jew will not trouble himself and forge a different seal, as cheese is generally inexpensive and the small profit he might make is not worth such an effort, this reasoning should apply to milk also, as he will not trouble himself and forge a new seal in this case either. Rav Kahana said: Remove the term: Milk, from Rav’s statement, and enter instead: A piece of fish that has no sign of kashrut.