שכח והכניס משקין לתוך פיו בלא ברכה בולען ואינו מברך עליהם ברכה ראשונה: הגה וי"א דמברך עליהם וכן נראה עיקר: [הרשב"א בשם הראב"ד והרא"ש פ"ג שאכלו וכן דברי רמב"ם פ"ח מהלכות ברכות]:

One who drinks or eats without a blessing. Containing 2 paragraphs If one put a drink in their mouth without saying a blessing, they should swallow it and not say a first blessing (a before blessing). Rema: And there are those who say a blessing and so it seems is the core practice [Rashba, in the name of the Raavad, and the Rosh Chapter 3 and also Maimonides Chapter 8 laws of blessings]

If one put food in his mouth without reciting a beracha and it would not be repulsive to remove it he should spit it out and recite the beracha. If it would be repulsive he should move it to the side of his mouth and recite the beracha with the food in his mouth