In literature and film, the consumption of certain foods is associated with character epiphanies. Cookies and milk are one such combination: they relate to childhood memories and comfort, and an adult consuming them may have powerful realizations about his life. Milk of course is associated with childhood and nurturing, the bond of mother and child.
In The Seventh Seal, the crusade knight consumes this simple rustic meal with the traveling family. For him, this meal is a symbol of life's simple pleasures, and since they have a baby, they also symbolize the vigor of youth and new beginnings. He is able to be happy in this moment, to appreciate what his life has meant, to understand that even if he cannot defeat Death, that simple moments among loved ones are what makes life worth living. He sacrifices his life to save them.
For Bergman, strawberries are a potent symbol; wild strawberries are hard to find and small, but highly prized. He titled one of his early films "Wild Strawberries" about an elderly man who confronts his youthful mistakes and loves.