He picked up a piece and put it in his mouth and chewed it slowly. It was not unpleasant. (Pg 58)
Here we see an example of a litote. According to literary devices.net, q litote is:
a figure of speech which employs an understatement by using double negatives or, in other words, positive statement is expressed by negating its opposite expressions.
This literary device functions similarly to understatement in that it gives emphasis to something by a blatant absence of emphasis. Here, the style seems very fitting for the old man, because it doesn’t utilize any posistive words, but uses two negative words to highlight the old man’s situation.
Another device can be seen a few lines later:
“How do you feel, hand?” He asked the cramped hand that was almost as stiff as rigor mortis.
Here, and in a few places in the chapter, Hemmingway utilizes personification. The old man is alone on the boat, and talks about companionship often. He talks to his hand as if it were a person to give him a little comfort. The fact that he was so desperate that he talked to his hand shows us the degree of lonliness that Santiago was feeling.