When dairy products are frozen, fat separation occurs, causing the fat to separate from liquid. This fat separation is what gives ricotta a grainy texture when it's defrosted. Fat separation also means that the watery parts of ricotta form ice crystals. When that ice defrosts, the consistency of ricotta becomes more watery and less creamy. Fat separation is why it's essential to whip or stir defrosted ricotta to bring back a creamy consistency, although it's sometimes impossible to avoid the grainy texture.