Drink just after blending, and make only what you plan to drink straight away. Papaya milk can turn bitter even when refrigerated because papaya enzymes break down milk proteins. If serving is delayed, cover and refrigerate immediately at 4°C (40°F) or below. Discard any smoothie left unrefrigerated for more than 2 hours, or 1 hour if the temperature exceeds 32°C (90°F).