The crop should be allowed to empty completely at least once every 24 hours. This is because, due to the location of the esophageal opening, the crop empties from top to bottom. Therefore, if you put fresh food on top of old food, the fresh food will move out first, leaving the old food in the crop longer. The food should be fed at a temperature of around 104 - 105 degrees, not below 100 deg. Above 110 can burn the delicate crop membrane, and food that is too cold will not move out of the crop well.
Humidity is essential.
Sour crop basically means that the food in the crop has soured, or spoiled. Most often the yeast organism, Candida, which is normally present in small amounts, will take advantage of the situation to grow out of control, causing a Candida infection of the crop. Another possibility is bacterial infection of the crop due to the growth of bacteria in the spoiled food. Either way, this baby will need to be taken to the vet to have the crop flushed out and medication begun. This baby will die of starvation if the condition is not corrected.