The average orange weighs 5 oz. (140 g). Like all citrus fruits, oranges are protected externally by a thick crust, which makes them quite resistant to transport. The concentration of water in the fruit, in most commercial varieties, varies from 70% to 92%, which of course depends on the available moisture and conditions of the trees. Orange and lemon trees are perfect examples of cases in which a plant leaf draws water from the fruit, when the needs of the foliage in water cannot be met through the root system. During the summer months and under average conditions of Southern California, the leaves of lemon start to draw water from the fruits around 6-7 am in the morning and continue to do so until 5-6 pm in the afternoon.
The main acid of citrus fruits is citric acid, which is found mainly in the fruit juice, while malate, malonate and oxalate are found mostly in the cortex. The juice of oranges usually contains 1-1.3% citric acid, however this amount can vary from 0.5 to 1.3%. The acid with the second largest concentration in citrus juice is malic acid. In oranges, the concentration ranges from 1,4 to 1,8 mg/ml of juice.