Long, slow cooking and slow cooling, especially with exposure to oxygen, causes greater vitamin C losses than does rapid heating and cooling, for example in a pressure cooker (Marks, 1975). However, whatever the method, minimum losses of about 50% can be expected.
Studies investigating the effect of cooking method and storage on the vitamin C content of Malaysian leaf vegetables, showed that holding leaves for 24 hours after harvest in the normal high temperature/high humidity environment resulted in losses of up to 85% of the vitamin C present in fresh produce. When leaves were boiled in water, cooked in coconut milk, or fried, their average vitamin retention was 45%, 56% and 66%, respectively. Vitamin retention in leaves was reduced due to the leaching of the vitamin into water over time when cooked vegetables were allowed to stand. However, even in the drained and fried leaves, only 30% of the initial vitamin C was retained after 4 hours. In the same study cassava leaves were first boiled to remove hydrocyanic acid and then cooked in coconut milk. The result was an average of 30 mg vitamin C per 100 g of cooked weight compared to 358 mg per 100 g of fresh leaves. Although the cooked vegetable contained less than 10% of the vitamin C of fresh leaves, this still represents a significant amount. Thus, in a Malaysian setting, vitamin C losses during cooking can be reduced by frying vegetables, cooking them in coconut milk (total retention in leaves + liquid was 70%) and consuming the coconut milk with the vegetables, and reducing as much as possible the time between cooking and eating.
Nicol (1958) looked at vitamin C in the diets of rural populations in Nigeria and concluded that the commonly used methods of cooking starchy roots did not destroy all the vitamin C present at the time of purchase. Yam and cassava tubers lost, on average, 50% of their vitamin C content during cooking. In the case of sweet potatoes a loss of 75% occurred if they were boiled after peeling, but only 56% if they were cooked without being peeled. Yam and cassava flour, both prepared by first soaking the uncooked tubers in water, and then pounding, sun-drying and grinding them, contained little vitamin C and the cooked food made from the flour contained only traces of the vitamin.
Watson (1976) studied the effects of cooking on the vitamin C content of plant foods in Ghana. Starchy roots and tubers and starchy fruits were found to retain some vitamin C after cooking, while the greatest loss occurred in leafy vegetables when boiled. There was only a 20% loss of vitamin C in yams after cooking compared to findings by Nicol (1958) who reported a 50% loss. Many of the leafy and other vegetables are incorporated into soups, stews and sauces which, because they are often prepared in large quantities for use over time, are subject to reheating before being consumed. Although this may lead to additional losses of the vitamin, heating food thoroughly is of course a vital food safety measure.
Hallberg et al. (1982) demonstrated the negative impact on vitamin C content of prolonged warming of food. The vitamin C content of food kept at 75°C for 4 hours was markedly lowered, with the greatest reduction occurring during the first hour. The study showed the importance of either preserving or restoring the vitamin C content of meals prepared on large a scale, e.g. in schools and other institutions. In the study vitamin C was restored by sprinkling vitamin C powder onto the cooked food.