Grilled potatoes, stir fried coulis flower with onions and tomatoes, stewed ground nuts, cabbages and cassava have nearly boiled for two hours, but will still be part of a huge buffet in the local restaurants. Daily customers will complain only when variety lacks. This kind of feeding is acceptable to many. Nutritionists and food therapists recommend consumption of fresh vegetables for maximum micronutrients such as mineral and vitamins.

Although consuming raw vegetables comes at a cost of palatability, aroma and taste, it is preferred for health benefits. Therefore, when it comes to vegetables, taste rarely meets good health. 

"Eating foods high in vitamin A, iron, vitamin E and potassium, boosts the body’s nutrient content as well as its defence mechanism. These nutrients are found in carrots, legumes, oranges, tomatoes and other leafy vegetables,” says Solange Umubyeyi, a public health specialist at Dama Clinic in Remera.

"Vegetables and fruits are essential toward healthy growth and development and should be part of our daily diet,” Umubyeyi adds.

A study by the same Institute that monitored dietary and health habits of about 110,000 men and women for 14 years found that the higher the average daily intake of fruits and vegetables, the lower the chances of developing cardiovascular diseases like heart attack and stroke. 

 As a comparison, individuals who averaged eight or more servings of vegetable and fruits a day were 30 per cent less likely to have a heart attack or stroke than those in the lowest category of fruit and vegetable intake (less than 1.5 servings a day) 

The Food and Agriculture Organisation (Fao) says Rwanda consumes a lot of vegetables, mainly cassava leaves.

Robert Kwizera, a cook in a local restaurant in Giporoso, Remera, says making the leaves soft requires boiling for two hours but with low heat, before mashing it. 

 "Rapid cooking yields a tough product that is not very nice, that’s why we cook for long using low heat. To achieve soft cabbages, I also boil them for about one hour, unless when they are to be used as salads that when we serve them raw,” Kwizera adds.

Fao studies show that although the vitamin content of the leaves is high, the processing techniques for consumption can lead to huge loss of these vitamins. Prolonged boiling involved in making African soups or stews results in a considerable loss of vitamin C.

"Nutrients in vegetables are only available to the body under certain environmental and processing conditions,” says Audrey Mutabazi, a director of Gasp, a food science consultancy firm in Gikondo. 

"While few people would probably consume vegetables in their raw form, processing using heat may have adverse effects on the nutrients.”

How much is too much heat?

According to the Journal of Food Science, some vitamins are more stable than others. In fact, water soluble vitamins (B and C) are more unstable than fat soluble vitamins (K, A, D and E) during vegetable processing and storage.  

Just like Umubyeyi warns, frying vegetables can lead to loss of vitamin A. This is because fat leaches out some of the vitamin. Frying also unfavourably affects vitamin B and C content.

 "Since these vitamins are very heat-sensitive, cooking at high temperatures, especially for a long period of time, leads to vitamin loss, that’s why opting for raw cabbages, spinach, tomatoes, carrots and onions is nutritionally valuable,” Umubyeyi adds.

Although other nutritional studies indicate that soaking beans improves absorption of nutrients due to the breakdown of phytic acid which inhibits absorption, the  reverse holds for vegetable studies discouraging boiling and soaking as they lead to greater vitamin loss, followed by poaching, frying and stewing.

Cooked vegetables lose more vitamins compared to other foods, research indicates.

The American Dietetic Association suggests that, cooking methods that use little or no water are the best. Unless the vegetable water will be used for a stew and, afterwards, consume the nutrient laden water. 

Objectively fat-soluble vitamins, such as A, D, E, K and carotenoids, are less affected by this factor.

Unavoidable as it might be, heat processing remains a big contributor to nutrient degradation in food stuffs. It is thus important to retain as much of the good nutrients, simply by limiting the amount of time vegetables are exposed to heat. 

The longer you cook the vegetables, the more they’ll be affected by the nutrition-destructing qualities of heat and water. 

Other methods can perform nutritional wonders, oven-roasting scores high on the zero-water front but low on the cooking-time front. But cooking is not so bad after all, besides it results into destruction of pathogens. 

Despite the slight decrease in nutrition with heat and water, for some vegetables, cooking can actually make them healthier. Thick cell walls in broccoli are broken down by heat to release the nutrients.