FRUITS: NUTRITIONAL VALUES

SAIMA IBRAHIM
(feedback@pgeconomist.com)

Jan 17 - 23, 2011

The nutritional value of fruit is because of good vitamins and enzymes. Obviously, different fruits vary in their specific attributes .Most fruits supply fresh vitamins, minerals and enzymes and the nutritional value of fruit lies mainly in its ability to supply these things abundantly.

Fruits and vegetables are a major source of macronutrients such as fiber and micronutrients such as minerals and vitamins C, thiamin, riboflavin, B-6, niacin, folate, A, and E. Phytochemicals in fruits and vegetables, such as polyphenolics, carotenoids, and glucosinolates, may also have nutritional value. While many fruits and vegetables are consumed primarily in their fresh state, some produce such as tomatoes, snap beans, corn, peaches, nectarines, and pineapples are also consumed to a significant degree in their processed state.

Most fruits and vegetables are composed of 70-90 per cent water and once separated from their source of nutrients (tree, plant, or vine) undergo higher rates of respiration, resulting in moisture loss, quality and nutrient degradation, and potential microbial spoilage.

The commonest vitamin in fruit is vitamin C and this happens to be a particularly important vitamin because our bodies neither store nor manufacture it. Vitamin C protects against heart disease and the "free radicals" which play a part in ageing and cancer. The best fruits for Vitamin C are the citrus group - oranges, lemons, grapefruit, and tangerines to name a few. Other excellent sources include kiwi fruit, mangoes and papayas and many of the soft fruits such as blackcurrants and strawberries. Vitamin A is another important vitamin for the immune system and for good vision and bone growth. It also helps regulate some hormones and promotes healthy teeth and hair. In fruit, it generally turns up as beta carotene, which the body then uses to make vitamin A. Fruits of a deep yellow or orange colour are usually good sources of beta carotene. A related vitamin is lycopene which is found in red coloured fruits such as tomatoes, guava, watermelon, pink grapefruit and chilies. Lycopene has an important role in protecting us against cancers and heart disease. It is a powerful anti-oxidant. Some fruits also contain useful amounts of some of the B vitamins and some vitamin D and E. However, most of these nutrients are better supplied in other foods.

Fruit is a rich source of essential minerals. Good quality fresh fruits supply minerals such as potassium and magnesium, iron and calcium.

Potassium is particularly in demand for many of us because of our sodium-rich western diets. Salt (sodium) and potassium are in a balance in the body cells and, (although the body maintains that balance through thick and thin,) a chronic lack of potassium can cause a number of problems from muscle spasm and pain through to cancer and heart disease. Potassium helps the body to cleanse itself of impurities and also helps maintain a powerful energy system. One of the symptoms of potassium deficiency is chronic fatigue. Good sources of potassium include oranges, bananas, blackberries, and tomatoes. Avocados are incredibly rich in both potassium and Vitamin A.

Phosphorus is not generally too important as it is found elsewhere in most people's diets. Even junk food is rich in phosphorus.

Calcium is a very important mineral because it is needed for healthy bone development and maintenance. It is also not especially well supplied in many people's diets. This is because although milk and cheese products are calcium rich, the absorption rate is often low from such sources. So it as well to have some decent back up. Blackberries, strawberries, oranges, kiwi fruit and tomatoes are all relatively rich in calcium.

Magnesium is important for bones and for nerves. It also helps us to extract energy from food. It is generally well supplied in most people's diets. Fruit and whole grains are good sources of magnesium.

Iron is another essential mineral, which is commonly found in fruit. Women and girls need around 15 mg per day for optimum health - men and boys rather less. Strawberries, blackberries, kiwi, tomatoes, grapes, and bananas are all good sources. Raisins are particularly rich but the concentrated sugars contained in them may be a problem for some people.

Fresh uncooked fruit supplies living enzymes, which our systems can use to great advantage. For example, many fruits supply "superoxide dismutase" - otherwise attractively named "SOD" for short. This enzyme has a role in protecting us against ageing. It is present in virtually all raw foods and it has a protective role against free-radicals and other damaging rogue molecules. It acts as an anti-oxidant. SOD also has a role in protecting the joints and the cardio-vascular system and respiratory system from age-related damage. Some scientists maintain that SOD is largely broken down by the digestive juices. Further, if we are healthy, we produce our own SOD. While this may well be true, SOD in foods can be assimilated to some extent and this process is helped by a natural nutrient found in wheat. One type of SOD has an effect upon the mitochondria in our body cells.

SOLUBLE AND INSOLUBLE FIBRE

Fruit is an excellent source of natural fibre. The main source of fibre present in fruit is not actually fibrous. If you eat apple cores and pear hearts, you will get some actual fibre but most of nutritional fibre is found in the soft pulpy parts of fruit too. Fibre is basically one form or another of carbohydrate. The indigestible plant fibres such as bran and cellulose are called "insoluble fibre". Soluble fibre consists of matter such as gums and pectins found inside the cells of plants. Fruit is a particularly good source of soluble fibre. Soluble fibre also has a role in reducing cholesterol in the body. One of the benefits of high fibre foods are that they make you feel full and keep you feeling satisfied for a good while. While too many watery fruits alone will not fill you up, a good balance of fruits with other foods can provide a sustaining and nourishing source of energy. Good sources of dietary fibre include blackberries (around 7 grams per serving), apples (4 grams) and avocado (10 grams). Most fruit will supply a useful amount. Both types of fibre are beneficial. An important nutritional value of fruit is its ability to supply both types of fibre.

STORED AND REFRIGERATED FOODS

In some instances, fruits and vegetables may be harvested immature to reduce mechanical damage during harvesting and transportation. Because intact fruits and vegetables are still alive and respiring, temperature and relative humidity must be carefully controlled to maintain low rates of respiration, prevent moisture loss, and maintain eating quality.

Changes in nutrient composition from harvest to consumption depend to a certain degree on the particular nutrient, the commodity, and the postharvest handling, storage, and home cooking conditions. In addition, initial nutrient content is affected by the particular cultivar (e.g., Red Delicious and Fuji apples), soil type, production system (conventional, organic, etc.), and weather conditions (temperature, humidity, daylight hours, etc.) during growth.

Nutrient retention is optimized if fruits and vegetables are gently handled and stored at high relative humidity and refrigerated. Some commodities such as apples and pears, are stored for up to 12 months under controlled atmosphere conditions that utilize low oxygen and high carbon dioxide levels to slow down respiration.

Most perishable commodities, however, are stored under refrigerated conditions, and storage life may range from 8-10 days for highly perishable fruits like berries to 810 weeks for less-perishable commodities like squash, pumpkin, apples, grapes, and pears.

PROCESSED FRUITS AND VEGETABLES

Many fruits and vegetables only grow in specific regions of the world, in a particular type of soil, under certain temperature and humidity environments, and at limited times of year, and in many countries refrigeration is not available. Processing allows fruits and vegetables produced in remote regions of the world, such as coconuts, durian, mango, and papaya, to be stabilized and transported to distant locations for consumption. In many instances, fruits and vegetables such as pumpkin, clingstone peaches, and some leafy greens and squashes may be too tough or bitter to consume C content in fresh, frozen, and canned vegetables that are further stored and then home cooked. Canning exposes fruits and vegetables to high temperatures, which degrade vitamin C and may cause leaching into the canning medium.

Losses in vitamins B also occur during transportation and storage of fresh fruits and vegetables, but less literature is available on these nutrients. The B vitamins, thiamin, and vitamin B-6 in particular, are quite sensitive to heat and light and reported losses as a result of canning range from 7 to 70 per cent for various vegetables. The B vitamins are also sensitive to blanching and freezing, with losses in the range of 2060 per cent.

The water-soluble polyphenolic compounds are found primarily in the skins of commodities such as peaches, pears, and apples. Therefore, removal of the skin will significantly change the levels of these compounds. Polyphenolics generally decline with storage of fresh fruits and vegetables and as a result of canning and blanching.

Fat-soluble nutrients such as vitamins A and E and the carotenoids (including lycopene) are sensitive to heat, light, oxygen, and pH. However, because these compounds are fat soluble, there is little leaching into cooking water or canning medium. Refrigerated storage for 14-16 days resulted in a 10 per cent increase in the beta-carotene content of carrots and a 10 per cent loss in green beans. Processed tomato products have higher lycopene content than fresh, most likely due to the heat-induced release of this nutrient from its cellular matrix. Therefore, processing does not cause the synthesis of more carotenoid compounds but merely allows scientists to detect greater amounts of them due to their increased extraction. Compared to the water-soluble vitamins, the carotenoids and lycopene appear to be relatively stable during processing, storage, and cooking, but there is a lack of information on these components.

Mineral and fiber content is found to be similar in fresh, canned, and frozen fruit and vegetable products; this is expected, since these nutrients are relatively inert and are not sensitive to degradation by thermal processes used in food preservation.

Fiber is relatively insensitive to thermal processing or freezing, so the fiber content is very similar in fresh and processed fruits and vegetables. Brassica crops, such as cabbage and broccoli, have a naturally occurring enzyme called myrosinase that produces nutritious isothiocyanate compounds. Processing destroys this enzyme, but it exists in the intestine as well, so the same nutritious compounds are produced there when cooked broccoli is eaten.