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Vegetables

Vegetables have a considerable proportion of water in their composition (75-95 %), and the dry matter consists mainly of carbohydrates (among which cellulose takes an important place). The proportion of carbohydrates varies from species to species as follows:

  • carbohydrate content between 1-5 %: lettuce, squashes, tomatoes, aubergines, cucumbers, leafy greens;
  • carbohydrate content between 5 and 10 %: green beans, onions, okra, carrots, leeks, turnips, celery, green peas, Brussels sprouts, carrots;
  • carbohydrate content between 15 and 20 %: potatoes, horseradish, cooked dried beans, lentils and dried peas,

Carbohydrates occur in the form of glucose, fructose, sucrose, starch and cellulose.

Proteins are generally present in small amounts, except in dried pulses (beans, soya beans, peas, chickpeas, lentils and so on), where their proportion is raised. Pulse proteins are of good nutritional quality, but they are unbalanced with regard to the sulphur-containing amino acid methionine, which is why combining them with cereals is advisable (cereals are low in the amino acid lysine). Through this combination the proteins are balanced and have high biological values, close to those of animal origin. Pulse proteins have positive effects on the cardiovascular system and are not accompanied by atherogenic saturated fats.

Lipids occur only in a small proportion, except in oilseeds, where they make up a high proportion.

Vitamins and minerals are present in an important proportion in most green vegetables, as follows:

  • Vitamin C (ascorbic acid): green leaves (leafy greens), some fruits (peppers, tomatoes), tubers (asparagus, potatoes), bulbs (raw onions). The richness in ascorbic acid is inversely proportional to the content of ascorbinase (the enzyme that turns vitamin C into compounds with no vitamin activity – cucumbers contain raised amounts, which is why their vitamin C content is low).
  • Carotene: green leaves, carrots, carrots, asparagus
  • Vitamin K: green leaves (spinach, chard, nettle, lettuce), cabbage, cauliflower
  • B group vitamins: beans, green peas, potatoes, beetroot, cabbage, spinach, lettuce
  • Vitamin E: spinach, green peas and cabbage
  • Potassium: carrots, radishes, potatoes, tomatoes, lettuce, cucumbers
  • Calcium: spinach, orache, beetroot
  • Iron: beans, green peas, dandelion, parsley, nettles, spinach, Brussels sprouts
  • Phosphorus:
  • Sodium: carrots, artichokes, radishes, spinach, beetroot, parsley leaves, beans and peas, potatoes, courgettes, cabbage

Various substances with a so-called antinutritive effect (which destroy the nutrients they are found with) have been identified in vegetables: antivitamin substances (ascorbinase), antimineral substances (phytic acid and oxalic acid – they bind calcium, magnesium and iron into insoluble complexes that prevent their absorption and use in the body), antiproteinogenic substances (an inhibitor of pancreatic trypsin – in beans), antithyroid substances (which prevent the uptake of iodine in the thyroid gland – cabbage, cauliflower, kale).

What vegetables add to the diet:

  • plenty of water and little energy: radishes, lettuce, cucumbers, chicory;
  • potassium: asparagus, leeks, celery, spinach;
  • flavour without salt: garlic, onions, leeks;
  • fibre: cabbage, carrots, spinach, leeks, onions;
  • vitamin C: red cabbage, peppers, broccoli;
  • bitter compounds and aroma: artichoke, chicory, rocket.

Cooked vegetables have a higher digestibility because of the partial hydrolysis of the cellulose structures. Raw, they contain natural simple sugars in a matrix of fibre, minerals, vitamins and phytonutrients. They travel quickly through the digestive tract into the small intestine, where the simple sugars are absorbed directly and the fibre passes into the large intestine with beneficial effects. When they are combined with fats or starches (doughs, puddings, creams and so on), this combination makes them enter into fermentation and putrefaction, drawing other foods into these processes as well.

Vegetable products preserved by freezing are of very good quality compared with the same products preserved by other methods. The changes in quality can be physical, chemical and microbiological in nature. Changes of a physical nature are: structure – texture, which becomes looser because of ice crystals that can break cell membranes in the case of migratory recrystallisation, an increase in volume of 10–15 % during freezing and a reduction of 20–40 % during thawing.

From a chemical point of view, plant products lose a range of nutrients during freezing, particularly vitamin C. The oxidation of vitamin C is more intense in green beans and cauliflower and slower in peas and broccoli.

With minerals, the greatest losses occur during preparation, freezing and storage. Comparative studies have shown that freezing retains 2-3 times more niacin, thiamine, riboflavin and vitamin C.