Factors involved in quality and nutritional value of fruits and vegetables during processing
Organoleptic quality and nutritional value of fruits and vegetables are characterised by some specific features. These include colour, texture, chemical components and antioxidant properties that are highly affected during the processing. The processing methods are the key factors that significantly contribute to maintaining/degrading the quality of the final products.
The colour and texture of fruits and vegetables mainly depend on the acidity of the cooking medium, pH of heat-treated plants, pigment compositions, time/temperature of cooking, enzymatic reaction and presence of metals. If green vegetables are heated as quickly as possible, the colour does not /slightly change. Introducing fresh vegetables into the boiling water initially brings about discolouration which is normally in response to the degradation of intercellular spaces.
Both volatile and non-volatile acids are likely to be expelled during the heating. The released acid from the cell vacuole has no effect on the colour if they are neutralised by the cooking medium. The pH of the medium is reduced when carbonic acid is formed or when bicarbonate is converted to carbonates (resulting from the release of carbon dioxide or the loss of hydrogen sulphide). The pH neutralisation relies on the alkalinity and volume of the medium. Although the addition of baking soda can increase the alkalinity of cooking water, it is not recommended because of the possibility of excessive soda. Some proportion of sodium bicarbonate may be unable to be neutralised by the acids in the medium. This can result in undesirable effect on the flavour and texture of the vegetables.
Pectin compounds that form water-retaining gels are responsible for the structure of the plants. The mushiness of heat treated vegetables depends on solubility and extractability of their pectin compositions. Hard water contains calcium ions that are able to form cross links between pectin molecules and make them less soluble. Therefore the texture of the plants becomes tougher in hard water during the heating process. Heat treatment of fruits and vegetables in hard water requires longer time to achieve the optimum softening. Prolonged heating results in colour degradation. If the water utilised for the heating medium contains insignificant amount of calcium ions, optimum soft texture can be obtained during shorter cooking time.
Carotenoid compounds of plants are slightly affected by the heat treatment. For instance, the desirable orange colour of carrots turns yellow or tomatoes change colour from deep-red to orange-red. High percentage of unsaturation in carotenoids makes them liable to oxidation during the dehydration treatment. Decrease of colour shade can be in consequence of the reaction of peroxides and free radicals or the oxidation of lipids with carotenoids. Carotene can be immune from oxidation during dehydration by the blanching of vegetables or the sulphuring of fruits.
Anthocyanins are not highly affected during the cooking due to their acidity. Red cabbage is steamed or boiled in tap water, the colour will turn blue. The colour may be changed to a desirable reddish colour by the addition of acid during the cooking e.g. vinegar. Anthocyanins can be degraded by oxidation, hydrolysis, or polymerisation. Time and temperature of heating used for the preparation of fruit juice (such as red currant and raspberries) can greatly affect the tannin content of the juice. Tannin compounds increase the degree of bitterness and astringency of the plants.
Blanching facilitates the prevention of enzymatic browning via inactivating certain types of enzymes including polyphenol oxidase (the enzyme responsible for the oxidation of phenol compounds of fruits and vegetables). The pH of the blanched vegetables and fruits is increased during blanching which is contributory to the colour changes of the cooked plants.
Canning process has noticeable effect on the colour change in green vegetables. This is usually attributed to the conversion of chlorophyll to pheophytin. During the heating process, the choloroplasts progressively become clumped and shrunk in the centre of the coagulated protoplasm. At this stage, the remaining chlorophyll in the chloroplasts is no longer protected by the plastid membranes from the acid-containing cell sap. As a result, the dull olive-green pheophytins are formed.
Freezing method helps preserve significant proportion of pigments such as chlorophyll. The extent of the preserved pigments is dependent on the pH of the plants. Vegetables with higher pH including spinach and peas can retain more pigment compared to those with lower pH, e.g. green beans and brussel sprouts.