Vegetables and Fruits

India is the second largest producer of vegetables (59 million tonnes i.e.12.94% of the world production of 456 million tonnes) and also India is the second largest producer of fruits (27.3 million tonnes i.e. 7.37% of the world production). Vegetables are susceptible to the action of a variety of microorganisms, which leads to substantial decay, losses during post harvest handling.

Vegetables and fruits have a limited shelf life and hence it warrants great scope for processing. Processing of vegetables includes following steps.

Cleaning

Thorough washing in water is essential. If vegetables are consumed raw extra care is needed in cleaning them. Cauliflower flowerets are washed with hot salt water to remove the insects. Greens are put in a large basin of water. The impurities like dirt and sand settle to the bottom and leaves are lifted from the top. Vegetables can be washed under running tap water.

Peeling

Roots and tubers are peeled before cooking. It is better to remove the skin after cooking because

the nutrients would seep inside during the process of cooking. Skin can be removed easily. Loss of colour can be prevented and sweetness of the carrots is better retained if the skin is removed after blanching.

Blanching

This is done to destroy microorganisms, denature enzymes, to remove the skin easily and to improve the colour.

Apart from these preliminary preparations, cutting, grating, grindingand stringing are also done before cooking.

Effect of pickling

Pickles are usually low in pH and anthocyanins change to deep crimson red in colour.

Changes during cooking

Vegetables are cooked to improve the colour, flavour and texture by which over all palatability is improved. Digestibility is also improved. The fibre becomes softened, starch gets gelatinised and protein gets coagulated. Cooking vegetables adds variety to the diet. Cooking destroys microorganisms.

Effect of method of cooking

Anthocyanins are water solube cell sap pigments, which can be leached from a vegetable by the cooking water. Cooking in a steamer or in a pressure pan which limit the contact of the vegetable with water are better methods than boiling in water. To retain the red colour in red cabbage, the cooking water should be acidified. Otherwise the pigment will change to a dull and unappetising blue.

Storage of vegetables

Most fresh vegetables retain their top quality for only a few days. All green vegetables of high water content are best when fresh. If allowed to stand long after gathering the vegetables becomes wilted and tough through loss of moisture. The flavour is also impaired because of enzyme action and the conversion of sugar to starch. Mature vegetable particularly roots tubers and bulbs deteriorate less in storage than do fresh immature vegetables. One reason for the short storage life of many vegetables is that they are rapidly respiring.

Most fresh green vegetable may be kept fresh and crisp in covered containers or plastic bags in the refrigerator. If they are washed before storing they should be drained thoroughly because too much moisture can increase the possibility of a spoilage and decay. Seeds such as can peas can remain fresh longer it left in the pots. Tubers and bulbs can be stored in a cool place without refrigeration. Many vegetables, especially the leafy ones, spoil quickly after harvest. CFTRI, Mysore has developed technology to keep them fresh for extended periods.

Factors affecting storage life

Loss of water

Transpirational loss of water is one of the main process that affect the commercial and physiological deterioration of vegetables after harvest. The moisture loss adversely affects the appearance, texture, flavour and weight of the products. Most noticeable effect of moisture is the softening of the tissues caused by loss of turgidity.

Respiration and metabolism

Vegetables are living commodities and continue to respire even after harvest. Respiration uses the stored food, leading to its depletion and consequently the loss of quality. Hence, storage life of vegetables is influenced by rate of respiration and is associated with biochemical activity.

Microbial spoilage

Role of vegetables in cookery

Vegetables are used universally in all recipes.

They are used in curries salads and in sambar.

They are used as garnishing agents e.g. shredded carrot and coriander leaves.

They are used as Stuffing in samosa and parathas.

They are used as thickening agents in gravies and soups.

They are used as in of chutneys (onion) and pickles (tomato, onion).

As part of recipes like pulao, aluvadi and in nonvegetarian dishes.

Fruits

Nutritive value of fruit

Composition

Fruits also contain pigments like chlorophyll, carotenoides, anthocyanins, anthoxanthins etc. The water content of fruit is 75-90%. The framework of the fruit is made up of cellulose which forms the walls. Apart from this pectic substances are found in cell walls of fruits.

Effect of canning or preserving

Whether canned in tin or glass, the colour of the fruits containing anthocyanins deteriorate on storage for extended periods. Deterioration is delayed by storage in a cool dark place.

Post harvest changes and storage of fruits

Numerous physical changes continue in fruits and vegetables during storage. Bulbs, roots, tubers,and seeds become relatively dormant during storage whereas the fleshly tissues of fruits and vegetables normally undergo ripening after maturation and then continue to senescence.

Only a few fruits mainly apples can be stored for an extended period of time in cold or cellar storage. Each fruit must be stored at its own optimum temperature. Proper air circulation will ensure successful storage of fresh fruit. Low temperature close to 0⁰C with a preferred relative humidity of a about 85% furnish satisfactory conditions for commercial storage of fruits. If large quantities of fruit are to be stored, air circulation must be assured. Crates of fruit are stacked so that air can circulate around the sides, tops and bottoms. Fruits stored in the home refrigerator tend to lose moisture. Some measures taken to prevent this loss include the use of ventilated covered containers. Fresh fruits absorb and emit odours. Strong flavour fruits can be stored in tight containers.

Enzymatic browning

The rapid darkening of the cut surface of apple. (brinjal and potato) and banana are examples of enzymic browning. Normally the natural enzymatic compounds present in intact tissues and do not come into contact with the phenol oxidases present in some tissues. When the tissues are cut, or injured and the cut surface is exposed to air, phenol oxidase enzyme released at the surface, act on the polyphenols present, oxidising them to orthoquinones. The orthoquinones rapidly polymerise to form brown pigments. Tyrosine, chlorogenic acid, the various catechins and several mono and dihydroxypehnols are among the many compounds that can serve as substrates for oxidation by polyphenonloxidase to cause browning or other discolouration in these foods. Phenolase is also known by other names as polyphenol oxidase, tyrosinase or catecholase, each of is specific of for certain substrate. The optimum pH for the activity of the enzyme polyphenolase is between 5 and 7.

Prevention of enzymatic browning

• Temperature

The most commonly used method is blanching. This method has several limitations. The enzyme is fairly heat stable and requires to be heated at 100⁰ C for 2-10 minutes for complete inactivation. This may not be possible in practice, as cooking for long periods will affect the flavour and texture of the fruits. The optimum temperature for browning is 43⁰ C- 50⁰C. If food is kept at higher or lower temperature browning is reduced.

• Change in pH

The optimum pH for polyphenolase activity is between 6.0 and 7.0 lowering of the pH to 4.0 by the addition of citric and inhibits the phenolase activity. It is also possible cirtric and reacts with the copper present in the enzyme. Malic acid also has been found to be effective. Lemon juice contains both citric and ascorbic acid and both are effective.

• Use of antioxidants

Sulphur dioxide, sulphites and bisulphites inhibit effectively browning. A sulphite concentration sufficient to maintain a free sulphur dioxide concentration of 10 parts/million will completely inhibit phenolase. Being a reducing agent sulphite has an additional benefit of preserving the ascorbic acid level. Pineapple juice has a relatively high percentage of sulphydryl compound which are active antioxidants. Fruits can be dipped in dilute solution of sodium metablisulphite. If the sliced fruit is then immersed for a short time in a solution of dipotassium phosphate, the odour of sulphur is less pronounced and the fruit remains crisp and does not brown subsequently.

• Prevention of contact with oxygen

Contact with oxygen can be reduced by immersing the fruits in water, or liquid like milk, curd, fruit juice or honey after cutting or by covering with a wet cloth.

Addition of sodium chloride or sugar to the cut fruit prevents browning. Due to osmosis, fruit is covered by leached solution and prevents contact wit oxygen.

Ascorbic acid browning

Ascorbic acid is responsible for the development of browning reactions in fruits juices and concentrates and in canned vegetables. Mixtures of ascorbic acid and amino acids develop brown colour more rapidly than mixtures of reducing sugars and amino acids. Dehydro ascorbic acid is highly reactive and can react with amino acids. In the decomposition of ascorbic acid or dehydro ascorbic acid furfural and osone of L-xylose are formed which are highly reactive.

Serving fruits

Raw whole or cut fruit or a mixture of cut fruits is frequently served as an appetiser, as a salad or for dessert. Fruits to be used as appetisers should be somewhat tart and need little or no sugar. Mixed cut raw fruits are usually chosen with a view to harmonious combination of appearance, flavour and texture.

Medicinal value of vegetables and fruits

• Raw onion shifts the ratio of good-to-bad cholesterol, replacing a significant amount of destructive LDL cholesterol with heart protectvie HDL. The HDL boost is greatest from raw white and yellow onions-the sharper the better and lessens with cooking.

• Lycopene, a carotenoid pigment present in tomatoes can protect human cells from nitrogen dioxide given off by tobacco smoke and diesel fumes. It may also prevent people from developing some cancers. Carotenoids present in carrot may also prevent cancers.

• Flavonoids present in apple and onions lower the risk of death from coronary heart disease. They decrease LDL levels. Grape fruit pectin too contains an anticholesterol substance.