Introduction to fruits and vegetables

 

VEGETABLES

Vegetables are plants or parts of plants that are used as food. The term vegetable has through usage come to apply in a more narrow sense to those plants or parts of plants that are served raw or cooked as a part of the main course of a meal. Vegetables supply many nutrients besides providing variety to the diet. They make the food attractive by their color, texture and flavor. Various parts of plants vary in their water, protein, vitamin, mineral and carbohydrate contents.

Classification of vegetables

Vegetables/protective foods are classified according to the parts of the plant consumed or color of the vegetable or according to the nutritive values.

Group	Examples
Roots	Carrot, beet root, radish, turnip, colocasia
Tubers	Potatoes, sweet potatoes, tapioca
Bulb	Onion, garlic, leeks
Leaves	Cabbage, lettuce, spinach, amaranth, fenugreek leaves, coriander leaves, mint leaves.
Flowers	Plantain flower, cauliflower, neem flower, broccoli
Fruits	Tomatoes, brinjal, lady’s finger, pumpkin, cucumber gourds (ash gourd, bottle gourd, ridge gourd), capsicum, drumstick, plantain.
Legumes ( pods and seeds)	Peas, beans, chowli, broad beans, French beans, double beans, Bengal gram tender, red gram tender.
Stems	Plantain stem, ginger, amaranth stem, celery stem, lotus stem.
Seed sprouts	Grean gram, Bengal gram, soyabean sprouts
Fungi	Mushrooms
Algae	Spirulina

 

Nutritionally, vegetables are classified mainly as:

·         Green-leafy vegetables

·         Roots and tubers

·         Other vegetables.

 

Nutritive value

Ø  Nutritive value of green-leafy vegetables: The leaves in consequences are low in carbohydrates and energy but they are good sources of β- carotene, calcium, riboflavin, folic acid, ascorbic acid, iron and vitamin K. They are excellent in carotenes which are converted to vitamin A.

Greens are good sources of B-vitamins particularly riboflavin and folic acid. Drying and withering reduce B-vitamins.

Green-leafy vegetables also contain vitamin C and can be used as substitute for fruits if needed. Green leafy vegetables are also rich in iron and calcium. Greens generally are high in moisture and easily withered and need to preserve properly. Greens are not good sources of protein, fat and carbohydrate and hence they do not contribute to the energy value of food. Greens are good sources of fiber which help in preventing degenerative diseases.

 

Ø  Nutritive value of roots and tubers

Roots and tubers give more calories as they contain more starches.

Roots and tubers are fairly good source of vitamin C.

They are poor source of calcium, iron and B- vitamins.

They are poor source of protein.

Ø  Other chemical compounds occurring in vegetables: Vegetables contain several groups of chemical compounds such as:

Ø  Pigments

Pigments present in vegetables include the water insoluble plastid pigments, the chlorophyll and carotenoids and the cell-sap soluble pigments- the anthocyanin, flavones, flavonols, and similar substances. These pigments may change in color as a result of a change in physical state or of chemical reaction with metals, metallic salts, acids, alkalis, oxygen etc.

The bright colors of many vegetables contribute much to their appeal. The colors result from the various pigments contained in their tissues. The chief pigments of vegetables and fruits can be classified as water insoluble and water soluble.

Water-Insoluble Pigments

Chlorophyll: The green pigments of leaves and stem are usually held close to the cell wall in small bodies called chloroplasts along with some carotenes and xanthophylls. Chlorophyll-a is intense blue green in color and chlorophyll-b is dull yellow green in color. Chlorophylls are mostly insoluble in water and dominant in unripe fruits. This pigment is present in green leafy vegetables, capsicum, beans, peas and chillies.

Carotenoids: In greens though carotenoids are present the color is masked by the chlorophyll.

 

In plants, carotenoids are present as α- carotene, β- carotene, ϒ- carotene, xanthophylls and cryptoxanthin. β- Carotene is valuable in the synthesis of vitamin A.

Food	Pigments
Yellow corn	Cryptoxanthin
Tomatoes	Lycopene, β-carotene
Red capsicum	Cryptoxanthin, capsorubin, β- carotene, violaxanthin, capxanthin.
Green capsicum	Lutein, β- carotene, violaxanthin, neoxanthin.
Carrots	β- carotene, α- carotene,¥- carotene, lycopene, xanthophylls.

 

Water soluble pigments

These pigments are not membrane-bound molecules but are dissolved in the cell sap of epidermal cells of these parts. Flavonoids are classified into anthocyanins and anthoxanthins.

Anthocyanins:

Anthocyanins are highly water-soluble pigments that range in colour from red to purple Cherries, red apples, various berries blue and red grapes, pomegranates, and currants archive their color appeal because of predominance of anthocyanins.

Betalins: Anthocyanin pigments are absent in some families and are replaced by highly different compounds, betacyanins and betaxanthins. These two pigments together are known as betalins.

Anthoxanthins:

Anthoxanthins: These are colorless or pale yellow pigments closely related to anthocyanins depending on the pH, Anthoxanthins give colour to cauliflower, onion and spinach or other leafy vegetables.

 

Organic acids: Vegetables contain a number of organic acids, metabolic products of the cells. Formic, succinic, citric, acetic, malic, fumaric, tartaric, and benzoic acids are present in fruits and vegetables. The concentration of acid is lower in vegetables than in fruits. Tomatoes and vegetables with the concentration of acid have a pH Ranging from 4 to 4.6. Foods like lemon, mango green, tamarind (tartaric acid), gooseberries, raw citrus fruits and grapes have low PH. Most of the vegetables have pH 5.0 to 5.6. Potatoes and peas have PH 6.1 to 6.3, more neutral in taste.

 

Polyphenols and tannins

Ø  Polyphenols and tannins

These include anthocyanins, flavones, flavonols and tannins. They are responsible for the astringent taste of some vegetables and for the astringent taste of some vegetables and for the discoloration of some vegetables after cutting, cooking or processing.

Ø  Enzymes

Enzymes are found in plants cells. They function as chemical reactions. Enzymes bring ripening of tomatoes and bananas. If this enzymatic reaction continues the fruit gets spoilt. They also bring browning in potato. Two of enzymes namely ascorbic oxidase and phenoloxidase are important as the former oxidizes ascorbic acid and the latter cause’s oxidation of polyphenols and enzymic browning in vegetables.

Raw papaya contains an enzyme called papain. Papain is used as a stabilizer in beer and other beverages and as a meat tenderizer.

Enzymes also can cause detrimental changes in anthocyanin pigments. Peroxidases and phenolases naturally present in some fruits and vegetables can catalyze oxidative reactions that are harmful.

 

Ø  Flavoring principles

The flavor of fruits and vegetables are extremely important to their acceptance in the diet.

Sweetness may result from the presence of glucose, galactose, fructose, ribose, arabinose and xylose.

All fruits and vegetables naturally contain a small amount of salt, which is detected in the overall taste impressions contributing to flavor.

The natural flavor of vegetables is due to mixtures of aldehydes, alcohol, ketones, organic acids and sulfur compounds. Some fruits and vegetables have an astringent taste attributed to phenolic compounds or tannins.

Two types of vegetable have strong flavours resulting from the presence of various sulphur containing compounds. Allyl sulphide found in onions, garlic and leeks. Brussle sprouts, broccoli, cabbage, turnips, cauliflower, kale and mustard are members of the family cruciferae, which also contain prominent volatile sulphur compounds.

Vegetables	Precursor	Reaction with treatment	Final volatile compound
Garlic	Alliin s-2-Propenyl (allyl) cysteine sulphoxide	Cutting/ Crushing results in allicin formation This undergoes nonenzymatic decomposition to disulphide and thiosulphinate	Disulphide further decomposes to a complex mixture of mono-sulphide and tri-sulphide – characteristic flavor
Onion	S-1-Propenyle cysteine sulphoxide	Cutting/ Crushing results in formation of sulphenic acids which is unstable and undergoes rearrangement	Thiopropanal-S-oxide- lachrymatory factor
Brassica family- Cabbage, cauliflower	S-methyl-cysteine sulphoxide and thioglucosides	Cooking	Dimethyl sulphides and isothiocyanates- give off-flavour

 

An amino acid s-methyl 1- cysteine sulphoxide is also present in raw cabbage and appears to be a precursor of cooked cabbage flavor.

Ø  Bitter compounds

Potatoes that have been exposed to light, mechanical damage and sprouting may become green due to rapid increase in the concentration of certain toxic steroidal glycoalkaloids such as solanin and chacomine. These impart to the potatoes a bitter taste. Gastrointestinal and neurological symptoms may develop due to solanin poisoning. These alkaloids are not destroyed during the process of cooking, baking and frying. The best way to minimize the hazards from these toxins is taking appropriate precautions against damage/ insect attack during growing, harvesting, storing and processing potatoes.

 

FRUITS

Fruits are produced from flowers and they are the ripened ovary or ovaries of a plant together with adjacent tissues. Fruits are fleshy or pulpy in character often juicy and usually sweet with fragrant, aromatic flavors.

Classification

Fruits are divided into groups depending upon the shape, cell structure, and type of seed or natural habitat. One system classifies them into the following groups.

Berries :               Strawberries, gooseberries, blackberries, raspberries, blueberries, cranberries.

Citrus fruits:       Sweet limes, oranges, tangerines, sour oranges, lime, lemon, grape fruit.

Drupes :               Apricot, sweet cherry, peach, plums.

Grapes  :               Green grapes, black grapes, seedless grapes.

Melons :               Musk melon, water melon.

Pomes  :               Apples, pears.

Tropical and subtropical fruits   :               Amla, avocado, banana, dates, guava, Jack fruit, mango, jambu fruit, papaya, passion fruit, pineapple, pomegranate, sapota, seetaphal.

 

Nutritive value

·         Fruits are very poor source of protein and fat. Avocado is the exception containing 28 per cent fat.

·         Fruits are not very good sources of calories. Fruits like bananas give fairly good amount of calories. Ripe fruit contains a higher percentage of sugar than unripe fruit and the sugar is chiefly in the form of sucrose, fructose and glucose.

·         Generally fruits are poor source of iron.

·          Excellent source of carotenes such as mangoes. Also, a source of vitamin C.

·          Apples, pears, cherries, grapes and citrus fruits contain flavonoids which act as antioxidants.

·         Like vegetables, fruits also contain different pigments.

Ø  Chlorophyll: Guava, gooseberry, country apple.

Ø  Carotenoids : Mango, papaya, orange, watermelon (lycopene), musk melon (β- carotene), jackfruit, peaches, (violaxanthin) tomatoes, grape pink (lycopene, b- carotene) pine apple (violaxanthin b- carotene)

Ø  Anthocyanins : Grapes, blueberries, plums, cherries.

Ø  Anthoxanthins: Guava, apple, gooseberry, pears, custard apple, banana.

Ø  Anthocyanins: Sometimes, strawberry jam changes gradually from the pleasing red to a dull reddish brown occurs if such factors as a high pH, oxygen in the head space and or a high storage temperature are present.

Ø  The metal iron precipitates anthocyanin. This reaction may cause ‘pin-holing’ of cans used for foods containing anthocyanins.

 

·         Water: Fruits contain 75 – 90% water. Dissolved in the water, the bulk of which is found in the vacuoles are soluble substances like sugar, salts, organic acids and water-soluble pigments. Substances unable to dissolve in water are colloidally dispersed in it.

·         Cellulose and pectic substances: The framework of the fruit is made of cellulose which forms the walls of the plant cells and in which large amounts of water are held. Apart from cellulose and hemicelluloses, pectic substances are found in cell walls and between the cells. They act as a cementing substance and bind cells together. Pectic substances include protopectin, the insoluble parent molecule, pectinic acid or pectin and pectic acid. The change in solubility of the pectic substances occurs because of ripening or the influence of heat. This makes tissues disintegrate. Acids make the structure more firm where as alkalies tend to disintegrate the fibres.

·         Flavor constituents

Volatile flavor compounds are esters, aldehydes, acids, alcohols, ketones and ethers. Some fruits contain essential oils which are also important flavor contributors.

Polyphenols: Polyphenols or tannins comprised catechins, the leucoanthocyanins and hydroxyl acids. Skin and seeds contain high amount of tannin. When a fruit is pressed, such as apples in the preparation of cider or grapes in making juice or wine, the tannins flow out in the juice.

Effects of polyphenols on fruit quality

 

The cause of the undesirable astringency in some fruits and desirable astringency in ciders and wines is due to polyphenols. Astringency is the feeling of puckering that occurs in the mouth when certain compounds, notably flavanols are present.

The formation of troublesome haze and precipitates in apple juice, beer and wine has been attributed to the interaction of proteins and phenolic polymers.

Development of brown discolouration occurs in cut fruits due to oxidation of phenolic substances (chlorogenic acids, catechins) by the polyphenolase enzymes.

The formation to dark-colored complexes with iron due to sequestering action of the dihydric and trihydric phenolics, which result in undesirable appearance of canned foods.

The development of pink to pinkish brown colour in fruits like pear, guava, apple, banana, litchi has been attributed to leuocoanthocyanins.

When bananas ripen, the excessive quantities of flavanols found in the unripe fruit are reduced. A warm and clear environment during ripening of peaches results in a lower content of polyphenols than is developed when cloudy weather prevails. The lower polyphenolic content imparts a reduced astringency.

Bitterness in fruits

Bitterness in citrus fruits can be attributed to compounds of two classes, the limonoids (triterpenes) and the flavanone glycosides (flavonoids).

The Precursor of Limonin is naturally present and stable in intact citrus tissue but when the fruit is squeezed to yield juice, the active compound of the precursor in combination with the acidic pH results in the formation of limonin.

The principal flavonoid bitter tasting component of citrus fruit is the flavanone glycoside, naringin. It is the major flavonoid of grape fruit.

Certain citrus products when heated can develop a bitter off flavour. The compound thought to be responsible is limonin which is intensely bitter tasting triterpenes dilactone which can also be naturally occurring in citrus products.

 

Pectin

Pectic substances: Pectic substances are derivatives of carbohydrates. They are methylated polymer of galacturonic acid. They are present in the primary cell wall and the middle lamella of fruits and vegetables. Protopectin is the water-insoluble form of pectic substances occurring in immature fruits and to a less extent in vegetables. Protopectin gives firm texture to unripe fruits. As fruit ripens, some demethylation and hydrolysis occur along the protopectin molecules due to the enzymes pectinesterases.

The transition takes place from a methylated water-insoluble polymer-protopectin-to a shorter methylated compound capable of dispersing easily in water-pectin. Pectin forms gel on heating with acid and sugar.

Pectic substance and gel forming ability is given in Table.

Pectic substance	Occurrence	Chemical characteristic	Capacity to form gel
Protopectin	Raw fruits and vegetables	Insoluble methylated polymer of galacturonic acid	Cannot form a gel
Pectinic acid	Slightly ripened fruits	Polygalacturonic acids containing more than a negligible proportion of methyl ester groups	Form a gel with very little sugar
Pectin	Optimum ripe fruits	Water-soluble shorter methylated compound	Forms firm gel with acid and sugar
Pectic acid	Over ripe fruits	Polygalacturonic acids free from methyl esters	Forms firm gel with acid and sugar cannot form gel

 

 

As the degradation of pectin continues the molecules gradually become shorter and lose all of their methoxyl groups. These shorter polymers of galacturonic acid are designated as pectic acid. Pectic acid is found in over ripe, very soft fruits and vegetables. This type of pectic substance has lost the gel forming ability characteristic of the longer methyl esters of galacturonic acid polymers.

Cell wall components undergo changes after harvest as a consequence of the action various enzymes. The pectic substance in cell walls and the middle lamella undergo degradation as a result of the increasing levels of two types of enzymes, pectinesterases and polygalacturonases. Other enzymes include hemicellulase and cellulase. As a consequence of the reactions catalysed by these enzymes, some sugars are released from the complex polysaccharides constituting the cell walls. The result is ripened fruits increase in sweetness despite the fact that they may have little or no starch to serve as a potential source of sugar.

 

 

 

POST HARVESTCHANGES IN FRUITS AND VEGETABLES

Loss of water: Transpiration loss of water is one of the main processes that affects the commercial and physiological deterioration of vegetables after harvest. The moisture loss adversely affects the appearance, texture, flavor and weight of the products. Most noticeable effect of moisture is the softening of the tissues caused by loss of turgidity.

Respiration and metabolism: 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. The low respiring commodities have a long shelf life in contrast to the short life of high respiring commodities.

During storage, the fleshy tissues of fruits and vegetables normally undergo ripening after maturation and then continue to senescence. Senescence occurs quite rapidly with an accompanying loss of palatability. Certain types of biochemical activities occur in all fruits and vegetables, including respiration, protein synthesis and changes in some constituents of cell walls.

Respiration rate varies with the stage of maturity and ripening in many fruits, with the rate increasing to a maximum just prior to full ripening. This phase is called the climacteric. Those fruits that exhibit this increase in respiratory rate just prior to senescence are termed climacteric fruits. They are distinguished by their ability to continue to ripen when they are harvested at the time that they are horticultural mature but not yet ripe. Citrus fruits and grapes are non- climacteric. Their respiration rate does not accelerate after harvesting. Non climacteric fruits are best when ripened before harvesting.

Microbial spoilage: Vegetables are susceptible to the action of a variety of micro-organisms, thereby leading to substantial decay losses during post-harvest handling.

 

Preservation measures

1.       Use of low temperatures

Refrigerated storage: Living organisms have an optimum temperature for growth and lower temperatures greatly retard metabolism and near the freezing point, rate of respiration is reduced. If foods are stored at temperatures near 32 to 340F the storage life may be prolonged as not only respiration rate is decreased but also growth of many spoilage micro-organisms is also retarded but not totally inhibited. In this type of storage, in addition to temperature the relative humidity should be in such a way that there should not be too much moisture losses which cause wilting or too much moisture which may decay fruits and vegetables.

Cold storage: Many vegetables can be stored from 14-250 days under refrigerated storage conditions. In general, tropical vegetables require slightly higher temperature of storage 42-55 0 F while vegetables grown in temperate climates or winter months in the tropics require lower temperature (32-38 0 F).

Frozen storage: Many vegetables can be stored under frozen storage conditions at 0 0F for several months. The vegetables are cut and packed in suitable containers, viz., metal foil or plastic bags and rapidly frozen to – 40 0F and stored at 0 0F.

 

2.       Waxing of fruits and vegetables:  Wax coating prevents moisture loss, maintains the appearance, delays ripening and decreases the rate of decay and sprouting. This method of storage has been applied in the case of mango, banana, citrus fruits and potato.

3.       Modified atmosphere packaging (MAP): MAP is the method for extending the shelf life of perishable and semi-perishable food products by altering the relative proportions of atmospheric gases that surround the food.