FOOD CHEMISTRY

Module 11. Vegetables and fruits

Lesson 26
CLASSIFICATION, GENERAL COMPOSITION, CHEMICAL CHANGES DURING RIPENING AND STORAGE

26.1 Introduction

Vegetables are defined as the fresh parts of plants which are consumed as such or cooked. More than ten thousand plant species are consumed as vegetables in the world. It provides essential nutrition to human in the form of minerals, dietary fibers, fats, vitamins,proteins, carbohydrates and energy. Fruits of perennial trees are not considered to be vegetables. Ripe seeds are also excluded (peas, beans, cereal grains, etc.).

Fruits include true fruits and spurious fruits, as well as seeds of cultivated and wild perennial plants. Fruits are commonly classified as pomaceous fruits, stone fruits, berries, tropical and subtropical fruits, hard-shelled dry fruits and wild fruits.

26.2 General Classification of Vegetables

They are classified in following different groups with examples along with their consumption pattern.

1. Mushrooms (edible species): Fungi are commonly known as mushrooms. They are nutritious and provide characteristic flavour.
e.g. Button, Flats, Truffle, Wood ear, etc. - Steamed, fried, dried, pickled or salted

2. Algae (seaweed)

1. Sea lettuce - Eaten raw as a salad, cooked in soups
2. Sweet tangle - Eaten raw in salads, cooked as a vegetable

3. Rooty vegetables: Usually a long or round-shaped taproot.

1. Carrot - Eaten raw or cooked
2. Radish – roots are eaten raw; used as a salad
3. White (Irish) potato - Cooked, fried or deep fried in many forms
4. Red beet, beetroot – Used as a salad

4. Tuberous (rhizomatic) vegetables: These grow underground on the root of a plant.

1. Sweet potatoes - Cooked, fried or baked
2. Cassava (manioc) - Cooked or roasted

5. Bulbous rooty vegetables: Usually grow just below the surface of the ground and produce a fleshy, leafy shoot above ground. Bulbs usually consist of layers or clustered segments. Only onions and garlic have prominent bulbs.

1. Garlic – Consumed raw, cooked as seasoning
2. Onion - Consumed raw, fried as seasoning, cooked as a vegetable

6. Stem (shoot) vegetables: The edible stalks of plants when the stalk is the main part of the vegetable.

a. Bamboo roots -Cooked for salads

1. Leafy vegetables: The edible leaves of plants.

1. Cabbage - eaten raw in salads; cooked as a vegetable
2. Spinach - Cooked as a vegetable

2. Flower head vegetables: The edible flowers of certain vegetables.

1. Cauliflower - Cooked as a vegetable or used in salads
2. Broccoli - cooked as a vegetable

3. Seed vegetables: It is also know as legumes. Seeds are usually obtained from pods. The pod is sometimes eaten along with the seed.
1. Green beans - cooked as a vegetable, steamed or cooked for salads
2. Green peas - cooked as a vegetable, steamed or pickled for salads

4. Fruity vegetables: Vegetable fruit are fleshy and contain seeds.

1. Cucumber – Consumed raw in salads; cooked as a vegetable or pickled
2. Capsicum - cooked as a vegetable
3. Pumpkin - cooked as a vegetable
4. Tomato - eaten raw, in salads, cooked as a vegetable, used as a paste or seasoned puree; immature green tomatoes are pickled and then eaten as salad

26.3 General Composition

The composition of vegetables can vary significantly depending on the cultivar and origin.

1. The amount of dry matter in most vegetables is between 10-20%. Some tubers and seed vegetables have high starch content and so a higher dry matter content.
2. Vegetables contain an average of 1-3% N-compounds. Of this 35-80% is protein and rest is amino acids, peptides and other compounds. The protein fraction consists to a great extent, of enzymes which may have either beneficial or detrimental effect on processing. They may contribute to typical flavour or to formation of undesirable flavour, tissue softening and discoloration. Enzymes of all the main groups are present in vegetables.

3. The carbohydrate content is in the range of 3-20%. The predominant sugars in vegetables are glucose, fructose and sucrose. Starch occurs widely as storage carbohydrate and is present in large amounts in some roots and tuber vegetables. Other polysaccharides viz. – cellulose, hemi-cellulose and pectin are there. The pectin fraction has a distinct role in tissue firmness of vegetables.

4. The lipid content of vegetables is generally low (0.1-0.9%). In addition to triacylglycerols, glycolipids and phospholipids are also present.Carotenoids are occasionally found in large amount.

5. The organic acids present in highest amounts in vegetables are malic and citric acids. The content of free titratable acids is 0.2-0.4 g/100 g fresh tissues. The pH is 5.5-6.5.
6. Mineral content of most vegetables is 1%. Potassium is the most abundant constituent followed by Ca, Na and Mg. The major anions are phosphates (PO42-),chlorides (Cl-) and carbonates (CO₃^2-).
7. Vitamins, flavouring compounds and dietary fibers are important secondary constituents. Plant pigments other than carotenoids and anthocyanins viz. chlorophyll and betalains are of great importance in vegetables.

26.4 Storage of Vegetables

The storability of vegetables varies greatly and depends mostly on type, but also on vegetable quality. While some leafy vegetables, such as lettuce and spinach as well as beans, peas, cauliflower, cucumbers, asparagus and tomatoes have limited storage time, root and tuber vegetables, such as carrots, potatoes, kohlrabi, turnips, red table beets,celery, onions and late cabbage cultivars, can be stored for months. Cold storage at high air humidity is the most appropriate. The relative air humidity has to be 80–95%. The weight loss experienced in these storage times is 2–10%.Ascorbic acid and carotene contents generally decrease with storage. Starch and protein degradation also occurs and there can be a rise in the free acid content of vegetables such as cauliflower, lettuce and spinach.

26.5 General Classification of Fruits

The most important fruits are classified as below along with their uses.

1. Pomme fruits

1. Apple - Fresh, dried, purée, jelly, juice, apple cider, brandy
2. Pear, - Fresh, dried, compote, brandy, jelly

2. Stone fruits

1. Apricot - Fresh, dried, compote, jam, juice, seed for persipan, brandy
2. Peach - Fresh, compote, juice, brandy

3. Berry fruits

1. Blackberry - Fresh, jam, jelly, juice, wine, liqueur
2. Strawberry - Fresh, compote, jam, brandy
3. Raspberry - Fresh, compote, jam, brandy
4. Gooseberry - Unripe: compote; ripe: fresh, jam, juice
5. Grapes - Fresh, dried (raisins) juice, wine,brandy

4. Citrus fruits

1. Orange - Fresh, juice, marmelade
2. Grapefruit - Fresh, juice
3. Lemon – Juice

5. Other tropical/ subtropical fruits

1. Pineapple – Fresh, compote, jam, juice
2. Banana –Fresh, dried, cooked, baked
3. Avocado – Fresh
4. Date – Fresh, dried
5. Guava – Compote, juice
6. Mango – Fresh, compote, juice
7. Watermelon – Fresh
8. Papaya - Fresh, compote, juice

6. Shell(nut) fruits

1. Cashew nut - Roasted
2. Peanut – Roasted salted
3. Almond - Baked and confectionary products
4. Pistchio - Fresh, salted, sausage flavoring, decoration of baked products
5. Wallnut - Fresh, baked and confectionary products, unripe fruits in vinegar and sugar-containing preserves

7. Wild fruits

1. Rose hips - Jam, wine
2. Sea buckthorn - Jam, juice
3. Elderberry - Juice, jam

26.6 General Composition

Fruit composition can be strongly influenced by its variety and ripeness.

1. Dry matter: Dry matter content of most fruits (except nuts) varies between 10-20%. Major constituents are sugar, polysaccharides and organic acids, while Nitrogen-compounds and lipids are present in fewer amounts. Minor constituents include pigments and aroma substances of importance to organoleptic quality, and also vitamins and minerals of nutritional importance.

2. Nitrogenous Compounds: Fruits contain 0.1-1.5% Nitrogen-compounds of which 35-75% is protein.The protein fraction varies widely with fruit variety and ripeness. This fraction is primarily enzymes. Free amino acids are also widely distributed and are on an average 50% of the soluble N-content. A number of aliphatic &aromatic amines are found in various fruits.

3. Sugars: Glucose and fructose occur but in varying ratios in fruits. Sucrose is the dominant oligosaccharide. Some fruits like cherry, grape and figs have nosucrose. D-sorbitol is the sugar alcohol that is most abundant in pomme and stone fruits but absent in berries, citrus fruits, pineapple and banana. All fruits have cellulose, hemi-cellulose and pectins. Pectin fractions of fruits are especially affected by ripening. Starch is present mainly in unripe fruit sand generally its content decreases to a negligible level as ripening proceeds,with the exception of bananas.

4. Lipid: Lipid content of fruits is generally low (0.1-0.5%) with the exception of nuts. The fraction consists of TAGs, glyco- and phospholipids, carotenoids, triterpenoids and waxes. The presence of carotenoids is widespread in many fruits and in a number of fruits, viz.citrus fruits and peaches, their presence is the main factor determining the colour. The presence of carotenoids also forms the basis of fruit classification. The triterpenoid fraction contains bitter compounds, viz. Limonoids and cucurbitacins. The fruit peel is often coated with a waxy layer.

5. Organic Acids: L-Malic acid and citric acid are the major organic acids in fruits. Malic acid is predominant in pomme and stone fruits, while citric acid is abundant in berries, citrus and tropical fruits. Tartaric acid occurs only in grapes.

6. Phenolic compounds: Phenolic compounds occur in most fruits and most of them contribute to colour and taste. They can form complexes during processing, resulting in discoloration of fruit pulp.

7. Vitamins: Many fruits are important sources of vitamin C; Pantothenic acid and biotin are present in some fruits, viz. Citrus fruits, figs, black currant; Vitamin B12,Vitamin D and tocopherols are found in trace amounts.

8. Minerals: The most important cation is K+, and the most important inorganic anion is PO^2-.

26.7 Physico-Chemical changes during Ripening of Fruits

It has been recognized for many years that fruits continue to undergo chemical changes after harvest until finally spoilage occurs. Ripening of fruits involves highly complex changes in chemical and physical properties. There are quite a number of intricate changes taking place in a complex bio-chemical system.

The most striking changes related to ripening are:

1. The cell-wall constituents are profoundly modified during ripening leading to softening.
2. The build-up of other cell constituents like starch and sugar are affected along with the disappearances of astringent compounds, which ultimately lead to increase in sweetness.
3. There is a formation of flavour and aroma compounds as well as changes in color due to breakdown of green chlorophyll pigment, whereby the yellowish color pigment of the shows up.

26.8 The Changes Taking Place are

1. Changes in respiration rate

The respiration rate is affected by the development stage of fruits. A rise in respiration occurs with growth. This is followed by a slow decrease in respiration rate until the fruit is fully ripe. In a number of fruits, ripening is associated with a renewed rise in respiration rate soon after picking, until it reaches a climax, which is known as climacteric respiration. This increase in respiration is referred to as climactericrise. This is followed by a steady decrease in respiration rate i.e.senescence. Depending on the fruit, this can occur before/after harvesting.Maximum CO₂ production occurs in climacteric stage.
The climacteric rise is so specific that fruits can be classified into:

a) Climacteric fruits: apples, banana, pears, mango, papaya, tomato
b) Non-climacteric fruits: pineapple, oranges, strawberry, grapes and lemon.

It should be emphasized that non-climacteric fruits generally ripen on plants and contain no starch.

2. Changes in metabolic pathways
Metabolic shift may occur in several fruits during ripening e.g. during ripening of banana, it appears that the Embden-Meyerh off Pathway (Glycolysis) becomes dominant and Pentose Phosphate Pathway is suppressed during ripening.

3. Change in individual constituents of fruits

Carbohydrates: During ripening, significant changes occur in the carbohydrate fraction. In green fruits, usually have abundant starch and less soluble sugars. On ripening,starch content decreases, while sugar content increases, which gives the ripe fruit its sweetness. It has been assumed that sugars are produced at the expense of starch. It also appears that in addition to starch, other sugars are also available for conversion. A decrease in hemi-cellulose content observed during the ripening of banana suggests that they can be a possible source of sugars. Additionally, organic acids may also be a possible source.

Another obvious change in the fruits is the alteration of texture. The softening of the fruit tissue, during ripening is associated with remarkable changes in pectin fraction. Insoluble protopectin is increasingly transformed into soluble forms. Protopectin is tightly associated with the cellulose in the cell wall matrix. So, this conversion can decrease rigidity of the matrix. Additionally a decrease in the degree of methylation of pectin (from ~80% to ~40%) and the decrease in the degree of polymerization of pectins have also been observed in fruits, viz. bananas, citrus fruits, mango,melons, etc. All these together contribute to an increase in softness of the ripe fruit. Moreover, the soluble pectins bind the polyphenols and there by quench their astringent effect and thus contribute to the mild taste of ripe fruits.

Proteins: During ripening of some fruits,although the total nitrogen content is constant, an increase in the protein content is observed, which is mainly due to biosynthesis of enzymes. During ripening, a shift also occurs in the amino acid and the amine fraction. These shifts are, however, not uniform and are affected by type and ripening stage of fruits.

Lipids: Little is known about changes in lipids. Changes have been found in the composition and quantity of lipids, esp. in phospholipid fraction.

Acids: There is a decrease in the acid content of fruits during the ripening with the exception of lemon. There can be the changes in the proportion of various acids. e.g. in ripe apples, malic acid is the major acid, while in unripe ones, quinnic acid is the major one. In many fruits, synthesis of ascorbic acid takes place during ripening.

Pigments: Ripening of fruits is usually accompanied by a decrease in colour. The transformation from green to other colour is due to the degradation of chlorophyll and the consequent appearance of the concealed pigments. In some fruits, the change is more due to synthesis of other pigments, e.g. lycopene content of tomatoes greatly increases during ripening.

Aroma compounds: Formation of typical aroma compounds occurs during ripening.In bananas, for example, noticeable amounts of volatile compounds are formed 24hrs after climacteric stage haspassed. Aroma buildup is affected by external factors, viz. temperature and day-night variations.

Water: Living parts constantly transfer H₂O to the surroundings. This loss of water gradually results in visible shriveling. This is especially because when fruit is plucked, H₂O flow into fruits is discontinued, eventhough H₂O loss continues. This loss is high at high temperature and dry atmosphere. This H₂O given off through physiological forces that remain active even after harvesting is called transpiration. Water is also formed due to respiration. Most of this water formed is removed through evaporation along with readily accessible surface water. However, some fruits have a waxy layer on the skin to check the loss of H₂O.