Introduction
Various parts of different plants can be used as fruits or vegetables for human consumption. They can be roots, tubers, bulbs, stems and shoots, flowers, leaves and fruits, or pods and seeds.
It is crucial to study and learn about the plant cell and compounds responsible for flavour, texture and colour to obtain reliable information about the effects of different treatments on these quality characteristics.
Structure of Plants:
Cells
Cell components are divided into two categories;
protoplasmic and
non-protoplasmic.
The protoplasm is the living active section of the cell and it contains certain components. The nucleus than controls the metabolic activities of the cell.
The cytoplasm is an undifferentiated constituent of the protoplasm surrounding the nucleus and creating a relatively thin layer inside the cell wall.
The plasma membrane (plasmalemma), is a thin membrane on the external surface of the cytoplasm. The plasmic membranes are permeable and can help separating and transporting metabolites and allow the enzymes to distribute regularly.
There are some arranged bodies within the cytoplasm called plastids which are classified in three groups; leucoplasts, chloroplasts and chromoplasts.
Leucoplasts are poor in pigmentation and are related to food storage. Many of them produce and store starch. The chloroplasts are present in green plants containing chlorophyll.
Chromoplasts include xanthophylls or carotenes which are normally in orange or yellow colour. They appear in some vegetables such as carrots and sweet potatoes.
The non-protoplasmic components of the cell contain cavities called vacuoles incorporating cell sap. The cell sap is a watery material containing several substances like sugars, salts, organic acids, polysaccarides, phenolic derivatives, flavones and the red or blue pigments (anthocyanins). The substances in the cell sap are nutrients consumed by the protoplasm or metabolism products.
The liquid in the vacuole of the cell is accountable in terms of texture of fruits and vegetables.
Cell walls
Components of the cell are covered by a wall that is responsible for the texture of the tissue. Cells are attached together by intercellular layer or middle lamella. This layer that has a cementing function consists of pectin in one or more of its forms.
In immature cell, the outer (primary) wall is created initially. Soft tissues that occur in some fruits contain only the primary walls.
The primary wall is made up of cellulose, hemicellulose, and some pectin. In some tissues secondary wall is produced inside the primary wall.
Properties of the cell wall constituents:
Cellulose – occurs in high extent and it is important in terms of the firmness of the cell wall. Cellulose is polysaccharide comprising glucose units.
Hemicelluloses – do not highly resemble cellulose. They are insoluble in water and soluble in alkali. They can be easily hydrolysed by alkali which may result in mushiness of vegetables following the heat treatment in water containing baking soda. The level of hemicellulose in vegetables is decreased by cooking process.
Hemicelluloses are polysaccharides in which xylans, galactans, mannans, glucomannans can be found. The hemicelluloses present in apples, tomatoes, pears and citrus fruits possess xylose. The cell walls of many fruits contain mannans.
Lignin – this is also one of the main constituent of some cell walls. It is largely present in wood. Some vegetables that are mature and virtually firm also contain lignin. Lignin molecules are polymers of phenylpropene derivatives.
Gums – is another component of cell wall carbohydrates. Gums may arise as a result of microorganisms present or by the occurrence of disease or mechanical damage to the cell. Gums may consist of a mixture of several sugars or sugar derivatives. They can swell many times their initial volume in water.
Pectic substances – Apples and the albedo of citrus fruits that contain large quantity of pectic material. The pectic substances are pectic acid, pectinic acid, pectin and protopectin.
Changes during cooking and processing:
Heat treatment of fruits and vegetables is a crucial method of preservation. When the pectic substances in cell walls decompose, softening of wall cells and subsequently separation may happen.
Divalent ions can increase firmness of canned fruit, canned tomatoes and cooked carrots. The divalent ions form cross-link between carboxyl groups of pectinic acid molecules, will enhance the firmness of the middle lamella and primary cell wall.
When monovalent ions are present the cross-link formation is stopped and consequently rigidity will decreased.
As plant tissue is heated, intracellular ions such as calcium and magnesium may come in contact and react with cell wall components e.g. free carboxyl groups to create bridges that toughen the tissue so that it may withstand degradation during heating process.
Another significant aspect in loss of firmness during the heating process is the pH f the heating medium. E.g. as the pH increases from 3 to 8, the rigidity of carrots decreases since the cell separation has increased.
The heating effects varies with the type of tissue, such as phloem and xylem.
Colour
Colour may be the most important factor influencing the attractiveness of the fruits and vegetables. There have been many studies to determine and obtain the optimum techniques to minimise the undesirable effects on quality of fruits and vegetables including colour.
Chlorophyll –
This is the green pigment of plants involved in the chloroplasts. Chlorophyll occurs in the leaves as their large surface area is suitable for absorption of the sun’s rays and the exchange of gases required for photosythesis.
Chlorophyll is soluble in fat and solvents such as ethyl ether, ethanol, acetone, chloroform, carbon disulfide and benzene.
All higher plants and many lower plants possess two types of chlorophyll, (a) and (b), in the ratio of approximately 3 parts of chlorophyll (a) to 1 part chlorophyll (b). chlorophyll (a) is blue-green in colour, chlorophyll (b) is yellow-green.
Changes during heating process:
The changes that arise in green pigments of vegetables may be in connection with the properties of chlorophyll.
Chlorophyll of raw vegetables can be protected from acid in the cell sap by its position in the chloroplasts. The initial colour change can be identified when the green vegetable is dropped into boiling water resulted in brightening of the green colour that may be as a result of expulsion of air and collapse of the intercellular spaces.
During cooking, the choloroplasts shrink and get clumped in the centre of coagulated protoplasm. In 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 may be formed.
The level of colour change is dependent on the acidity of the cooking medium, the pH of the vegetable, the chlorophyll content, and the time and temperature of cooking.
The pH of all common vegetables is less than 7 because of the acids that are present in the cell sap.
Both volatile and nonvatile acids are discharged during the cooking of vegetables. Released acids from the cell vacuole during heating is not able to influence the colour of vegetables if they are neutralised by the cooking water.
The pH of water can be reduced by boiling if carbon dioxide is released (dissolved in water to produce carbonic acid), or if bicarbonates are converted to carbonates with the release of carbon dioxide, or if hydrogen sulphide is lost.
The quantity of acid that can be neutralised by an alkaline cooking water depends on the water’s alkalinity and volume. High extent of water can give a desirable green colour because they neutralise or at least dilute plant acids.
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Baking soda can be added to increase the alkalinity of cooking water, however this is not recommended because of possibility of excessive addition of soda.
Sodium bicarbonate not neutralised by the acids in the cooking water can have an undesirable effect on the flavour and texture of the vegetables.
In blanching process the acidity can decrease and the pH of the vegetables is increased.
The sensitivity of green vegetables to colour change during cooking is influenced by their chlorophyll content and their pH.
Frozen vegetables contain more chlorophyll as they initially preserved a more percentage when cooked.
In addition, vegetables with high pH, spinach and peas can retain more pigment than green beans and brussel sprouts which have lower pH.
If green vegetables are heated as quick as possible, the colour would not / slightly change.
Canning process has noticeable effect of colour changing in green vegetables resulted from conversion of chlorophyll to pheophtin.
Carotenoids; the yellow and orange colour and some of the red colour of fruits and vegetables are caused by carotenoids that are situated in the chromoplasts of the cells.
The carotenoids are insoluble in water but soluble in fats and organic solvents.
They are classified in two groups.
Carotenes contain only hydrogen and carbon and are soluble in petroleum ether.
Xanthophylls; oxygen-containing carotenoids, are soluble in alcohol.
Changes during preparation, processing and storage:
Usual cooking methods have slight effect on the colour or nutritional value of carotenoids. The pigments are affected at a very low extent by acid, alkali, the volume of water or the cooking time. Extreme retention of the carotene can be achieved in peas heated in water in a conventional oven and with or without water in a microwave oven.
However there can be a shift in the visualised colour. For instance, the orange of carrots may turn yellow and the red of tomatoes may turn orange-red.
The high portion of unsaturation of the carotenoids can make them liable to oxidation that gives loss of colour, after the food has been dried. Change / decrease of colour may be in consequence of the reaction of peroxides and free radicals, oxidation products of lipids with carotenoids.
Carotene can be immune from oxidation during dehydration by the blanching of vegetables or the sulphuring of fruits.
Flavonoids; They involve two main groups of related compounds; the anthocyanins and the anthoxanthins.
Anthocyanins – Many of the red, purple, and blue colour of fruits and vegetables are due to anthocyanin pigments. In some fruits like different cherries, apples and plums the anthocyanins are present in the cells of the skin but not in the flesh.
Other fruits and vegetables like raspberries, blueberries, grapes, strawberries, peach skins, red potato skins, radishes, red cabbage and aubergines also contain anthocyanins.
Carotenoids or chlorophyll may be present in tissue containing anthocyanins.
Changes in preparation and storage:
The colour is very dependent on the pH conditions. The colour is the deepest when the pH is very low Francis. The molecule is uncharged at neutral pH and is violet in colour. In alkali condition, the anionic form is present which gives a blue colour.
Following the addition of alkali, the juice of some fruits and vegetables will turn greenish. Anthocyanins can be degraded by oxidation, hydrolysis, or polymerisation. The main aspects affecting degradation are temperature, pH, other cell components, enzymes, and the existence of metals.
Pigment damages can be intensified with higher pH, e.g. in strawberry products such as preserves and grape juice.
Overall in terms of colour change, the anghocyanins are not highly affected during the cooking due to their acidity. Red cabbage is the anthocyanin containing vegetable usually cooked. When it 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.
Anthoxanthins – consist of a series of compounds including flavones, flavonols, and flavonones. The pigments are relatively colourless or pale yellow. They are water-soluble and present in the vacuoles of the plant cells.
They may appear alone in light-colour vegetables such as potatoes and yellow-skinned onions, or with other pigments such as anthocyanins.
Blackening of potatoes after coking is related to the formation of a dark-coloured complex between iron and chlorogenic acid in potatoes with low organic acid content. Cauliflower can turn discoloured since flavonol glycosides complex with ferrous or stannous ions.
Enzymatic Browning; The term tannins is used for a group of polyphenolic compounds that serve as substrates for enzymatic browning and also contribute to astringency of foods. These compounds act as the substrates for the enzyme ortho-diphenol (oxygen oxideo-reductase). The enzyme is usually called polyphenol oxidase or polyphenolase.
Three components including substrate, enzyme and oxygen are essential for the first reaction of browning.
There are certain methods to prevent enzymatic browning. Polyphenol-oxidase is denatured and inactivated by heat treatment e.g. blanching.
Flavour
The volatile flavouring components are crucial including organic acids, aldehydes, alcohols and esters.
Phenolic compounds – e.g. catechin and leucoanthocyanins are responsible for the astringent taste of some foods. Time and temperature of heating during preparation of juice extracts from e.g. red currant and red raspberry fruits can highly affect the tannin content of the juice. As tannin portion increases, the degree of bitterness and astringency will also increase.
Sugars – are the main contributory factor to the desirable flavour of freshly harvested vegetables. The concentration of sugar can change resulting from the metabolic process. During storage the sugar content can significantly change depending on the type of fruit or vegetable.
Acids – All fruits and vegetables are acidic. Acidity is very dependent on the maturity of the plant (constantly decreases as the fruit ripens).
Sulfur compounds – are believed to be responsible for the flavour of two groups of vegetables; the Allium genus of the onion family and Brassica genus of the Cruciferae family.
The odour characteristic of garlic indicates the presence of allicin which is formed by the action of allinase (enzyme of garlic).
Typical flavour of raw cabbage is believed to be in association with allyl isothiocyanate produced by the hydrolysis of sinigrin, a glucoside, and related coumpounds.
Conclusion
Basically the extent of colour and texture change depends on the acidity of the cooking medium, the pH of the vegetable, the chlorophyll content, and the time and temperature of cooking.
Pectins present in vegetables form water-retaining gels that help give vegetables their structure. Pectins become soluble and are extracted into the cooking water making the cooked vegetable become mushy.
Calcium ions ca2+ found in hard water can form cross link between pectin molecules making them less soluble and keeping the vegetable tough.
The calcium ions content of water can change the colour of the cooked vegetables as well as their texture but indirectly by its effect on pectin molecules.
Since most vegetables require a certain level of softening, cooking in hard water means that longer time is required to achieve the optimum softening. During this longer cooking time more chlorophyll is converted to phenophytin and the colour of green vegetables becomes browner.
To reduce this effect, in order to achieve optimum texture and pleasant colour of vegetables in addition to shortening the period of heating and cooking process, water should also contain low level of Ca2+.
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