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.
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 following heat treatment, cooking in hard water means that longer 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 (cooked for as short time as possible), water should contain low level of Ca2+
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