Structure and Composition
This page describes what vegetables are, and the different components that go together to form them. Where the vegetable comes from on the plant affects its postharvest qualities, especially how it changes after harvest. Vegetables are composed of plant cells. These come together to form the different types of structures that we grow and the components that make them good to eat.
What is a vegetable?
Unlike fruit, which are derived from flowers, vegetables cannot easily be defined. Vegetables are derived from many different parts of plants. Some are actually fruit; others are buds, leaves, stems, roots or storage organs. Where the vegetable comes from on the plant affects how well it stores and many of its sensory qualities.
Vegetables can generally be grouped into three categories:
1. Bulbs, roots and tubers
2. Flowers, buds, stems and leaves
3. Fruit, seeds and pods
The origin of vegetables affects how they are best handled after harvest. For example, vegetables from above the ground are more likely to have waxy skins to protect them from water loss. Roots and tubers do not have such protection, so may require high humidity to limit dehydration. Seeds necessarily contain stored energy in the form of sugars or carbohydrates. Buds and shoots generally have few stored reserves and are developing rapidly, so need careful handling to preserve them after harvest.
Changes at harvest
When a fruit is harvested, it could be seen as continuing its natural ‘purpose’. That is, fruits are produced by plants in order to be eaten, and will eventually detach by themselves if not actively harvested.
Unlike fruit, most vegetables are not adapted to exist apart from the whole plant. Once harvested, vegetables are cut off from their source of water and nutrients. Without sunlight, they are unable to photosynthesise and are usually removed from the protection of soil or foliage. Many fruiting vegetables − such as cucumber and eggplant − are harvested while immature. These ‘baby fruits’ lack the sugar reserves and/or protective skins of fruit such as apples or bananas.
With the exception of vegetables which are mature fruit, bulbs or storage organs (pumpkin, onions, sweetpotato, etc.), most vegetables are on the path to senescence and death from the moment they are harvested. The role of postharvest management is to delay deterioration, maintaining freshness, nutrition, flavour and texture until vegetables reach the end consumer.
Like other living things, vegetables are made up of cells. There are many different types of cells within even a simple vegetable, and each has different functions within the plant.
Plant cells have a fairly rigid cell wall, composed mainly of cellulose, lignins and some proteins. Calcium is critical in the formation of cell walls. If calcium is deficient (due to low soil levels or growth faster than the rate of transport within the plant) the growing tips of fruiting vegetables such as zucchini and eggplants will lack structural integrity and can break down. This is the cause of blossom end rot.
The cell wall is permeable to water and solutes. Inside the cell wall is the plasmalemma, which acts like a liner. The plasmalemma helps maintain pressure inside the cell, keeping it turgid. It is this turgidity that keeps vegetables firm and crisp.
Adjacent cells are glued together by a layer of pectins. A series of channels (plasmodesmata) allows exchange of various substances between cells.
The cell contains a liquid ‘soup’ called cytoplasm. Within the cytoplasm float various structures and organelles, each of which has a specific purpose.
- The nucleus, which contains the cell’s DNA and acts as the control centre.
- One or more vacuoles. These are reservoirs of liquid containing sugars, acids and other materials. Some plant products (such as phenols) are stored inside the vacuoles to keep them apart from enzymes with which they would otherwise react.
- Mitochondria, which are the cell’s powerplant, converting breakdown products from sugars into energy the cell can use.
- Chloroplasts are found in green parts of the plants. They contain chlorophyll and are responsible for photosynthesis.
- Chromoplasts develop from chloroplasts once chlorophyll breaks down. They contain the red and yellow pigments (carotenoids) that give some vegetables their colour.
- Amyloplasts, that contain grains of starch.
If the cell walls are broken—by freezing or physical injury—then the turgidity of the cells is lost and the contents leak out, resulting in soft, water-soaked tissue.
Breaking or crushing cells also allows compounds normally held in separate parts of the cell to mix. For example, phenols held in the vacuole mix with oxidising enzymes in the cytoplasm, turning them brown. This is what causes the brown colour in bruises and on cut surfaces. Lowering the pH (with citric acid, for example) inhibits this reaction on cut surfaces, reducing browning.
Reducing oxygen (O2 ) to a very low level can also reduce browning reactions.
Plant cellular structure
Plant tissues contain many different types of cell, as well as air spaces. These include:
- Veins, consisting of:
- Xylem vessels – which transport water and minerals from the roots to the leaves.
- Phloem vessels – which transport sugars from leaves to the roots.
- Stomata and lenticels – openings that allow gas exchange between the inside and outside of the plant tissues.
- Stomata are actively controlled by ‘guard’ cells; these open to allow gas exchange or close to reduce water loss. They usually close at harvest in response to a small drop in water pressure inside the cells. Leaves can have large numbers of stomata spread across their surface.
- Lenticels are sunken openings that cannot be closed. They are typical of storage organs such as sweetpotato as well as cucumbers and other fruit vegetables.
- Epidermal cells – the plant’s skin. These can develop a waxy cuticle, protecting the underlying tissue against water loss. In some storage organs, such as potatoes, several layers of epidermal cells form a corky layer protecting the underlying cells.
- Parenchyma cells – non-specialised cells that carry out functions such as photosynthesis and form most of the internal tissue (mesophyll).
- Collenchyma cells – stretchable cells rich in pectin and cellulose that help give the plant strength, e.g. celery strings are mainly collenchyma.
The structure of the epidermis and the number of lenticels and stomata govern how quickly a vegetable will lose water. For example, solanaceous fruiting vegetables (eggplant, capsicum, tomato, etc.) do not have lenticels or stomata. This reduces the rate of water loss from the fruit. Cucumbers have numerous lenticels, while many leafy vegetables have large numbers of stomata across their surface, so both lose water rapidly. The thin epidermis of a zucchini presents little barrier to water loss compared to the thick epidermal layer and waxy cuticle of a pumpkin.
The veins (vascular bundles) that transport water and nutrients can also conduct diseases. Browning of the vascular system due to bacteria or fungus can result in stripes or spots. Brown striping of celery stems, or reddish brown streaks inside a cut-open eggplant, show where the veins have been infected.
Vegetables are mostly made of water. Lettuce, cucumbers and leafy vegetables contain about 95% water, so only 5% of their mass is dry matter. Hard vegetables like carrots and pumpkins have around 12–15% dry matter, sweetpotato are close to 20% dry matter, and cassava is one of the highest at 35–45% dry matter.
Harvesting when water content is highest is an obvious way to both increase yield and ensure products have firm texture.
After water, the next most important component in vegetables is carbohydrate. Carbohydrates can be present as starch, sugars and dietary fibre. Starch is mainly found in storage organs, such as potatoes and sweetpotato. Starch is used by the plant as a concentrated, ‘long-term’ fuel source. It needs to be broken down into sugars in order to provide energy to cells (See the section on Temperature for more on this).
The main sugars that are present are glucose, sucrose and fructose, although not every product contains all three. Although more usually associated with fruit, sugars are an important component of flavour in vegetables such as carrots, sweet corn and peas.
Dietary fibre compounds include cellulose, lignin, pectins and other substances. Dietary fibre may be soluble or insoluble, with soluble dietary fibre now believed to have particular benefits for health . Dietary fibre cannot be readily broken down by the digestive system, and most is simply excreted. However, consumption of dietary fibre is now considered an important health marker and is actively promoted by many health professionals.
Carbohydrates can be used as fuel by humans, as well as by plants. The speed at which carbohydrates are absorbed by the body is defined by the glycaemic index (GI). A low GI indicates that the carbohydrates are absorbed slowly, so these foods tend to be satisfying for longer. Most starchy vegetables are low to medium GI (score 50 – 55). This is less than many grain based starch sources e.g. bread or white rice (score ~70 and ~85 respectively).
Protein is also found in vegetables, although in small amounts. Brassica vegetables and legumes (peas and beans) are generally the best protein sources, containing around 3–5% protein.
Other components include vitamins, minerals, phenolic compounds, volatiles and anti-oxidants. Although also present in relatively small quantities, these compounds are likely involved in many of the health benefits demonstrated for diets rich in vegetables. For example, beta-carotene (found in carrots but also sweetpotato, pumpkin and many leafy greens) converts to vitamin A in the body and has benefits in terms of healthy skin, eye health and improved immune system. The sulphur compounds found in brassicas and onions have been associated with reduced risks of certain cancers, as has lycopene, responsible for the red colour of tomatoes.