Vegetables and fruits

Post 4 of a series of digests from the magisterial ‘On food and cooking’ by Harold McGee. I may augment these posts as I come across nuggets of information from other sources.

Plants produce lignin, the major component of wood, in secondary growth away from the main shoot. We rarely encounter lignin in plants we eat, but where we do, say the base of mature asparagus, there is no alternative but to remove it.

The other important factor in plant texture is the inner water pressure, or turgor. of the individual cells. When the cells are fully hydrated we experience a crisp texture. If the tissue has lost water, the cell membranes draw away from the cell walls and the vegetable become limp. The vegetable compartment in your refrigerator is designed to maintain a higher humidity to preserve turgor.

Cut vegetables and fruit turn brown due to an enzyme that oxidises phenolic compounds in the tissue and causes them to condense into brown polymers. Browning can be arrested by cold or very hot temperature, immersing in water or better brine, and especially treating with lemon juice or ascorbic acid (vitamin C).

Flavours:

Bitter – astringency is caused by tannins and is the sensation caused by the ‘tanning’ of proteins in the saliva and mucous membranes of the mouth.
Sweet – sugars. In most fruit sugars supplied by photosynthesizing leaves are stored as starch, which is then converted back into sugar as ripening commences.
Sour – various acids.
Odor. The essential oil of a plant is the set of all the compounds that can be distilled from that plant and that contribute to its characteristic aroma. Only a few plant families have strongly accented oils – our spice rack is basically a collection of our favorite essential oils. The volatile oils in fruits are usually concentrated in the skin in specialised oil glans.

Vegetables generally have mild flavours when raw but develop stronger ones when cooked, cabbage being a famous example. The onion family is one example that goes the other way.

The darker the colour of a leafy vegetable, the more nutrients it contains. The inner leaves of a lettuce have 1/30th as much vitamin C as the outer leaves. The situation is similar with the skin of fruit.

Poisonous plants to avoid. Potatoes that started to sprout and turn green contain alkaloids that can only be removing by peeling the outer surface away.

Fruit are stimulated to ripen by ethylene gas, which they produce themselves. Nowadays many fruit are picked hard, shipped and gassed with ethylene to prepare them for the counter. This helps explain the anaemic flavour of much supermarket produce – it has been removed prematurely from its source of nourishment.

In many ways vegetables deteriorate more quickly than fruit after harvest and fresh vegetables are sweeter, more flavorful and better textured. Most fruits and vegetable take well to refrigeration as their biochemical activity is slowed. Fruits native to the tropics are an exception; bananas, avocados, citrus, pineapples, melons, eggplants, squash, tomatoes, cucumbers, green peppers, beans are best kept at 10’C.  Fruits and vegetables are still alive and respiring. If denied oxygen the plant cells switch to anaerobic respiration, which results in the accumulation of alcohol in the tissue and spoilage. Therefore do not store them in closed plastic bags.

Plant tissue suffers less in freezing than meat. However freezing does not eliminate enzymic activity and it is preferable to blanch vegetables before freezing, to inactivate the enzymes. Frozen vegetables are best cooked straight from the freezer and not thawed as this will given microbes more of a chance to degrade the food.

Preserves are the technique of boosting the sugar content of fruits and some vegetables in order to kill micro-organisms responsible for spoilage. Sugar also helps fruit retain its shape and texture by interacting with the cell wall hemicelluloses and pectin. Preserves get their smooth consistency from pectin, present either in the fruit itself or added in powder form. Coaxing a pectin to gel requires a delicate balance of acid, sugar and pectin and this explains why we refer to the ‘art’ of making jam.

There are two basic types of pickles: adding acid, usually in the form of vinegar, and fermentation where acid-producing bacteria are encouraged to grow. In the first the vegetable is cooked to the desired consistency or put in a brine for a short time to draw out moisture that would dilute the vinegar. Then it is immersed in the vinegar, often with spices. Bacteria are inactive in this environment, although the surface must be covered. With fermentation the vegetable is put in a brine solution strong enough to prevent the growth of undesirable bacteria but weak enough to allow several species that produce lactic acid.

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