英文科學(xué)讀本 第六冊(cè)·Lesson 38 Plants Useful for Food (Ⅵ)

Lesson 38 Plants Useful for Food (Ⅵ)

Sugar

Sugar is one of the most important products of the Vegetable kingdom. The juices of many plants contain sugar, each variety having its own special characteristics. The various sugars are arranged in three groups—grape-sugar, cane-sugar and manna, or leaf-sugar.

The first of these—grape-sugar—takes its name from the white, crystalline, sugary substance found in the inside of raisins and currants, which, as you already know, are dried grapes. This white substance is really sugar; we call it grape-sugar. It is the source of the sweetness in the raisins and the currants. But fruits in general, as well as the grape in particular, owe their sweetness to this same kind of sugar—grape-sugar. The apple, pear, plum, gooseberry, cherry, etc., have at first a sharp, sour taste, and gradually pass from sour to sweet as they ripen. This is owing to the gradual formation of grape-sugar in them. Even when fully ripe most of them are a little sour, and it is the mixture of sour and sweet which gives fruit its pleasant flavor. This grape-sugar of the fruits yields wine when fermented. We have frequently had occasion to notice the conversion of starch into sugar via the saliva, in the work of digestion. The kind of sugar thus formed is this same grape-sugar which is found in fruits.

By mixing some starch in water with a little sulphuric acid, and boiling the mixture in a shallow dish over the spirit-lamp, we may easily effect this conversion for ourselves. When the liquid has cooled it will be found to have acquired a sweet taste. The acid will have converted the starch into sugar—grape-sugar. The saliva, you may remember, is an acid fluid too, and has a similar action on the starchy matters of the food. We could, by adding a little lime to the boiled solution, separate the acid, and then if the liquor were boiled down, we should get actual sugar.

Potato, wheat, barley, rice, maize, or sago starch can be readily made to yield sugar in this way. The sugar obtained from starch is known as maltose, and is made into beer by the process of brewing.

Manna-sugar is a peculiar and distinct product, differing from both grape-and cane-sugar in many respects. It is obtained from the sap of a variety of plants. In some it exudes from the surface of the leaves, and is hence known as leaf-sugar; in others it is obtained by making incisions in the stem. It is produced in very small quantities, and only in a few parts of the world. It is chiefly used for medicinal purposes.

A species of ash, which grows in Sicily and Calabria, yields a valuable kind of manna-sugar. It is obtained by making cross-cut incisions in the stem. The sap, exposed to the air, hardens as it flows, and concretes into a solid gum-like mass round the slits in the bark. When fresh gathered it is very nutritious, and enters largely into the food of the people. It soon acquires, however, a slight purgative property, and this, while it unfits it for food, renders it valuable for medicinal purpose. As a medicine England imports annually no less than 11,000 lbs. of this manna-sugar, most of it coming from Sicily.

We have already dealt at some considerable length with the varieties of cane-sugar, their preparation and properties. It will be quite unnecessary to do more now than call attention once more to the fact that these sugars are not all of them the product of the sugar-cane. The class known as cane-sugars include, in addition to that, beetroot-sugar, maple-sugar, maize-sugar, and palm-sugar.

Sugar has become almost a necessity of life. It is estimated that people consume, on the average, no less than 70 lbs. of sugar per head of the population in the United Kingdom each year. The U.K. is by far the largest consumers of sugar in the world. Upwards of one million tons of sugar are annually imported into England. The total yearly production from the sugar-cane all over the globe is said to be upwards of 5000 millions of pounds, and by far the greater part of this comes from British dominions, chiefly the East and West Indies and the Mauritius.

Think of the millions of men in various parts of the world who are employed in cultivating the plants, extracting and preparing the sugar, and conveying it to this and other countries, to say nothing of the traders and others through whose hands it must pass before it reaches the actual consumer. Thousands of ships are employed in carrying this one article of commerce.

The preparation and sale of beetroot-sugar, too, is largely increasing every year. England imports, as a matter of fact, more beetroot than actual cane-sugar, but it is not all consumed at home. Much of the sugar imported is sent out of the country again as a manufactured article.

The topmost shoots of the date-palm yield a juice which, when boiled down, gives a brownish, raw sugar, commonly known as East India date-sugar, palm-sugar or jaggery. The juice is obtained by piercing the tender shoots, so as to cause the juice to flow. It is produced by the populations of India and other tropical regions where the palms grow, and mostly for their own exclusive consumption.