To All,
Intellectuals rave about the glory that was Greece and the grandeur that was Rome. As we have seen, however, ancient Greece and Rome were impoverished in that they did not have chocolate (16 May 2015), coffee (18 Jul 2015), or tea (25 Jul 2015). Nor in that benighted age did they have sugar, the topic of this e-mail.
By "sugar" I mean table sugar. I will not get into the classification of the compounds that organic chemists include under the rubric of "sugar" but will just say that table sugar is sucrose. Sucrose is a disaccharide, which means that it is composed on two monosaccharides, namely fructose and glucose. Below are two depictions of the structure of sucrose; in both cases the glucose portion of the molecule is on the left and the fructose portion on the right.
An unusual feature of sugar that distinguishes it from most agricultural products is that it can be obtained from two unrelated plants, sugar cane and the sugar beet. Actually, sugar can be obtained from a variety of plants, including many fruits and vegetables, but none of these other plants provide it in economic quantities. Sucrose is common in plants since, as Nelson and Cox [p. 244, also see pp. 783, 792, 708] state, "Sucrose is a major intermediate product of photosynthesis; in many plants it is the principal form in which sugar [i.e., energy] is transported from the leaves to other parts of the plant body." What distinguishes sugar cane and sugar beets from other plants is explained by Evert and Eichhorn [p. 19]: "The sucrose we consume as table sugar is commercially harvested from sugar beets (enlarged roots) and sugarcane (stems), where it accumulates as it is transported from the photosynthetic parts of the plant." In short, we are lucky that in these two plants sucrose accumulates instead of being immediately passed on and used. (To provide context. let's compare animals and plants. The main form in which energy is shipped around the body is glucose (a monosaccharide) in animals and sucrose (a disaccharide) in plants. Long term storage of energy in animals is in glycogen (a polysaccharide) and fat, and in plants is in starch (a polysaccharide) and fats and oils [Evert and Eichhorn, pp. 20-22]. There are many exceptions, but my aim here is just to give you the big picture.)
Currently, roughly two-thirds of the world's sugar comes from sugar cane and one-third from sugar beets. Typically, the percentage of sucrose is 10 percent for sugar cane and 17 percent for sugar beets [USDA, p. 12]. Sugar cane must be grown in tropical and sub-tropical regions, while sugar beets do not tolerate extreme heat and are grown in regions with temperate, Mediterranean, or arid climates. The first map below shows where sugar cane and sugar beets are grown worldwide; the second provides more detail for the U.S., where red indicates sugar cane and green sugar beets (the yellow is corn). In the U.S. about 55 percent of the sugar production is from sugar cane and 45 percent from sugar beets [USDA, p. 10].
Sugar Cane: The Plant
Sugar can be produced from several species of giant grasses in the sugar cane genus Saccharum. There are four domesticated species of Saccharum that are cultivated, but by far the most important is S. officinarum [Mintz, pl 231]. By "sugar cane" this e-mail means S. officinarum unless otherwise stated. (This site says there are 35-40 species of Saccharum. This site has a list of species and varieties. See this Purdue site for more on the other three cultivated species.)
The sugar cane plant, which is a perennial in the grass family, has great growth potential, e.g., it can grow an inch a day for six weeks [Mintz, p. 21], but to exploit this potential it must be grown in a frost-free environment with sufficient rainfall to support its growth. Compared to sugar beets, sugar cane tolerates hot climates better but needs four times as much water.
Sugar cane consists of a unbranched stem that can typically reaches 10 to 15 feet in height. Roughly speaking, the stems range in diameter from one to two inches. Each internode, i.e., the length of stem between two leaves, is impregnated with a sugary sap. The stem has a tough rind that surrounds a soft pith that contains the sugary juice [Macinnes, p. 144]. Here are pictures of the plant.
- A sugar cane farm in Africa.
- A field of sugar cane, from the side.
- A field of sugar cane, from the end, which shows the spaces between rows, which allows space for cultivation.
- Sugar cane plants with people for scale.
- Realistic drawing of an individual plant.
- Schematic drawing that shows the parts of the plant.
The leaves are usually between one and two feet long and have saw-toothed edges that can cut the skin.
The plant matures after 22-24 months and produces as an inflorescence a fluffy, pinkish plume at the top of the stem (see pictures below). Sugar cane is usually harvested before the plants flower since flowering decreases the sugar content. Sometimes it is possible to grow a cultivar that will not flower at all because its has photoperiod requirements for flowering that are not met where it is grown. Another way to exploit the photoperiod requirement is to interrupt the night with flashes of light [Langer and Hill, p. 111]. (The "photoperiod requirement" is a plant's genetically programmed instructions that say that it will not flower until it has experienced nights of a specified length.)
Saccharum officinarum's main method of propagation in nature is that rhizomes under the soil send up new shoots. (See the discussion of rhizomes in the e-mail of 13 Nov 2015.) Though it can reproduce from seeds, it only produces a small number and they lose viability after a short period [Langer and Hill, pp. 110, 111]. Therefore, in a commercial setting, propagation is virtually always accomplished by using cuttings, i.e., by planting a segment of stem [Langer and Hill, p. 110, Mintz, p. 231-32]; this produces a clone, which means that a desired cultivar in reproduced in a controlled way without genetic alteration. When a crop is harvested, the stumps can several times be allowed to sprout and produce another crop [Langer and Hill, p. 111]. "Ratoon" is the word used to describe re-growth from stumps of harvested sugar cane [Langer and Hill, p. 111]. The advantages of ratooning are that the cost of preparing the field is reduced and the next crop matures earlier than a fresh planting. The disadvantages are that the resulting canes are thinner with lower sugar content and are less disease resistant. Two or three crops might be raised before replanting is necessary.
As we have seen repeatedly (27 Mar and 16 May 2015), clonal crops like sugar cane are genetically identical and especially susceptible to disease. While seed cannot be used to propagate commercially, it is fortunate that a few seedlings can usually be produced. This gives scope for crossing different varieties to increase genetic diversity. In fact, varieties from various Pacific islands have been crossed in this way. (This paragraph is from Macinnes, pp. 143-45].) Activities like this have allowed numerous cultivars to be "... bred for sugar production, disease resistance, maturity, varying soils, dry conditions and flooding."
[Continued in Part 2]
[Continued in Part 2]
