The Fruit Explorer Ponders Sugar, Part 2 of 6

[Continued from Part 1]

Sugar Cane: Harvesting and Processing

A key fact concerning sugar cane is that there is a narrow window during which the ripe cane must be harvested, and once harvested it must be rapidly processed. As Mintz [p. 21], states, "...cane must be cut when ready so as not to lose the juice or the proportion or sucrose in this juice; and once it is cut, the juice must be rapidly extracted to avoid rot, desiccation, inversion, or fermentation." The rule of thumb is that the juice needs to be extracted within 24hours after cutting. One implication is that the laborious harvesting that needed to be precisely timed, the complexity of the processing, and the need to process quickly after harvesting combined to mean that sugar production from cane was until the 20th century a highly labor-intensive crop [Mintz, p. 26]. 

A second implication is that the initial processing of the cane must be done close to the fields where it is grown. The practice that has grown up is that the cane undergoes field processing close to where the cane is grown since this is unavoidable; the remainder of the processing, which is called refining, has come to be performed primarily in the consuming countries. The input to field processing is sugar cane, and the output is called raw sugar; the input to refining is raw sugar and the output is what in this e-mail is called sugar, or table sugar.

The common sense essence of processing of sugar cane is to isolate the sucrose in sugar cane juice by evaporating the water in the juice to leave a residue of sucrose, which crystallizes. Wikipedia describes the harvesting, field processing, and refining:

The crop is harvested mechanically or by hand, chopped into lengths and conveyed rapidly to the processing plant. Here, it is either milled and the juice extracted with water or extracted by diffusion. The juice is then clarified with lime and heated to kill enzymes. The resulting thin syrup is concentrated in a series of evaporators, after which further water is removed by evaporation in vacuum containers. The resulting supersaturated solution is seeded with sugar crystals and the sugar crystallizes out and is separated from the fluid and dried. Molasses is a by-product of the process and the fiber from the stems, known as bagasse, is burned to provide energy for the sugar extraction process. The crystals of raw sugar have a sticky brown coating and either can be used as they are or can be bleached by sulfur dioxide or can be treated in a carbonatation process to produce a whiter product. (Links and footnote omitted)

More detail will now be provided for those who desire it.

Harvesting and Field Processing

The main steps in harvesting and field processing are as follows. (Much detail and alternative processes are omitted.) 

• Preparation for harvesting: Ideally the cane is burned before harvesting to remove the leaves and drive out the snakes that like this habitat; see pictures below. Also, burning can prevent the cane cutters from contracting leptospirosis, which is a disease spread by rats [Macinnes, p. 175]. Some localities consider burning to be a nuisance and don't allow it; for example, this article in the Nation of Change Bullhorn describes attempts to outlaw the burning of sugar cane fields in South Florida. Harvesting can be by hand or machine. The last picture shows an example where the leaves have been chopped off rather than burned.

               

• Hand harvesting: The stalks are cut at ground level, any remaining leaves are lopped off, and the stalks are tied into bundles for transport. This is a hard, dirty job. Sugar cane is harvested by cutting the cane but leaving the roots, which can be used to regrow to produce the next crop if desired.

               

• Machine harvesting: Machine cutting is only possible on flat land. Many sugar estates do not use machine processing because of a shortage of capital cost or the need to provide jobs. Machine harvesting differs from hand harvesting in that the stalks are cut into short lengths. The first two pictures below show sugar can being harvested by machine, and the last shows a John Deere sugar cane harvester.

      

• Transport to the mill. The freshly harvested cane must be transported expeditiously to the mill. The pictures below shows cane being trucked to the mill in India and Egypt.

   

• Extracting: The juice can be extracted from the cane in various ways, but the essence is that that cane is crushed so that the juice flows out. The first picture below shows the huge rollers used to crush the cane. The second picture illustrates the counter-current process typically used to concentrate the juice. In this picture the cane moves from left to right and the water from right to left. This means that the water becomes increasingly concentrated as it moves through the system since it first encounters the cane from which much of the juice has been removed and last encounters the cane from which no juice has yet been removed. (Counter-current processes are common in biology. For example, a counter-current process is used to moisten the air that you inhale (this keeps it from drying your lungs) and to dry the air that you exhale (this keeps you from losing water.)) The juice produced by this process contains dirt from the field, small fibers from the cane, green parts from the plant, and other impurities. Typically extraction begins within 24 hours of cutting to avoid loss of sucrose content due to evaporation and enzymatic degradation of the sucrose [McCallum. p. 6]. The remaining cane after the juice is crushed out of it is called bagasse (accent on the second syllable), which is typically burned to power the various steps in the process. In this way, sugar cane provides its own renewable power [USDA, pp. 38-39]. Bagasse can also be used to make paper or as animal feed.

   

• Cleaning: Slaked lime is dropped into the juice; as it falls, it gathers up and removes the impurities. 
• Evaporating: The juice is boiled to remove the water. This step represents the main power use by the process. A thrifty factory will tune the evaporation process so that the energy requirement matches what can be obtained by burning the bagasse. This means, on the one hand, that external energy need not be purchased and, on the other hand, that there is no excess bagasse to dispose of.
• Boiling: The reduced juice is boiled again until the resulting solution is supersaturated.
• Crystallizing: Sugar is added to the supersaturated solution to initiate crystallization.
• Separating: The sugar crystals are separated from the remaining water with a centrifuge, which is much like the spin cycle on a washing machine. The sugar produced is called raw sugar and is a sticky brown clump (see picture below). It is often sent to the consuming country for refining, as explained below. The remaining juice is called cane molasses or blackstrap molasses, depending on exactly at what step of the process it is extracted, and is used to make cattle food or sent to a distillery where rum is made. 

   

Refining

The discussion above has shown how to use sugar cane or sugar beets to produce raw sugar. The next step is to refine the raw sugar to make table sugar. This involves cleaning and purifying the raw sugar. The highly simplified steps are as follows.

• Affination: The raw sugar is brown since the sugar crystals, which are white, are covered with a sticky brown substance. The raw sugar is mixed with a sugar syrup that will remove the brown layer but not dissolve the sugar crystals. The resulting mixture, which is called "magma" since is it is extremely viscous, is centrifuged to separate the brown layer from the sugar crystals, which, however, still retain some of the brown substance and other impurities. The brown liquid that is removed is further processed, and one of the products is refiners' molasses.
• Carbonatation: This steps removes some of the non-sugar solids from the magma. In one popular process lime is used to achieve this. The solids removed are called mud, and this lime mud is spread on fields as fertilizer.
• Decolorization; One of various chemical processes are used to remove the remaining impurities that detract from the natural whiteness of sugar. The result is a clear, lightly colored liquor that is for the most part a dilute solution of water and sucrose.
• Boiling: The liquor from the previous step is boiled until the solution is supersaturated, and sugar crystals are then introduced to initiated crystallization. The crystallized product is dried and is then ready to be packaged and sold as table sugar.

The entire process is summarized in the diagram below.

Sugar Beet: The Plant

The plant Beta vulgaris, which is in the amaranth family, has many cultivars, including beetroot or garden beet (a root vegetable), chard (a leafy vegetable), mangelwurzel (a fodder crop), and, what is relevant for our purposes, the sugar beet [Langer and Hill, pp. 148-51, Cooke and Scott, pp. 5-7]. (Formerly, beets were placed in the goosefoot family, but in 2003 this family was incorporated into the amaranth family.)  Beets were first domesticated in ancient times probably in the Eastern Mediterranean [Langer and Hill, p. 142] and originally were grown for their leafy, spinach-like greens [Cooke and Scott, p. 1]. Beets are mentioned in line 898 of the play "The Acharnians" by Aristophanes. By around 1500 the root had become favored over the leaves as a vegetable, in part because any plant that tasted even slightly sweet was highly valued [Cooke and Scott, p. 2].

The sugar beet is a biennial. In the first year it develops a basal rosette of leaves and the root accumulates sucrose and swells. The seed might be planted in March; leaf development is emphasized until perhaps August and then root development until perhaps November, depending on location and conditions. The root is harvested at the end of the first year of growth; the growing period is 170-200 days. If the plant is allowed to grow a second year, after being exposed to cold ("vernalization") it uses the energy stored in the root to send up a shoot and to produce flowers and seeds. A plant that flowers in its first year is called a bolter; this can happen if it experiences cold soon after being planted or if the seed is exposed to cold while still on the mother plant. (This paragraph is based on Cooke and Scott [pp. 37-38, 60] and Draycott [pp. 30, 31].) Here are some pictures.

• A field of sugar beets showing their first year basal rosettes of leaves. These beets are ready to be harvested.
• Sugar beets ready to harvest, with the dirt scraped away to show the entire plant.
• Harvested, cleaned roots.
• Drawing of a plant.
• A second year plant that has sent up a stem and is flowering.
• Drawing of a second year plant in flower.
• A bolter.
• Sugar beet jack-o'-lanterns.

                     

Sugar beets are propagated by seed, which in the U.S. is most efficiently produced in Oregon's Willamette Valley (home of Dave and Gwenie!) since winter temperatures are low but not freezing.

The rotation in which sugar beets are planted is described by Britannica: "The sugar-beet crop is produced every four to six years; several other kinds of crops are also raised during this period. Sugar beet is usually planted after corn (maize) or wheat in order to lessen the damage due to Rhizoctonia root rot or nematodes" [Links omitted].

Sugar beets are seen as being ecologically friendly. As Cooke and Scott [p. xviii] state, "... it is a most effective scavenger of nitrogen fertiliser, leaving little in the soil at harvest to then escape into the groundwater.... By-products of the crop (tops, insoluble root material and molasses) are extensively used as animal feeds...."

One of the main goals in sugar beet breeding is to produce a beet that will reduce the amount of soil that adheres to the harvested root. Not only does this removed soil reduce the amount of fertile soil available for the next crop but it also increases transportation costs, cleaning costs, and disposal costs. The ideal sugar beet root would be without grooves or branches. The standard table beet provides the desired geometry; experiments that cross-breed table beets and sugar beets, however, have not yet been successful since the progeny have too low a sucrose content [Cooke and Scott, pp. 49-50].

Sugar beets thrive in cold climates since cool fall nights cause a rapid increase in the amount of sucrose in the root. Sugar beets are grown on all the populated continents except Australia, but most are grown between 30 and 60 degrees north, e.g., from Cairo to Helsinki [Cooke and Scott, p. xiv]. See the range map for Europe below.

In the U.S. sugar beet factories that process the sugar beets into sugar are usually cooperatives that are owned by the farmers in the area. There is a close relationship between the farmers and the cooperative, which I witnessed when I lived in North Dakota in the fall of 1978. During the winter some of the farmers who needed more income had jobs at the local sugar beet cooperative.

On 6 Feb 2011 the U.S. Department of Agriculture announced its deregulation of Monsanto's Genetically Modified Organism (GMO) sugar beets. The unexpected action received relatively little attention since the public was distracted by riots in Egypt and the upcoming Super Bowl. (My understanding of GMOs is negligible; this is a sign that the Fruit Explorer should write an e-mail on genetically modified plants.)

Sugar Beets: Harvesting and Processing

Since the bulk of sugar beets are grown in Europe, Japan, and the U.S., the processing of sugar beets typically occurs as one continuous process rather than being split it two as for sugar cane. The processing of sugar beets is summarized by Langer and Hill [p. 149-50]:

The roots are delivered to the factory where the beets are washed, sliced, and the sugar extracted by diffusion in a battery of large tanks filled with hot water. The raw juice is purified in a number of stages, decolourised and finally crystallised to produce sugar.

Another short but somewhat fuller summary is provided by Wikipedia. 

For those wanting more detail, here are the steps followed to produce sugar from sugar beets.

• Harvesting: The beets are dug from the ground and trucked to the factory. Unlike sugar cane, sugar beets can be stored for several weeks (or longer under favorable conditions) without losing sucrose content, so there is no frenetic need for immediate processing as there is with sugar cane.
• Cleaning: Since the beets come out of the ground, they are typically covered with dirt and must be carefully washed. The roots, which contain the sucrose, must also be separated from rocks, beet leaves, and other detritus; this step will be immediately understood by those of you who have heard me describe my career as a clodpuller.
• Extracting: The beets are sliced into thin chips; this increases the surface area, which makes it easier to get the sucrose out of the beet. The chips are placed in hot water for about an hour and the sucrose diffuses from the beet into the water, much as tea diffuses from a tea leaf into the hot water. The resulting water mixed with sucrose and other chemicals from the beet is called juice.
• Pressing: The chips are pressed to remove whatever water remains in them, and this water is added to the juice. The exhausted chips are typically turned into animal feed.
• Purifying: The impurities are removed from the juice with lime. This step is called carbonatation.
• Concentrating: The very dilute juice needs to be concentrated. For technical reasons it is first evaporated and then boiled in huge pans that hold 60 tons or more of juice until the juice is supersaturated with sucrose.
• Crystallizing: Sugar crystals are added to the supersaturated solution to initiate crystallization.
• Separating: A centrifuge is used to separate the sugar crystals from the water. The resulting sugar is white and ready for use in the kitchen. The left-over water still has some sucrose in it, and it can be made into a by-product called beet molasses, which is of lower quality than cane molasses; humans find it unpalatable. Beet molasses can be turned into animal food; it is suitable for making some forms of alcohol but not rum.

[Continued in Part 3]