Saturday, 12 August 2017

A plant's sugar high

I love breakfast food. I especially love those Ma and Pa breakfast diners where one can get two giant pancakes, hashbrowns, and eggs for about $5. In my mind's eye, I sit down in one of these places, the server brings over the coffee, and there's this little container on the table that has small, pastel colored packets of sweeteners, and maybe a brown packet with raw cane sugar. If I want to treat myself, I tear open one of the brown packets and empty the small crystals into my morning cup. Where do these crystals come from, and why are they brown? Today: sweeteners from plants (volume 1)!
Figure 1: Sugar cane plants. Here, the beautiful colors on the canes are from the fields having been burned to remove excess foliage. Though this reduces the amount of manual labor required to harvest the plants, it releases substantial amounts of carbon dioxide into the atmosphere.
Cane sugar comes from Saccharum officinarum (Figure 1), a plant with Southeast Asian origins, and was the earliest sweetener used on a grand scale. This grass species is now cultivated extensively in its home territory and in South and Central America, having been brought there as part of the Columbian exchange, and accounts for around 70% of the world's sugar production [1], most of the rest coming from sugar beets.

Sugar cane is relatively unique in how it stores sugar - Virtually all plants operate by photosynthesizing to harness the energy of the sun to convert CO2 gas into the solid chemical glucose. Plants then stash this high-energy glucose away in a storage unit for use during the night, a later season, or to give their children a better chance in the dangerous wild. For plants like the potato, their storage unit is a big fat tuber (which is actually a modified stem!). They link many glucose units together into long polymers or chains called starch - these are easy to pack together for storage (Figure 2). Many plants hoard starch in their roots, fruits, or grains. However, unlike most plants, sugar cane stores each of its glucose units linked to just a single partner to make sucrose (these two together are also called a disaccharide), and it stores the sucrose in its stalk. It is not entirely clear why Saccharum does this, but it could be because it allows the plant to continue rapid growth immediately after photosynthesis stops, while its competitors need time to get glucose out of starch storage, giving Saccharum an advantage in the short-term.

Figure 2: Plant sugar storage. Most plants store sugar as a large polymer, many glucose units linked together (top). Sugar cane, together with sugar beets and sweet sorghum, is an exception in that it stores sugar as sucrose, a disaccharide (bottom).
Cane sugar is purified by crystallization - By forcing sugar cane stalks through a press, a watery liquid containing the disaccharides can be obtained. This liquid is then heated to kill enzymes that might break down the disaccharides (and destroy the sweet taste!), and finally boiled to get rid of some of the water and concentrate the sugars into a juice. Ever notice how salt or sugar dissolve most fastest in boiling water? This is because the water molecules are vibrating really quickly and can easily smash apart the salt or sugar crystals. However, when they are vibrating so quickly, hot water molecules can bite off more than they can chew: they dissolve more sugar than they would be able to if they were cold. So, when sugar cane juice becomes very concentrated while boiling, lots of sucrose is dissolved in a relatively small amount of water. When the juice cools, the water can no longer dissolve all the disaccharides it could when it was hot, and the sucrose molecules coordinate and form themselves into crystals that can be collected in a filter. Though purification by crystallization is an elegant chemical process, in practice, at least historically, it was difficult, dangerous, and carried out by slaves at a high cost to human life.
Figure 3: Cane sugar crystallization. Sucrose crystals can be obtained from the liquid of pressed sugar cane by heating and boiling it to concentrate it to a point where the ratio of sucrose to water is very high, then allowed to cool so the sugar crystallizes and can be isolated by filtration.

Other plants also accumulate disaccharides - Sugar beets and sweet sorghum also defy the starch trend and belong to the disaccharide club. Instead of storing their disaccharides in their stalks though, a sugar beet's cache is, of course, in its roots. For processing, these roots are chopped into small pieces and extracted with water, which can then be concentrated and the sucrose obtained by crystallization in a process roughly similar to that used for cane sugar. Of course, after filtering crystals from either cane or beet processing, the liquid is left over (Figure 3, far right, blue liquid). This liquid can be subjected to another round of recrystallization to obtain more crystals, and after that the remaining liquid itself can be concentrated into the brown molasses sold in grocery stores. This is most often done during sugar cane processing, as the molasses obtained from beet processing is not as palatable. Sugar crystals that contain trace amounts of molasses have a brown color.

Some plants are sweet without creating lots of sucrose - There are several other plant-based sweeteners that are not related to sucrose that are beginning to be used widely in U.S. consumer products. These have the advantage of not having the negative health effects associated with sucrose. However, discussion of these phytochemicals will have to wait until next time. See you then!

[1] Lakshmanan O, Geijskes RJ, Aitken KS, Grof CPL, Bonnett GD, Smith GR. 2005. Sugarcane biotechnology: the challenge and opportunities. In Vitro Cellular and Developmental Biology-Plant 41, 345–363.
[2] A. J. McCormick, D. A. Watt, and M. D. Cramer. 2009. Supply and demand: sink regulation of sugar accumulation in sugarcane. Journal of Experimental Botany, Vol. 60, No. 2, pp. 357–364