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Friday, August 23, 2013

Munching on tree trunks

Malaysian sago starch cookies.
In a prior post, I described the relationship between simple sugars, and more complex carbohydrates, like starches and glycogen. To summarize, starches are basically by linking together the smaller sugar molecules. The different types of starches (mostly a mix of amylose and amylopectin) differ primarily on how the sugar molecules are arranged. Glycogen is pretty much the same idea, except it's what animals make.

However, simple sugars can also be hooked together into a form that can't easily be broken down. When organized in such a fashion, these complex carbohydrates fulfill a structural function: cellulose. Due to it's strong and indigestible nature, cellulose is the primary component of wood. So, in short, the basic molecule of glucose (aka - corn syrup) can be hooked together to make either paper or bread.

The main reason to make starches, from the point of view of the plant, is for energy storage. After all, starches need to first be degraded into simple sugars before they are further digested for energy. That's why our most common sources of starch tend to come from grains (wheat, rice, corn). The starch there is primarily energy stored for the seedling to use when it germinates. Conversely, we can't digest cellulose, so the plant parts that aren't seeds are rich in fiber (indigestible cellulose that still serves an important "regular" function), and aren't starchy. But there's always an exception to the rule.

The sago palm (Metroxylon sagu) is harvested primarily for the starch content stored in the pith of its trunk. Sago starch remains an important dietary carbohydrate in Southeast Asia and Papua New Guinea, and may even predate rice as the dietary carbohydrate in China. Harvesting sago is a laborious process where the trunk has to be split, the inner pith is shredded, soaked in water, beaten to release the starch granules, and then dried. Sago starch has some optimal gelatinization properties between classic grain based starches and potato starch, and is remarkably free of protein - perhaps an important ingredient for those seeking gluten-free starches. 

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