Sugar and other carbohydrates

Photo by Jason Leung on Unsplash

The name carbohydrate derives from ‘hydrated’ carbon (carbon combined with hydrogen/oxygen) — shortened to carbohydrate.

However, our dietary carbohydrates are not random combinations of carbon, hydrogen and oxygen; there is a basic building block, known as a simple sugar (or monosaccharide), of which carbohydrates are made. Some carbohydrates, such as table sugar, are composed of only 2 simple sugars; others, such as starch, can contain thousands.

Simple sugars

The simple sugars come in 3 forms that may be familiar: glucose, fructose and galactose. They all have the same chemical composition (C₆H₁₂O₆ — 6 carbons, 6 oxygens and 12 hydrogens), but arranged in different shapes.

Glucose is an important energy source for the body. We burn (oxidise) glucose to release energy. It is produced in plants by photosynthesis. It is also synthesized in the liver and kidneys from sources other than carbohydrates. Thus, a low carbohydrate diet does not necessarily affect glucose availability in the body,

Fructose is a very sweet simple sugar, much more so than glucose or galactose and sweeter than table sugar. It occurs naturally in fruits and honey.

Galactose is found in milk and sugar beets. It is rapidly transformed into glucose when digested and can also be manufactured by the body itself.

So, those are the 3 main simple sugars that are the basic building blocks of all dietary carbohydrates. The complexity and diversity of carbohydrates come from how many of these are used and how they are arranged.

Dietary sugars

What we think of as ‘sugar’ is a carbohydrate consisting of 2 simple sugars. Three of the more common sugars:

Sucrose (table sugar) consists of glucose bound to fructose.

Lactose (milk sugar) is glucose bound to galactose.

Maltose (malt sugar) consists of 2 glucose.

Larger (complex) carbohydrates

Our taste receptors are activated by the small simple sugars, registering sweetness in the brain. However, as carbohydrates get more complex by adding more building blocks, they get too large to activate the receptors and we no longer perceive them as sweet, even though they are constructed almost entirely from simple sugars.

This is perhaps even more surprising because the very large carbohydrates, such as starch and hemicellulose, are constructed virtually entirely of glucose.

As we eat complex carbohydrates, such as starch, our body readily breaks them apart to get at the glucose. This is why starchy foods such as potatoes and rice can have very high glycemic indices (GI=95) — they release a sudden rush of glucose into the blood stream. Table sugar (glucosefructose) has a slightly lower GI (90) because fructose is low GI (that doesn’t mean it’s good for us). The GI is an estimate of the rise in blood glucose associated with ingestion of a food, and glucose itself is assigned a GI of 100. A weakness of the GI is that it does not take into account the relative percentage by weight of carbohydrates in the food. Glycemic load (GL) does this and is perhaps a more useful measure.

Presumably, we are attracted to carbohydrates such as rice, pasta and sugar because they provide a ready source of blood glucose. However, too rapid a rise in glucose triggers hormonal responses (principally insulin-mediated), initiating a cascade of reactions that can end in fat storage, or retention (oppose burning fat for energy).

Some carbohydrates, such as cellulose, cannot be broken apart by our digestive system (although gut bacteria can ferment them into smaller components) — they aid digestion by providing dietary fibre.

Think of glucose as the Lego pieces of the larger carbohydrates. They can be combined in different ways to produce a staggering range of structures and fuel sources (from tree trunks to potatoes). But the components remain the same. This is also why the rule of thumb, that there are ~4 calories per gram of digestible carbohydrate, holds regardless of the carbohydrate — weight-for-weight, they are all essentially glucose.


Syrups are a mixture of simple sugars, not a chemical compound of them.

Honey is a syrup, as are maple and palm syrups (duh) and agave nectar. Syrups can also be manufactured by splitting sugars into their simple sugar components. High fructose corn syrup (HFCS) is a manufactured mixture of glucose and fructose.

Invert sugar is produced by the action of the enzyme invertase (found in yeast) on sucrose, yielding a glucoseand fructose mixture. Yeast doesn’t digest sucrose, but it comes with an enzyme to break it up and release the glucose. The freed fructose sweetens yeast breads and the yeast digests the glucose.

Sucrose can be split into glucose and fructose by hydrolysis, which just means heating in the presence of water. This is known as simple syrup — mix equal quantities of sugar and water, bring to boil, simmer briefly and cool. Typically used in cocktails, or to increase sweetness in baked goods.


Most of our table sugar comes from sugarcane (a tropical/temperate grass originating in South Asia) or sugar beets (cooler European climates). As you might expect, there is a rich (and at times disturbing) history underlying the production of sugar from these two sources, taking in Columbus, the new world, slavery, Venetian dominance, the usual human greed and more. Not always a history to be proud of, and too much for this post. I will concentrate on the manufacture of table sugar from sugarcane.

The first step usually occurs at processing plants close to the cane fields. Sugarcane is crushed to extract the juice. The juice is mixed with lime, which settles out more of the pulp. The juice that is left is boiled down (usually in a vacuum to reduce the boiling point). Eventually, enough water has been removed for sugar crystals to form. The crystals are spun in a centrifuge to drive off further moisture. The resulting brown sugar mass, which arguably is true raw sugar, is unfit for human consumption.

So, it is transported to specialized food-grade refineries where it undergoes this process repeatedly (with carbon filtering and decolourisation replacing the lime). At each cycle, the moist syrup separated out by the centrifuge is captured as molasses (of various grades) and the crystals steadily become whiter until they are pure sucrose.

Sugars such as Turbinado and Demerara are sugar crystals extracted early in this process while they still have some molasses attached to them.

However, most supermarket ‘raw’ sugar is crystalline white sugar that has been sprayed with molasses to colour and flavour. Brown sugars are white sugar mixed with molasses. You can make your own: mix one tablespoon of molasses and a cup of sugar. It will be as good as shop-bought brown sugar.



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