Glucose, a monosaccharide, tastes sweet while cellulose, a polysaccharide, does not. Are all monosaccharides sweet? What about disaccharides? What determines whether something tastes sweet or not?
There’s a general pattern of monosaccharides being sweeter, but it’s not absolute.
Sucrose, a disaccharides combination of glucose and fructose, is usually assigned the reference value of 1.0. By comparison, monosaccharides:
fructose - 1.73
galactose - 0.32
glucose - 0.74
and the other disaccharides:
lactose (glucose + galactose) - 0.16
maltose (glucose + glucose) - 0.32
So some disaccharides like glucose average out the components, others are less than either.
It all has to do with the way the molecules fit into the taste buds.
I once read in Organic Gardening that we should throw out the soak water when preparing to cook dry beans. The soak water, they said, contains most of the polysaccharides that become farts. Up until today, that was the only time I ever saw the word “polysaccharides.”
How are these determined?
Probably by determining the amount necessary to make a solution of water (or something else) reach a desired level of sweetness. To make it more precise, they would probably look for the minimum detectable level of sweetness. If it takes 1.00 grams of sucrose to make 100 mL of water taste just barely sweet, and it takes 1.35 grams of glucose to do the same, then glucose is 0.74 times as sweet as sucrose.
The molecular basis of sweetness is structural. To taste sweet, a compound must have a hydrogen bond donor (such as a hydroxyl, -OH, group) and a hydrogen bond acceptor (such as an oxygen atom) separated by about 2.5 to 4 angstroms. (see here). Additionally, a non-polar section of the molecule increases its sweetness dramatically. Natural sugars do not really have this property, which probably explains why it is possible to make artificial sweeteners that are vastly sweeter than natural ones. Among the natural sugars, different molecules have different distances between the hydrogen-bond donor (an -OH group) and the acceptor (the -O- group in the cyclic form or the -CHO or -C=O in the straight-chain form). This explains why, for example, fructose is sweeter than galactose. All this is hard to illustrate without images, unfortunately, though a search should find some.
Disaccharides are generally less sweet than monosaccharides, and polysaccharides are hardly sweet at all. The complex sugars are larger molecules, which means that a) there is less of them in a given mass and b) they will be less able to bind to a receptor.