Carbohydrates in coffee.
 

Carbohydrate content in green coffee represents a significant portion of coffee's matter making up about 50% of coffee's total dry base. The composition is made up of a complex mixture of both soluble and insoluble matter including mono, oligo and polysaccharides.

With the exception of sucrose - whose concentration is almost double in arabica than robusta - much of the carbohydrate profile remain quite similiar for both species. Refer to Table 1 below. 

Table 1a: Carbohydrate content in Green coffee (% dry base)
Compound
Arabica
Robusta
Monosaccharides
0.2-0.5
0.2-0.5
Sucrose
6-9
3-7
Polysaccharides43-45
46.9-48.3
   arabinose3.4-4.03.8-4.1
   mannose21.3-22.5
21.7-22.4
   glucose
6.7-7.8
7.8-8.7
   galactose
10.4-11.9
12.4-14.0
   rhamnose
0.3
0.3
   xylose
0-0.2
0-0.2
 a: Illy, A. Espresso Coffee, 2nd ed. p151

As expected the sucrose concentration varies with degree of ripening and should always be considered when making analytical comparisons. This is particulalry true for defective beans where differences between sucrose concentration can vary significantly. See Table 2. 

Table 2b: Monosaccharide content in green coffee.
Monosaccharide Arabica Robusta
Arabinose
3.7
4.0
Mannose22.0
22.0
Glucose7.2
8.2
Galactose
11.0
13.1
Total
43.9
47.3
b: Illy, A. Espresso Coffee, 1st ed. p29

During roasting, sugars play a critical role with many of the reducing sugars participating in the Maillard reaction. While the simple sugars - sucrose, hexoses, pentoses - are almost all lost producing water, carbon dioxide, color and aroma1. A portion of the carbohydrate also go into the production of aliphatic acids such as acetic and formic acids1.

Interestingly, researchers have found higher levels of reducing sugars in robusta than arabica which may be responsible for differences in cup quality.

References:
1. Parliment, T. What Makes that coffee  smell so good? CHEMTECH, August 1995.