4.1 Classification and structure of carbohydrates

Simple Carbohydrates

Monosaccharides

Monosaccharides are the simplest carbohydrates, each consisting of a single sugar unit. They share the general formula $C_n(H_2O)_n$, and they can't be broken down into smaller sugars by hydrolysis. In food science, two monosaccharides matter most:

• Glucose is the most common monosaccharide and the primary energy source for cells. It's found naturally in fruits, vegetables, and honey. When you see "blood sugar" on a nutrition label or in a health context, that refers to the concentration of glucose in the bloodstream.
• Fructose, often called fruit sugar, occurs naturally in fruits, vegetables, and honey. It's noticeably sweeter than glucose at the same concentration, which is why it's widely used as a sweetener in processed foods (most commonly as high-fructose corn syrup).

Both glucose and fructose have the same molecular formula ($C_6H_{12}O_6$), but their atoms are arranged differently. Glucose is an aldose (it contains an aldehyde group), while fructose is a ketose (it contains a ketone group). That structural difference is what gives fructose its greater sweetness.

Disaccharides

Disaccharides form when two monosaccharide units join through a glycosidic bond, a covalent link created by a condensation reaction that releases one molecule of water.

• Sucrose (table sugar) = glucose + fructose. Plants produce it naturally, and commercially it's extracted from sugar cane or sugar beets. It's the most widely used sweetener in food products and beverages.
• Lactose (milk sugar) = glucose + galactose. It's the primary carbohydrate in milk and dairy products. Some people are lactose intolerant, meaning they produce insufficient amounts of the enzyme lactase needed to hydrolyze lactose. Undigested lactose then ferments in the gut, causing bloating and discomfort.

Complex Carbohydrates

Oligosaccharides

Oligosaccharides contain 3–10 monosaccharide units linked by glycosidic bonds. They sit between simple sugars and large polysaccharides in size and complexity.

• Raffinose (found in beans) is a trisaccharide made of galactose, glucose, and fructose. Stachyose (found in soybeans) is a tetrasaccharide that adds one more galactose unit to raffinose. Humans lack the enzyme $\alpha$-galactosidase needed to digest these sugars, so gut bacteria ferment them instead, which is why beans are associated with gas production.
• Some oligosaccharides, such as fructooligosaccharides (FOS), function as prebiotics. They resist digestion in the upper GI tract and selectively feed beneficial bacteria (like Bifidobacteria) in the colon.

Polysaccharides

Polysaccharides are large carbohydrates composed of many monosaccharide units joined by glycosidic bonds into long chains or branched structures. Their size and bonding patterns determine their function in foods.

Starch is the main storage polysaccharide in plants and a major dietary energy source found in potatoes, rice, and wheat. It has two components:

• Amylose is linear. Glucose units are linked by $\alpha$-1,4 glycosidic bonds, forming a helical chain. Amylose typically makes up about 20–30% of most starches.
• Amylopectin is branched. It has the same $\alpha$-1,4 backbone, but every 24–30 glucose units a branch point forms through an $\alpha$-1,6 glycosidic bond. This branching makes amylopectin more readily digestible than amylose because enzymes have more endpoints to attack.

Cellulose is a structural polysaccharide that forms the rigid cell walls of plants. It's the most abundant organic compound on Earth.

• Cellulose is made of glucose units linked by $\beta$-1,4 glycosidic bonds. That $\beta$ linkage is the critical difference from starch. It forces the glucose chains into straight, flat ribbons that pack tightly together through hydrogen bonding, creating strong fibers.
• Humans lack the enzyme (cellulase) needed to break $\beta$-1,4 bonds, so cellulose passes through the digestive tract largely intact. This makes it an important source of dietary fiber, which aids digestion by adding bulk.