FOOD CHEMISTRY: CARBOHYDRATES
Diagram
Two glucose chains, two jobs
Starch
Storage carbohydrate of plants. Alpha 1,4 bonds. Digested by human enzymes.
Cellulose
Structural carbohydrate of plant cell walls. Beta 1,4 bonds. Not digested by humans.
Figure 1: Starch and cellulose are both chains of glucose, but the bond between the units decides whether the chain is food or fibre. Drawn in our own style from the polysaccharide discussion in the FTN Food Chemistry notes, Unit 3.
Introduction
Starch and cellulose are both long chains of glucose units, and both are polysaccharides. The difference is the bond that joins the units: starch is built with alpha bonds that human enzymes can break, while cellulose is built with beta bonds that human enzymes cannot break. One small difference in a bond decides whether a carbohydrate is food or fibre: starch is the storage carbohydrate of plants, and cellulose is the structural carbohydrate of plant cell walls.
Quick comparison
Table 1: Starch compared with cellulose
| Aspect | Starch | Cellulose |
|---|---|---|
| Built from | Glucose units | Glucose units |
| Bond | Alpha 1,4 bonds (with alpha 1,6 branch points in amylopectin) | Beta 1,4 bonds |
| Job in the plant | Storage, packed into granules in seeds, roots, and tubers | Structure, in plant cell walls, packed into fibres by hydrogen bonds |
| Parts | Amylose (mostly straight) + amylopectin (branched) | One straight chain type, packed into fibres |
| Dissolves in water | Granules swell in hot water and thicken; does not dissolve easily cold | Does not dissolve in water |
| Digested by humans | Yes: human enzymes break alpha bonds | No: human enzymes cannot break beta bonds, so it is dietary fibre |
| Food role | Main thickener and gelling agent; swells in hot water, thickens a sauce, sets a gel on cooling | Gives plant foods structure and fibre; adds bulk and holds water in the gut |
| Sweet taste | Not sweet | Not sweet |
Scientific explanation
Definition
Starch is the storage carbohydrate of plants, packed into granules in seeds, roots, and tubers. It has two parts: amylose, a mostly straight chain of glucose joined by alpha 1,4 bonds, and amylopectin, a branched chain. Cellulose is the structural carbohydrate of plant cell walls, a straight chain of glucose joined by beta 1,4 bonds, packed into fibres by hydrogen bonds.
Working principle
When two sugar units join, the bond forms at the ring carbon, and whether the bond is alpha or beta decides whether the chain can be digested. Starch is built with alpha bonds and can be digested by human enzymes; cellulose is built with beta bonds and cannot, which is why cellulose passes through as fibre. In hot water, starch granules swell, thicken a sauce, and set a gel on cooling. Cellulose stays as fibre and does not dissolve.
Apparatus
The laboratory test for starch uses boiling with acid, then a copper reagent test for the glucose formed. No separate apparatus is named in the books used here for cellulose, so no further apparatus is listed.
Measurement procedure
Starch is estimated by acid hydrolysis: the sample is boiled with acid so the starch breaks down to glucose, and the glucose is then measured as a reducing sugar, by heating with a copper reagent and reading the colour or the titration. The glucose found is multiplied by 0.9 to give the starch, because a glucose unit loses a water molecule when it joins the starch chain, so starch weighs 0.9 times the glucose it came from.
Factors affecting readings
Acid strength, boiling time, and temperature decide whether the starch is fully broken down to glucose. If the hydrolysis is not complete, the starch reading is low. The source of the starch also matters, because the amylose to amylopectin ratio changes how the starch swells and thickens.
Formulas and calculations
Starch from glucose. Starch is reported as glucose x 0.9. Worked example: if acid hydrolysis of a sample gives 20 g of glucose, the starch present is 20 x 0.9 = 18 g.
Glucose from starch. A starch chain is built from glucose units that have each lost one water molecule, so a repeat unit weighs 162 g per mol (C₆H₁₀O₅), while free glucose weighs 180 g per mol (C₆H₁₂O₆). The mass factor is 180 / 162 = 1.111, so 100 g of dry starch gives 111.1 g of glucose when fully hydrolysed.
Industrial applications
- Starch: thickening sauces, soups, and gravies, and gelling desserts and pie fillings.
- Starch: the base of glucose syrups and maltodextrins, made by breaking starch down.
- Starch: baking, where it swells, thickens, and sets the crumb on cooling.
- Cellulose: dietary fibre in cereals, vegetables, fruits, and legumes, adding bulk and holding water in the gut.
- Cellulose: structure and texture in plant foods, and a fibre ingredient in foods.
Advantages and limitations
- Starch, advantage: a strong thickener and gelling agent, digestible energy, and a base for syrups.
- Starch, limitation: it thickens only in hot water, and on cooling and storage it retrogrades, squeezing water out and making bread go firm and stale.
- Cellulose, advantage: dietary fibre that adds bulk, holds water in the gut, and slows the digestion of starch.
- Cellulose, limitation: it gives no energy to humans, because it cannot be digested, and in very large amounts or without enough water it can bind minerals such as iron and calcium and cause bloating.
FAQ
Are starch and cellulose both made of glucose?
Yes. Both are long chains of glucose units. The difference is the bond: alpha bonds in starch, beta bonds in cellulose.
Why can humans digest starch but not cellulose?
Human enzymes can break the alpha bonds of starch, but they cannot break the beta bonds of cellulose.
Does cellulose dissolve in water?
No. It does not dissolve in water, and it stays as fibre.
What is dietary fibre?
The wider nutritional idea of all the plant parts the body cannot digest, including cellulose, hemicellulose, pectins, and resistant starch.
Why is starch multiplied by 0.9 when it is measured as glucose?
A glucose unit loses a water molecule when it joins the starch chain, so starch weighs 0.9 times the glucose it came from.
References and further reading
- FTN subject: Food Chemistry, Unit 3: Carbohydrates.
- Zeece, Michael. Introduction to the Chemistry of Food. Academic Press, 2020.
- Coultate, Tom. Food: The Chemistry of Its Components. 7th ed., Royal Society of Chemistry, 2023.
- Damodaran, Srinivasan, and Kirk L. Parkin. Fennema’s Food Chemistry. 5th ed., CRC Press, 2017.
- Wang, Dongfeng, Hong Lin, Jianqian Kan et al. Food Chemistry. De Gruyter, 2015.
- Google Books pages read for this article:
- Fennema, Owen R., ed. Food Chemistry, 2nd ed., Marcel Dekker, 1985: carbohydrate chapter on monosaccharides, disaccharides and polysaccharides (Google Books search result, full text read via the search record)
- Provost, Joseph J. et al. The Science of Cooking. John Wiley and Sons, 2016 (Google Books page read)
