Rate of digestion
Digestion of food involves chemical reactions that break up large food molecules into their ‘building block’ components. There are a number of factors that affect the rates of these reactions.
Surface area
Chewing breaks the large into the small. The mechanical digestion of food in the mouth involves converting bite-sized pieces of food into smaller particles. This has the effect of increasing the surface area of the food particles and allowing saliva to be thoroughly mixed with them.
Saliva is a watery fluid containing a slippery substance called mucin, which lubricates the food for easier swallowing, as well as a starch-digesting enzyme called amylase. When chewed, any starch present in the food is then readily exposed to amylase, speeding up the digestion process.
Surface area increases as particle size decreases for a given bite-sized piece of food:
Cubes of food
The diagram illustrates how surface area increases as particle size decreases for a given bite-sized piece of food. Although the particles in 3 are smaller than in 1, the total surface area available is nearly 3 times larger.
If a cube of food has 1 cm sides, the surface area of 1 face is 1 x 1 cm = 1 cm2 and the total surface area of the cube = 6 x 1 cm2 = 6 cm2.
If the cube of food is divided into 8 cubes with 0.5 cm sides, the total surface area of 1 cube is 6 x 0.5 x 0.5 cm2 = 1.5 cm2, and the total surface area of 8 cubes is 8 x 1.5 cm2 = 12 cm2.
If the cube of food is divided into 27 cubes with 0.33 cm sides, the total surface area of 1 cube is 6 x 0.33 x 0.33 cm2 = 0.6534 cm2, and the total surface area of 27 cubes is 27 x 0.6534 cm2 = 17.64 cm2.
Surface area is a key factor in determining the rate of a chemical reaction, so to aid digestion, food should be thoroughly chewed before swallowing.
Temperature
Most chemical reactions proceed at a faster rate as temperature increases. Food digestion reactions follow this general principle, but there is an upper temperature limit of about 40°C.
Rate of reaction and temperature
The graph indicates how the rate of digestion of starch using saliva changes as temperature changes.
Based on a graph by Dr Huw Parry.
Above 40°C, the enzyme present in saliva and responsible for the digestion of starch is deactivated. The enzyme is a protein, and at high temperatures, the shape of the protein is altered preventing it from performing its function. It has been denatured.
Denatured enzyme
The enzyme is a protein, and at high temperatures, the shape of the protein is altered, preventing it from performing its function. It has been denatured.
pH
The rates of chemical digestion processes involving enzymes are dependent on the pH of the surrounding fluids.
pH is a measure of the acidity or basicity of a given solution. The scale covers the range 1–14, with pH 7 being neutral, pH 1–6 acidic and pH 8–14 basic.
Enzyme activity and pH
The activity of amylase changes as the pH changes. A higher enzyme activity will result in a faster rate of reaction. Other digestive enzymes have a similarly shaped graph but with the peak either as shown or displaced to the left (acidic optimum) or to the right (basic optimum).
The enzyme amylase, which is present in saliva, has its maximum activity at pH 7. The activity of amylase changes as the pH changes – a higher enzyme activity will result in a faster rate of reaction.
Enzymes are globular proteins with a characteristic shape. This shape can alter with changing pH, the result being a decrease in activity.
Related content
Discover more about Digestive enzymes – the globular proteins that control biological reactions.
The article Catalysing chemical reactions with enzymes includes an animated video outlining in detail how enzymes work.
