What is starch?
Starch is a form of dietary carbohydrate and is the form in which glucose is stored in plants [1,2]. Chemically speaking, it is a polysaccharide which is made up of monosaccharides (glucose/sugar molecules) that are linked together. Starch is stored in compact granules consisting of two components:
- Amylose – a linear molecule consisting of linked glucose units. Amylose is soluble.
- Amylopectin – a branched molecule of glucose units that makes up a more significant proportion of starch and is insoluble .
There are two different crystalline structures of starch which have differing amounts of amylopectin. They are simply classified as type A and B. Type C is a mixture of A and B. The proportion of amylopectin and amylose will determine how digestible a starchy food is .
Starch that is digestible is broken down in the small intestine by amylase, glucoamylase and sucrase-isomaltase. The glucose is released and absorbed .
What is resistant starch?
Resistant starch is resistant to digestion by the pancreatic enzyme, amylase. This means that most of the starch makes it past the small intestine and through to the large intestine where it is used in bacterial fermentation . Resistant starch is great for colorectal health! However, when the starch is heated in water, its crystalline structure is disrupted and it forms a gel (gelatinisation) – this makes it easily digested .
Interestingly, ‘recrystallisation’ can occur to a certain extent after cooking. This is known as retrogradation. This will return some of its former resistant properties.
What does this process look like?
- Raw starch is made up of amylose and amylopectin in a crystalline structure.
- When heat and water is applied, the structure is lost as the starch granules swell and the amylose leaves the granule, causing it to collapse.
- The amylose forms a gel surrounding the collapsed granules.
- Upon cooling, the starch gels reform crystals causing it to harden and regain its digestion resistant properties 
Why is some starch resistant to digestion?
- Digestive enzymes cannot penetrate the cell wall of plants. Some starch granules are packed so tightly within the plants strong cell walls that they are trapped . This is the case for some cereal grains such as wholegrain oats and wholegrain bread – those cereal ingredients that are coarsely ground and therefore the cell remains intact. Amylase is unable to break down the wall of the cell and it therefore passes through to the large intestine .
- Starch is protected by the granule it is tightly packed in. These granules are insoluble unless they are subjected to water and heat in the cooking process. This starch is found in unripe bananas and uncooked potatoes. For the granule structure to be disrupted, these foods will need to reach temperatures of approximately 70 – 80 degrees Celsius . As previously discussed, this causes a gel to form and increases digestibility.
Interesting fact: Geneticists have developed a type of maize (Hi-maize) that has a higher content of amylose that will not gelatinise during cooking. This starch is then added to white bread to make it ‘high fibre’ when it would otherwise have a much lower fibre content [2, 3].
Interesting fact: The resistant starch content in bananas decreases as the banana ripens and the resistant starch turns in to sugar.
- Starchy foods that have been cooked and then cooled. The gelatinised starch – produced upon cooking – becomes insoluble and therefore indigestible once cooled . This is as a result of that retrogradation process mentioned earlier. This occurs in cooked and cooled potatoes, rice and bread.
Interesting fact: This type of resistant starch could be responsible for the low rates of bowel cancer among Asian populations who don’t have as high an intake of other types of fibre .
The table below outlines the different classifications of resistant starch and further summarises where they are found and how their resistance is reduced 
Table sourced from ‘Health Properties of Resistant Starch’ 2005 
Resistant starch content of food
The table below was developed by the CSIRO and really emphasises the difference cooking/ ripening can make to the starch content of different food.
Table sourced from: CSIRO 
The health benefits of resistant starch are numerous:
- Decreased risk of colon cancer and promotion of good bowel health  – increased transit time, larger faecal output, prebiotic effects, lower colonic pH.
- Improved glycaemic responses 
- Improved lipid profile – promotes cardiovascular health 
- Increases satiety and therefore aids in weight loss/ maintenance. Also lower in energy than digestible starch e.g. approximately 8kJ/g vs. 15kJ/g .
- Suspected to enhance the absorption of certain minerals e.g. calcium 
- Benefits to immune function
- Mann J, Truswell S. Essentials of Human Nutrition. 3rd Oxford: Oxford University Press; 2007.
- Nugent A. Health Properties of Resistant Starch. British Nutrition Foundation. 2005;30:27-54.
- Wahlqvist M. Australia and New Zealand Food and Nutrition. 2nd Crows Nest: Allen & Unwin; 2002.
- The Hungry Microbiome: Resistant Starch Feeds the Beneficial Bacteria of The Large Intestine. CSIRO.
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This article was written by our dietitian Belinda Elwin who is a Dietitians Association of Australia member and Accredited Practising Dietitian and Nutritionist. If you have questions about healthy eating, make an appointment. We‘ll provide you with a simple and effective routine targeted to your concerns.
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