What is starch?

amylose1Starch belongs to a group of chemical compounds called carbohydrates. They are called this because they contain only carbon, hydrogen and oxygen. Pure dry starch is a white granular powder. Wheat flour contains 70-73% starch and most commonly anywhere between 8 -14.5% protein. If you look at flour under a microscope you can see lots of brick like structures called cells. In each cell you will see a granule of starch surrounded by glassy looking protein. Different types of starch have different structures. Potato starch is oval in shape, wheat starch is oval or round but smaller than potato starch, and maize starch has a "rocky" look.

Starch is called a complex carbohydrate because it is made up of many sugar molecules linked together. It has two main parts: amylose and amylopectin. Amylose is a straight or linear chain of sugar molecules linked together. Amylopectin is a branched chain of sugars.

amylose2Starch is a storage carbohydrate of plants such as cereals (wheat, maize, oats, rice and barley), tubers (potatoes, cassava and taro) and pulses (peas and beans). In whole wheat grains it makes up 60-70% of the grain. It is found in the endosperm which is the part of the grain that white flour is milled from.

Starch and the products derived from it are used in the food, brewing, pharmaceutical, paper, textile and adhesive industries.

In the food industry starch is used as a thickener, filler, binder and stabiliser in products such as soups, custard powders, pie fillings, sausages and processed meats, ice cream, sauces and gravies, baby foods, bakery products and baking powder.

The pharmaceutical industry use it in the manufacture of pills. It is used as a filler because it is bland and odourless. The textile industry uses starch for coating the fibre before weaving, and the dye pastes used for printing have starch in them. In the paper industry a starch solution is applied to surfaces of paper to increase the strength of the paper and give it a better finish. Starch also makes a very good adhesive or glue and is used to make cardboard cartons, boxes and containers. The gum used on the back of stamps and on envelope flaps is also made from starch.

Starch in Breadmaking and Baking

When starch is heated with water, granules absorb the water, and swell. Eventually they burst and the inner part of the granule spills out to form a thick gel. This is what occurs when you make a gravy or sauce. This process is called gelatinisation. In bread making not as much water is added as when making a sauce or gravy, and gelatinisation isn't completed - the starch granules swell, and many don't burst to form a gel. This forms a network of bloated starch granules all touching at the edges.

Starch also interacts with gluten during baking. The gluten breaks down and gives up water which is quickly taken up by the starch. This makes the gluten set and become rigid, which is why our loaves of bread don't collapse when they come out of the oven.

This scanning electron microscopic view of dough rising shows gluten strands forming two ways, diagonally down and across the photograph. Starch and yeast granules can be seen randomly amongst the gluten. The smallest granules are yeast.

Starch also provides "food" for the yeast to feed on during fermentation. As explained, alpha- and beta-amylase work together to build starch into sugar. It is this sugar that feeds the yeast in fermentation. The yeast produces carbon dioxide which helps the bread dough rise and gives bread its finished texture.

Starch, gluten and the gas from yeast fermentation all work together to produce what we know as bread, with it's crumb and gas bubble texture.

Starch is important for holding water in baked products like cakes. For certain cakes, cake flour treated with chlorine is used. The chlorine alters the starch's properties and the baker can include more sugar and fat (like butter) in the recipe. A soft, low protein wheat flour is usually preferred as less starch damage occurs, which gives better volume and a softer crumb.

Biscuits are high in sugar and fat and low in moisture (water). These factors inhibit starch gelatinisation which therefore does little to contribute to the structure of a finished biscuit.


Handy Hint:
To freshen slightly stale bread simply reheat it for a short time in the oven. The starch granules reabsorb water, swell again and produce a "fresh" loaf. If the bread is very stale you could try pouring milk over it first.