Technical Talk – January-February 2010

Continuing our series on the basic ingredients for baking, what is starch and what is the role of natural and modified starches in the formulation of baked goods?

A major component of wheat and other flours, starch plays an important functional role in the production of baked goods. As such, its presence contributes to the functionality of these flours. Beyond the natural presence in flours, isolated starches from a variety of sources are available as ingredients for formulating baked goods. In addition to its importance as a functional formulating ingredient, in recent years resistant starch has been promoted as a dietary fibre delivering health benefits.

Chemically, starch is composed of long chains of repeating d-glucose units (molecules). It’s formed in plants during their growth by the linkage of the glucose units forming this complex polysaccharide. It is normally found in storage parts of the plant such as seeds, tubers and nuts.

There are two naturally occurring, basic types of starch polymers. The first is a linear starch composed of many glucose molecules – sometimes hundreds to thousands of units in a single chain – called amylose. The second – amylopectin – is more complex; made up of many branches of chains of glucose units, it has a treelike structure. The number of glucose units in each branch normally ranges from 20 to 30.

Amylose and amylopectin are normally contained within the starch granule, a structure present along with protein in the endosperm of the wheat and corn kernel, potatoes, seeds, roots and other plant materials. The shape and size of starch granules are relatively characteristic of the different species of plants. Starch granules from potatoes are fairly large (100 microns in length), and rice starch granules fairly small (two to six microns), while wheat starch granules are intermediate in size.

The two types of starch have varying characteristics and behaviour when present in different food systems. In fact, the ratio of amylose to amylopectin in a given flour plays an important role in its functionality in baked goods.

Starches from different sources based on the granule characteristics and amylose and amylopectin content behave differently in regard to swelling, gelatinization temperature, viscosity and tendency to retro-gradate. In the presence of heat and water the granules swell and gelatinize, providing different functionalities to the food formulation such as viscosity and textural changes.

Over time, starch granules might revert back to their original granular or crystalline form. This behaviour is called retro-gradation and plays a key role in the staling of baked goods and specifically bread. Enzymes and other ingredients are available that can interact with starch and minimize the effect of staling, thus increasing the shelf life of breads and other baked goods. Gelatinization of starch granules also makes the amylose and amylopectin accessible to different amylase enzymes and allows the breakdown into shorter chains (dextrins) or glucose units that can be fermented by yeast or other micro-organisms. In industrial processes, starch that has been gelatinized will be used in production of syrups, alcohol and other products. In addition, it’s used in conjunction with various physical, chemical and enzymatic treatments to produce the myriad modified starches available in the ingredient market.

Starch plays a critical role in the baking process. As a component of wheat flour it constitutes more than 65 per cent of the flour. Much research has been carried out to elucidate the role of starch in baking. This research has shown that the presence of starch has an effect on the baking process but the mechanism of this action is still being investigated. It’s been demonstrated that starch influences the loaf volume and internal characteristics  of bread, and several properties of the starch affect its behaviour during the baking process.

Starch from different varieties of wheat has a different effect on the baking process and final products. In addition to the influence due to genetic differences of the varieties, seasonal variations of wheat crops result in starches with different behaviour. Experiments with other sources of starches demonstrated that wheat starch boasts a superior performance in bread baking. This superiority is attributed to characteristics of the granule that allow better interaction with gluten, the gelatinization behaviour and the ratio of amylose to amylopectin.

One aspect of how starch influences the baking process can be demonstrated by the starch damage of flour during the milling process. Starch granules in wheat flour, depending on the tightness of the milling rolls, can be damaged. Damaged starch granules will absorb more water and influence the gluten behaviour, thus affecting dough characteristics. The starch damage of wheat flour is normally reported using the Farrand method, and flour specifications often include an allowable limit. Starch damage can also be observed by looking at the absence of the characteristic “maltese cross” of the granules under the polarized light microscope.

Isolated starches from various plant sources are important ingredients for the formulation of many food products, and starch manufacturers and suppliers can provide a host of modified starches that can perform many functions. In addition to their functionality in food systems, recently some starches have been promoted as beneficial to health because of their resistance to digestion in the small intestine. Resistance to digestion in the small intestine means these starches behave like dietary fibre. They may be fermented in the large intestine and thus can play the role of prebiotics, food source for probiotics (beneficial bacteria), which in turn provide health benefits.

For these reasons, resistant starches are recognized as a dietary fibre in certain countries. In Canada, however, this is not the case.
Many choices of modified starch ingredients are available. Suppliers of these ingredients will provide usage guidance and information on their regulatory status in Canada and countries to which your food products might be exported.

Funding for this report was provided in part by Agriculture and
Agri-Food Canada through the Agricultural Adaptation Council’s
CanAdvance Program.

Dr. John Michaelides is Guelph Food Technology Institute’s director of
research and technology. For more information, or fee-for-service help
with product or process development needs, please contact GFTC at
519-821-1246 or  gftc@gftc.ca.