Over the last decade a significant amount of research has focused on the effects of low GI versus high GI diets on health.
So far, most studies have been able to demonstrate that low glycaemic index (GI) foods help to treat type 2 diabetes and are associated with lower risk of developing cardiovascular disease and cancer. The effect this knowledge has had on the food industry has been tremendous, resulting in a flood of foods low in GI being marketed for health benefits.
The food industry is also looking at novel ways of preparing foods so that the end effect is a low GI food – for example, lowering the GI of bread by adding seeds.
Not all carbohydrate foods are created equally. In fact, each behaves quite differently in our bodies. The GI describes this difference by ranking carbohydrates according to their effect on our blood glucose levels. However, it is not just the carbohydrate content of a food which will affect its GI ranking.
The following factors are known to affect the GI of foods:
1. The type of starch present. The enzyme amylose in starch absorbs less water as the molecules form tight clumps, slowing the rate of ingestion. Sources include low-GI kidney beans, with high levels of amylose. By contrast amylopectin, the other type of molecule in starch, absorbs more water. Amylopectin molecules are more open and lead to a higher rate of digestion. Potatoes, a high GI food, is an example of a food high in amylopectin
2. The physical form of the food. The riper the fruit the quicker the absorption will be.
3. The amount of cooking and processing. Cooked or processed food is absorbed more quickly.
4. The amount of water-soluble fibre present. Water-soluble fibre found in some beans, oats and fruit slows down the absorption of carbohydrates.
5. The type of sugar. For example, sucrose, is broken down at a slower rate than glucose.
6. Fat and protein content. High fat and protein content delays the absorption of carbohydrates. For instance, the GI of plain cornflakes can be lowered by adding milk as milk is high in protein.
7. The acidity of food. For example, absorption can be delayed by adding lemon juice or sour milk to food.
Bread is a potentially nutritious and rich source of low GI carbohydrates. As a staple in many diets, lowering the GI of bread has the potential to positively affect the health of the population.
GI values have been determined for nearly 200 different breads. Research suggests that adding parboiled or whole grains or increasing the dietary-fibre content can decrease the GI of bread.
Little is known about whether the rising methods and leavening agents used in the production of bread affect the GI. Researchers at the department of Human Nutrition at the University of Otago in New Zealand looked to find the answer.
Their study, published in the December 2010 edition of the Asian Pacific Journal of Clinical Nutrition, tested eleven types of bread between three groups of 10 volunteers each. The first two groups were exposed to two different rising methods: the no-time method (short, 30 minute rise and prove) and the bulk-fermentation method (long, 50 minute rise and prove). The breads used in these two groups were made using three different types of flour – rye, oats and whole wheat.
The breads tested by group three were selected based on differing leavening agents used in the fermentation process. Four out of five of these breads were made with whole-wheat flour, but were leavened using yeast, yeast and desem (Dutch for leaven), sourdough and, or sourdough and oats.
Yeast is the traditional rising agent used in the production of bread. Yeast organisms produce carbon dioxide which is released when the dough is left to rise in a warm environment. The gas production helps the dough to rise and also assists in the development of the gluten network.
Desem is a fermented dough that is added to standard bread ingredients to help the dough rise. It is usually made from flour, water, salt, yeast and, sometimes, honey that ferments to produce carbon dioxide.
Sourdough is made from a starter similar to desem but with a more porridge-like consistency made from flour and water only. This slurry is left to capture wild yeasts from the environment. These yeasts breed in the slurry which can then be used as a rising agent. The development of the sourdough starter can take up to two weeks and extra ingredients must be added regularly to maintain the viability of the culture.
Generally desem and sourdough are less potent rising agents than commercial baker’s yeast. One commercially produced bread (Swiss Rye) claiming to be ‘naturally fermented’ was also tested by group three.
Contrary to expectations, eight of the eleven breads were found to have high GI values. None of the long fermentation breads had GI values that were significantly different from the GI values of the short fermentation breads. This result would suggest that rising methods do not influence GI, or that changing the rising method to increase the rising time by approximately 20 minutes is not enough to impact on the GI of bread.
In addition, no trends were observed to suggest a relationship between leavening agent and GI. Of interest, however, is that the plain sourdough bread was found to produce an unexpectedly high GI response. These are surprising results as it has been previously proposed that acids used in the production of sourdough bread reduce the GI because they limit gastric emptying rate.
The results of this study suggest that the density and the percentage of whole grains in bread may have more influence on GI than the rising methods or leavening agents used in its production.
The GI is a ranking of carbohydrate foods from 0 to 100. Carbohydrate foods are those that largely contain starches and sugars, like cereals, potatoes, bread, fruit and milk. GI is derived from measuring the blood glucose levels in people after they have eaten an amount of food that contains 50g of the carbohydrate (starches and sugar).
Carbohydrate foods that break down slowly release glucose gradually into the blood stream and have a low GI. Carbohydrate foods that break down quickly produce a faster and higher rise in blood glucose levels and have a high GI. Pure glucose produces the fastest and highest rise in blood glucose levels. Therefore glucose is given a GI of 100 (or 100%) and is used as a reference for measuring against all other foods. For example, spaghetti raises blood glucose levels by 41% compared to glucose, so it has a GI of 41.
Reference: Fredensborg, Monica Hardman; Perry, Tracy; Mann, Jim; Chisholm, Alex and Rose, Meredith. Rising Methods and Leavening Agents Used in the Production of Bread Do Not Impact the Glycaemic Index [online]. Asia Pacific Journal of Clinical Nutrition, Vol. 19, No. 2, 2010: 188-19.
