March 30, 2011

Using Sourdough to Lower the Glycemic Index of Everyday Baked Goods

This post is a departure for me–more technical than usual, and intended to address a specific health issue: the blood sugar spikes that come with eating everyday baked goods. I based it on experiments conducted in my own home kitchen, and I have written it for those with diabetes or blood sugar issues, or those interested in lowering the glycemic index of carbohydrate-rich foods. As I have no immediate plans to publish additional posts on this topic, please contact me by email if you would like more information. If there is enough interest, I will look at ways to disseminate instructions and recipes for using sourdough in baking.

Background

Both sweet and savory baked goods can pose health concerns for a variety of reasons:

  • When we eat baked goods made with white flour, blood sugar can spike upward after eating (the glycemic index, or carbohydrate component, is too high).
  • When we eat baked goods made with whole-grain flour, blood sugar remains relatively stable, but phytic acid* stored in the bran may prevent us from absorbing such minerals as iron, calcium, magnesium and zinc. This can lead, over time, to mineral depletion and bone loss.
  • When we bake with white or whole-grain flour and allow the batter to rest (soak) for a period of hours before baking, phytic acid is reduced, but grains become super-digestible and the result is a dramatic spike in blood sugar (the glycemic index is again too high).

The problem, then, is circular in nature, with each solution creating a new problem.

As a sourdough bread baker and teacher, I knew that sourdough creates an ideal carbohydrate: bread with a low glycemic index and almost no phytic acid due to the dough’s long rest before baking. I therefore wondered if I could use sourdough to improve the healthfulness of everyday baked goods by adding it to a batter and then allowing it to rest for a period of hours, just as when making sourdough bread. I believed this approach would lower the glycemic index of baked goods and reduce their phytic acid, a simple and elegant solution to the challenges listed above. This approach might be useful to those with insulin resistance or diabetes, those with mineral deficiencies (like low calcium or iron), and those who simply wish to lower the blood sugar impact of the baked goods they consume.

The rest of this post describes my tests and their promising results.

A Graphic Summary of Test Results

Sourdough is an ancient leavener, created when wild yeasts and beneficial bacteria feed on a mixture of flour and water. Sourdough preserves, moistens, and leavens baked goods. It also reduces or eliminates phytic acid and improves the nutritional value of foods. Since sourdough is easy to maintain in a home kitchen, the goal of my experiments was to explore whether I could use sourdough in everyday baking to lower the glycemic index of baked goods and to reduce their phytic acid.

Test Results

I conducted four series of tests** centered around two recipes, one savory (buttermilk biscuits) and one sweet (banana muffins). In each case, I prepared the recipes using stone-ground flour, which may be important. The results of these tests were consistent and clear.

  • The top chart summarizes the results of adding sourdough to a savory biscuit recipe. The whole-wheat sourdough biscuit–soaked for 24 hours–caused only a modest rise in blood sugar and gave the most impressive result (see the red bottom line on the graph). These biscuits had the lowest blood sugar impact after 30 minutes, which is when blood sugar typically spikes upward, and provided the most substantial satiety and blood sugar stability. You can see this in the low arc, and the slow, steady blood sugar decline. This last factor illustrates why lessening blood sugar impact can also prevent hunger and overeating. In contrast, soaking a whole-wheat batter with buttermilk instead of sourdough causes blood sugar to spike and then plummet (the blue top line).
  • The bottom chart summarizes the results of adding sourdough to a sweetened banana muffin. The whole-wheat sourdough muffin–soaked for 12 hours–gave the best result (see the blue line on the graph). These muffins caused a moderate increase in blood sugar, coupled with sustained blood sugar levels over a full two hours and a slow, steady decline. Interestingly, in this experiment the same sourdough batter soaked for 24 hours  (the yellow line) yielded the worst results. More research would be needed to understand why.
  • While not illustrated in the charts, I conducted further tests to determine the quantity of sourdough needed to achieve these same results. Using just one quarter the amount of sourdough worked, proving that where a sour taste is not desired, less sourdough can be used. (I have so far experimented with using as little as 1 ounce of sourdough starter to 31 ounces of  savory biscuit dough to achieve this same result.)
  • In each case, unsoaked whole-wheat batters traced a gentle glycemic curve but, without soaking, phytic acid in the finished baked good may remain an issue.
  • Test results also indicated that the inclusion of sourdough in a batter–but without a soak–has no impact on the glycemic index. A soak is required, and soaking time seems to matter.

Conclusion

These tests demonstrated that we can indeed use sourdough to reduce the glycemic index of everyday baked goods and to reduce their phytic acid, as long as the flours we use are stone-ground (see below), and as long as we allow the batter or dough to rest before baking. In these experiments, a 12-hour sourdough soak rendered the best results in sweetened baked goods. For savory baked goods, a 24-hour soak worked better. Soaking with buttermilk, and without sourdough, caused blood sugar to spike dramatically and then plummet downward. In recipes where a sour taste is not needed, sourdough can comfortably be cut to a quarter of the amount initially tested.

There is more to learn. Most importantly, what is the ideal amount of sourdough and length of soaking time needed to give the optimum result? Also, studies by others show that stone-ground flour is necessary for lowering blood sugar. My own tests confirmed this, but a closer look is needed. Finally, in what other varieties of baked goods can we use sourdough to lower glycemic response, and where is this a palatable approach and where is it not? From a culinary perspective, I have used sourdough in other sweetened baked goods (cakes–including chocolate cakes, cookies, cornbread, muffins and quick breads) and achieved a very good result, often better than without. On the savory side, I have made sourdough pizza, pancakes, English muffins, and a variety of breads. All this is to say that sourdough offers delicious potential to lower the glycemic index of everyday baked goods.

Copyright 2011, Ellen Arian, Ellen’s Food & Soul

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* Phytic acid is a nutrient inhibitor found in the outer layer–the “bran”–of whole grains. It can inhibit our ability to absorb calcium, magnesium, iron and zinc, as well as digestive enzymes such as pepsin and amylase.

** I conducted these tests in my home kitchen, and I have normal insulin responses. My measurements were exact and portions were controlled with tests conducted at the first morning meal following a 12-hour overnight fast. I consumed only the baked good being tested and water. Using a True2Go blood glucose monitor, I measured blood sugar before eating, and again at 30-60 and 120 minutes after eating.

  • The first tests determined how savory and sweetened baked goods, prepared with both white and whole-wheat flour, affect blood sugar. I conducted these tests on just-prepared batters, and on batters allowed to rest for 12 and 24 hours before baking. I added no sourdough to these batters.
  • The second tests explored whether the addition of sourdough, both with a rest and without, has an impact on glycemic response.
  • The third tests explored whether an acid other than sourdough–in this case, buttermilk–might impact glycemic response.
  • The fourth tests attempted to determine how much sourdough is needed to lower glycemic response.