Sourdough starters smell and taste sweet due to the high activity of amylase enzymes in the starter that converts the complex starch molecules found in flour into simple sugars; starches smell and taste neutral while the simple sugars produced by the amylase enzymes smell and taste sweet.
Amylase enzymes are important as the simple sugars that they excrete is consumed by the yeasts which ferments the dough to produce alcohol and carbon dioxide gasses; the alcohol produced contributes to flavor and the carbon dioxide gives bread its rise and volume.
However, when a sourdough starter is too sweet, it can have a negative impact on the bake; the crust will darken excessively in the oven, your crumb will be gummy and flavor is compromised.
There are 5 reasons why your sourdough is sweet
Amylase enzymes is naturally present in the flour that we use for baking. In the presence of water, amylase enzymes break down long starch molecules in the flour into simple sugars which is consumed by yeast during fermentation.
When amylase enzyme activity is very high, the sourdough starter and subsequently the sourdough bread will smell and taste dominantly sweet and fruity.
1. High proportion of Rye flour in the mix
Rye in particular contains a high amount of amylase enzymes, and it is quite commonly used in the beginning stages of building a sourdough culture. Sourdough cultures made with rye flour contains high levels of amylase enzymes which converts a large amount of starch into simple sugars that gives off a particularly sweet and fruity smell and taste.
2. High proportion of salt in the mix (more than 2%)
Salt inhibits yeast activity by slowing down the rate of sugar consumption by the yeast. When there is too much salt in the starter or the final mix, yeast consumes less sugar (that is produced by the amylase enzymes) and subsequently there will be more residual sugar (sugar that is not consumed by the yeast) which makes the starter and final mix smell and taste sweet.
3. Flour with a high percentage of damaged starches (more than 10%)
Amylase enzymes prefers to metabolize damaged starches first because it is easier to process than non damaged starches. When the flour has a high content of damaged starches, amylase are very active during the beginning stages of fermentation, which produces a greater amount of sugar that contributes to the sweet smell and taste.
Wheat grain goes through a harsh process when being milled into flour and it is unavoidable that some of these starch granules to get split open. The split and broken starch granules are damaged starch particles.
American flour has 8-9% damaged starch while European flour has about 7% damaged starch. When damaged starch is more than 10%, it may cause your sourdough to smell and taste sweet.
4. Flour with a low falling number (less than 225)
The falling number measures the activity of amylase enzymes in flour. The falling number is determined by the time a measuring pole takes to descend through a fixed sample of mixed dough, a low falling number corresponds to a high amylase content in the flour.
The dough sample for determining the falling number is made with 7g of flour and 25g of distilled water. A pole is placed on the paste and time is recorded for how long it takes the pole to descend through the paste, hence the ‘falling’ number.
If amylase content is high, starches is converted into sugar rapidly, which breaks down the viscous paste and the pole descends quickly. A flour with low falling number has a high content of amylase, which contributes to the sweetness of your sourdough.
5. Excessively Malted flour
Some flour have insufficient amylase content, and it is corrected by the addition of diastatic malt (0.1% to 0.2% weight of the flour). However, when too much malt is added, it can introduce too much amylase into the mix which contributes to excessive sweetening of the starter and dough.
Excessively sweet sourdough is prone to a dark crust, gummy crumb and compromised flavor
We need some amylase enzyme and its sugary byproducts for the coloration of the crust as well as to provide nutrients for yeast to undergo fermentation. However, when too much amylase is present in your starter, it can cause excessive darkening of the crust, a gummy crumb and compromised flavor.
The crust goes through the Maillard reaction and takes on a brown color in the presence of heat, amino acids and reducing sugars (from the byproducts of amylase activity) on the surface of the loaf. The sweeter the dough, the faster the crust darkens in the oven, and when the crust is overly dark or burnt, it results in an unpleasant off flavor. A way to circumvent this problem is to monitor the loaf closely in the oven, and reducing temperatures by 5-10C when the crust is darkening too much too quickly.
Another problem with excessively sweet sourdough is that it can cause gumminess in the crumb. Amylase enzymes break down starches into simple sugars; the starches absorbs water under heat and gelatinizes, contributing to the formation of the crumb structure, but the simple sugars do not contribute to the formation of the crumb structure.
Hence, when there is too much amylase enzymes, a high proportion of the structural starch molecule is converted into the non-structural sugar molecules and causes a breakdown of the crumb. The starch molecules that initially held water is broken down by the amylase enzymes, releasing all that water back into the crumb, causing it to be gummy. This is known as ‘starch attack’ in rye bread.
When there is too much amylase activity in the sourdough, sugar is produced by the amylase at a rapid rate, which is consumed at a rapid rate by the yeast; fermentation is completed at a rapid rate over a shorter duration which leads to compromised flavor and potential for collapse of the loaf.
Fermentation produces the best flavors when the dough is left to mature at a slow to moderate rate over a longer period, that is why some sourdough formulas call for overnight fermentation in the refrigerator (aka retarding the dough) to improve the flavor of the baked loaf. When fermentation happens too rapidly, flavor is compromised, and there is an increase risk of collapse in the loaf as a result of over proofing if the dough is not monitored closely.
Tips to prevent an excessively sweet sourdough
Amylase inhibitors are substances that reduce the activity of amylase enzymes. Barley, corn, millet and peanuts are natural amylase inhibitors which you can try to introduce to your sourdough that intervenes with the metabolism of starches by the amylase, and hence reducing the effects of excessive amylase activity and sweetness in the sourdough.
We can also use less proportion of rye flour in your starter and mix, to reduce the amylase content of the formula.
Reducing the amount of salt used increases the rate that yeast consumes sugar, which reduces the residual sugars in the mix, and hence a less sweetened mix.
Using flour with a lower content of damaged starches will help with excessive sweetening as the amylase enzymes metabolizes undamaged starch particles at a slower rate.
Using flour with a higher falling number (lower amount of amylase content) will directly reduce the sweetening of the mix.
Reduce the amount of malt added to your mix as malt contains a high amount of amylase, which contributes to the sweetness of the mix.
An excessively sweet and fruity sourdough starter and mix is indicative of an overly active amylase activity, which can give rise to a number of problems in particular the excessive darkening of the crust, gumminess in the crumb and a compromised flavor.
We can improve our bake by controlling the amount of amylase content in the mix by reducing the proportion of rye flour used, using flour with lower percentage of damaged starch, using flour with a higher falling number and reducing the amount of malt used.
Salt can also be reduced to improve the rate yeast consumes the sugars in the mix, which reduces the amount of residual sugars present at the time of bake, slowing down the coloration and darkening of the crust.