Why does bread sourness increase overnight after baking?

Why does bread sourness increase overnight after baking?

Submitted by Doc.Dough on July 25, 2015 – 8:23am.

I have noticed over many years that the flavor intensity of sourdough seems to increase over time after the bread is baked. Immediately after baking it may be relatively mild, and increase as it cools off, then increase even more over a long meal, and perhaps be even more acid the following day. It is a well understood part of aging sourdough rye bread but the underlying phenomenology seems obscure.

Until recently I was not equipped to make the measurements, but I am now able to assess the degree to which my sense of sourness is supported by objective measurements, and i am surprised by the clarity and significance of the initial data.

A batch of bread baked yesterday was measured six hours after it came out of the oven at a TTA of 13.7.

It was left on the counter overnight and a sample from an adjacent slice was measured this morning (8 hrs after the first measurement) at a TTA of 15.7.

While the objective of the last month of experimentation has been to increase the sourness of the bread, this result (perhaps because it is the first objective data confirming my sensory judgement) seems noteworthy. Yet I am at a loss for an explanation of the underlying chemistry. What changes over 8 hrs that would release that much acid or what buffering agent would break down to produce this result. While there is some loss of water through evaporation, and perhaps some additional loss due to staling, I don’t have a sense of the mechanism.

Does anybody have an idea what is going on?

The formulation was as follows:

680g of high gluten white flour + 378g of water @ 140°F (mixing bowl was preheated with 172°F water), 472g of starter @ 86°F (composed of 24g of mother all-white starter, 228g water, 120g KA white whole wheat, 108g high gluten white, 20g granulated fructose, fermented for 9 hrs 2 86°F); autolyse 20 min in 100°F combi oven at low fan speed, 16g salt, mix 4 min (final dough temperature – 94°F); bulk ferment 1:30 in 100° combi oven; divide into 6 parts, rest 15 min, shape into demi-baguettes; retard @ 60F for 4:20; return to counter at 80°F for 2:00, bake with steam.

The testing procedure was to measure 15g of crumb (15.038g for the first measurement and 15.015g for the second test), add 100g of distilled water, blend with immersion blender and rinse the residual slurry on the blender with 5ml of distilled water back into the container. The pH was monitored with an ISFET probe using an IQ Scientific IQ150 digital pH/mV/temperature meter after a two-point calibration using laboratory standard calibration buffers of 4.01 and 7.00 pH. The mixture was titrated with 0.1N NaOH to an indicated pH of ~6.70. The probe was again checked against the calibration buffers to measure offset drift during the titration process and the last two measurements interpolated using the post calibration values to an end pH of 6.60. Total testing error is estimated to be less than 0.1 ml of 0.1N NaOH.

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