Though most flour comes from milled wheat grains, flour also refers to ground powder from rye, barley, rice, nuts, pulses or even root vegetables. For that reason, and because of different additives, flour pH levels vary. The pH value of a substance refers to its level of alkalinity or acidity. The lower the pH number, the more acidic it is. Higher pH means greater alkalinity. A neutral pH equals 7.0. In flour, different pH measurements may result in different tastes.
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Wheat flour usually has a pH between 6.0 and 6.8, according to the authors of "Pearson's Chemical Analysis of Foods." That makes most flour slightly acidic, but close to neutral in terms of pH. White flour is often bleached using chlorine. Chlorine is very alkaline. Bleached flours therefore usually have higher pH numbers than unbleached varieties and may have a slightly more bitter taste.
Though many modern, bleached flours with additives keep fresh for a long period, some unbleached raw flour turns more acidic over time. In the book "Experimental Cookery From the Chemical and Physical Standpoint," author Belle Lowe lists the average pH of fresh flour as 6.12. However, as the flour matures it drops in pH, right down to 5.29 at its most mature point. Some bakers believe that this positively affects baking taste. For this reason, bakers use this type of flour to create sourdough.
Extra ingredients also play a role in the pH of flour. For example, the flour used to make the flat bread lavash may contain added baking soda, or bicarbonate of soda. Baking soda is alkaline, with a pH as high as 12.0. This raises the pH of the lavash flour, making it a more alkaline bread. Other bread and flour varieties, such as sourdough or macademia flour, have varying pH levels.
The addition of yeast and water to flour causes the pH to drop as the yeast release acids during fermentation. Contaminated flour that contains some live yeast or other bacteria therefore may have a lower pH because of the extra biological activity. In general, you should avoid using flours with pH levels below 5.5 in case of contamination, though obviously it's not always possible to perform a check.