Week 7: Pentosans, amylase, acid

The week 7 lecture — why rye cannot build a gluten network, what holds the loaf up instead, and what the acid in a rye sour is actually for

5 April 2027

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Rye flour contains prolamins and glutelins, the same two protein classes that make gluten in wheat. Wet it and mix it and you get no gluten. The reason is a group of carbohydrates called pentosans, or arabinoxylans: 6 to 8% of rye flour by weight against 2 to 3% in wheat, and able to hold something like ten times their own weight in water. They hydrate first, they hydrate fast, and they leave the proteins short of water and physically in the way of each other. The protein chains never get close enough, or mobile enough, to polymerise into a continuous network.

So the structure of a rye loaf is made of something else: a viscous gel of hydrated pentosans, plus starch that gelatinises during the bake and sets around it. Both of those are consequences of heat and water, not of work. This is why nothing you learned about developing a dough transfers. There is no windowpane because there is no film to pull. Kneading does not build strength, it only tears the gel and smears the starch. An 80% hydration rye is a paste, and it is supposed to be.

That leaves a problem the gluten network was quietly solving for you. Rye carries far more active alpha-amylase than wheat does, and rye starch begins gelatinising near 55 °C, a few degrees below the point at which wheat’s sets the crumb of a week 1 loaf. Put those together and there is a window during the bake in which the starch has opened up and become digestible while the enzyme is still alive and working. In that window amylase cuts starch into dextrins — short, sweet, water-holding fragments that never set. The loaf bakes, browns, sounds right, and has a sticky unset core that a knife will not get through cleanly. It is not underbaked. It has been eaten from the inside.

Acid closes that window from both ends. Dropping the dough to around pH 4.3 suppresses alpha-amylase sharply, and it raises the temperature at which rye starch gelatinises, so the starch becomes available later and to a much weaker enzyme. That is the whole argument for a rye sour, and it is why the sour is not optional the way a wheat levain is. In week 6 you chose a starter for flavour and timing; here you are choosing it to stop the loaf destroying itself. Commercial bakers who skip the sour acidify with lactic acid or a dose of acidified dry sour instead — same mechanism, bought in a bag.

The rest of the method follows from the first two paragraphs. A sour of 20 to 40% of the total flour, held at 26–28 °C for 12 to 16 hours, is what reliably gets the whole dough acid enough. A paste is handled with wet tools and put in a tin because there is nothing to shape. A dense, wet crumb that conducts heat poorly needs an hour or more in the oven. And the crumb finishes setting well after the loaf leaves it, as the starch gel retrogrades and binds the water still loose in it, which is why the 24-hour wait before cutting is part of the bake rather than good manners.

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