The Alveograph uses air pressure to inflate a thin sheet of dough, simulating the carbon dioxide bubbles that are present in bread dough, that cause dough to stretch when rising.
The Alveograph indicates how pliable a dough a flour can make, by measuring how much pressure and how much time is necessary to cause and burst a large air bubble in the dough.
The measurements are not greatly used in North America, though they are routinely done in Europe.
To use an Alveograph, a set amount of dough is mixed. The standard amount of flour usually used is 250 g (½ pound.) A liquid consisting of 97.5% water and 2.5% sodium chloride is added (how much liquid is added will depend on how moist or dry the flour used was.) The flour and water are mixed for 8 minutes, then the machine extrudes the dough in small sheets. The dough is allowed to rest, then it’s moved to the Alveograph, which then inflates the dough until it bursts, and measures the point at which it burst.
Different values are extracted from the analysis of the bursting point. To simplify the values:
- P — stands for maximum pressure that was withstood. Shows how strong the dough was;
- L — stands for the height of the bubble that was achieved, measured from where the slope of the bubble started to the top of the bubble. Shows how flexible the dough was;
- P/L — for the ratio of P to L;
- G — stands for the square root of the volume of air in the bubble that was possible before the bubble burst. Shows how flexible the dough was;
- W — stands for the energy needed to make the bubble as big as it could go before it burst. Shows the strength of the flour.
The analysis is done by computer.
W Values of flours
W is the factor most widely used, quoted and paid attention to. You will see it referred to as the “W value” of a flour.
The W factor is almost never shown on flour bags, even in Europe. It is only shown on the very large bags (65 pounds / 30 kg and up) sold through wholesalers. Consequently, despite Alveograph measurements being done in Europe, Europeans have to rely, as North Americans do, on the indicated protein content of the flour, which will be shown on the bags, as an indication of strength.
- < 130 — the flour is not useful at all for bread;
- < 170 — the flour will absorb water equivalent to about 50% of its weight. Good for biscuits, small pastries;
- 190 to 220 — weak flour;
- 180 to 260 — the flour will absorb water equivalent up to 65% of its weight . Good for pizza and similar doughs;
- 180 to 320 — good for rapid rising;
- 230 to 290 — normal strength flour;
- 280 to 350 — the flour will absorb water equivalent up to 75% of its weight. Good for brioche and pastries raised with yeast;
- > 300 — strong flour, ideal for bread;
- > 350 — the flour will absorb water equivalent up to 90% of its weight. In Italy, flour of such strength is usually North American wheat-based. This flour is usually mixed with other, less-strong flour for bread, or used to reinforce other flour as needed;
- 300 to 370 — good for normal length of time allotted for rising;
- 380 to 450 — good for long rising times (e.g. good for breads made from sourdough starters, bigs or poolishes.)
The Alveograph was developed in France in 1920 by a man named Marcel Chopin. He called his invention an “extensimeter.”