Bread is typically made by first dissolving preserved yeast (a microscopic biological organism that consumes sugars and emits CO₂ as a waste product) in water, then adding other ingredients, including flour, sugar, fat (usually butter or shortening), and salt. After the ingredients are combined, the dough is "kneaded," or mixed to promote the formation of a protein network from two proteins (gliadin and glutenin) present in wheat flour. This network is what strengthens the dough and allows it to stretch elastically without breaking. The dough is then allowed to rise in a process called "proofing," in which the yeast consumes sugar and releases CO₂, which inflates air pockets in the dough that are subsequently filled with air. Finally the dough is baked; the gas pockets expand due to the temperature rise and evaporation of water, the starches from the flour are dehydrated (dried), and the yeast dies.

A good French bread has an open, porous structure. The pores must be stabilized by the protein network until the bread is dried sufficiently to hold its shape. The bread collapses if the protein network fails prematurely.

(a) Rouille et al. investigated the influence of ingredients and mixing conditions on the quality of frozen French bread dough. Each loaf was initially formed roughly as a cylinder with a mass of 150 g (including essentially no CO₂), a diameter of 2.0 cm, and a length of 25.0 cm. Determine the specific volume of a bread dough proofed for two hours at 28°C from which 1:20 cm³ gas/min per 100 g dough evolves as bubbles within the dough. State your assumptions.

(b) During proofing, the increases in volume of a series of control loaves were monitored along with the mass of CO₂ evolved. Rupture of the protein network during proofing can be detected when the volume of the dough no longer increases at the same rate as the production of CO₂ from the yeast. Data from one of these experiments are shown in the table below. Plot the specific volumes of CO₂ (per 100 g dough) and dough as a function of time. If the preferred proofing time is such that the dough achieves 70% of its total volume before collapse, specify the proper proofing time for this formula.

(c) The referenced study found that the parameter with the most significant influence on dough quality was mixing time, with an extended mixing time producing a stronger protein network.

Why might extended mixing times not be desirable in commercial production of bread?

(d) Suggest causes for the following undesirable bread-baking outcomes: (i) a flat, dense loaf; (ii) an overly large loaf.

(e) Suggest why the period during which the dough rises is called "proofing." Remember that yeast is a biological organism.

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20 t(min) AV(cm³ dough) gas evolved (g CO2) 0.0 37.2 63.2 68.8 126.3 192.7 234.8 315.8 385.4 515.0 578.1 657.5 | 745.0 120 40 60 80 100 140 160 180 200 220 240 322 334 155 247 305 20 60 80 0|0 336 115 198