Chapter 5, Problem 5.31P

Bread is typically made by ?rst dissolving preserved yeast (a microscopic biological organism that consumes sugars and emits CO₂ as a waste product) in water, then adding other ingredients, including ?our, 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)15 present in wheat ?our. 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 "proo?ng," in which the yeast consumes sugar and releases CO₂, which in?ates air pockets in the dough that are subsequently ?lled with air. Finally the dough is baked; the gas pockets expand due to the temperature rise and evaporation of water, the starches from the ?our 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 suf?ciently to hold its shape. The bread collapses if the protein network fails prematurely.

(a) Rouille et al.16 investigated the in?uence 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 25.0cm. Determine the speci?c 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 proo?ng, 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 proo?ng 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 speci?c volumes of CO₂ (per 100g dough) and dough as a function of time. If the preferred proo?ng time is such that the dough achieves 70% of its total volume before collapse, specify the proper proo?ng time for this formula.

(c) The referenced study found that the parameter with the most signi?cant in?uence 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 ?at, dense loaf; (ii) an overly large loaf.

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

t(min)	0	20	40	60	80	100	120	140	160	180	200	220	240
cm3 dough)	0	0	20	60	80	115	155	198	247	305	322	334	336
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