Hardening index

It is convenient to introduce a quantity A, which will be called the hardening index, defined by... 

The hardening index /I is a nondimensional scalar which has the same value... 

A similar argument leads to the result that, in stress spaee, the normal veloeity of the elastie limit surfaee is given by = fjn where is the magnitude of the normal veetor = d//ds. Consequently, = R(vJvJ where R = njn is a positive sealar, and the hardening index A has the same sign as the ratio of the outward normal veloeities of the elastic limit surfaees in stress spaee and strain spaee, respeetively. 

Another expression for the hardening index A is obtained by inserting the stress rate relation in the form (5.4) into the expression for the loading func-... 

The magnitude of the derivative is called the strain hardening index. A good correlation is observed between its value (between 0.5 and 2.5 in most doughs) and the strain at rupture. There is also a good correlation between the index and the gas holding capacity of the bread, since early coalescence of the cells leads to loss of gas to the atmosphere. 

Figure 7.4 Stress-strain relationships for dough of good and poor bread-making flours (left) and the relationship between strain hardening index and baked loaf volume (right). (Data from Dobraszczyk, B. J. 1999. In Bubbles in food, ed. G. M. Campbell, C. Webb, and S. S. Pandiella, 173-182. St. Paul, MN AACC International.)...

The higher the value of n, the greater the strain hardening will be and the more likely it is that the bubble walls will be stable. This is reflected in the higher baked volumes for the flour with higher strain hardening index (Figure 7.4). 

Riband. The strong relationship between failure strain and strain hardening index found by Dobraszczyk and Roberts (1994) is shown in Figure 7.6. 

Test-bake loaf volume and dough strain hardening index for two base flours with 1% addition of gluten protein fractions were plotted as a function of the percentage of the total polymeric protein that had been extracted up to each point. The graphs are shown in Figure 7.7 Two... 

Figure 7.7 Strain hardening index and bake-test loaf volume as a function of the percentage of polymeric protein extracted from flour of variety Jagger. (Sroan, B. S. 2007. Ph.D. thesis, Kansas State University, Manhattan, KS.)...

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