Figure 6 Effect of proofins temperature on fermentation rate. 0% sucrose, standard yeast 3%.

two ions per molecule and the osmotic pressure can be twice that of a glucose solution also containing 1 mmole/L (180 mg/L).

In a dough, osmotic pressure mainly depends on the proportions of sugar and salt used. It can be very high, as much as 35 to 100 atmospheres. This explains the slow-down in the activity of yeast in which metabolism cannot take place normally because water has leaked outside the cell and fewer fermentable sugars can enter. In bread-making, sugar, often added with fat, is responsible for a reduction in dough consistency. It then becomes necessary to decrease dough hydration, which accentuates the concentration phenomenon in the medium. The baker compensates for the drop in fermentative activity by increasing dough and fermentation temperatures, reducing the quantity of salt and increasing the amount of yeast. The Rheofermentometer shows the fermentation rate of yeast with different levels of sucrose, or salt (with or without calcium propionate added in the dough) (Figs. 7,8). For a standard French yeast, there is a low level of sugar stimulating fermentation and prolonging it; from 5% of sugar upwards, the rate of fermentation is slowed down and tends to decrease. The rapid accumulation of alcohol in the medium, to which the yeast is very sensitive, could be the reason for this. The higher the fermentation temperature, the more obvious this phenomenon is.

There are some bread-making processes in which the concentration of sugars is such that the methods described above cannot guarantee the dough rising within a reasonable time using ordinary yeasts. This has led to the development of osmotolerant strains (Fig. 9) which are used in France (Vendee brioche); the United States of America (Danish pastries, sweet doughs, Hawaiian sweet bread), Indonesia (roti mani), Japan (kashipan) and so forth, where the levels of sugar vary from 20 to 50% based on flour weight. The osmotolerance of these strains is associated with various characteristics, including:

A low level of invertase so that sucrose is gradually converted to glucose and fructose, resulting in fewer dissolved molecules at a given moment. Obviously, this ability does not apply to sugars such as glucose/fructose syrups.

0 30 60 90 120 150 180 210 240 270


0 30 60 90 120 150 180 210 240 270


Fiaure 7 Effect of sucrose dosage on fermentation rate. Standard yeast 3%. T = 27°C.

Figure 8 Effect of salt and calcium propionate on fermentation rate. 0% sucrose, standard strain 3%. T = 27°C.

The capacity to synthesize molecules such as glycerol or trehalose, so that the exit of water from a cell is counterbalanced when the concentration of the medium outside the cell is too high.

The propagation conditions on which, regardless of strain, the osmotolerance of a yeast depends.

3. Effect of Yeast Dosage

At a given temperature, the rate of fermentation depends on the quantity of yeast used (Fig. 10). We can see a depression in the curves in the first hour because the sugars already present are being used up more quickly.

If no sugar is added, the quantity of maltose resulting from amylolysis is a limiting factor. When the amount of yeast is too high, a flour with low levels of damaged starch or

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