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Riboflavin (B2) Pyridoxine (B6) Niacin (PP)

Vitamins of which Thiamin (B1)

Riboflavin (B2) Pyridoxine (B6) Niacin (PP)

The minerals—including phosphorus which is essential as it is involved in the formation of nucleic acids, molecules with a high energy potential (ATP), and membrane phospholipids.

Due to high levels of proteins in deactivated yeast, with its presence of all the essential amino acids in addition to phospholipids, minerals and vitamins, it is regarded as a top-quality food supplement.

III. BEHAVIOR OF YEAST DURING BREAD FERMENTATION A. The Roles of Yeast in Bread-Making

1. Dough Rising

This is the most obvious phenomenon for anyone who is unfamiliar with bread manufacture. Air is incorporated into dough during mixing and the yeast is able to establish respiratory-type metabolism. A few minutes after mixing, all the oxygen that has been introduced is used up by the yeast. Consequently, due to the anaerobic conditions, yeast metabolism is geared towards fermentation. The carbon dioxide produced firstly dissolves in the free water in the dough. When it reaches saturation point, it accumulates in gaseous form, exerting internal pressure on the impermeable gluten network. The latter, which is elastic and extensible, enables the dough to rise, while the external structure is maintained. Contrary to what is generally thought, the pores in the crumb of bread have nothing to do with the distribution of yeast cells but correspond to the dilatation of the CO2 they produce. This diffuses into the air bubbles, which are incorporated and dispersed in the dough during mixing and the various mechanical operations.

2. Acidification

The formation of carbon dioxide and organic acids results in a lowering of pH and an increase in the total titratable acidity (TTA) of the dough during fermentation, in spite of the high buffer capacity of proteins in the flour. This acidification confirms that bread fermentation is working properly. It is often measured (pH and TTA) in routine checks in industrial bakeries using prefermentation processes as water brew, stiff, or liquid sponge.

3. Flavor Production

The alcohol formed, the lowering of pH, and the release of metabolites from secondary fermentation are directly involved or act as precursors in the development of bread taste and flavor (7-9). Long fermentation times, slightly low dough temperatures, and sensible quantities of yeast result in a bread with excellent organoleptic properties. This is on condition, of course, that the dough has not been subjected to excessive oxidation which is the result of high-speed mixing and the presence of bean or soya flour.

Particular genuses and species of yeast have metabolisms that result in bread products with very characteristic flavors.

4. Change in Dough Rheology

Apart from the physical changes dough undergoes during the various operations of mixing, dividing, rolling, or molding, its viscoelastic properties are transformed throughout the fermentation process. This is a very familiar phenomenon to the craft baker, for whom the effects of bread fermentation do not simply mean the inflation of dough (10). The dough strengthening to which bakers refer, and which is occurring during bulk fermentation, is a perfect illustration of this. It involves a reduction in gluten extensibility combined with an increase in its elastic resistance (Fig. 3).

Extensibility

Unyeasted dough Dough with 3% yeast

Figure 3 Effect of fermentation on dough strengthening.

The causes are known but it is difficult to find a scientific explanation for the mechanisms because of the complexity of the dough system. It involves:

First, a purely mechanical effect that is the result of gluten development; namely, its extension and organization into a three-dimensional network under gaseous pressure;

Second, the formation of physicochemical bonds, strengthening the cohesion of the gluten network. The lowering of pH, reactions with the different metabolites produced by secondary fermentation, and variations in surface tension between the different dough phases seem to play a part in these phenomena. According to K. Hoseney, the phenomenon is essentially oxidative, involving the production of oxygen peroxide by the yeast (11).

Understanding the notion of dough strength is complicated by certain factors that can cause confusion, particularly in long fermentation processes. After a long first fermentation, or the fermentation of a sponge, the dough softens, which seems to go against this idea of strength. This effect, which is real, is heightened by the under-mixing inherent in this type of dough. In fact, it is linked to the gradual hydration of gluten, and to the action of enzymes in the flour: the amylolytic activity, which contributes to the release of water previously fixed by the starch, a slight proteolysis affecting the gluten; and possibly also the reducing effect of the glutathione excreted by some types of dried yeast.

These changes in no way preclude the strengthening of dough elasticity, which takes place in the gluten and is evident during the molding operations. Too much strength will affect the external and internal characteristics of the finished product, so the baker's skill lies in knowing how to control fermentation processes to fit the characteristics of the flours and the type of mixing used. This skill is somewhat restricted by high-speed mixing methods and the use of dough conditioners. The fact remains that in reality the technology is complex, making it difficult for bakers and technicians or engineers in the world ofindustry or applied research to communicate. The only way forward is for the latter to ''get their hands dirty'' if they wish to establish a fruitful dialogue based on mutual trust.

B. Use of Fermentable Substrates: The Enzymes Involved (12)

Monosaccharides, simple sugars with six carbon atoms (such as glucose, fructose, and galactose), are preferentially used by S. cerevisiae. Nevertheless, the assimilation of galactose depends on the concentration of glucose, with the latter exerting catabolic repression. It is generally acknowledged that glucose and fructose can penetrate the cell by facilitated diffusion, involving phosphorylation.

Disaccharides can be assimilated after enzymatic hydrolyses.

The sucrose already present in flour or added to the ingredients is converted to glucose and fructose by invertase in the yeast. This reaction takes place in the periplasmic space between the wall and the cytoplasmic membrane. The two hexoses then diffuse into the cytoplasm where they are metabolized. Invertase acts very quickly, practically doubling the osmotic pressure in the region next to the cell. Maltose, which mainly comes from the conversion of starch by action of the a-alpha and h-beta amylases in flour, is split into two glucose molecules by maltase, an enzyme in yeast cells. However, the maltose must have previously been carried inside the cell by maltopermease.

Not all strains of yeast have the same ability to ferment maltose. Strains used in the USA and Japan have adaptive maltopermease and maltase. The synthesis of these enzymes is catabolically repressed by glucose and induced by maltose, so the cell can only produce the two enzymes that enable it to use the maltose present in the medium after the glucose has been exhausted (13,14).

In Europe, most industrial baker's yeasts have constituent maltopermease and maltase. These strains were developed in Great Britain in the early 1960s for the Chorleywood Bread Process. They were then used on the continent where high-speed mixing was catching on. These types of strains were able to rapidly adapt to maltose in bread-making processes with no added sugar, because mechanical dough development replaced first fermentation in straight doughs, or sponge in sponge and dough processes. The risk of exhausting the fermentable sugars in the dough was thus to be eliminated by systematically supplementing flours with malt or a-amylases.

There are also small quantities of tri- and polysaccharides, which are fermentable to varying degrees.

C. Fermentative Profiles

This ability to ferment maltose is easily demonstrated by the Rheofermentometer, an equipment for recording the variations in a yeast's rate of fermentation as a function of time (for example, CO2 released per minute). The dough used must have no sugar added to the flour (so-called normal dough).

Depending on the quantity of yeast, its fermentative capacity, or the fermentation temperature, a ''depression of adaptation to maltose'' varying in size can be seen on the curve. This depression in fermentation speed is linked to the exhaustion of directly fermentable sugars already present. If the yeast contains constituent maltase and malto-permease, the depression is much less pronounced, or nonexistent, depending on the above-mentioned conditions.

If there is added sugar (sucrose, glucose/fructose syrup), the depression disappears as yeast preferentially uses these substrates (Figs. 4 and 5).

D. Influence of Various Factors on Fermentative Activity

For a strain with a given biochemical composition, the conditions of the medium affect the rate of fermentation of yeast. The effects must be taken into account in baking, not only for the sake of productivity and economics but also for technological reasons. For example, there are the problems of variations in dough density when passed through a volumetric divider, or variations in dough strengthening between the start and end of molding (the larger the dough mass being divided, the greater the difficulty).

Although numerous factors have a bearing on the fermentative activity of yeast, we should remember that the baker judges this activity by the rising of the dough. This is the result of the force exerted by the increase in internal pressure (impermeability + CO2 production) and the resistance of the dough to deformation. Extensibility, elastic resistance, permeability, viscosity of dough (cold or warm, soft or firm) are so many parameters that

minutes

Figure 4 Strains: fermenting profiles. 0% sucrose, yeast dosage 3%, T = 27°C.

minutes

Figure 4 Strains: fermenting profiles. 0% sucrose, yeast dosage 3%, T = 27°C.

affect the rate of dough rising, regardless of the activity of the yeast itself. This is clearly demonstrated by observing a dough piece as it rises in a mold, in a fermentation chamber at 43°C and 85% relative humidity. The dough ''moves'' slowly at first, but its volume grows more and more quickly. Over a final fermentation period of 1 hour, the last 5 minutes are crucial as, in that time, there is a spectacular increase in the height of dough in the mold. This is related, among other things, to a reduction in internal pressure as the dough swells, as can be seen on the alveograph curve.

45 60

minutes

Figure 5 Fermenting profiles of strains A and B. 7% sucrose, yeast dosage 3%, T = 43°C.

45 60

minutes

Figure 5 Fermenting profiles of strains A and B. 7% sucrose, yeast dosage 3%, T = 43°C.

Enzyme activity in the yeast depends on the temperature of the medium. Within a range of 20 to 40 °C, an increase in dough temperature of jC results in an increase in fermentation speed of 8-12% according to the type of yeast (Fig. 6). In production conditions, therefore, it is vital to check and control dough temperatures very accurately at the end of mixing, and particularly in the laboratory when comparative studies are being carried out.

This explains the variations of dough and fermentation temperatures required for various applications. Cooler doughs are needed for baguette production (22-24°C) in order to restrict dough strength, which would hinder molding, and to encourage certain flavors to develop. When working with raw frozen doughs, temperatures of between 18 and 20j C restrict the startup of yeast activity. At 4°C, fermentation is held up which retarded dough process to take place. Final fermentation temperatures of 30-35°C are used for tinned sandwich bread in France, but the general average in the United States is 48 °C, which increases productivity but at the expense of the overall quality of the finished product. In the United Kingdom, fermentation temperatures of 55 °C for soft bun manufacture are quite common, and can be as much as 75°C in the case of industrially produced pizza crusts, all at relative humidity of 95%. It should be noted that yeast is very quickly killed when the internal dough temperature exceeds 55°C.

2. Effect of Osmotic Pressure

The semipermeable cytoplasmic membrane of the cell delimits two compartments. When there is a difference in concentration between the medium inside and outside the cell, water gradually moves from the less concentrated medium to the more concentrated one so that equilibrium can be established. This transfer is influenced by a force that exerts a virtual pressure linked to the semipermeability of the membrane. This movement of water can be prevented by exerting hydrostatic pressure. The difference in pressure between the two media defines the osmotic pressure. This is in proportion to the number of dissolved particles per unit of volume. A solution containing 1 mmole/L of sodium chloride (58 mg/L) will release

|-p-Proofing temperature 27 °C -■— Proofing temperature 43 QC |

|-p-Proofing temperature 27 °C -■— Proofing temperature 43 QC |

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