Sourdoughs

A number of different sourdoughs and methods of fermenting them have been described in the literature (6,27-33). Common to all sourdoughs is the proliferation of lactic acid bacteria that accompanies the formation of organic acids, predominately lactic acid and acetic acid. The properties of the respective sourdoughs are the result of that metabolic activity, which in turn can be influenced by production conditions.

The way in which microorganisms and the process variables of sourdough fermentation affect the principal properties of sourdough and the subsequent bread quality are illustrated in Fig. 4. The figure shows that the properties of the sourdough are not only affected by the type and number of species of microorganisms but also by the total number of microorganisms and their phases of development in the sourdough. The effect of the microorganisms develops as a result of the metabolic activity, which is coupled with a considerable degree of proliferation. The formation of metabolites is therefore a direct function of the number of microorganisms. Lactic acid bacteria predominate, both as far as the number of species and their total number of metabolic active cells are concerned, although various species of yeast also occur in naturally fermented sourdoughs.

The umbrella term ''microorganism flora'' is used to denote the different species of microorganisms present in sourdough. In naturally fermented sourdoughs, the development of the composition of the microorganism flora depends on the type of microorganisms with which the substrates are contaminated. The result is a biotope in which the composition of species is in a steady state determined by the generation time of the microorganisms. The generation time depends on the process variables used to produce the sourdough.

The principal process variables are fermentation temperature and time, flour-to-water ratio, and type of flour. The fermentation time is the most important process variable because it directly determines the extent of the metabolic performance of the microorganisms. It therefore governs how sourdough fermentation is integrated into the overall bread-making process. The other process variables mentioned are further mutually dependent parameters that control the development of the properties of the sourdough (e.g., acidity, lactic to acetic acid ratio, viscosity, density).

Substrates, which consist essentially of milled products and water, are converted to sourdough primarily as a result of the reduction in the pH following the formation of organic acids and the simultaneous formation of carbon dioxide. Endogenous enzymes in

Figure 4 Influence of microorganisms and process variables of sourdough fermentation on bread production.

the milled product are either activated or inactivated during the conversion of the substrate, resulting in the degradation of polymer substances, in particular of nonstarch carbohydrates (e.g., pentosans). Some amino acids and peptides are also released. Some of the products of degradation are digested by the microorganisms; others act as precursors of flavor components (34).

The enzymic degradation of polymer carbohydrates in conjunction with the change in the density of the dough owing to the formation of carbon dioxide results in a marked reduction in the viscosity of the sourdough. As metabolic performance and the accompanying substrate conversion depend on the proliferation of microorganisms, sourdough production generally takes longer than all the other process steps required to make common bread varieties. Production of the well-known San Francisco sourdough bread is an exception, the length of time it takes to produce the San Francisco sourdough corresponding roughly to the time that the dough takes to rise after it has been divided into loaves (35,36).

It is crucial that the properties of the sourdough are transferred to the bread dough and fulfil their functions until the baking process has been concluded. For example, when making bread from rye milled products it is important to reduce pH (below pH 4.6) of the bread dough by adding sourdough in order to inactivate the a-amylase and thus prevent excessive liquefaction of the leavened dough piece as a result of starch degradation during the early stages of heat transfer in the baking process. In addition, the bread dough absorbs more water owing to the extensive swelling of the polymer substances in the sourdough, resulting in an increase in dough water content, with the effect that bread baked from these doughs exhibits an improved freshness and a retarded staling (6). Finally, it should be mentioned that more precursors of flavor components are formed in sourdoughs that are fermented over a long period of time (8 hr) than in those that are produced over a shorter period (2-4 hr) (37).

A. Principles of Sourdough Preparation

This survey of the functions of sourdoughs raises the question as to which fundamental criteria should be applied when adapting sourdough preparation to modern methods of producing baked goods. As already mentioned, the main criterion for the modern-day production of baked goods, whether in small or large bakeries, is how to increase productivity. The greatest advances in productivity have been achieved by mechanizing production processes which requires optimization of product flow schedules during each process step. For sourdoughs, optimization is limited by the laws of microbiology that govern the proliferation of microorganisms and the accompanying metabolic performance.

In bread-making, the growth phases and metabolic performance of the microorganisms are a quantity that can be calculated in a similar way to manpower, which is subject to legislation regarding working hours. This applies, for example, to the way in which working hours are organized, whether in a single shift or several daily shifts. Thus, three shifts per day, 6 days a week, permit uninterrupted production during that period. Sourdough production, being governed by the laws of microbiology, can be fitted into uninterrupted production processes far more easily than into production rhythms comprising only a single shift per day over the same period (38).

However, the aim of both types of production is to make bread of a consistent quality day by day in order to ensure success on the market. This presupposes that the quality of each of the raw materials used does not change and that all sections of the production plant operate uniformly. Achieving this aim is rendered more difficult by the chosen range of bakery products and the subsequent raw material requirement and use of plant. The oper ation of small and plant bakeries alike therefore places especially great demands on both internal and external logistics and the data processing required.

The production of sourdough plays an important part in the system of logistics as its use is crucial to the quality of the bread. It must also be borne in mind that a constant bread quality can be achieved only if the sourdough, as one of the main ingredients in recipes, also has uniform quality characteristics. It is therefore necessary to ensure that the most important properties of the ripened sourdough remain constant. Those properties are essentially a constant leavening capacity, characterized by a constant pH and acidity, and a constant flavor, which depends in turn on constant lactic acid and acetic acid contents. In order to maintain these properties, which constitute the quality characteristics of ripened sourdough, the process variables must be controlled in such a way that reliability of production is ensured.

Both in small bakeries and in plants it used to be possible to ensure constant leavening capacity and flavor by employing methods that had been developed by bakers specifically with that purpose in mind. However, when it came to increasing productivity, these methods were shown to lack the flexibility required if the use of manpower or the product flow were to be rationalized. The crucial breakthrough as regards technical advances in the production and use of sourdough did not come until it was realized that it is possible to produce sourdoughs on the basis of high dough yields (200%).

The main difference between modern and traditional methods, such as the method of producing San Francisco sourdough (wheat sourdough) or German three-stage sourdough (rye sourdough) is thus the viscosity. Unlike classic high-viscosity sourdoughs, low-viscosity sourdoughs with high dough yields are pumpable, which is a considerable advantage as regards technological developments (Fig. 5). This paved the way for the continuous production of sourdough and for storing sourdough that is ready for use in both daily and weekly production rhythms.

Figure 5 Distinction of sourdoughs by their viscous properties.

B. Principles of Continuous Sourdough Fermentation

The principle of the most advanced continuous sourdough fermentation consists of pumping a dough through a fermenter in plug-flow pattern (Fig. 6). This enables a gradient to be established for the changes that occur in the sourdough between fermenter inlet and outlet, the magnitude of which results from the residence time of the sourdough in the fermenter. Continuous sourdough fermentation requires the gradient to be constant with respect to the number of metabolic active cells. The bacteria count must be high enough to permit a degree of acid formation adequate for the intended purpose of the sourdough (38).

During sourdough fermentation, the gradient is established by feeding a portion of the fermenting dough taken from the outlet back into the inlet of the fermenter. This dough serves as an inoculum that is mixed with fresh sourdough to maintain a constant mass flow. The condition for upholding continuous fermentation requires that with a constant bacteria count at the outlet of the fermenter, the gradient or the ratio between the respective bacteria counts at the inlet and outlet of the fermenter remains constant. The residence time of the sourdough in the fermenter depends on the generation time resulting from the fermentation conditions. Consequently, the generation time determines the productivity of the fermenter, which is defined as the mass of sourdough that can be taken from the fermenter within a unit of time. Assuming a constant generation time, the maximum productivity of the fermenter is equal to half the mass flow through it. The mass flow of sourdough that can be withdrawn from the fermenter decreases as the ratio of the inoculum to the total amount of dough diminishes.

Figure 6 Principle of the tube fermenter.

Sourdough requires high acid formation by lactic acid bacteria. This is possible only if there is a high bacteria count. When the bacteria count is high, acid formation follows the development of the bacteria count (38,39). For the operation of the fermenter it is of considerable practical advantage that proliferation is self-regulating in its transition phase. This can be demonstrated theoretically on the basis of a logistical proliferation model (40).

In this model (Fig. 7), the theoretical development of the proliferation of a lactic acid bacteria population is presented for different initial bacteria counts in the area of the transition phase, assuming constant values for the generation time and the maximum attainable count (39). For example, a relatively high initial population can no longer double within a defined period. With an initial inoculum to total dough ratio of 1:1, the bacteria count is halved for each new sourdough set. The new set has a smaller initial population than its predecessor. Due to the smaller initial population, the microorganisms multiply more rapidly during the fermentation time so that the population doubles. As long as the residence time is the same as the shortest generation time in the exponential proliferation phase, the culture cannot be washed out of the sourdough in continuous operation of the fermenter.

It was this invention that provided the crucial breakthrough for all subsequent methods of continuous sourdough fermentation based on the same principle. The first system to

Figure 7 Theoretical consideration of the influence of the initial bacteria count on the proliferation of the lactic acid bacteria.

be based on it was the Ankerbrot-Reimelt system (ARS) (41,42) which was developed to produce large quantities (20 tons/day) of sourdough. The system comprises a fermentation tube and fermentation tank. The principal function of the fermentation tank is to multiply the sourdough.

The ARS has two specific advantages. First, sourdough can be cooled in the fermentation tube to bridge interruptions in bread production. Second, a short period of time (less than 4 hr) is required to prepare a large mass flow of sourdough for breadmaking. The ARS is well suited to operations with a large sourdough requirement but with only small variations in production and a small assortment of bread.

However, these are also disadvantages. The system cannot handle variations in daily and weekly production rhythms, and a variety of sourdoughs cannot be prepared simultaneously. Therefore, a new course had to be followed to meet these requirements, which led to the development of the Paech-TUB-Reimelt system (PTRS). PTRS is based on the principle of combining continuous and batch-wise sourdough fermentation to vary production in response to hourly, daily, or weekly production rhythms (43).

The system consists essentially of two fermenters, between which an insulated tank is located (Fig. 8). A heat exchanger is situated downstream of the first fermenter (43). This fermenter is a narrow, cylindrical, insulated tank through which sourdough flows continuously. The tank downstream of this fermenter is also cylindrical but has a considerably larger diameter. This tank functions as a storage vessel that is filled from above and emptied from below. The tank is fitted with a stirrer to facilitate the withdrawal of the cooled dough, which does not flow as easily as freshly fermented dough owing to its relatively higher density and higher viscosity. The second fermenter is constructed as a segmented tank, the segments of which are also filled from above and emptied from below.

Continuous Fermentation
Figure 8 Schematic diagram of the most advanced continuous sourdough fermenter (Paech-TUB-Reimelt system).

The advantage of the PTRS is that the first stage can be operated continuously over a long period of time. The sourdough in the storage tank can be used to meet the different requirements for the supply of sourdough. Different quantities can be taken quickly from the storage tank to prepare and propagate fresh sourdoughs. In the PTRS system, fermentation in the second stage is intermittent because the quantities of sourdough required for each type of dough are too small for continuous production to be profitable.

The system can also be operated with just the first continuous stage in which the continuously fermented sourdough, which is cooled as required, does not necessarily need to be propagated but instead can be used for making bread immediately. The advantages of propagation to produce a variety of sourdoughs are lost in this case. However, sourdoughs can be supplied from the storage tank in widely varying quantities at virtually any time. Just as so-called ''no-time'' wheat flour doughs can be transferred directly from the mixer to the divider, these ready-to-use sourdoughs may be referred to as''no-time'' sourdoughs. This is the second key breakthrough for the present-day preparation and use of sourdough because it ensures highly flexible bread production in terms of bread varieties and quantities per time unit.

The sole use of the first continuous stage has now become the established way of preparing sourdoughs. Although the advantage of being able to flexibly produce different types of sourdough is forfeited, many plant bakeries in practice mostly require only a single type of sourdough in day-to-day production, although it may be combined with other preliminary doughs as necessary to maintain a wide assortment of bread.

As regards the design of the plant, a fermentation tube has been substituted for the fermentation tank. This became necessary as the tube enables sourdough fermentation to be integrated into the bread-making process more readily than the tank. This is explained by the fact that the small cross-section of the fermentation tube enables the dough in the tube to be cooled rapidly and effectively by means of a cooling jacket in order to inactivate the metabolism of the microorganisms. The poor thermal conductivity of the frothy sourdough aside, the sourdough cannot be cooled as effectively in the tank owing to the latter's diameter, which is four times larger than that of the tube. Tanks must therefore be emptied completely and cleaned whenever operation is interrupted, but the tube can remain filled.

Although cooling the dough results in an increase in its density, as shown by the drop in the filling height over the cross-section of the tube, there are no disadvantages for restarting operation of the tube fermenter. The increase in the density of the dough due to cooling enables heat to be transferred within a short period of time when the operating temperature at which fermentation takes place has been reached so that the metabolic activity of the microorganisms is quickly set in motion. This results in a renewed volumetric expansion of the sourdough and in the cross-section of the tube being filled. In addition, the tube cross-section is rapidly refilled by sourdough being pumped into it, resulting in the required plug-flow. The tube fermenter is therefore fully functional as soon as the sourdough has been heated to its operating temperature.

This type of sourdough plant is suitable for fermenting both large (>10 tons/day) and small (<1 ton/day) quantities of sourdough. It has hitherto been designed mainly to achieve a high level of performance of the continuously operating tube fermenter owing to the relatively high level of investment required to integrate it into the bread-making process. Above all, the investment is worthwhile, considering the immediate availability of sourdough whatever the quantity required, the increased reliability of production, and the new possible realization of manpower. The resultant economic advantages of continuous sourdough production for bread-making are so great that they outweigh the cost of producing around 25% more sourdough than required for discontinuous three-stage sourdough production in troughs for the same quantity, of bread. As has already been explained, the larger quantity of sourdough is the result of the differences in the acidity of the two types of sourdough.

These advantages aside, one of the disadvantages of this type of continuous sourdough fermentation is the limitation of the final acidity (total, titratable acidity = TTA) in the cooled sourdough to a maximum of around 12 due to the fermentation time being linked to the generation time of the microorganisms. Thus, the leavening potential of the milled product used, which depends first and foremost on the latter's phytate content, is nowhere near exhausted. Owing to the low acidity of this type of sourdough, the proportion of flour that needs to be leavened when making bread dough is around 25% greater than that used to ferment a three-stage sourdough when compared with the same level of acidity in the final dough.

Another difference between the two types of dough is that the pH decreases more slowly during the third stage of the batch-type fermentation of the three-stage sourdough than during continuous sourdough production. This is due to the higher content of lactic and acetic acids (around 0.33%) in the inoculated continuously proliferated sourdough (42) as compared to the lower content of these acids in the inoculated third stage (around 0.16%) of the three-stage sourdough (23). In case of the continuously proliferated sourdough, the large quantity of inoculum (50% of the total sourdough) leads to the development of a microorganism biotope in the tube fermenter. The biotope remains stable for a long period of time (years) and is thus similar to a three-stage sourdough, which is started with a set-aside portion of the previous third stage. Consequently, the sourdough does not need to be reinoculated with a sourdough starter. In this respect, it is similar to three-stage sourdoughs, although it differs from these in ripening times, which may affect the aroma profile of the end products.

C. Batch-Type Production of ''No-Time'' Rye Sourdoughs

All classical methods of sourdough production were developed on the basis of a daily rhythm of bread production. As it is not possible to exhaust the buffering capacity of common rye flours within a 24-hr period in any type of sourdough production, it was never considered that it might be possible to reduce the proportions of sourdough required by bringing about the highest possible degree of acidity in the sourdough. Such thoughts are also inconsistent with the ripening regimens for rye sourdoughs, such as the regimen that has been developed to perfection for three-stage sourdough, for example, which focuses on the production of precursors of flavor components.

Figure 9 shows a three-stage sourdough fermentation method that was used in a bread factory in Berlin. It shows that it took more than 20 hr to ferment the sourdough needed for bread dough production. Bulk fermentation took place during this time, and the dough was proliferated in three stages. The batchwise preparation of sourdough, which requires many vessels in a large factory, is time-consuming and difficult to automate and integrate into computer-integrated manufacture.

It was not until mechanization of production processes and rationalization of manpower began to displace this type of sourdough production, particularly in small bakeries, that the possibility of raising the acidity of sourdough was considered. The invention of Isernhager rye sourdough provides an astoundingly simple solution, which has become an important breakthrough in sourdough production (Fig. 10). The Isernhager process focuses on producing a high acidity (TTA = 28-32) in liquid sourdoughs (dough yield: 200-250%). The idea behind the invention is to produce a high level of acidity by exploiting the buffering

* % dry matter of total dough dry matter ** % acids of total acids in the final dough

Figure 9 Course of a three-stage sourdough fermentation.

* % dry matter of total dough dry matter ** % acids of total acids in the final dough

Figure 9 Course of a three-stage sourdough fermentation.

Figure 10 Isernhager sourdough fermentation.

capacity of the substrate. Isernhager rye sourdoughs are produced in a fermentation tank fitted with a stirrer. The fermentation tank is not cooled. A siphon through which the sourdough is pumped out is fitted on the base of the fermentation tank, which can have a capacity of up to 2 m3 (without a cooler). The height of the siphon outlet determines the quantity of sourdough remaining in the tank, which serves as an inoculum when the fermentation tank is refilled each week. The maximum capacity of the fermentation tank and the final density of the sourdough determine the maximum quantity of sourdough available each week. The fermentation tank is designed in such a way that its surface releases the heat generated by fermentation into the environment (44). The sourdough is also stirred continuously to aid the cooling process. At the same time, the volumetric expansion of the sourdough that accompanies the formation of CO2 is limited. It takes around 48 hr for fermentation to be completed. The sourdough then has such a low pH (3.6-3.8) that the metabolic activity of the microorganisms is largely reduced to the basal metabolic rate (45). Sourdough in this form is microbiologically stable. It can therefore be stored for weekly breadmaking rhythms as its acidity remains virtually constant.

We were able to demonstrate that Isernhager sourdough prepared in accordance with the process specifications over a relatively long period of time (2 weeks) is not only microbiologically active but is also stable and will keep throughout its intended storage period (46). For the purposes of verification, the TTA and the lactic acid bacteria and yeast counts (colony-forming units [CFU]) were determined during fermentation. The metabolism of the microorganisms present in the sourdough during prolonged storage was calculated from the measured values (Eq. 1).

In Eq. 1, bE is the coefficient of acid formation irrespective of the growth of lactic acid bacteria, CFU is the lactic acid bacteria count per gram of sourdough, and dTTA/dt is the increase in the TTA with time (TTA rate). The TTA rate was calculated from the measured values using a linear regression equation (Eq. 2) in which b is a constant.

Figure 11 shows that the CFU of the lactic acid bacteria and yeasts decreased only slightly during the storage period. The coefficient bE of the metabolism during prolonged storage ranged from 3.53 to 6.20 x 10"12 (TTA x hr"1 x CFU-1) for the sourdoughs concerned. The results demonstrate that the lactic acid bacteria in the sourdough are active during prolonged storage, that the sourdough will keep owing to the high TTA and low pH, and that the sourdough is stable by virtue of the low acid formation accompanying its low metabolism activity during prolonged storage. The lactic acid production rate during prolonged storage was virtually constant as shown by the upward slope of the curves (Fig. 12). However the TTA rate, which corresponds to lactic acid production, was different for each of the sourdoughs under investigation (2.50-3.23 x 10"2 (TTA hr"1). From this it follows that various species of lactic acid bacteria also exhibit different levels of metabolic activity during the phase of prolonged storage on an otherwise constant substrate.

Storage is facilitated in particular by its high acidity, which is twice as high as that of the ready-to-use three-stage sourdough. The quantity of sourdough needed to make sourdough bread with Isernhager sourdough is therefore half that required when using three-stage sourdough.

Figure 11 Development of the microorganism count.
Figure 12 Development of the total titratable acidity during constant metabolism activity.

The quantity of sourdough remaining in the fermentation tank and used as an inoculum for the weekly production of Isernhager rye sourdough ranges from 7% to a maximum of 30% of the total amount of sourdough produced in a single batch (45). The quantity remaining in the fermentation tank can be used as an inoculum several times over. A special starter (Isernhager GmbH & Co.) is substituted for the inoculum in the fermentation tank whenever the metabolic activity diminishes to such an extent that the final acidity cannot be achieved rapidly enough. This happens when the vitality of the microorganisms decreases and the spectrum of microorganism flora changes to the detriment of the dominant species of microorganism. The starter is proliferated on milled products and contains a mixed culture in which lactic acid bacteria species dominate. The dominant metabolic activity of these starter species results in bakery products with a mild flavor. In addition to this species, the lactic acid bacteria species present in the flour also proliferate in the starter (45).

Owing to the rapid formation of acid and the low pH after the final acidity has been reached, the metabolic activity of the microorganisms is reduced to the basal metabolic rate and the activity of the endogenous enzymes in the flour is limited to such an extent that formation of precursors of flavor components is virtually halted. Furthermore, as only half as much Isernhager sourdough as three-stage sourdough is used in the total bread dough and the microorganisms in the Isernhager sourdough do not exhibit any significant metabolic activity during the dough and loaf ripening times, Isernhager rye sourdough contributes less to the formation of the flavor profile of bakery products than the three-stage rye sourdough.

This disadvantage has been overcome by another invention that also constitutes a breakthrough in sourdough production. The invention consists of including bread (either whole loaves or sliced bread) in the fermentation substrate and utilizing the stirrer designed with cutting blades together with a basket-like feeding device in the fermentation tank to cut up the bread. This results in a pulpy sourdough containing, in particular, the flavor substances present in the bread crust. In addition, this type of sourdough is enriched with the gelatinized starch present in the crumb. It can therefore also perform the same function as preliminary doughs containing gelatinized starch.

In addition to this, including bread returns in the sourdough recipe is particularly hygienic and economical. Under the conditions specific to the manufacturing process, the bread is broken up and converted into a homogeneous mass that can be used to make well-leavened bread with an aromatic flavor that keeps its freshness well. These advantages are obtained in particular when there is a high concentration of bread (50%) in the sourdough (45).

D. Modern Small-Scale Wheat Sourdough Production

While it is essential to use rye sourdoughs in order to achieve the required baking performance of bakery products containing rye flour, this does not apply to the use of wheat sourdough for bakery products made of wheat milled products. Wheat sourdough is included in recipes primarily to increase the flavor of the bakery products (panettone) or to emphasize a particular flavor profile (Sanfrancisco sourdough bread). Sourdoughs with a very low dough yield (<150%) are used for this purpose. The advantage of this type of production is the long fermentation time and low level of acid formation (TTA =12) that are essential for the development of the typical characteristics of this type of sourdough, with the precursors of flavor components governing the formation of the quality characteristics. The use of yeast leavens with long fermentation times, such as sponges, has the same objective. Although not brought about intentionally, sourdough bacteria proliferation and acid formation takes place in sponges, the extent thereof depending on the fermentation time and conditions.

Liquid sponges for use in dough fermentation have the considerable advantage of being pumpable, which sets them apart from the firm, nonpumpable wheat sourdoughs. This major criterion for the mechanization of sourdough production has recently led to the development of wheat sourdough plants. The designers of such plants have applied findings resulting from the use of rye sourdough plants and pumpable sponges. Gluten formation in wheat flour doughs with dough yields greater than 200% presented a considerable problem for the development of such plants, which has only been solved empirically so far.

The aim was to mix flour with water in such a way as to obtain a dough in which the gluten develops only to the extent that, together with the starch, it forms a homogeneous and stable dispersion that is also maintained on lactic acid formation. When dispersions of this kind are subjected to an excessive input of mechanical energy, the elastic properties of the gluten can develop to such an extent that the gluten agglomerates and releases starch granules. The latter separate out and, as a result, the dispersion cannot be maintained and loses its pumpability. The risk of changes in the rheological properties of the dispersions is the main obstacle to the construction of wheat sourdough plants. The successful design for example by the Isernhager Landkost company of fermentation plants in which flour-in-water dispersions remain stable throughout the fermentation time in each stage of sourdough production can be regarded as a major breakthrough in wheat sourdough production.

The wheat sourdough plant comprises a mixing vessel (volume: 0.5 m3) and two temperature-controlled fermentation tanks (volume: 2 m3) operated in tandem. The mixing vessel is fitted with a specially designed propeller mixer and is used to fill the fermentation tanks during batch-wise operation. The inoculum—in this case a wheat sourdough starter— is added to the first batch. The quantity of inoculum depends on the quantity of sourdough in the fermenter, which in turn depends on the quantity of sourdough required for producing baked goods on the following day.

Operating the fermenters on alternate days requires foresighted planning in the same way that production of three-stage rye sourdough does, for example. Once the operating schedule has been laid down, it cannot be altered. Such operating schedules for the sourdough plant therefore rule out any flexible response to sudden variations in the sourdough requirement. However, it is possible to store a certain amount of sourdough as the filling volume of the fermentation tanks is exploited to the full and the sourdough can also be cooled to 10 °C. Unfortunately, the maximum storage period is 48 hr for both mechanical and microbiological reasons: gluten agglomeration on the one hand and the fact that metabolic activity is not completely suspended at 10°C on the other hand (43).

Each fermentation tank is fitted with a wall-scraping anchor stirrer. The fermenting dough is mixed at a low energy input, mixing being halted completely at intervals. The sole purpose of mixing is to ensure that the fermenting sourdough is distributed evenly, has the same temperature throughout and that its volumetric expansion is limited. The sourdough in the fermentation tanks can be heated or cooled by means of the temperature-controlled jacket. Heating serves mainly to reach and maintain a constant operating temperature, and cooling reduces the metabolic activity so that the sourdough can be stored. The wheat sourdough in the fermentation tanks takes at least 16 hr to ripen.

This type of wheat sourdough is a '' no-time'' sourdough intended for storage within a daily production rhythm. The basic principle for its production originated in the Isernhager tank fermentation for sourdough. On the base of its two fermentation tanks, the plant can be operated semicontinuously to produce sourdough for bread-making. Sourdough produc tion is restricted owing to the technical limits placed on the size of the fermentation tanks fitted with stirrers.

Such limits do not present any problems as long as bakery products contain only a small proportion of sourdough and the bakeries involved have small capacities for producing individual types of baked goods. The proportion of this type of sourdough added to baguettes, for example, is normally less than 10% in relation to the quantity of flour used. From this it also follows that there are as yet no wheat sourdough plants for large bakeries making products in which a large proportion of wheat sourdough is used, such as San-francisco sourdough bread.

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