The temperature of the sourdough is influenced by the temperature of the flour, the water, and the mother sponge, and it is often adjusted/regulated by the water temperature. In practice, the temperature increases 6 to 8°C during fermentation on an industrial scale if the temperature is not thermostatically regulated, so it is important that the temperature of the water is not too high. The temperature of the sourdough greatly influences the microbial propagation and production of acids, as the optimal temperature for growth and acidification varies for the different species of LAB. Spicher (80) found that the lowest generation time was 20 min for L. brevis at 35°C and L. plantarum at 40 °C, and 60 min for L.fructivorans at 30°Cand L.fermentum at 40 °C. Changes in the fermentation temperature from the optimal conditions increased the generation time considerably, and the generation time for L. fermentum was prolonged to 120 min at 40 °C and 140 min at 25 °C.

The optimum temperature for growth of the LAB is close to the optimal temperature for acid production, and most LAB have temperature optima between 30 and 35°C (21). In general, the final pH is reached more quickly at higher temperatures (30-35°C) compared to lower temperatures (20-25°C) (21,79,81). Some species, mostly heterofermentative, can grow below 15°C, such as L. farciminis, L. plantarum, L. rhamnosus, L. brevis, L. fructivorans, Lb sanfranciscensi. The highest temperature for growth is between 45 and 55°C, and most species that can tolerate high temperatures are homofermentative, such as L. acidophilus, L. amylovorus, and L. delbrueckii. However, also the heterofermentative species of L. pontis, L rhamnosus, L. fermentum, and L. reuteri can grow above 45°C (45).

The optimum temperature for growth of sourdough yeasts has not been intensively investigated, but it seems to be lower than for the LAB. The optimum temperature for growth of C. milleri was determined to be 27°C (44); C. milleri and S. exiguous do not grow at temperatures above 35°C (56). The minimum temperatures for growth of LAB and yeast are important when sourdoughs are stored by cooling because the sourdough should not develop during the storage. The minimum temperature for growth of most sourdough yeasts has been found to be 8°C (55).

The content of acids produced in sourdough increases with increased fermentation temperature due to higher production of lactic acid, whereas the production of acetic acid is only negligibly influenced by the temperature (21,81). This confirms the general rule that the relative content of acetic acid is higher in cold sourdoughs compared to warmer sourdoughs (21).

Investigation of the influence of the fermentation temperature on the production of flavor compounds in rye sourdoughs showed that the starter cultures themselves produced few volatile compounds, whereas the production of iso-alcohols and ethyl acetate increased considerably with higher temperature in sourdoughs fermented with homofermentative LAB due to activity by the propagating yeasts (81).

E. Amount of Mother Sponge

The amount of mother sponge to be mixed with flour and water for a new sourdough should be so high that the content of LAB in the sourdough is able quickly to decrease the pH to inhibit the growth of the gram-negative bacteria in the flour. The amount of mother sponge influences the pH-lowering capacity in a sourdough, as low pH is reached more quickly when the amount of added mother sponge is high (79). However, higher levels of acids are produced when a lower amount of mother sponge is added, as the fermentation time is longer before the pH drops to the critical pH level (20). The recommended amount of mother sponge is generally 10-20% for both rye and wheat sourdoughs (21,79). The San-francisco sourdough is rebuilt every 8 hr or at least two to three times a day, 7 days a week. The amount of mother sponge used in preparing a new sourdough is 25-40% of the sourdough (15). This high amount of mother sponge makes the sourdough very stable, and this sourdough has been continued for more than a century.

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