During storage of bread, several different physical and microbiological changes occur, lowering the quality of bread. The bread crumb becomes hard, the bread crust changes from crispy to leathery, and the characteristic and favorable bread flavor disappears. All these changes are characterized as the staling process. Within few days the bread might be spoiled due to contamination and growth of molds on the surface or development of rope in the bread crumb caused by Bacillus spp. Addition of sourdough in the bread recipe can be used to retard the staling process of the bread, prevent the bread against ropiness and prolong the mold-free period. Sourdough addition is the most promising procedure to preserve bread from spoilage, since it is in agreement with the consumer demand for natural and additivefree food products.
Mold is the most frequent cause of bread spoilage. Addition of sourdough in the bread recipe increases the mold-free period for rye bread (21,32) and wheat bread (78,108). The length of mold-free period was prolonged from 4 days in wheat bread to 6 to 8 days in sourdough bread (78). No correlation was found between pH and bread shelf life (108).
The mold-free period was prolonged 1 to 3 days in slices of sourdough rye bread inoculated with Aspergillus glaucus when the sourdough was fermented with heterofermen-tative LAB compared to homofermentative LAB, or bread without addition of sourdough (32). The antimicrobial effect of the heterofermentative LAB was supposed to be the result of their production of acetic acid.
Using agar-well-diffusion assay, 232 strains of sourdough LAB belonging to nine different species were screened for production of anti-mold substances against Aspergillus niger, Fusarium graminearum, Penicillium expansum, and Monilia sitophila (109). The anti-mold activity varied very much among the strains and was mainly detected within obligately heterofermentative LAB. L. sanfrancisensis had the largest spectrum of anti-mold activity. Not only the acetic acid had inhibitory effect, but the LAB produced also formic, propionic, butyric, n-valeric, and caproic acid, and a mixture thereof was responsible for the anti-mold effect.
Ropiness is spoilage of wheat bread noticed as an unpleasant odor similar to that of overripe melons, followed by the occurrence of a discoloured sticky bread crumb and sticky threads that can be pulled from the crumb. This bread spoilage is caused by heat-resistant strains of
Bacillus and occurs particularly in summer when the climate favors growth of the bacteria.
It is mainly caused by Bacillus subtilis, formerly referred to as B. mesentericus, because the heat-resistant spores can survive the baking process, sporulate, and multiply in the baked bread. The rope symptoms can be recognized when the level of Bacillus in bread crumb is 108 bacillus/g (110). Its incidence has increased during the past decade, presumably because most bread is now produced without preservatives and often with the addition of raw materials such as oat products, wheat bran, and sunflower seed with a high contamination level of Bacillus spores (110). Even a low level of the heat-resistant spores (101-102 bacillus/ g) in raw materials resulted in a level of 107 bacillus/g bread in 2 days.
One potential way to prevent development of rope is to include sourdough in the bread recipe. Addition of 10% sourdough inhibited the natural Bacillus contaminants in wheat dough, but it was insufficient to inhibit the bacillus strains inoculated at a level of
106 spores/g (40). Addition of 15% sourdough was more efficient as the strains of rope-producing Bacillus were effectively inhibited by sourdough fermented by strains of L. sanfranciscensis, L. brevis, L. maltaromicus, or by three different strains of L. plantarum. In this investigation, B. subtilis tended to be inhibited if the TTA value in the sourdough was more than 10 and when the pH of the bread crumb was below 4.8. Rocken (29) demonstrated that sourdough effectively decreased the heat resistance (D97-value) of a rope-producing strain of Bacillus. He found that the heat resistance was reduced from 143 min without the addition of sourdough to 5.9 min and 6.9 min with the addition of 10% and 20% sourdough, respectively.
Bread becomes firmer during storage, and retrogradation of starch towards a more crystalline form is considered to be the primary cause of this bread staling. Several sourdoughs have been investigated for their potential effect on delaying the development of bread firmness and staling rate of wheat bread, but most investigations did not find any influence on staling rate by the sourdough compared to yeast- and sponge-leavened bread (108,111). However, delayed staling rate has been observed in sourdough bread (112). The rate of starch retrogradation was not influenced if the acidification was rather low, whereas a standard sourdough (L. sanfranciscensis 57, L. plantarum 13, S. cerevisiae 141) was able to retard the staling rate. The staling rate was mostly influenced if the starter culture had amylolytic activity (L. amylovorus or a genetic modified strain, L. sanfran-ciscensis CB1 Amy).
In some investigations, the addition of sourdough resulted in lower bread firming. However, sourdough wheat bread has higher bread volume (31,86,88) and the measured resistance will thus be lower.
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