The spoilage of bakery products is mainly caused by molds and yeasts and only occasionally by bacteria. This is due to a reduced, water activity and for some products, also a reduced pH. The spoilage fungi are fastidious organisms, and a shelf life of only 2-3 days may be expected for products like unpreserved wheat bread, especially if the hygiene in the factory is not sufficiently high. Besides the repelling sight of visible growth, fungi are responsible for off-flavor formation and the production of mycotoxins and allergenic compounds. These compounds may be formed even before growth is visible.
In some cases, bacteria may cause a serious spoilage problem in bakery products. In white and wholemeal wheat bread, the heat-resistant endospore-forming Bacillus subtilis is known to cause ropiness (1-3), especially if the bread is produced without preservatives or sourdough (4). Other Bacillus species such as B. licheniformis and to some extent also B. cereus, may survive the baking process despite their lower heat resistance, but they will not cause ropiness (4). B. cereus is capable of causing food-borne infections and intoxications, but B. subtilis and B. licheniformis have also been implicated in food poisoning (5). Another and more serious spoilage organism is the anaerobic bacterium Clostridium botulinum which may grow and produce toxins in high-moisture bakery products like English-style crumpets (pH 6.5 and aw 0.99) when packaged in modified atmosphere (6). Crumpets were toxic after 4 and 6 days respectively, when inoculated with 5 x 102 bacterial spores per gram after or before baking. The packaging conditions did not influence toxin production. Consequently, high-moisture bakery products contaminated with C. botulinum spores could pose a public health hazard when stored at room temperature. Bakery products with unbaked cream fillings are sensitive products and the production and handling of these products in small bakeries with insufficient hygiene have been responsible for many cases of food poisoning (7). These are, however, all related to the cream filling and will thus not be discussed further.
Most vegetative cells and mold spores are expected to be killed by the high temperature in the baking process, thus after-contamination is the source of spoilage problems. Some heat-resistant molds can survive the bread-making process in some cases, but these organisms have not been reported as spoilage organisms on bread. Contaminants of wheat bread are mostly Penicillium species (90-100%) and, to a lesser degree, Aspergillus and Clodosporium species (3). The most important mold species associated with wheat bread are P. commune, P. crustosum, P. brevicompactum, P. chrysogenum, P. roqueforti, A. versicolor, and A. sydowii (8). On rye bread, P. roqueforti is the major contaminant (9-11). In a 4-year investigation of rye bread in Denmark, P. roqueforti (27%), P. corylophilum (20%), and Eurotium sp. (15%) (E. repens, E. rubrum) were identified as the most important species. Looking at all these isolates of the important spoilage organism, P. roqueforti, it was clear that they formed three distinct groups. These were further investigated by chemical and molecular methods and shown to be three distinct but closely related species: P. roqueforti, P. paneum and P. carneum (12). Of these P. roqueforti and P. paneum dominate on bread, P. carneum on meat.
The yeasts known as chalk mold are also common on bread but mostly on sliced bread: in one survey, they accounted for 5-30% of the spoilage organisms (3). On rye bread, the chalk molds Endomyces fibuliger and Hyphopichia burtonii are the dominant yeast species
Rychlik and Schieberle (13) studied mycotoxin production by a less common bread spoilage funga, P. expansum, on wheat bread. They found high concentration of the mycotoxin patulin at the surface and were able to detect it down to 4 cm from the infected spot. Other toxins may be able to diffuse even longer in bread, as demonstrated for aflatoxin that may be produced by A.flavus on bread (14). The production and diffusion rate in bread, are however, not know for the toxins more likely to be found in bread: roquefortin C from P. roqueforti and P. chrysogenum; PR-toxin from P. roqueforti; cyclopiazonic acid from P. commune; patulin and penitrem A from P. carneum; patulin from P. carneum; and sterig-matocystin and nidulotoxin from A. versicolor (15). Despite that, it is still evident that it is of great importance to hinder mold growth on bread and to discard bread when it becomes mouldy and thus avoid the use of infected bread for human consumption or cattle feed.
Cakes typically have a much lower water activity than breads (aw 0.70-0.85 as compared to 0.92-0.97) and a much higher pH (6.0-8.0 as compared to 4.4-4.8 for bread started with sourdough and around 5.7 for wheat bread started with yeast alone) (13,16). This will of course, favor another group of fungi. Thus, the fungi associated with cakes, also termed the associated funga of cakes, will typically consist of Wallemia sebi, Eurotium spp., and Aspergillus spp. such as A. flavus and A. niger (5,17,18). The osmophilic yeast Zygosaccharomyces rouxii has also been isolated from spoiled cake with high sugar content. A predictive model for growth of this organism in bakery products has been developed by Membre and coworkers (19). They used a mixture of 30% glucose and 70% sucrose to simulate the mixtures commonly used in bakery goods and found that growth rate decreased linearly with increasing sugar content up to 950 g/L, corresponding to aw 0.788. The optimal pH for growth was in the range 3.5-5 (19).
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