Biological spoilage

Microbiological spoilage of bread is most often associated with fungi, and occurs when fungal mycelia are visible to the consumer. Some strains of Bacillus subtilis, Bacillus mesenteri-cus, and Bacillus licheniformis can spoil high-moisture breads via production of an extracellular capsule material that gives the infected bread a mucoid or ropy texture.There are also wild yeasts capable of causing flavor defects in bread after baking; however, bacterial and yeast spoilage of bread is relatively rare, and it is fungi that are, by far, the most common micro-bial cause of bread spoilage (Table 8-5). In large part, this is because the water activity of bread is usually less than 0.96, which is below the minimum for most spoilage bacteria. In addition, baking ordinarily kills potential spoilage bacteria, the exception being spore-forming bacilli mentioned above.

The baking process kills fungi and their spores.Thus, when molds are present in bread, it is invariably a result of post-processing contamination. Fungal spores are particularly widespread in bakeries due to their presence in flour and their ability to spread throughout the production environment via air movement. When the baked breads leave the oven, their transit through the cooling, slicing, and packaging operations leave plenty of opportunity for infection, either indirectly by airborne spores or directly by contact with contaminated equipment. Fungi are aerobic and ordinarily grow only on the surface of loaf bread. However, slicing exposes the internal surfaces to mold spores, enabling growth within the loaf. Once packaged, moisture loss in the bread is negligible; thus, the water activity remains well within the range necessary for growth of fungi. Bread that is packaged while still warm is very susceptible to mold growth, due to localized areas of condensate that form within the package.

Although there are many different species of fungi associated with bread spoilage (Table 8-5), the most common are species of Penicillium, Aspergillus, Mucor, and Rhizopus. Bread serves as an excellent growth substrate for these fungi; visible mold growth may appear within just a few days.What one actually sees is a combination of vegetative cell growth (the mycelia), along with sporulating bodies (Figure 8-10).The latter are responsible for the characteristic blue-green or black color normally associated with mold growth.The ability of fungi to grow on bread and which species predominate depend on several factors, including bread pH and water activity, and storage temperature and atmosphere. Finally, it is important to recognize that some mold strains not only can grow on bread, causing spoilage and economic loss, but, under certain conditions, specific strains can also produce mycotoxins. Fortunately, visible mold growth ordinarily precedes mycotoxin formation, so in the very unlikely event that a mycotoxin were present, most consumers would reject the product before ingesting it.

292 Microbiology and Technology of Fermented Foods Box 8—6. Fresh Ideas for Controlling Staling of Bread

The precise mechanisms responsible for bread staling are complex. According to current models (Figure 1), staling occurs when unstructured and gelatinized starch (mainly amylopectin) that is formed during baking reorganizes or retrogrades, first after cooling and continuing during aging, into rigid, crystalline structures (Gray and Bemiller, 2003). As starch retrogrades, intramolecular hydrogen bonding between amylopectin branches occurs, causing additional structural rigidity. In addition, the outer chains of adjacent amylopectin molecules may form double helices. Efforts to reduce staling, therefore, necessarily involve reducing the rate at which these crystallization and hydrogen-binding reactions occur.

Because staling is influenced by many factors (Table 1), no single approach aimed at delaying or preventing staling is likely to be completely effective. Rather, bread manufacturers have adopted a more multifaceted strategy that considers the entire bread-making process, from ingredient selection and product formulation to processing and packaging steps (Zobel and Kolp, 1996). In the discussion below, the role of ingredients, enzymes, and processing conditions on controlling staling are described.

Table 1. Factors affecting staling.

Factor

Effect

Ingredients

Flour

Fats

Surfactants Enzymes Moisture Packaging Manufacturing Baking temperature Storage temperature

High protein flours maintain crumb softness Delay staling by increasing loaf volume Increase crumb softness

Amylases (mainly a) hydrolyze starch and reduce retrogradation

Low moisture increases staling

Prevents moisture loss and crumb firming

Long fermentation times increase loaf volume and softness

High temperature, short time baking increases staling rates

Refrigeration increases staling, freezing decreases staling rates

Ingredients

Several of the ingredients that are added to bread dough either retard staling rates and/or soften bread crumb and crust. Included are surfactants and other emulsifying agents, dextrins, and selected oligosaccharides. Mono- and diglycerides are probably the most common surfactant-type agents used by the bread industry and the most effective as anti-staling agents.They appear to function by binding to amylose and amylopectin and forming complexes with lipids, thereby reducing starch retrogradation rates (Knightly, 1996).

Enzymes

Enzymes are widely used by the bread industry.Amylases, in particular, are often used to hydrolyze starch and to increase the concentration of fermentable sugars in the dough.These sugars can also contribute to desirable flavor and color changes via the Maillard reaction. However, specific a-amylases are now also used for their anti-staling properties.

How amylases actually delay staling is not clearly known, although several explanations have been proposed (Bowles, 1996). It could be that the simple sugars released by a-amylases could act directly by interfering with crystal formation; however, this seems unlikely since sugar addition to breads has no effect on retrogradation. Instead, it appears that the anti-staling function of a-amylases is due to the hydrolysis of amylopectin, especially near branch-points.This would reduce the number of cross-links that contribute to crumb and crust firmness.

Box 8—6. Fresh Ideas for Controlling Staling of Bread (Continued)

amylose (amorphous)

Mucoid Ropy Texture Breads
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