Replicating traditional methods of preparing buns, bagels and rolls on an industrial scale is a task with an extensive list of challenges, to achieve high-volume, consistent results. Flexibility and innovation joined in dedicated production lines help provide the answers for endless product variations.
Bread is intrinsically a safe food that does not normally support the growth of food poisoning microorganisms. Three factors limit how long bread remains fit to eat: moisture loss, staling of the crumb, microbial spoilage. Tackling each of these can help improve the shelf-life of bread products; but, how is this done in practice?
Here, I’ll put my many years’ worth of experience in working on bakery products at Campden BRI to outline the techniques that will allow these factors that limit shelf-life to be addressed (much of the focus when establishing useable shelf-life is placed on slowing mold growth).
Moisture loss
Bread packaging mostly takes the form of polyethylene (PE) bags with twist ties or sealing tape to create a partial seal. Waxed paper is another material that offers similar performance to PE. Products intended to be consumed the same day or the next tend to use cardboard and other plastics for their packaging. PE in bread bags provides a physical barrier to protect the bread but is limited in terms of the shelf-life it offers. Moisture loss takes place slowly by transmission through the thin PE material over a few days resulting in the crust drying out. As it does this, water is drawn from the crumb making the bread firmer. This is often confused with the firming of texture that happens over a similar period but is caused by chemical staling of the starches rather than physical moisture loss.
Longer shelf-life breads such as flatbreads use modified atmosphere packaging (MAP) to extend their useable life. This requires thicker multilayer plastic packaging with barrier properties to slow the moisture loss and prevent oxygen from getting into the packaging and diluting the carbon dioxide atmosphere. There is also a need to chemically preserve longer life products.
Staling of the crumb
All bread will become stale over time as the starches change their configuration after baking. This is a chemical change that involves the starch molecules attempting to convert their amorphous gelatinized form back into the original crystalline form. This makes the crust firmer and is easily confused with the firming that happens as the crumb dries out.
Staling can be slowed down by adding a type of amylase enzyme that changes the structure of the starch so it cannot recrystallize as easily. This group of bread softening amylases has attracted considerable attention from enzyme manufacturers and bakers. As enzyme technology has advanced, each generation of amylases works more effectively than the previous. Breads produced with these amylases have a softer and more elastic crumb and enable bread to retain its desirable textural properties beyond two-three days. Most packaged bread with a shelf-life over five days now contains bread softening enzymes. The amylases are active during proofing and early stages of baking and are inactivated by the high temperatures reached in the bread. This prevents them from being active in the packaged bread.
Unfortunately, there is not a natural enzyme solution that retains the crumb softness for longer. Certain ingredients can be used to improve the initial softness. Higher levels of fat or oil, increased water content, and increased sugar will perform functions that retain softness for a bit longer. Fats and sugar, however, are not ideal for health reasons or as effective as softening enzymes.
Microbiological growth
Most microbiological issues relate to post-bake contamination and these are very unlikely to be with food poisoning bacteria because of the limited handling of the baked products, particularly with larger-scale plant-produced bread. In addition, the times and temperatures achieved within the bread crumb during baking exceed those required to kill vegetative food poisoning bacteria such as Salmonella, Listeria and E. coli. Mold is the primary factor affecting the shelf-life of packaged bread, particularly in warmer conditions when mold will grow more quickly.
Figure 1: Strands of ‘rope’ in bread, caused by Bacilli breaking down starches to sugars
Microbiological growth
Most microbiological issues relate to post-bake contamination and these are very unlikely to be with food poisoning bacteria because of the limited handling of the baked products, particularly with larger-scale plant-produced bread. In addition, the times and temperatures achieved within the bread crumb during baking exceed those required to kill vegetative food poisoning bacteria such as Salmonella, Listeria and E. coli. Mold is the primary factor affecting the shelf-life of packaged bread, particularly in warmer conditions when mold will grow more quickly.
Molds can produce ‘spores’ as part of their life-cycle which are resistant to heat, light, dryness, acids and alkalis. They are released into the air in vast numbers and some will likely land on bread prior to wrapping. Conditions within packaged bread are ideal for the spores to germinate and grow. As they grow, they extend root-like threads into the loaf, extracting moisture and nutrients, helping them to grow into a ‘colony’. Each colony can produce many thousands more mold spores.
Control of mold is essential in bakeries, strategies include:
+ Attention to bakery hygiene: Regular cleaning, equipment maintenance, hygienic design and positive air pressure within the bakery all help to control mold. Lower spore levels landing on bread surfaces reduce the number of visible colonies that will form, as not all mold spores are able to grow.
+ Product formulation: Chemicals based on propionic acid (or fermented alternatives) successfully retard mold growth. The effectiveness of these can be increased by reducing the pH of the product (e.g. by the use of vinegar).
+ Post-bake technologies: Treatment with UV or high-intensity pulsed light can provide a surface clean-up before packaging. MAP in combination with preservatives can extend shelf-life beyond 10 days.
Despite mold growth on crusts being the most common microbiological issue with bread, bacterial and yeast spoilage do occur on occasion. Spores from various Bacillus species are heat stable to the extent they will survive the baking process within the crumb. Bacillus subtilis is one of these and causes the spoilage condition known as ‘rope’. Rope occurs when, because of spore germination and growth, the amylases released by the growing Bacilli convert the structural starches into liquid sugars, breaking the crumb into a stringy mass (see Figure 1). A sweet fruity odor is associated with ‘ropey’ bread.
Control of rope requires vinegar and/or calcium propionate to be used in the recipe if its use is permitted in the type of bread being made which in general is only prepacked sliced bread. Vinegar lowers the pH below the level that Bacilli can grow, and propionate inhibits their growth. It is often thought that propionate is used to increase the mold-free shelf-life of bread but it is actually added to control Bacillus spore germination and growth. Its effect on slowing mold is a secondary benefit.
Bacilli will spoil the bread but not cause food poisoning. With a typical retail shelf-life of five to seven days, the systems used by the industry are proven to be effective against all bacteria. However, as shelf-life is extended beyond this, there is a need to consider protection against more heat resistant and pathogenic bacteria, including anaerobic Clostridium botulinum. Longer life bread products tend to use MAP in carbon dioxide to control mold growth, however, it does not protect against anaerobic bacteria.
Flatbreads are a popular MAP product with market growth for clean label products and with the addition of herbs sprinkled on top, sometimes fresh. Our team at Campden BRI is currently setting up an industry project to challenge test the growth of C. botulinum in these flatbreads. The preservation system in bread is complex, requiring control of pH and preservatives effective against mold, yeast and bacteria. Natural preservatives must contain sufficient levels of active preservatives to control all the relevant microorganisms. It is important to know which ones can grow in bread with no protection and design the preservation system to prevent the growth of microorganisms.
Yeast spoilage of bread is rare but can occur from cross-contamination during bread cooling or slicing. It is confusing that this type of spoilage is known as chalk mold because of its chalky appearance (Figure 2) since the organism responsible is a yeast. Hyphopichia burtonii, Wickerhamomyces anomalus, and Saccharomycopsis fibuligera are spoilage yeasts that can cause chalk mold. Good attention to hygiene conditions around cooling and slicing is required to prevent this from happening.
Shelf-life determination
With bread being technically a safe product to eat (in most cases) after expiry, its shelf-life is determined as a best before date (BBD). The target BBD is set as the last test date at which no mold is observed using the incubation temperature appropriate for the country of sale. In the UK, this is taken as 21°C, although a duplicate test at 25°C can be useful to represent home storage conditions during the summer months. The growth of rope-forming bacteria, should this be a possible issue, is likely to be faster at 25°C.
The Federation of Bakers, who represent the UK baking sector, has a protocol for setting the BBD for bread. Before testing starts, a target BBD must be set, which is the anticipated date the bread remains mold-free. Twenty loaves are packed, preferably in clear bags, with loaves examined both visually and by opening according to the scheme in Table 1. It involves examining four loaves each day to look for visible mold colonies. The test continues for the BBD +1 day or when one or more loaves are moldy.
Ideally, three consecutive shelf-life evaluations should be carried out to confirm the same BBD. In the event of mold being found on or before the BBD, the shelf-life should be reduced or further testing carried out following investigations into product formulation and plant hygiene. Table 2 shows example results where the target BBD was six days after production (P+6 days).
By Gary Tucker, research fellow, Campden BRI
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