Control of Bakery Products Microbial Contamination along its shelf-life

The distribution of fresh bread is a problem. The consumer expects a certain shelf life of the bread. Bread hermetically packed and sealed, can be kept mould-free for a couple of days at room temperature. After a few days the staling phenomenon (changes in softness and taste) will occur but the bread will also start to get mouldy. It is possible that certain bacteria start to grow in the crumb (especially when the bread has been cut) in view of the high moisture content.

There are number of techniques one can use to extend the mould-free shelf life of bread:-

• Use of preservatives
• Use of modified atmosphere packaging
• Freezing
• Pasteurization

In this overview we left out the drying of bread because in that case on gets a fundamentally different product. But drying to make for instance rusks is nothing else than a method to extend the shelf life of the bread.

By using one or more of the above techniques, it is possible to obtain a bread that has a rather long mould-free shelf life i.e. 3 – 9 months. In such cases the shelf life does not depend anymore on keeping the bread mould free, but on the loss of softness or the development of off-flavors.

A number of factors play a role in the growth of micro-organisms (and the speed at which they develop):-

• temperature at which the products is stored
• pH of the product
• moisture content of the product
• the aw value of the product
• the type of micro-organism that is present
• the initial contamination of the product
• the presence of products which slow down the development of moulds (typical example are raisins which contain a natural preservative)

Bacteria are classified in 3 different groups depending on their resistance to heat: -

• Psychrophilic bacteria have a temperature range of 5°C tot 30°C. Some of them are even active below 0°C. These bacteria can grow on food even if it stored in the fridge.
• Mesophile bacteria have their optimum temperature between 15° and 50°C. Most bacteria belong to this group including pathogenic bacteria which have an optimum temperature between 35°C and 40°C.
• Thermophile bacteria which grow best between 50° and 60°C. They found in geysers and some of them survive temperatures up to 90°C.

Also, in with regards to pH tolerance one can make sub-categories:

• Acidogenic bacteria are bacteria that produce acids. The pH of the product will go down as a result of their metabolism. Yeasts and lactic acid bacteria belong to this family.
• Acidophilic bacteria can grow at a low pH but they prefer a neutral pH although they can tolerate a pH-range from 5 to 8.

Water activity or aw-value

What is the aw-value of a product?

The aw-value or water activity is a measure for the amount of free water present in a product. By definition the aw-value of water is 1. The aw-value is measured with a hygrometer. To determine the aw-value of a product, the relative humidity of the air around the hermetically sealed product is measured when an equilibrium is reached between the product and the air. In other words, the product does not lose moisture anymore to the air around the product or vice versa.

The aw-value a product is determined by a number of factors:-

• The amount of components dissolved in the free water
• The molecular weight of the components dissolved in the water
• The temperature at which the product is kept
• The conditions in which the product is stored
• The barrier characteristics of the packaging material.

Microbial growth in bread:

By baking the dough most bacteria get killed. Practically the bread comes out of the oven sterile. Theoretically that’s not 100 % correct as the temperature in the center of the bread does not go above 100°C.

On the other hand, we know that a bread kept in sterile conditions, immediately after it leaves the oven, will not get mouldy. It is also a known fact that bread cooled, sliced and packed in a clean room or “white” room (i.e., filtered air combined with an extremely hygienic environment) will not get mouldy in the first 20 days.

So, it is extremely important to remember that the contamination of the bread happens after the baking during the cooling and packaging operations. And although an initial contamination cannot be excluded, is the reason why the bread gets mouldy in most cases unhygienic conditions after baking. Spores are present in the bakery, nothing to do about it. They just are there and they will whirl down on the surface of the bread.

Anyway, by slicing the bread spores are transported by the slicing operation between the various slices and within 4 to 5 days the spores will develop and show the typical “hairy” mould growth. Hygiene in the bakery is an absolute must to avoid the growth of mould in the packed bread.

The moulds one normally finds on the bread, belong to one of the following families:

• penicillum (corylophilum, expansum, frequentans, roqueforti)
• aspergillus (flavus, niger, paecilomyces)
• eurotium (amsteldami, crustosum, chrysogenum, verucosum, herbariorum, chevalieri)

Physical methods to extend mould free shelf life:

Bread can be treated in a number of ways to avoid mould growth or to slow down the appearance of moulds. The most important systems are:

• Pasteurization
• Use of modified atmosphere packaging

Pasteurization

The aim of pasteurization is to produce a bread without the addition of preservatives. Pasteurization means that the bread, after baking and packaging, is heated during a certain time at a certain temperature so all present micro-organisms are killed. This can happen with hot air as well as with microwaves.

Modified atmosphere packaging (MAP)

As mentioned before, the bread when it leaves the oven is practically sterile. From a pure theoretical point of view that is not correct, but for all practical purposes one can assume that the bread is sterile immediately after it left the oven. If the bread is cooled and packed in perfectly sterile conditions, there will be no microbial contamination and hence no moulds or bacteria will appear on the product. This would be an ideal condition.

Practically speaking additional precautions are taken by using modified atmosphere packaging. By using this technology very little or no oxygen remains in the packed and no mould will grow because most moulds and bacteria need oxygen to grow. So if the air in the packet is replaced by another gas, the moulds and the bacteria don’t get a chance to develop. To have success by using this technology, two conditions have to fulfilled:-

• The rooms in which the bread is cooled and packed have to be kept as sterile as possible. This is called “clean room technology”. This also means that the personnel working in these areas need to follow a set of strict rules to keep the room as clean as possible (washing hands before entering the room, wear protective clothing and mouth masks). These areas are also put on overpressure so no outside air can enter the room. The air entering the room has to be filtered using “absolute filters” so no micro-organisms can enter the room through the air.
• The packaging material need to have a barrier for O2 so no outside air can enter the packet but it must also create a barrier for the gasses inside the packet so they cannot escape from the packet.

MAP and Calcium propionate:

An interesting study has been done. In this study the shelf life of bread was examined in function of a number of variables:

• Bread made with and without 0,36 % (on flour weight) of calcium propionate.
• Moisture content of the bread
• pH of the crumb (5,4 – 6,0)
• composition of the gas mixture

With regards to the composition of the gas mixture, the following compositions were used in the test: 100 % nitrogen, 80:20 nitrogen – carbon dioxide, 50:50 nitrogen – carbon dioxide and a standard test in which the product was packed under normal conditions with air.

Samples packed without protective atmosphere

Sixty percent of the bread packed without protective atmosphere, produced without preservative and stored at 22 - 25°C showed mould growth after 8 days and all breads were molded after 13 days. Also, breads kept at a lower temperature (15 - 20°C) were also all molded after 13 days. Samples that were made with calcium propionate were all molded after 20 days when kept at 22 - 25°C and all of them were molded after 34 days when kept at 15 - 20°C.

Samples packed with 100 % nitrogen, gave the following results: -

Bread that did not contain calcium propionate: after 13 days 100 % of the samples were molded when kept at 22 - 25°C. Also, at lower temperature all samples were molded after 13 days. There was no difference with the breads packed with air.

Breads that contained calcium propionate: after 26 days all samples were molded when kept at 22 - 25°C and after 52 days only half of the samples were molded when kept at 15 - 20°C.

Samples packed with 80:20 carbon dioxide – nitrogen

All samples that did not contain calcium propionate were molded after 20 days if kept at 22 - 25°C (so compared to air or pure nitrogen, it took 7 days more for the bread to get molded). At lower temperature it took 52 days before all the samples were molded.

With calcium propionate only 60 % of all samples showed mould growth after 26 days if kept at 22 - 25°C and 50 % if kept at 15 - 20°C.

Finally bread packed with a mixture of N2:CO2 (50:50)

Hundred percent of the bread without calcium propionate was molded after 26 days when kept at 22 - 25°C (so compared to air and pure nitrogen, it took 13 days longer before all the bread was molded). At lower temperature, after 52 days 1/3rd of the samples were not molded. So practically speaking the mould free shelf life had doubled.

With calcium propionate all breads were free of mould after 26 days at 22 - 25°C and after 52 days at 15 - 20°C.

Packaging material:

It doesn’t matter which technology is used. In both cases a foil with good barrier characteristics is a must. To create these barriers multiple-layered foils are used. EVOH (ethylene vinyl alcohol) is a copolymer of ethylene and vinyl alcohol. EVOH copolymer is defined by the mole % ethylene content: lower ethylene content grades have higher barrier properties; higher ethylene content grades have lower temperatures for extrusion.

The plastic resin is commonly used as an oxygen barrier in food packaging. It is better than other plastics at keeping air out and flavors in, is highly transparent, weather resistant, oil and solvent resistant, flexible, moldable, recyclable, and even printable. Its drawback is that it is difficult to make and therefore more expensive than other food packaging. Instead of making an entire package out of EVOH, manufacturers keep costs down by coextruding or laminating it as a thin layer between cardboard, foil, or other plastics.

Chemical methods to extend the mould free shelf life of bread:

One can a number of preservatives to bread to avoid the development of micro-organisms. In this case one should remember that yeast is also a micro-organism and that proofing times will be longer when chemical preservatives are added to the dough.

Temperature and moisture favor the development of moulds and bacteria. Sugar and salt will reduce the effect of moisture because both substances lower the aw-value of the bread. Fiber will also absorb water but has little influence on the aw-value. As a result, products with reduced salt and/orsugar content (so called “light” products) will mould more readily than products containing more salt and/or sugar.

Also, the pH will play a role in the development of moulds. The pH of the product will be lower when introducing longer proofing times or by adding organic acids. Most preservatives use pH reduction for their anti-microbial properties.