3.5. The microbiological shelf life of bread.

3.5.1.Introduction

The relatively high moisture content of bread encourages the development of mould on the bread. Moulds get killed during the baking process but spores and moulds present in the air of the bakery, are the cause that the bread gets polluted after it comes after the oven. Although strict application of good manufacturing practices with regards to re-infection can and will limit mould growth, a 100 % mould-free environment is difficult to achieve.

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:

In this overview we left out the drying of bread because in that case on gets a fundamentally different products. 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. 2 – 3 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-flavours. One can indeed wonder whether or not a bread to which alcohol was added to extend the microbiological shelf life is still nice to eat after 20 or 30 days. Let alone the aspect of using alcohol in a product that is also consumed by children. No need to say that from an organoleptic point of view, the bread loses its goodness. In case of par-baked products the development of off-flavours is masked by the reheating process.

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

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

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

3.5.2. Water activity or aw-value

What is the aw-value of a product? The term aw-value was introduced in 1953 by W.F. Scott. 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 following table gives an overview of the aw-value of a number of products:

aw-value
product moisture content aw-value
fruit
88 – 95 %
0,98 – 0,99
wheat
13 %
0,70
milk powder
3 %
0,70
farine
14 – 15 %
0,68 – 0,70
sugar
3 %
0,60 – 0,70
raisins
17 %
0,60

aw-value of some bakery products
water
1,00
bread
0,96 – 0,98
donuts
0,96 – 0,98
cake
0,85 – 0,87
biscuits
0,78 – 0,81
apple pie
0,95 – 0,98
crackers
0,20 - 0,30

We mentioned that there is a relationship between the aw-value and the microbial growth. When the aw-value goes down, it will be more difficult for the moulds and/or bacteria to grow in the product. Next table gives an overview at which aw-value a certain micro-organism will not grow anymore. Obviously this is linked to the storage temperature. So the table has to be read as follows: if a product has an aw-value of 0,90 of more and it is stored at 27°C of more, Saccharomyces cerevisiae (so the table refers to yeast) will develop.

aw-values and microbial growth
micro-organism
aw-value (min)
temperature
rhisopus
0,96
?
mucor spinosus
0,96
25°C
penicillum cyclopium
0,82
32°C
penicillum patulum
0,82
23°C
aspergillus flavus
0,78
43°C
aspergillus ochraceus
0,77
25°C
aspergillus niger
0,78
43°C
aspergillus echinulatus
0,62
25°C
monascus bisporus
0,61
25°C
saccharomyces cerevisiae
0,90
27°C

3.5.3. 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 centre 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. The crust however is not a very good substrate for the micro-organisms to develop because it contains very little water (about 6 – 8 %). This is of course not true for a par-baked bread or if the crust absorbs humidity from the air. In these instances the crust contains more moisture and it is “normal” occurrence that gas packed par-baked bread gets mouldy or that tiny white colonies develop on the surface. These colonies are not really moulds but it is yeast that develops in such products even if they are packed in a mixture of CO2 and N2.

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:

There are also two particular cases which have to be mentioned.

B. subtilis in sporulatieThe first case is Bacillus subtilis of which there are many different kinds. The one known best in the bakery is B. subtilis mesentericus. If that bacteria starts to grow in the bread, at the beginning the crumb will change colour to a light yellow-creamy colour and it will a have fruity (pineapple – peaches) smell. However after a day or so, the crumb will become slimy and gooey and have a real bad smell. This phenomenon is called rope. Unfortunately there is no microbiological test to ascertain that the bacteria is present in the bread. On top of all misery this bacteria can also survive the baking process. To make matters even worse, this bacteria can be found on the wheat so it is probably present in every bakery as it enters the bakery with the flour. It is also pretty complicated to combat rope and it is not unheard of that complete production runs have to be destroyed.

The phenomenon has been studied in great detail. There is a microbiological test which can be used to find out if the bacteria is present or not in the flour.

A suspension of flour in water gets heated for 10 minutes at 80°C. After cooling down the slurry an inoculation is done in a petri plate with an adequate medium. The petri plate gets incubated for 48 hours at 37°C. The presence of a maximum of 20 colonies of B. subtilis in 100 g of flour is considered acceptable.

Of course the baker can take a number of precautions:

In case there a rope problem in the bakery, there is only one efficient way to get rid of it and that is the use of vinegar.

Apply this procedure for 3 days. I mean not only adding vinegar to the dough but also washing down all equipment etc.

A second instance that happens rather often is the development of a red mould on the bread. The red colour is caused by red pigments present in a bacteria called Serratia marcescens. This bacteria grows quite easily on products that contain a lot of starch. There are also two moulds that cause a red colour: Monilia stiophila and Oidium geotrichum aurantiacum.

3.5.4. 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:

Pateurisation

The aim of pasteurisation is to produce a bread without the addition of preservatives. Pasteurisation 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.

The standard method for heating the product (the packaged bread is put in some kind of cabinet where it gets heated by using hot air or steam) did not have a lot of success. One of the reasons is because it is an expensive and long (90 – 120 minutes) process. The spongy structure of the crumb to not favour the heat transfer within the bread. Hence the long time that is needed to reach the desired temperature in the core of the bread. Subsequently a number of organoleptic changes are taking place in the bread which do not improve the overall quality of the product. With this technique the surface of the bread is heated to about 130°C while the centre only reaches about 60 - 70°C. This temperature could be acceptable. It all depends how long this temperature is maintained in the centre of the product. On top of that, to avoid the organoleptic changes, if one tries to accelerate the process, the bread won’t be pasteurised at all in the centre. Another is aspect is the quality of the packaging material Because of the high temperature, the material can be deformed leading to which seems to be packed carelessly and certain by-products from the plastic might be created. For this reason rather expensive packaging material has to be used.

To avoid the negative aspects of pasteurisation with steam or hot air, one can use microwaves. The energy created by the electromagnetic field, is transformed in the centre of the bread into heat. By using microwaves the centre of the bread will reach, depending on the size and shape of the bread, a core temperature of about 100°C in about 2 – 5 minutes. To achieve the desired pasteurising effect, the bread must be kept at 100°C, for about 5 to 10 minutes. Afterwards the bread is ready to be cooled down.

The high speed with which the bread reaches the desired temperature when using microwaves, has to do with the open structure of the crumb. Because of the non-compact structure of the crumb, the microwaves can easily penetrate in the centre of the bread. With typical German rye breads, which have a rather compact structure, the heating time is considerably longer than the time needed for a traditional white loaf. It goes without saying that the packaging material must be transparent to microwaves. That is also the reason why the packaging material doesn’t heat up. It will increase slightly in temperature because of the physical contact between foil and product.

The cooling down of the bread should take place rather quickly. There is however a side-effect, the so-called after-pasteurisation. Because of the heat captured by or inside the product, pasteurisation goes on for a while even after the product has been removed from the pasteurisation equipment. However this consideration can also have a negative impact. In case not all spores are destroyed by the pasteurisation process, the cooling down might be the reason for the spores to start growing again. Also the pH of the product plays a role in this process. Breads which are slightly acid (such as rye breads or wheat breads made with sourdough) are more readily pasteurised as also the low pH has an anti-microbial effect.

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:

Normally the following gasses are used:

In order to be complete, I also want to mention that in some cases oxygen (O2) is added to the packet. This happens preliminary in the meat sector. After all the air is removed, mixtures of nitrogen and carbon dioxide are, together with a small quantity of oxygen, are injected in the packet. The oxygen is used to keep a nice red colour of the meat.

In most cases a mixture of nitrogen and carbon dioxide is used. For bakery products these gases are normally mixed in a 20:80 ratio (20 % nitrogen + 80 % carbon dioxide). The CO2 is used for its anti-microbial properties while the N2 is used on avoid that packet would become vacuumed. Indeed as the CO2 dissolves into the water in the product, there is always less and less gas present in the packet (until some kind of equilibrium is obtained) and this might give the impression that the product is packed under vacuum.

Important to remember that mould growth is inhibited easier than the development of yeasts and bacteria. There is one particular yeast (Saccharomycopsis fibuligera also known as Endomyces fibuligera) which looks like a mould but in fact is a yeast which can develop in anaerobic conditions. It appears as small white colonies which have a pseudo-mycelium. So it is very important to work in absolutely hygienic conditions and one should remember that belts in coolers are difficult to keep clean.

Saccharomycopsis fibuligera grows well at 30°C and at an aw-value of 0,976. This yeast has a low capacity to metabolise sugars and produces small quantities of maltose, glucose and ethanol. Characteristic for this yeast is that it forms small round or cylindrical shaped cells which, in anaerobic conditions, are seen as small white colonies with a pseudo-mycelium. Xylose and galactose are not converted by this yeast but it converts starch rather easily into dextrin.

There are other types of yeast which have the characteristic of growing with a pseudo-mycelium when they find themselves in anaerobic conditions. One of them is for instance is Pichia burtonii. If the bread is contaminated with one of these micro-organisms, it will develop within 4 days approximately. Miller did a study on Saccharomycopsis fibuligera in 1942 and concluded that the speed with which it develops, depends on the temperature, the moisture content of the bread and the extend of the initial contamination. It develops in a temperature range of 15 - 35°C and a pH range of 4,2 – 5,0. The growth of S. fibuligera is completely stopped by the use of 0,20 % calcium propionate or 0,05 % sorbic acid.

Strangely enough Pichia burtonii is mainly found in the United Kingdom while Saccharomycopsis fibuligera is found on the European continent. This might seem strange but in fact it is not. It is linked to the different way bread is made in the UK compared to other countries on the continent. Main causes are different dough pH, the crumb structure of the bread and the type of wheat.

MAP and potassium propionate

An interesting study has been done by M. Rodriguez, L. Medina and R. Jordano (University of Cordoba) and published in the German scientific journal Nährung (number 44 in 2000). In this study the shelf life of bread was examined in function of a number of variables:

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 moulded after 13 days. Also breads kept at a lower temperature (15 - 20°C) were also all moulded after 13 days. Samples that were made with calcium propionate were all moulded after 20 days when kept at 22 - 25°C and all of them were moulded 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 moulded when kept at 22 - 25°C. Also at lower temperature all samples were moulded after 13 days. There was no difference with the breads packed with air.

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

Samples packed with 80:20 carbon dioxide - nitrogen

All samples that did not contain calcium propionate were moulded 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 moulded). At lower temperature it took 52 days before all the samples were moulded.

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 moulded 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 moulded). At lower temperature, after 52 days 1/3rd of the samples were not moulded. 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.

By using the technique of controlled atmosphere, the producer wants to achieve the following objectives:

In practice 100 % CO2 is only used when the baked goods have a moisture content of 20 % or lower. In all other cases a mixture of nitrogen and carbon dioxide is used. There are two techniques used for modified atmosphere packaging which are very different from each other.

Flow pack machines which use the technique of air flushing. While the packet is being sealed, the gas mixture is injected under a certain pressure into the packet. In this way the air is replaced by the gas mixture. This technique has the advantage that it does not require costly investments (equipment is relatively cheap) and these machines are capable of producing a high number of packets per hour. The disadvantage of this technique is that there remains anything up to 5 % of air inside the packet and therefore products will get moulded in a relatively short time. The amount of oxygen remaining in the packed is also a function of the roughness of the surface of the bread (captivities).

The second technique is totally different. In this case all air is removed before the gas mixture is injected in the packet. To remove the air a vacuum is applied so all air is out of the packet. Afterwards the gas mixture is injected in the packet. In this technology the residual oxygen can be as low as 0,40 – 0,70 %. The disadvantage is that the machines are very costly and slow (in a 4-row machine, one can do about 7 x 4 = 28 packets per minute).

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.

A number of aspects have to be kept in mind:

The EVOH layer determines the properties of the foil. If there is more ethylene present the barrier properties get worse. In practice 32 % or 38 % (and sometimes 44 %) EVOH is used in the multi-layered foils. It means that the EVOH contains 32, 38 or 44 % of ethylene. The permeability of the foil depends on:

In principle the barrier properties of the foil are less effective with increasing level of ethylene. This is true for a humidity content of about 95 %. When the relative humidity is more than 95 %, the opposite is true: foil containing 38 % of ethylene forms a more effective barrier than foil containing 32 % of ethylene. Also the temperature plays an important part in the permeability of the foil. As a rule of thumb one can say that with a 10°C increase in temperature the permeability increases by factor 2. Finally one should remember that the permeability of CO2 is about 4 times more than the permeability for oxygen and the permeability of nitrogen is about 1/4th of the permeability of oxygen.

The most popular foils are composed as follows:

The relative humidity in the packet will be high: an the aw-value of 0,97 means that the relative humidity in the packet is 97 %, while the air around the packet normally will be lower in RH. This means that there moisture will migrate from inside the packet towards the atmosphere around the packet. As polyamide has a low moisture barrier, the moisture will readily diffuse out of the packet and the EVOH layer will be kept dry (in this case of the bottom foil – type I foil). In the case of type II foil the opposite phenomenon takes place: there is no polyamide layer and for this kind of foil the EVOH layer becomes more moist and loses its barrier properties for O2 and CO2.

A number of measurements gave the following results concerning the barrier properties at 23°C and 0 % RH:

3.5.5 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 favour 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. Fibre will also absorb water but has little influence on the aw-value. As a result products with reduced salt and/or sugar 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. In this context one can distinguish 3 different types of preservatives:

However also here certain sourdoughs can be a solution. Wheat flour can be fermented not only with lactic acid bacteria but also with other types of bacteria which will produce a number of acids during the fermentation. The following table gives an idea of the type of acids that will be produced by the bacteria.

raw material
% lactic acid
% acetic acid
% propionic acid
fermented wheat flour
5,0 - 7,0 %
< 2 %
13,0 - 15,0 %

However certain sourdoughs can be a solution. Wheat flour can be fermented not only with lactic acid bacteria but also with other types of bacteria which will produce a number of acids during the fermentation. The following table gives an idea of the type of acids that will be produced by the bacteria.

raw material
% lactic acid
% acetic acid
% propionic acid
fermented wheat flour
5,0 - 7,0 %
< 2 %
13,0 - 15,0 %

Apart from the above mentioned acids also other acids such as butyric and malic acids are formed and there is a synergetic effect between all these organic acids. After fermentation the amount of propionic acid will be around 30 - 32 %. However during the subsequent drying operation, together with the fact that propionic acid is relatively rather volatile, the final product will contain 13,0 - 15,0 % propionic acid. And as we all know propionic acid is an excellent anti-microbial agent. One has to keep in mind that the effectiveness depends on the pH of the dough. A pH around 5,0 is ideal. At a pH higher than 5,2 the effectiveness declines. A dosage level of 1,5 - 2 % of the fermented wheat flour is normally sufficient to protect the bread on condition that also all other precautions have been taken to avoid contamination.

Another aspect of the usage of fermented wheat flour is the declaration. The product can be declared as fermented wheat flour or as sourdough. However if the bread is analysed, propionate will be found and one can wonder why the propionate is not mentioned in the ingredient list. Well I don't think it should be declared because it is also not declared in for instance cheese where the propionate is also produced in a natural way.

Also a word of caution. It is impossible to distinguish the propionate that was produced in the all natural way by fermentation and the one obtained from a chemical reaction. Some(Italian) companies offering so called 100 % natural fermented wheat flour and cannot be trusted. Insist on seeing the fermentation plant and if they refuse because of the so called "confidentiallity" of the process, just don't believe them and go and look elsewhere. These companies buy some products from China (which cannot always be trusted) and add chemical propionate. If necessary I can advise you about companies that can be trusted.

Propionates (E282)

Propionic acid and its salts are the most widely used chemical preservatives in bakery foods. Although some consumers consider propionates as “undesirable chemical additives,” it should be remembered that propionic acid occurs naturally at a level of about one percent in Swiss cheese. Propionates are used at a substantially lower level as preservatives in baked foods.

Calcium and sodium propionates are white, water soluble powders. They have a slight cheese flavour which is not detectable at low levels of use, but impart a characteristic somewhat bitter flavour when used at higher levels (0.5 to 0.7%) in English muffins.

The antimicrobial effect of propionic acid was reported as early as 1913 (8). Work published by Hoffman et al. in 1939 showed the effective antimicrobial action of propionic acid at pH 5.0-6.0.

Propionates are effective against a broad spectrum of moulds. They only mildly affect yeast activity at levels normally used in yeast leavened bakery foods. However, if used at higher levels, propionates can retard yeast activity and extend fermentation times.

Propionates have limited effectiveness against bacteria with one significant exception: they do retard the growth of Bacillus mesentericus, the organism responsible for “rope” in bread and other yeast leavened products.

It must be kept in mind that it is the propionic ion which is effective. Hence the pH plays an important role. At pH 4,8 (pKa value) about 50 % of the calcium propionate is dissociated in the Ca2+ ion and the propionic ion.

Ca(CH3-CH2-COO)2 -> Ca2+ + 2(CH 3-CH 2-COO)-

2(CH3-CH2-COO)- + 2H20 -> 2CH3-CH2-COOH + 2OH-

The slowing down of mould growth by calcium propionate depends heavily on the pH of the dough. If, for instance, the pH of the dough is 0,3 pH-units lower or higher, than the effectiveness of the calcium propionate is doubled or reduced to half. The reason for this is that the pH scale is a logarithmic scale and log 2 = 0,3. So a strict control of the pH of the dough is necessary. Extra vigilance is necessary when sourdough is used in the recipe. However to be effective, the pH should be lower than 5,4. To achieve this one can add some vinegar or citric acid (0,2 % normally is sufficient).

Sorbates (E200)

Both sorbic acid and potassium sorbate are effective mould inhibitors. The potassium salt is more soluble than the acid, but is only about 74 percent as effective. Sorbic acid and potassium sorbate are slightly more effective than propionates as mould inhibitors, and therefore may be used at slightly lower levels. When used at higher levels, sorbates will retard yeast activity and extend fermentation or proofing times of yeast leavened products.

Sorbates are more effective than propionates at slightly higher pH and might be preferred in higher pH chemically leavened baked foods. However, the sorbates are relatively ineffective at pH 7.0 and above. Like propionates, sorbates are effective against rope forming bacteria. The reason why sorbates are mainly used in chemically leavened product such as cakes or sponges is because the pH of the batter is normally around 6,5, while in the case of yeast leavened products the pH is normally lower (5,0 – 6,0). As a result calcium propionate is more effective than sorbates in yeast raised products.

MFSL with sorbic acid
trial number g E200 per 100 kg dough Mould free shelf life
1
187 g
47 days
2
192 g
58 days
3
200 g
63 days

Because of a possible adverse effect on yeast activity in fermented products, sorbates may be applied as a spray of a dilute (1 to 6%) aqueous solution to the baked product as it emerges from the oven. The heat of the product evaporates the water from the spray, leaving a residue of preservative on the surface where most post-baking contamination is likely to occur.

Sorbic acid has another side effect on dough systems. At a very low usage, 20 to 50 ppm, sorbic acid is an effective mix time reducer (as much as 25%) in some dough systems. At this level it has no effect on yeast activity.

Sorbic acid and sorbates are GRAS substances and may be used in bakery foods at levels not to exceed good manufacturing practices. They are free flowing products and essentially tasteless and odourless. Again in Europe legislation is more strict: the maximum level allowed is 0,1 % potassium sorbate on the flour.

Vinegar

We already mentioned that vinegar is an excellent preservative against rope. The anti-microbial effects of vinegar were already known in ancient times. Vinegar is regarded as a natural preservative.

The effectiveness of vinegar is mainly due to the fact that it lowers the pH of the product. Normally bigger quantities of vinegar have to be used to get the same pH-lowering effect as some other additives. There has to be at least 0,50 % vinegar present to have some kind of anti-microbial effect. It is also more effective against bacteria and less effect to combat mould development. By adding 0,3 – 0,5 % of vinegar to the dough, mould development will slow down considerably.

Finally one should also remember that vinegar can be used to disinfect the bakery. It is advisable to clean tables and utensils regularly with vinegar. One has to use about 90 ml vinegar of 8 % per square meter to be effective. If less vinegar is used one gets the opposite effect because moulds grow better in mildly acid conditions.

Alcohol

In southern European countries, especially Italy and Greece, packed bakery products (breads, croissants etc.) are injected with alcohol. In fact the use of alcohol in the packet is a particular case of modified atmosphere packing. The alcohol is present as alcohol vapour in the packet and it is a well-known fact that alcohol can be used as disinfectant (hospitals for instance or labs specialised in microbiological testing, use it all the time). According to the season and as rule of thumb, in winter 1 – 2 ml of alcohol is injected in the packet of a 400 g bread while in summer up to 4 ml is used. It is assumed that the alcohol also lowers the aw-value of the product.

Alcohol is very effective against moulds but not so much against yeast and sometimes white yeast colonies with a pseudo-mycelium will grow in the packet. Also alcohol will give a typical smell when the packet is opened and one can wonder whether or not this a good thing. This preservative, although perfectly safe, is more and more criticised and food manufactures wonder if it is such a good idea to add alcohol to products which are intended for children, let alone considerations with regards to religious objections.

One final remark. It also seems that alcohol slows down the staling process. The mechanisms is not understood but by using alcohol bread remains softer over time.


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Noël Haegens

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