What are the best conditions to use for proving bread dough

The main purpose of the proving stage in baking is to expand the dough piece and modify its rheology to obtain further expansion and structure development in the oven. To achieve this we need to generate carbon dioxide gas from yeast fermentation. So our first consideration is to provide the best possible conditions for yeast activity. Yeast will produce carbon dioxide gas over a range of temperatures running from around 0 °C. As the temperature rises gas production increases reaching a maximum at around 43 °C. By the time that the temperature has reached 55 °C all yeast activity has ceased and the cells are dead.

Usually we seek to achieve around 90% of our required product volume in the proved dough, leaving the last 10% or so to come from oven spring. The time that it takes for this point to be reached in the prover depends mainly on the proving temperature and the level of yeast, that is present in the dough. The greater the quantity of yeast, the shorter will be the proving time to a given volume. Thus, if our sole criterion for deciding on proving conditions is to leave the dough in the prover for as short a time as possible then we would choose a high yeast level and a temperature around 40-43 °C, and to a large extent this is the norm in most bakeries.

The other issue we have to consider is the relatively poor conductivity of heat by dough. The dough commonly enters the prover at a lower temperature than the air in the prover. As proving proceeds, the outer layers quickly warm while the dough centre remains cooler. If the yeast level is very high the outer layers will quickly become over-proved and lose their gas retention properties. Large temperature differentials in a dough piece by the end of proving tend to give poorer product quality, showing as lack of volume and uneven cell structure.

The other condition that we must pay attention to is the relative humidity of the air surrounding the dough. The dough relative humidity lies around 90-95% and so there is considerable potential for surface evaporation unless we take steps to raise the prover humidity. Typically we raise this to around 85% to minimise surface evaporation or skinning.

In summary the best proving conditions to use are the ones that are most 'dough-friendly'. This would suggest temperatures similar to those that we achieve in doughmaking but this would give extended proving times unless we raise yeast levels to such an extent that we may incur unacceptable flavour changes or unnecessary high ingredient costs. The practical compromise suggests temperatures from 35 to 40 °C with appropriate humidity control.

Further reading

CAUVAIN, S.P. and YOUNG, L.S. (2000) Bakery Food Manufacture and Quality: Water control and effects, Blackwell Science Ltd, Oxford, UK.

6.15 Can we freeze our unproved dough pieces and store them for later use?

The freezing and storing of unproved bread and other fermented doughs is perfectly possible but does require some attention to all aspects of dough production, processing and subsequent use on defrosting. The following guidelines highlight some of the most important areas:

• Use a no-time doughmaking process as periods of fermentation before freezing have an adverse effect on bread quality.

• Use ingredients and a dough formulation that give good products by scratch production. Freezing and thawing cannot improve product quality.

• Raise your recipe yeast level to compensate for the loss of gas production from yeast cells that are killed during the freezing and storage. Or use a yeast strain with a greater tolerance to freezing.

• Freeze the dough as quickly as possible after moulding to minimise gas production.

• You may need to adjust product dimensions before freezing as doughs may sometimes spread during freezing and fail to fit the pans when you take them out for thawing.

• Use a blast freezer but avoid air temperature less than —30 °C because of adverse effects on product quality.

• Ensure that products are fully frozen, with a core temperature of at least — 10 °C, before passing to storage to minimise quality losses.

• Expect progressive loss of final product volume as frozen storage time increases so compensate with increased proving times.

• Thaw the products using low temperatures and long times to minimise temperature differentials between the dough centre and its surface when it reaches the end of proving.

• Select carefully the products that you wish to make with frozen dough. Products with small diameters such as rolls and baguettes will be more successful than thicker products such as pan breads.

Further reading

CAUVAIN, S.P. (1998) Dough retarding and freezing, in Technology of Breadmaking (eds S.P. Cauvain and L.S. Young), Blackie Academic & Professional, London, UK, pp. 149-179. KULP, K, LORENZ, K. and BRUMMER, J. (1995) Frozen and Refrigerated Doughs and Batters, American Association of Cereal Chemists Inc., St Paul, Minnesota, USA.

STAUFFER, C.E. (1993) Frozen dough production, in Advances in Baking Technology (eds, B.S. Kamel and C.E. Stauffer), Blackie Academic & Professional, London, UK, pp. 88-106.

6.16 What happens when dough bakes?

In simple terms when dough enters the oven it expands and loses moisture, the crust darkens and the dough sets (Fig. 20). Behind this simple description are a great many different physical and chemical changes, which are summarised as follows:

• Gas production by the yeast continues as the dough temperature rises in the early stages of baking. When all of the dough exceeds 43 °C the rate of gas production falls and eventually ceases by 55 °C. While the dough surface is rapidly heated and yeast activity ceases there the poor heat conductivity of dough means that the centre continues to produce carbon dioxide gas for some time after the crust has formed. The force that is created by the expanding centre means that tin dough springs upwards, creating oven spring.

• The dough is also being expanded by steam pressure and the expansion of trapped gases.

Compression

Expansion-^

Lidded bread

Open top bread

Oven spring in cuts

Oven bottom bread

Fig. 20 Changes in bread dough during baking.

Oven bottom bread

Fig. 20 Changes in bread dough during baking.

• In order for the dough to continue to expand during baking it must be able to retain the gas that is being released. The stresses placed on the dough during the early stages of baking are much greater than those placed on it during proving and it is only in the oven that doughs that truly lack the correct gas retention properties are exposed. Commonly, lack of gas retention is seen as lack of oven spring or in more extreme cases as collapse.

• The dough loses moisture with increasing baking time. The moisture losses are greatest from the crust and this encourages the formation of a crisp eating, crusty layer.

• The Maillard reactions begin to develop the crust colour.

• The starch begins to swell and gelatinise. At this time more of it becomes susceptible to the action of any a-amylase enzymes present and the breakdown to sticky dextrins and maltose is accelerated by the higher temperatures.

• In the dough the gas bubbles present are separated from one another by a thin protective film. Since they are not connected with one another they are commonly described as a 'foam'. As baking proceeds, the loss of water makes the gluten protective film become more rigid and the pressures within the gas bubbles rupture the protective films. The foam in the dough is converted to a sponge, that is a system in which all the cells are open and interconnected. At this time the volume of the baking loaf falls slightly as the internal and external gas pressures are equalised.

• Moisture continues to be lost while the product remains in the oven.

• All of the necessary changes from dough to baked product are usually achieved by the time that the product centre reaches a temperature between 92 and 96 °C.

Further reading

CAUVAIN, S.P. and YOUNG, L.S. (2000) Bakery Food Manufacture and Quality:

Water control and effects, Blackwell Science Ltd, Oxford, UK. WIGGINS, C. (1998) Proving, baking and cooling, in Technology of Breadmaking (eds S.P. Cauvain and L.S. Young), Blackie Academic & Professional, London, UK, pp. 120-148.

6.17 Why do crusty breads go soft when they are wrapped?

When crusty products leave the oven the moisture content of the crust is much lower than that of the centre crumb. Typical values can be as widely apart as 12 and 42% respectively. From the moment of leaving the oven this moisture differential provides a driving force for moisture migration from the crumb centre to the crust. This moisture migration continues as the product begins to cool and carries on during subsequent storage. Eventually the crust moisture content rises to a level at which the product is no longer crisp or crusty.

The rate and extent to which the moisture migrates from the crumb to the crust depend on several different factors, including the storage temperature. The lower the storage temperature, the lower the rate of moisture migration, but note that the rate of non-moisture related firming will increase (see 6.18).

The usual process for the movement of moisture in crusty bread is from crumb to crust as nature tries to achieve moisture equilibrium between the two components. The moisture content of the crumb falls and that of the crust rises. If the product is unwrapped then the crust generally loses moisture to the surrounding atmosphere provided that the atmosphere relative humidity is lower than that of the loaf. In practice this is mostly the case and air draughts sweeping across the product surface carry the moisture away. This lost moisture is replaced by more migrating from the crumb and the whole product dehydrates and loses consumer appeal.

To prevent this dehydration bread is wrapped in a suitable protective film but if crusty bread is put in a moisture-impermeable film (e.g. a polyethylene bag) then the moisture that would have been swept away remains and the bread quickly comes to equilibrium with the atmosphere in the wrapper. The result is that crustiness is quickly lost. The alternative is to use a semi-permeable film to let some of the migrating moisture escape through the holes in the wrappers and help keep crust crispness for a longer time. Commonly perforated films are used for the purpose, the size and distribution of the perforations being used to control the rate of moisture loss.

A common cause of loss of crust crispness, even when perforated films are used, comes from wrapping bread too warm. In many bakeries bread freshness is only equated with the product being hot, and staff may be encouraged to wrap the product while still warm. This practice has three main disadvantages:

1. Moisture will be lost from the warm bread and condense within the wrapper. The moisture will be re-absorbed by the product crust and so in the case of crusty breads it encourages softening of the crust.

2. The loss of crust crispness leaves the bread susceptible to crushing on the shelf and in the shopping basket.

3. Condensation encourages the localised raising of product water activity and so encourages the growth of moulds.

Further reading

CAUVAIN, S.P. and YOUNG, L.S. (2000) Bakery Food Manufacture and Quality: Water control and effects, Blackwell Science Ltd, Oxford, UK.

6.18 We have been comparing our bread with that of our competitors and find that the crumb of our bread is firmer. Why?

There are a number of reasons why we can have differences in breadcrumb softness; some are related to the ingredients used while others are directly affected by the processing methods. The first element to look at is whether there are any differences in the moisture content of the crumb of the breads concerned: the higher the crumb moisture content, the softer the bread will appear to be (Cauvain and Young, 2000).

Bread softness is directly related to bread volume and the greater the volume of the bread, the softer it will be. Even when breads have the same volume we may still see differences in softness which are related to the density distribution in the crumb cross-section. If we want to make the crumb of pan breads softer then one possibility is to create greater expansion of the centre crumb to lower its density and resistance to compression. We can do this by increasing the gas retention in the dough using ingredients such as oxidants, fat, enzymes and emulsifiers, or by improving dough development during mixing.

However, producing a crumb that is less resistant to compression is only part of the answer to making fresher bread. We also need to make a crumb that will largely recover its original shape after the squeeze test. This again can be achieved by improving dough gas retention. In particular we would want to create a fine (small average cell size) crumb cell structure with thin cell walls. This can best be achieved by creating a gas bubble structure in the dough which consists of many small bubbles and expanding them uniformly without excessive damage to the dough during moulding.

All bread goes firmer during storage, even if moisture is not lost from the crumb. This firming process is the one most often referred to as ' staling' and is largely associated with the recrystallisation (retrogradation) of the starch in the bread (Pateras, 1998). A number of factors will influence the rate at which bread stales including the following:

• The storage temperature - bread staling increases as the temperature of storage falls, reaching a maximum at about 4 °C. Check the temperatures in your despatch and storage areas and see if they can be made warmer but watch out for greater microbial spoilage.

• The presence of emulsifiers in the formulation. Some emulsifiers work to improve crumb softness by improving dough gas retention or through reducing gas bubble size in the dough. Additions of GMS can be used to slow the starch retrogradation process but it is important that the GMS is added in its active alpha form, commonly as a hydrated gel.

• Maltogenic amylases can be added to the dough and these also affect starch retrogradation. In this case the bread will not only start softer but firm at a lower rate.

You should also look at your oven baking conditions. Generally softer bread is obtained if you can bake at a higher temperature for a shorter time but of course there are limitations. Also check your cooling process and whether you can shorten the time being taken. The shortest possible cooling time will be dictated by the temperature at which you can slice or wrap your product. You do not want to encourage condensation in the wrapper, which can encourage mould growth.

References

CAUVAIN, S.P. and YOUNG, L.S. (2000) Bakery Food Manufacture and Quality:

Water control and effects, Blackwell Science Ltd, Oxford, UK. PATERAS, I. (1998) Bread spoilage and staling, in Technology of Breadmaking (eds S.P. Cauvain and L.S. Young), Blackie Academic & Professional, London, UK, pp. 240-261.

6.19 We have been deep-freezing bread products and experience a number of problems with different products. With crusty products we observe that the crust falls off, but with some other products we find that longer periods of storage lead to the formation of white, translucent patches in the crumb which are very hard eating. Are the problems related to the performance of our freezer?

The first of your problems is commonly referred to as ' shelling' , that is the loss of the crust from frozen fermented items which may occur during storage but more commonly manifests itself when the product is defrosted. Similar problems may be observed with some part-baked, frozen products.

When all bread products leave the oven the moisture content of the crust region is much lower than that of the crumb. This differential in moisture content is much greater in crusty products than with many other types of bread, e.g. sandwich breads, and is an integral part of the character of the product. The difference in moisture content between crust and crumb is partly responsible for their differences in texture, with the low moisture crust having a harder, more rigid character than the higher moisture content soft crumb. The difference in moisture content also means that the salt concentration is higher in the crust region than in the centre crumb, which will lower the temperature at which ice forms in these regions.

The combination of different freezing points and structural architecture means that the crust and crumb will expand and contract at different rates. The stress that this places on the interface between the two regions may become so great that they become separated from one another. This phenomenon will occur under almost any freezing condition so it is unlikely that your freezer performance is directly to blame for the problem. You will have to accept that you are unlikely to freeze crusty products successfully because the only solution is to allow equilibration of moisture before freezing, but then the product would not be crusty anyway!

Your second problem could well be related to your freezer performance and is a phenomenon known as 'freezer burn' (Cauvain and Young, 2000). It comes from the loss of water from different regions of your product during frozen storage. It has been estimated that about 30% of the water in bread remains unfrozen in bread, even at —20 °C. This 'free' water may leave the product and enter the freezer or pack atmosphere where it eventually shows as ' frost' . The hard, translucent patches that you see are areas of crumb that have become dehydrated in the freezer.

The condition is exacerbated by any periods when the freezer has been allowed to warm to temperatures above the freezing point of the product. The higher temperatures accelerate moisture losses and the slow re-freezing that follows also contributes to the problem. We suggest that you look at your freezer performance and in particular any changes in conditions during the defrost cycle. Also look closely at your operating procedures and try to minimise the opening and closing of the freezer. This is a common cause of the problem because the cold air is lost and replaced by warmer air which raises the temperature of the products nearest to the door or lid.

Reference

CAUVAIN, S.P. and YOUNG, L.S. (2000) Bakery Food Manufacture and Quality: Water control and effects, Blackwell Science Ltd, Oxford, UK.

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