Bubbles in Bread
August 01, 1998
by Teresa Acklin
Physical structure and bubbles serve as defining characteristics in high-quality bread.
Bread comes in many different shapes, forms and sizes both within and between countries. Bread, for the purpose of this article, is described as a baked product made from wheat flour, yeast, salt and water. Other additives are employed depending on custom and legality in different countries, but in its final form, bread consists of two principal characteristics physical structure and bubbles.
Bread quality is a highly localized matter. The baker knows his market and he produces on a consistent basis day-by-day for his consumer. So, bread that is perceived as good bread in one locality is not necessarily acceptable in another.
Accordingly, bread of a larger or smaller volume, crumb structure, loaf shape, size and crust feature is aimed at the ultimate consumer together with the demanded organoleptic characteristics of appearance, eating quality, taste and aroma.
How do bubbles get into bread? The short answer is by fermentation in the main dough. The starting point of the process is the mixing of the principal ingredients to dough. At this point some air is drawn in, entrained and trapped. This is not of ultimate concern, except perhaps in mechanically developed dough processes.
Yeast is present to ferment the dough. This produces carbon dioxide for aeration and ethanol (alcohol) that is largely lost, though finished bread retains up to 0.3% carbon dioxide. The former is responsible for the general matrix of holes or bubbles in bread, the latter primarily for its aroma. However, it is not as simple as that.
Given that water, yeast and salt ingredients are in balance, the flour used has the significant role all else being equal. The miller needs to produce consistent flour for the baker. Without this factor the baker will not be able to optimize. The desired bread is influenced by the two main components of flour namely vital gluten (insoluble protein) and starch. Vital gluten has retained its vital full-natural functionality, i.e., extensibility and elasticity.
Gas production capability during fermentation relies primarily on yeast and amylase in flour; gas retention relies mainly on gluten quality for the particular fermented product desired. Other influences abound from flour, which include the degree of damaged starch, gluten and non-gluten protein quantity, alpha amylase activity, penotosans, proteolytic activity and fat content. The degree of purity (mineral content, color) and therefore the extraction level of flour from wheat is also important. The lower the mineral content the bolder the loaf and more pleasing the texture, including bubble formation.
So, wheat variety selection is important in gas retention. Even with high protein, some wheat will not retain or hold gas during fermentation by degree or comparison. This is difficult to measure or identify in the mill laboratory except by a baking test. This effect can be adjusted beneficially by the use of emulsifiers and fats in some cases, typically in the bakery.
In this regard it is well-known that each new harvest provides wheat of a different quality to the previous one even if it is only a small difference.
It is here that the improver or dough conditioner manufacturer can help by rebalancing additives, whether they are emulsifiers, fats, fungal enzymes, malt flours, oxidizing or reducing agents, lecithin, soya or bean (popular in France) flour.
Arrange trials with a sympathetic baker in the final event in order to eliminate doubt. Moreover, the same supplier can often aid both the baker and miller by producing a balanced improver that will allow an apparently poor wheat flour to perform satisfactorily but both practical and cost limitations do exist.
A correct level of damaged starch is needed. Too little will not attract enough water, too much will give a bias towards a lower loaf volume, gray crumb, waterlogged appearance and inconsistent or undesirable holes in bread. Higher protein flours result in elevated starch damage. Gluten will hold approximately twice its own weight of water, undamaged starch between one-third and half its own weight and damaged starch will hold roughly its own weight. When damaged starch is too high for the gluten quantity present, there is not enough gluten to wrap around the starch to allow proper gas bubble formation thus, relative collapse or lack of volume ensues.
Flour that is too weak but good in other respects will produce an unduly open hole texture, whereas too strong a flour has a tendency to show holes of an elongated nature. Excessive proving of the dough, where the dough has been allowed to stand either for a few minutes too long or at too high a temperature or both, is likely to produce coarse holes in bread as well as poor crumb color.
Incorrect molding tends to produce unwanted holes in the crumb. Reworked old dough, through skinning or chilling, is liable to show itself as unpleasing hard streaks or cores in the crumb.
Cold bread tins in the bakery have a similar effect where they are used. Another consistency upsetting factor in the use of tins may occur when the tins are jarred or knocked, especially after proof and before baking. The effect at this very sensitive point in the baking process is for the proved dough to drop because it lost some of its gas. As a result, the volume and bubble structure are spoiled.
Excessive alpha amylase activity in the flour provides a further source of trouble. This is evidenced by a sticky and darker crumb with a poor hole structure. Usually, this can only be corrected by a change of flour. Though not often a problem and easily corrected by the addition of fungal enzymes or diastatic malt flour, low alpha amylase activity appears as not only low volume with a pale crust color, but also an aerated structure that is closer or smaller than expected. Too small an amount of yeast for the particular bread making process in use as well as too much salt will also give rise to an irregular and uninvited hole matrix.
An example of various characteristics of the inside of a loaf of bread is shown on page 21. Both slices are from the same loaf. The legend highlights five separate and clear features with an explanation of each. Thus, it is apparent that the determination of the cause of any effect (or defect) in bread is a relatively complicated, though not impossible, matter.
Unwanted Bubble Structures
Accordingly, any abnormal practice in bread production will tend to give unwanted gas bubble structures, the use of an undue amount of dusting flour included. Dough, both “foam” at mixing and “sponge” around the proving stage and when rising in the oven, needs to be stable enough to allow regular production of gas bubble cells.
The formation, growth and containment of these cells has been, and still is, subject to much scientific research. Interesting approaches not only to the structure of bread, but also including bubble matrices, are constantly being tried. For example, the separation and reconstitution of individual flour components bit by bit in the laboratory has pointed the way by observation of the effects in bread making.
One of the surprising results of wet separation of gluten and starch from a poor baking variety of wheat was the excellent baking quality of the gluten separated. On the other hand, the A, B and C starches plus the solubles including pentosans were not good. The suggestion in this case was that other factors apart from gluten were harming the structure and bubble matrix of bread.
Taking whole wheat as opposed to white flour, neither crumb gas cell structures nor loaf volumes are as even nor as bold. The reason is the introduction of components from the outer layers of the wheat kernel. This includes the home of non-functional protein, high-mineral content, high-alpha amylase, high fiber, and of course, wheat germ (high-oil content).
None of these are helpful to bread making in performance when compared to white flour. Whole wheat breads can be enhanced for bolder structure and healthy bubble cells by the addition of vital gluten plus care and attention at the recipe and processing or bread making stage. Dough rheology, via the Brabender Farinograph, Extensograph or the Chopin Alveograph, as a routine mill laboratory test does not lend itself easily to determine or indicate those below par wheat flours. Dough rheology, by definition, deals with the dough step in the baking process, but not the baked product itself. It may or may not give an indication of a potential fault.
Accordingly, bread is a structure made up of cells created by gas bubbles during the process of making the loaf. A multiple set of factors come together to create the desired or indeed undesired effect. Some of those are swayed by the discipline additives and environment at the bakery. Others are clearly in the sphere of control of the miller. By these means the morphology of bubbles in bread, their size, shape and distribution, is decided.
David Sugden, independent consultant to the grain industries