The role of lipids in determining gas bubble retention in wheat dough

Bread is an essential dietary staple, which has a significant influence on the nutritional profile of the population in terms of energy intake, fat and salt consumption. Approximately 80 million loaves are produced in the UK each week in a business worth around £2.5 billion per year. The UK still imports significant amounts of wheat for bread making due to higher protein content and quality. Bread quality is determined by gluten strength and dough bubble stability, which have impacts on loaf volume and crumb structure, respectively. The gluten network formed in dough controls the elasticity of the dough which in turn controls the dough's ability to rise during proving, and its behaviour during baking. Bubble stability controls the extent to which bubbles coalesce during this time, enabling the fine texture typical of UK sliced bread. However, while dough strength is now well understood, with plant breeders routinely selecting for specific gluten proteins which confer high dough elasticity, bubble stability is as yet still poorly understood. Therefore if we can identify ways by which UK grown wheat can be improved to give better bubble stability, we would enable:-
- breeders to develop better quality wheats,
- manufacturers to produce better quality bread,
- reduced reliance on imported wheat
- development of healthier bread with reduced salt and fat
The proposal focusses on how the gas bubbles in dough are stabilised. Gas bubbles can be stabilised by proteins, surfactants or lipids forming a stabilising layer on the surface. The molecular properties of the stabilising layer will determine whether the bubbles burst or coalescence. This is particularly important as the dough rises or proves, because to increase the volume of the loaf, the gas bubbles in the dough expand, and eventually come into contact. It is at this point when they will either remain stable, producing a good quality bread with fine structure, or the bubble will coalesce with each other, leading to partial collapse of the dough, and poor quality bread.
Research has focused on both protein and lipid components in wheat flour, but the story is not clear, as dough is fragile but viscoelastic and therefore difficult to study directly without destroying the gas cell structure. However, the consensus is emerging that it is the wheat lipids largely control bubble stability.
Wheat flour contains a range of lipids, all of which will adsorb to the surface of the bubble, but their differing molecular structures will have different (positive or negative) effects on bubble stability. The lipid composition of the flour will therefore be critical for dough stability. Using state of the art lipid analysis techniques we will identify the lipids which stabilise gas bubbles in dough. Using novel surface and biophysical techniques we will determine how the different lipids stabilise the gas bubbles, and what their effect is on the stability of the dough and the quality of bread produced. We will determine the variations in the amounts of the different lipids occurring different wheat varieties to develop targets for breeders to improve the bread making quality of UK grown wheat.
In addition, improving the gas bubble stability in bread dough will allow manufacturers to reduce the levels of salt, fat and emulsifiers in bread. This is because salt is required to improve dough strength, and fat (as shortening) and emulsifiers are added to improve gas cell stability. Increasing the natural stability of the gas bubbles will reduce the need for the levels of salt and fat currently required to produce the quality of bread desired by consumers.

The proposal will exploit new analytical opportunities and approaches to clearly define the role of endogenous wheat lipids in determining gas bubble stability in dough and hence bread making quality, and exploit this knowledge by selecting improved wheat lines for breeding.
It will benefit from a unique combination of skills and facilities, including new analytical capabilities offered by the "lipidomics platform" at Rothamsted Research and biophysical methodologies including atomic force microscopy (AFM) at IFR. Baking and analysis of bread structure will be carried out at Campden BRI.
1. Fermented dough from a single variety of UK breadmaking wheat will be fractionated to isolate lipids associated with the gas bubble interface and profiled by electrospray tandem mass spectrometry (ESI-MS/MS). Surface tension and surface dilatational rheology will identify functional behaviour of individual lipids.
2. Mechanisms of gas bubble stabilisation by the lipids will be determined by foam microconductivity, interfacial tension, interfacial rheology and interfacial imaging using AFM.
3. The functional behaviour of individual lipids will be established by addition to dough systems. Proving volumes and the evolution and properties of gas cells will be determined by microscopy and image analysis of dough proofed against a glass plate. Gold-labelled lipids will be incorporated and their location established by SEM.
4. Full scale baking trials will determine the impact of lipid composition on bread making quality, and innovative image analysis techniques will quantify the impact on crumb structure of the final product.
5. Genetic variation in lipid composition and gas bubble stability will be determined by analysis of 20 modern wheat line
This knowledge will lead to the production of UK grown wheat lines with improved bread making capacities and facilitate the production of healthier breads with reduced levels of salt and fat.

About 6 million tonnes of wheat are milled in the UK, with most of the flour being used for breadmaking. Most of this is "home grown" but it is also necessary to import significant quantities of grain from the EU and third countries to maintain the quality of bread demanded by consumers. The volume of imported wheat ranges up to about 1 million tonnes a year depending on the quality of the UK harvest. Improved quality for breadmaking therefore remains an important target for UK wheat breeders and farmers.
The demonstration of a clear role of lipids in determining dough stability will therefore provide a new target for UK wheat breeders in addition to the current selection for glutenin alleles associated with dough strength. This will enable breeders to improve wheat quality in a more focussed and rational manner, producing wheat varieties tailored for particular end uses.
Farmers will benefit through a wider choice of wheat varieties, with improved, rationalised end product quality, ensuring premium prices for high quality crops, and be more competitive with the quality of imported grain.
Processors will be less reliant on expensive import costs to maintain flour quality, and will be able to supply manufacturers with a more consistent supply of UK grown wheat.
Improved baking quality will allow manufacturers to develop a wider range of products, including healthier products with reduced levels of salt and fat and reduce the use of synthetic emulsifiers.
Consumers will benefit from good quality, low cost products, due to reduced levels of imports. A wider choice of good quality, healthier products will also improve consumers ability to maintain a healthy lifestyle.
The nation as a whole will ultimately benefit through reduced reliance on imported products, enhanced competitiveness from UK breeders and growers.
Improving the intrinsic quality of UK breadmaking wheats will therefore accrue benefits throughout the food chain, including increased market shares for breeders, premium prices for farmers, reduced costly imports for processors and low stable prices for consumers.
Information from the project will have wider health benefits in enabling bakers to reduce the levels of salt in bread and other food products. Salt is currently included at about 1.4% in bread (corresponding to 17% of the UK recommended daily intake) and affects the functional properties of dough by interacting with the gluten proteins as well as contributing to flavour. Reducing the salt content of dough results in a requirement for greater dough expansion during proving (as the dough sets at a lower temperature during baking), which in turn requires a more stable bubble structure. Consequently, improving the stability of gas bubbles in dough is a crucial stage in reducing the salt content of bread to the current target of about 50% of the current concentration. The use of fat and emulsifiers in baking is also widespread to improve gas cell stability, and their levels will also be reduced by increasing the natural stability of bread dough.
Therefore this research will benefit consumers and the health of the nation in the longer term, through the reduced intake of salt and fat.