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Croissant Chemistry

Consider the chemical reactions that transform an innocuous white powder -- flour -- into delicious pastries.

Dr Juliet A. Gerrard
and Dr Siân E. Fayle

Understanding the chemical reactions that take place in food is a challenging problem. Test-tube chemistry has the reactants limited and conditions strictly controlled by the experimentalist. In contrast, the chemistry that takes place in food is complicated by the large number of ingredients and varying conditions of temperature and water level. This makes the reaction mixture incredibly complicated -- it is no surprise that the chemistry involved remains far from understood.

If you have ever made bread in your kitchen, you will have found it a very different product to the one typically sold in the supermarket. One of the main reasons for this difference is that in the kitchen only simple ingredients are used: flour, water, salt, sugar and yeast. If you look at the ingredients label for a commercial loaf, you will find various other additions, typically emulsifiers, ascorbic acid, improving enzymes etc. This cocktail of additives is included to improve the quality of the product, when manufactured on an industrial scale. They are generally described in the baking industry as "flour improvers".

Traditionally, most flour improvers were discovered empirically, and added to a dough because they achieved a desired effect. How they achieved this effect was less important to bakers, who were understandably focussed on the end product. When scientists looked at the chemicals that were successful flour improvers, such as potassium bromate, they noticed that they were often oxidising agents. So, how do these oxidising improvers work?

Research has shown that many improvers act by forming crosslinks between food proteins. In effect, the proteins have been "stitched together" by the improver, resulting in larger molecules. This has been put forward to explain the improvement in product quality at a molecular level. Most of the crosslinks identified in food have been disulfide bonds, formed from the oxidation of two cysteine residues in the food protein. This explains why many flour improvers are oxidising agents.

We postulated that any protein crosslink, not just disulfide bonds, might improve the performance of a flour, and explored other possible flour improvers that could potentially crosslink gluten proteins, but were not dependent on oxidation. Top of the list of candidates was an enzyme, transglutaminase (TGA), which was known to catalyse protein crosslinking and was potentially suitable for use in the baking industry.

Enzymes are popular food additives, since consumers perceives them as "natural" and, unlike most chemical improvers, they are catalytic, and need only therefore be included in a recipe at low levels. Transglutaminase was a particularly appropriate choice, since it forms pseudopeptide bonds, analogous to those found in natural proteins.

So, does transglutaminase act as a flour improver? The answer to this question was an emphatic yes! Extensive research on the effect of TGA on bakery goods has shown very exciting results. The enzyme improver has a dramatic effect on pastry and croissants, as we were able to show in tests comparing goods with and without TGA.

Croissant Chemistry Figure A (66KB)
Using transglutaminase, as at left, can do wonders for improving a croissant's lightness.

Transglutaminase has also been shown to have beneficial effects on bread. A common consumer complaint with commercially produced loaves is that slices of fresh bread rip when buttered -- the bread has poor crumb strength. An instrumental measure of crumb strength involves rupturing the bread with a probe, and measuring the maximum force at rupture. The crumb strength was measured as the dose of TGA was increased, and a marked improvement was seen. Remarkably, the treated bread was still soft -- the crumb strength had improved, but not at the expense of the fresh produce texture.

The commercial potential of transglutaminase as a flour improver is still under investigation, but clearly represents an exciting development for the baking industry. This discovery of a new class of flour improver emerged from fundamental studies of the chemistry of protein crosslinking in dough.

Dr Siân E. Fayle is with Grain Foods Research at Crop & Food Research.
Dr Juliet A. Gerrard is with Grain Foods Research at Crop & Food Research.