7 minutes reading time (1483 words)


    Dr Juliet A. Gerrard and Dr Siân E. Fayle
    Grain Foods Research
    Crop & Food Research Ltd
    Christchurch, New Zealand

    When leaving your local hot bread shop with a cream donut or a Danish pastry, have you ever stopped to consider the chemical reactions that take place and transform an innocuous white powder - flour- into the delicacy that you are about to eat? 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. This article is concerned with some recent research into the chemistry that takes place during the manufacture of bread and related foods.

    If you have ever made bread in your kitchen, you will have found 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 focused 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?

    Product quality is often associated with the protein component of flours, generally known as gluten. Gluten proteins contain thiol groups that are known to react with oxidising agents. The effect of oxidising improvers has, therefore, generally been attributed to the reaction of the improver with the thiol groups of the protein - this is shown in Figure 1. The important thing to notice is that the product of the reaction contains protein crosslinks . 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.

    Ascorbic acid (vitamin C) is a common food additive with considerable consumer acceptance and has been used as a flour improver for many years. It remains present in most of the commercially available flour improving formulations. Unlike the improvers discussed above, ascorbic acid is not an oxidising agent. How does it work?When mixed with the oxygen in the air, ascorbic acid is converted to dehydroascorbic acid, or DHA. Unlike ascorbic acid, DHA is an oxidising agent. It has therefore been assumed that ascorbic acid is converted to DHA during mixing, and that DHA acts by the mechanism described above for oxidising improvers, and crosslinks gluten proteins. It has been shown that if ascorbic acid is converted to DHA in a test tube, and then added to the dough, the improving effect still functions. This is summarised in Figure 2. One of the problems with this process is its reversibility. The commercial baking process therefore takes place in an oxidising environment, with intensive mixing and oxidising improvers present to ensure that the crosslinks remain intact.

    DHA is a very reactive molecule that could get involved with lots of other chemistry in a system as complicated as dough. This study focused on the protein component of a flour and asked the questions: does DHA react with proteins in any other ways, and, if so, do these reactions give us any insights regarding the action of ascorbic acid as a flour improver?

    Tracking chemical reactions in a dough is very difficult, so we modelled the reactions of DHA and gluten proteins in a test tube. We developed a method to study the chemical reactions of proteins that lead to crosslinking. Since the crosslinking reaction leads to changes in the size of the protein, the products of the reaction were studied using a technique that separates proteins according to molecular size, for example by using an electric current to move the proteins through a material that acts like a molecular 'sieve'. This method is known as electrophoresis.

    Figure 3 shows a typical result. The photograph on the left shows protein that has been 'cooked' in the absence of DHA, and then put through the 'sieve'. Samples removed at various times are all the same size, showing that no crosslinking reactions have occurred. The photograph on the right shows that a very different pattern results when protein is reacted with DHA. The size of the molecules increases with time. This is consistent with the proposed mechanism of DHA acting as a protein crosslinking reagent. 

     We also made the important discovery that the same pattern was found whether or not the thiol groups (shown in Figure 2) had been removed so that they couldn't react.

    We concluded that in addition to the proposed mechanism involving the thiol groups (Figure 2), other chemistry occurs that produces stable protein crosslinks. A very detailed analysis of the reaction lead us to propose a new mechanism, represented in Figure 4. This mechanism invokes the same sort of chemistry that is known to occur during the Maillard, or browning reaction, which is responsible for the colour, flavour and aroma of bread. We believe that these reactions are taking place, in addition to those shown in Figure 2.

     Whilst the molecular details of the crosslinking process may seem somewhat esoteric, they contain an important conclusion for the baking industry: flour improvement may be achieved without forming crosslinks between thiol groups.

    SO WHAT?
    Since any protein crosslink might improve the performance of a flour we explored other possible flour improvers that could potentially crosslink gluten proteins, but . unlike potassium bromate and ascorbic acid, were not dependent on oxidation. Top of the list of candidates was an enzyme, transglutaminase (TGA), that was known to catalyse protein crosslinking and was potentially suitable for use in the baking industry. The reaction that TGA catalyses is shown in Figure 5. Notice the similarity to the chemistry that we proposed in Figure 4.

    The answer to this question was an emphatic, yes! This research has led to an extensive study of the effect of TGA as a flour improver, with very exciting results. The enzyme improver has a dramatic effect on pastry and croissants, as is shown in Figures 6 and 7, where the only difference between the two samples is the addition of TGA to the flour.

    TGA 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, as shown in Figure 7.

    The commercial potential of TGA 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. Such improvers are not dependent on oxidation to take effect, and therefore represent a route to by-passing an important technological problem for the industry, and result in cheaper, more efficient breadmaking practices.ACKNOWLEDGEMENTS:The more fundamental aspects of this work were funded by the Foundation for Research, Science and Technology (FfRST) and the more applied aspects by the Baking Industry Research Trust. 

    Juliet was born and educated in England, completing her undergraduate and doctoral degrees at Brasenose College, Oxford. She left Oxford in 1992 to take up a position in the Grain Foods Research Unit at Crop & Food Research Ltd, Christchurch, New Zealand. Her research interests have focused on the structure and function of proteins, and the applications of this science in a variety of spheres. Juliet has recently taken up a lectureship in the Department of Plant and Microbial Sciences, University of Canterbury, Christchurch, New Zealand.

    Siân was born and educated in Christchurch, and completed her undergraduate study at the University of Canterbury. This article describes aspects of her doctoral research which she completed under Juliet's supervision. Siân is continuing her work on the Maillard reaction with a postdoctoral position in the Department of Food Science and Technology, University of Reading, UK.

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