Peter Shewry of Rothamsted Research and University of Reading and Alison Lovegrove and Till Pellny of Rothamsted Research investigate approaches to improving the contribution of bread and other wheat products to diet and health.
The role of wheat and bread in diet and health
Wheat is the major staple crop in temperate countries, with global production of about 700 million tonnes a year. A substantial proportion of this is consumed by humans, particularly after processing into breads, cakes, other baked goods, noodles and pasta. These wheat-based products are central to the diets of many countries; the range of product types is enormous and is strongly determined by historical and cultural factors. The ability to make acceptable products is highly dependent on the types of wheat, the growing conditions and the processing system. Consequently, the overwhelming interest in improving the quality of wheat for human consumption has been focused on processing quality. The high consumption of wheat in temperate countries results in a significant impact on the overall diet. For example, the National Diet and Nutrition Survey (NDNS) has reported that UK adults consume an average of 90g of bread a day (113g for men, 76g for women) [1], which contributes between 10 and 13% of their total energy intake and 17-21% of their total intake of carbohydrates, depending on age and gender. These significant contributions to energy and carbohydrate intakes are not surprising as starch alone accounts for about 70% of the whole wheat grain and for up to 80% of white flour. However, it is asurprise to many that wheat is also a major source of a range of other components which are essential or beneficial to health. For example, a study of bread consumption in the UK [2] reported that all types of bread (including rolls) contributed 12% of the adult daily intake of protein (8% from white, 2% from wholemeal), 20% of fibre (11% from white, 5% from wholemeal), 16% of iron (10% from white, 3% from wholemeal) and 11% of folates (vitamin B9) (6% from white, 2% from wholemeal). The same study and data from the NDNS also show significant contributions of breads to the daily intakes of other essential minerals (zinc, manganese, calcium, magnesium, zinc, copper and selenium) and other B vitamins (B1, B2, B3 and B6).
All of these components are important for diet and health but of particular interest is the contribution of bread to the intake of dietary fibre. Fibre is essential for human health and almost all of the fibre consumed by humans is derived from plant-based foods (with small amounts from fungal and algal sources). Cereals are major sources of fibre in the diet, contributing about 40% of the total intake in the UK, and have well established health benefits. Although the mechanisms of these benefits are not fully understood, the impact is clear. This is summarised by a recent report, [3] which presents the most detailed analysis of current evidence to date and concludes that ‘there is strong evidence that increased intakes of total dietary fibre, and particularly cereal fibre and wholegrain, are associated with a lower risk of cardio-metabolic disease and colo-rectal cancer.’ This conclusion indicates that increased consumption of breads and other wheat products would contribute to improved health outcomes for consumers, without the need for costly supplements or significant dietary changes. However, there is a major limitation to this strategy in that fibre and most other beneficial components (including minerals and B vitamins) are concentrated in the outer layers of the grain and the embryo (germ), which are removed when the grain is milled to produce white flour. Consequently, whereas wholegrain wheat contains about 11-15% dry weight of total dietary fibre, the fibre content of white flour is reduced to about a quarter of this (3 to 4% dry weight). Furthermore, despite the promotion of wholegrain products for their health benefits, the production of wholemeal flour remains below 7% of total flour production in the UK, with white flour for bread making being about 50% of the total (data from NABIN [4]). Changing the pattern of bread consumption by UK consumers to increase the proportion of wholemeal products will clearly be a significant challenge! There are two potential approaches to increasing the delivery of fibre and other beneficial components in breads for the UK market, with both posing challenges for wheat breeders and processors.
1. Improving the health benefits of white flour
The first option is to ‘improve’ the composition of white flour. Work carried out in our laboratory at Rothamsted and in collaboration with colleagues in other European countries has shown that there is significant variation in the contents of ‘beneficial components’ in white flour from different cultivars of wheat, including high yielding cultivars which have been grown commercially over the past few decades. Whereas the fibre content of whole wheat grain varies from about 11 to 15% of the dry weight, white flour contains only about 3-4% fibre. In both cases the major fibre component is arabinoxylan, a cellwall polysaccharide which accounts for about half of the total fibre in whole grain and about 70% in white flour. In addition, about 25% of the fibre fraction of white flour and up to 10% of whole grain fibre is β-glucan. This is related to the β-glucan fractions which are the major dietary fibre components present in seeds of oats and barley and have health benefits that are accepted by EFSA and the FDA for health claims. However, whereas the fibre in wheat bran is largely insoluble, a significant proportion of the fibre present in white flour is soluble in water. The contents of total fibre and soluble fibre in white flour both vary by over two-fold, with the proportion of soluble fibre ranging from about a fifth to half of the total. Soluble fibre is of particular interest asm it may confer viscosity to the food matrix during digestion, slowing digestion and nutrient absorption and hence reducing the glycaemic index. It may also be more actively fermented in the colon, when compared with insoluble fibre, resulting in higher production of beneficial short chain fatty acids. Although the weather conditions during cultivation affect grain composition, the influence on fibre appears to be less than on other grain components, with a high proportion of the variation in fibre being determined by genetic differences between the cultivars. This ‘high heritability’ means that increasing the fibre content should be achievable by conventional plant breeding.
We have also shown that increasing the content of total and soluble fibre in white flour does not necessarily affect the bread making properties (Figure 1). This is an important observation as it is crucial that increases in fibre content should not result in adverse effects on processing quality. In addition to exploiting variation in the content of fibre in modern wheat cultivars grown in the UK and Western Europe, we are also seeking wider variation in other types of wheat (Figure 2). Firstly, we have investigated lines adapted to other parts of the world, including modern cultivars and breeding lines. Secondly, we have carried out preliminary analyses of the Watkins collection of wheat landraces which are curated at the John Innes Centre in Norwich (UK) [5]. These lines were collected from farmers and markets in 34 countries in the 1920s and 1930s and therefore represent cultivated forms which have not undergone intensive selection as part of modern plant breeding. Our preliminary study showed that these lines had wider diversity in fibre content than modern cultivars and we are now exploiting this in collaboration with wheat breeders. Our third source of diversity is ‘historic’ wheat cultivars, which have been bred and grown commercially in the UK since 1790. About 40 such cultivars have been grown at Rothamsted over the past 2 years and these will be analysed for a range of grain components including dietary fibre.
2. Increasing the use of ‘high extraction’ and wholemeal flours
Many consumers find wholemeal wheat to be unattractive and unpalatable, due to the colour, flavour (bitterness) and texture (grittiness). However, it may be possible to achieve a significant increase in the content of fibre and other beneficial components by increasing the flour extraction rate by a limited amount with minimal effects on acceptability. The effect of increasing extraction rate has been demonstrated by a study of phenolic acids. This showed that the total phenolic acid content of flours was increased by a factor of 1.8 when the extraction rate was raised from 70% to 80% and by a factor of 4.2 when increased to 90% [6]. About 80% of the phenolic acids in wheat are bound to the dietary fibre fraction so effects of extraction rate on fibre content would also be expected.
Although the texture of fibre-rich milling fractions may be improved by ultrafine grinding, the colour and flavour are intrinsic characteristics of the major type of wheat which is used for bread making in the UK. This is called ‘red’ wheat due to the presence of a red pigment in the seed coat (Figure 3). This pigment is responsible for the colour and flavour of wholemeal flour and can be readily eliminated by plant breeders. However, when the pigment is eliminated, the grain becomes highly sensitive to sprouting while still in the ear if the weather is wet during the period leading up to harvest. This pre-harvest sprouting’ reduces the quality for bread making and is a major problem in the UK and many other wheat-producing countries. Hence, the range of cultivation of ‘white’ wheat is severely restricted and the cost correspondingly high. Intensive wheat breeding has failed to break the association between red colour and susceptibility to sprouting. However, the recent enzymes may allow the use of modern molecular approaches to block the pathway at an intermediate stage, preventing pigment formation while retaining the characteristics for inhibition of sprouting.
Impact on cost and availability
An important factor to bear in mind is the impact of improvements in wheat composition on the cost and hence availability of the product. Bread is an attractive vehicle for delivering benefits precisely because it is widely consumed and available in a range of types at affordable prices. Hence, it is crucial that ‘improved’ products should be available to all and not only affordable by higher socio-economic groups. The wider consumption of wholemeal breads will almost certainly result in increased cost as higher intrinsic quality, particularly gluten quality, is required. This is because the bran fractions do not contain components that are functional in bread making and have to be ‘carried’ by the flour. Similarly, any additional processing steps, such as ultrafine grinding, will have impacts on costs. By contrast, increasing the fibre content of white flour should be achievable without effects on either the yield of the crop or the processing properties of the flour and should not require major changes in the pattern of consumption.
