Читать книгу Whole Grains and Health - Группа авторов - Страница 46

The structure of cereal foods has for a long time been recognized as a parameter of importance for health benefits of whole grain foods that was pointed out among others by Frølich and Åman (2013). Studying flours comprising whole grains, cracked grains, course and fine whole grain wheat flour, it was found that the smaller the particle size of the food, the lower the satiety rating (Holt and Miller (1994). Also, glycemic responses are highly dependent on particle size (Tosh and Shu 2015).

Whole grain wheat bread, but not whole grain wheat pasta, has been shown to reduce appetite measures compared to refined wheat bread (Kristensen et al. 2010). This study indicates that the complexity and the interaction between the cereal source and the processing need to be raveled. Later studies have shown that preserving the natural initial fibrous network more or less intact, especially in wheat, rye, oats and barley kernels seems to be of importance for the physiological responses like satiety. Also formed compact structures, like in pasta and dumplings, can give positive responses on the same parameter (Riccardi et al. 2003).

Amount, molecular weight distribution, structure and conformation of fermentable dietary fibre are of importance for the physiological effects in the intestine (Poutanen et al. 2017). The solubility of the dietary fibre varies greatly between whole grain cereals and is as high as 37 % for rye but lower for the other cereals (Andersson et al. 2009). During wet processing, such as baking, endogenous β‐glucan degrading enzymes may reduce the molecular weight of the polymer. This will lead to changed properties such as reduced viscosity and modified gelling properties. Similar modifications may take place with extractable arabinoxylans, but this polymer is more resistant to degradation/fermentation than β‐glucan maybe due to a more complex structure requiring several enzymatic activities. It has been shown in a human study that a lower average molecular weight of the β‐glucan is less effective in reducing the cholesterol concentrations compared to more inert β‐glucan present in oats (Wolever et al. 2010). A scientific opinion by the European Food Safety Authority (EFSA) also states that oat β‐glucan may be degraded during purification and manufacturing of foods, affecting considerably its physiochemical properties (EFSA 2010). Consequently, the cholesterol‐lowering effect of oat β‐glucan in food may be weakened or may even disappear during processing.

Resistant starch is naturally present in some foods with intact botanical structures, like in intact barley and rye kernels, but can also be formed during heat treatment. During hydrothermal processing, resistant starch can be formed due to crystallization of amylose, resulting in a higher dietary fibre content in the cereal foods. It has, for example, been shown that preparation of whole grain rye flour porridge will increase the content of resistant starch compared to the raw ingredients (Rakha et al. 2011). Whole grain cereals, especially rye, are a rich source of fructan, but the content of fructan may be decreased during breadmaking. Also, sourdough baking reduces the fructan content, up to 62% (Andersson et al. 2009). Processes such as germination and fermentation will have a large impact on phytochemicals in grains, which may have consequences for the health effects of the grains (Koistinen and Hanhineva 2017; Singh and Sharma 2017; Fardet 2015). Whole grain foods are valuable sources of minerals. A high content of phytate in these products has been considered a factor for limited bioavailability of these important nutrients (Hunt 2003). Degradation of phytate may, however, result in an increased bioavailability of the minerals (Schlemmer et al. 2009). This could be done during food processing like soaking, germination, malting and fermentation. The acidity of the dough during breadmaking is of great importance for phytate degradation, both during scalding and sourdough fermentation.

To summarize, processing will highly influence that content and properties of certain bioactive components in whole grain foods. However, these changes will generally only have a smaller effect on total whole grain mass and therefore only have a minimal effect on whole grain definitions and contents.