Cereal grains are very important for human nutrition as they are usually the largest single component of the diet in energy terms, and mainly consist of starch, protein and non-starch polysaccharides (NSP), namely arabinoxylan (AX) and β-glucans. NSP’s are a major component of dietary fibre and are found in cereal endosperm cell walls, as well as the aleurone layer, the bran and the husk. The major nutritional properties of dietary fibre are linked to the extent of solubilisation (in reality including insoluble but swollen states). From a nutritional functionality viewpoint the soluble and insoluble forms of dietary fibre offer varying nutritional health advantages e.g.; promotion of beneficial microflora, the prevention of re-absorption of bile acids leading to lower blood cholesterol, and retardation of starch digestion leading to controlled glycemia. The risk of serious diseases like colorectal cancer, cardiovascular disease, and diabetes can be reduced through the long term consumption of a healthy diet incorporating adequate dietary fibre.
However, whilst these advantages of dietary fibre consumption are well established from a nutritional viewpoint, what is not known are the solubility and functional effects of cereal processing operations on AX and β-glucan. After all, cereal dietary fibre, particularly from the endosperm, is mainly consumed in a processed form. This thesis therefore reports a study of the effects of model food processes on three common food grains (wheat, rye, and hull-less barley), in order to establish structure-processing-nutrition relationships. The food processing conditions studied were fermentation 35˚C (dough), baking 200˚C (breads), extrusion pressures 6-18bar, and temperatures between 30˚C -130˚C (breakfast cereals) and “chemical”/boiling 100˚C cooking conditions using yellow alkaline noodles (YAN).
By examining and understanding the effects of various food processing conditions on the functionality and characteristics of dietary fibre, we will be able to gain a better understanding of how these food processes change the quantity, redistribution and composition of soluble and insoluble dietary fibre, and how and to what degree this affects the nutritional quality of finished foods.
However, in order to fully characterise the unprocessed and processed soluble (AX and β-glucan) and insoluble forms (cell wall) of dietary fibre, these fractions needed to be extracted, separated and purified whilst also maintaining their inherent nutritional properties. The extraction fractionation method that was developed to achieve this is detailed in the first experimental chapter 3 “Separation and Purification of Soluble Polymers and Cell Wall Fractions from Wheat, Rye and Hull-less Barley Endosperm Flours for Structure-Nutrition Studies”. The structural and microscopic characteristics of dietary fibre fractions obtained by this method, including soluble and insoluble AX and β-glucan fine structures are detailed in the second experimental chapter 4“Characterisation of soluble and Insoluble Cell Wall Fractions from Rye, Wheat and hull-less Barley Endosperm Flours”. In addition to the effects of food processing described in the fifth chapter, “The Effects of Food Processing Conditions on the Nutritional Functionality and Solubility of Wheat, Barley and Rye Endosperm Dietary Fibre”, the in vitro fermentation, short chain fatty acid (SCFA) end products, and gas kinetic properties of dietary fibre fractions before and after food processing are reported in chapter 6 “In Vitro Gas Kinetics of Soluble and Insoluble Processed and Unprocessed Dietary Fibre”.
Overall, the processed AX and β-glucan results show that the amounts of dietary fibre were not significantly affected. No major losses of β-glucan and arabinoxylans were found across the various forms of processed foods, except for the β-glucan found in the hull-less barley YAN broth, which gave losses of 22%. Solubilisation of AX and β-glucan from the cell walls of endosperm rye, wheat and hull-less barley occurred during cereal food processing. The most notable being approximately 19-25% insoluble dietary fibre (IDF) reduction from the cell wall in hull-less barley, rye and wheat, during bread baking and YAN production, and 22-29% in extruded food. Food processing may alter the phenolic ester bonding arrangements within the endosperm cell walls, thereby solubilising AX from the cell walls and increasing soluble AX amounts. Confocal images illustrate loosely held associations of β-glucan with the cell walls of processed foods, which is in contrast to the arabinoxylans which appear linked (possibly with ferulic acid) or more tightly held within the cell walls.
The in vitro fermentation results suggest that fermentation kinetics for either processed or non-processed dietary fibre were not significantly different. Therefore, processing of the endosperm dietary fibres and the resulting formation of a fused cell wall food matrix, does not seem to significantly impede microbial access to enzyme target sites of the more easily fermentable substrates, namely soluble AX and β-glucan. Fermentation end products (SCFA) were similar for the dietary fibre types (endosperm WEAX and cell wall) and process conditions. However, significant differences were observed between the varying cereal grains. Hull-less barley produced a slightly higher propionic acid level, wheat promoted a slightly higher acetic acid level, and rye a higher butyric acid production for both processed and non-processed dietary fibre. However, both the wheat and rye grains did produce more butyric acid than barley perhaps due to their higher amounts of AX. Therefore, it appears that differences in carbohydrate composition and structure within the cell walls of a grain play a more important role in fermentation kinetics and end product profiles than the conditions of food processing.