Über die Autorin bzw. den Autor

Bob Rastall, University of Reading, UK.

Glenn Gibson, University of Reading, UK.

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Prebiotics: Development and Application is a one-volume account of the emerging area of prebiotics – non-digestible food ingredients that stimulate the growth and activity of beneficial bacteria in the colon. Prebiotics can be incorporated into many foodstuffs to improve the health of the individual and are receiving much commercial interest.

The book takes a broad view of prebiotics from the conceptual stage, definition, production, evaluation of individual food products and their effect on microbial flora, and their potential relation to diseases. Contents include:

• an introduction to the prebiotic concept and its development
• the synthesis and manufacture of prebiotics
• testing for prebiotic effects
• fructans, galactans, lactulose and other prebiotic forms
• prebiotic intervention for improving animal and human health
• sectors for prebiotic foods
• development and commercialisation issues
• future developments

Prebiotics: Development and Application is an essential guide to this emerging technology for researchers, students and practitioners of food science, biotechnology, nutrition, microbiology, dietary health and food processing.

Aus dem Klappentext

Prebiotics: Development and Application is a one-volume account of the emerging area of prebiotics – non-digestible food ingredients that stimulate the growth and activity of beneficial bacteria in the colon. Prebiotics can be incorporated into many foodstuffs to improve the health of the individual and are receiving much commercial interest.

The book takes a broad view of prebiotics from the conceptual stage, definition, production, evaluation of individual food products and their effect on microbial flora, and their potential relation to diseases. Contents include:

• an introduction to the prebiotic concept and its development
• the synthesis and manufacture of prebiotics
• testing for prebiotic effects
• fructans, galactans, lactulose and other prebiotic forms
• prebiotic intervention for improving animal and human health
• sectors for prebiotic foods
• development and commercialisation issues
• future developments

Prebiotics: Development and Application is an essential guide to this emerging technology for researchers, students and practitioners of food science, biotechnology, nutrition, microbiology, dietary health and food processing.

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Prebiotics

John Wiley & Sons

Chapter One

Claire L. Vernazza, Bodun A. Rabiu and Glenn R. Gibson

1.1 Acquisition and Development of the Human Gut Flora

The human embryo is virtually sterile, but at birth microbial colonisation of the gastrointestinal tract occurs, with the neonate receiving an inoculum from the birth canal (Fuller, 1991; Zetterström et al., 1994). The microbial pattern that ensues depends on the method of delivery (Beritzoglou, 1997; Salminen et al., 1998a) and hygiene precautions associated with parturition (Lundequist et al., 1985). In addition to characteristic vaginal flora such as lactobacilli, yeast, streptococci, staphylococci and Escherichia coli, the neonate is also likely to come into contact with faecal microorganisms and skin bacteria during birth (Fuller, 1991). Furthermore, inoculation from the general environment and other external contacts may also be significant, especially during Caesarean delivery (Beritzoglou, 1997; Gronlund et al., 1999). During the acquisition period, some bacteria transiently colonise the gut whilst others survive and grow to form the indigenous microflora. Consequently, the neonatal gut experiences a rapid succession of microfloral components in the first days to months of development, selected for, initially, by luminal redox potential (Eh) but more frequently reported as being due to the feeding regime that follows birth (Zetterstrom et al., 1994). Initial colonisers utilise any available oxygen, usually by 48 h, creating an environment sufficiently reduced to allow succession by obligate anaerobes, mainly those belonging to the bifidobacteria, bacteroides and clostridia groups. At this stage, it appears that feeding methods have a significant influence on the relative proportions of bacteria that establish in the infant gut. Historically, breast-fed infants are thought to have relatively higher proportions of bifidobacteria than formula-fed babies of the same age, who possess a more complex composition (Fuller, 1991). Such purported differences have been linked with a lower risk of gastrointestinal, respiratory and urinary tract infections in breast-fed infants (Kunz and Rudloff, 1993). However, as the nature of commercial feeds has altered in recent times, the bifidobacterial predominance seen during breast feeding is less definitive. Nevertheless, such observations demonstrate the ability of diet to influence the gut microbiota composition and the possibilities for influencing health as a result. This has formed the basis for dietary intervention procedures that are extremely popular today (see later).

By the end of weaning there is a drop in the frequency of bifidobacteria. With the introduction of solid foods and by about 2 years after birth, infants start to adopt microflora profiles in proportions that approximate to those seen in adults (Fuller, 1991). The populations then seem to be relatively stable (>99 % anaerobic), aside for perturbations by diet and habit, until advanced ages when a significant decline in bifidobacteria, plus increases in clostridia and enterobacteriaceae are reported (Mitsuoka, 1990).

1.2 The Human Gastrointestinal Tract and its Microflora

Microorganisms occur along the whole length of the human alimentary tract with population numbers and species distribution characteristic of particular regions of the gut (Macfarlane et al., 1997). After the mouth, colonisation is markedly influenced, in part by luminal pH, and by the progressively slower transit of food materials towards the colon. The movement of digesta through the stomach and small intestine is rapid (ca. 4–6 h), when compared with a typical colonic transit time of around 48–70 h for adults (Macfarlane and Gibson, 1994). This allows the establishment of a complex and relatively stable bacterial community in the large intestine (Table 1.1). The near neutral pH and the relatively low absorptive state of the colon further encourages extensive microbial colonisation and growth (Macfarlane et al., 1997; O'Sullivan, 1996).

The human large intestine consists of the caecum, ascending colon, transverse colon, descending colon, sigmoid colon and rectum (Macfarlane and Macfarlane, 1997) (Figure 1.1). Through the microflora, the colon is capable of complex hydrolyticdigestive functions (Cummings and Macfarlane, 1991). This involves the breakdown of dietary components, principally complex carbohydrates, but also some proteins, that are not hydrolysed nor absorbed in the upper digestive tract (Macfarlane et al., 1992). Carbohydrate availability subsequently diminishes as dietary residues pass from the proximal colon to the transverse and distal bowel.

For persons living on Western-style diets, the microbial biomass makes up over 50 % of colonic contents. There are more than 500 different culturable species of indigenous bacteria present in the adult large intestine comprising around 10¹² bacteria per gram dry weight (Moore et al., 1978; Simon and Gorbach, 1984). A summary of the principal bacterial groups present is shown in Table 1.2.

In very general terms, intestinal bacteria can be divided on the basis of whether they can exert health promoting, benign or potentially harmful activities in their host (Gibson and Roberfroid, 1995). The most obvious pathogens are strains of E. coli and clostridia. Pathogenic effects include diarrhoeal infections and putrefaction whereas beneficial aspects may be derived simply by improved the digestion/absorption of essential nutrients. This leads towards a consideration of factors that may influence the flora composition in a manner than can impact upon health.

The multiplicity of substrates is probably the single most important determinant for dynamics and stability of species existing in the large bowel (Gibson and Collins, 1999). Whilst these are mainly produced by dietary residues, there is appreciable contribution from host secretions like mucins. The colonic microflora derive substrates for growth from the diet (e.g. nondigestible oligosaccharides, dietary fibre, undigested protein reaching the colon) and from endogenous sources such as mucin, the main glycoprotein constituent of the mucus which lines the walls of the gastrointestinal tract (Rowland and Wise, 1985). The vast majority of bacteria in the colon are strict anaerobes and thus derive energy from fermentation (Macfarlane and McBain, 1999). The two main fermentative substrates of dietary origin are nondigestible carbohydrates (e.g. resistant starch, nonstarch polysaccharides and fibres of plant origin and nondigestible oligosaccharides) and protein which escapes digestion in the small intestine. Of these, carbohydrate fermentation is more energetically favourable, leading to a gradient of substrate utilisation spatially through the colon (Macfarlane et al., 1992). The proximal colon is a saccharolytic environment with the majority of carbohydrate entering the colon being fermented in this region. As digesta moves through towards the distal colon, carbohydrate availability decreases and protein and amino acids become a more dominant metabolic energy source for bacteria in the distal colon (Macfarlane et al., 1992). Overall however, the principal substrates for bacterial growth are dietary carbohydrates. It has been estimated that about 10 to 60 g per day of dietary...