PART III PROCESS SAFETY
4. Prebiotics: Role as Bioactive Nutrients
fatty acids (SCFA) and gases and c) increase in faecal bacterial mass and faecal volume (Roberfroid, 1993).
Accumulating scientific evidence suggests that from the non-digestible carbo- hydrates specific oligosaccharides and polysaccharides can be selectively metabolised by the “beneficial” bacteria belonging to the genera of Lactobacillus and Bifidobacterium, thus resulting in an increase in their numbers and activity in the gut.
Such carbohydrates are known in the scientific and commercial literature with the term “prebiotics” and could be regarded as nutritional tools for the beneficial modulation of the composition and metabolic activity of the host’s intestinal microflora.
From the three principal macronutrients of foods (i.e. proteins, carbohydrates and fats) only carbohydrates are currently represented in the prebiotic concept.
However, as it has been the case with carbohydrates, the potential of proteins and fats as well as of their respective oligomers to modulate the composition and metabolic activities of the intestinal microflora is under examination. Recently the antimicrobial and bifidogenic properties of major milk proteins (e.g. lactofer- rin, κ-casein glycomacropeptide) have been reviewed (Mountzouris et al., 2002a).
Prebiotics are non-viable food/feed ingredients and have been defined as:
“non-digestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon, and thus improve host health” (Gibson and Roberfroid, 1995).
Therefore they should not get confused with probiotics that have been defined as live microbial feed supplements that are beneficial to health (Salminen et al., 1998).
According to the concept of prebiotics, introduced by Gibson and Roberfroid in 1995, a food ingredient in order to be classified as prebiotic it should: a) be nei- ther hydrolysed or absorbed in the upper part of the gastrointestinal tract, b) be a selective substrate for one or a limited number of beneficial bacteria commensal to the colon, which are stimulated to grow and/or are metabolically activated, c) consequently, be able to alter the colonic flora in favour of a healthier composi- tion and d) induce luminal or systemic effects that are beneficial to host health.
Therefore prebiotics could also act as dietary bioactive components beyond their role as selective substrates for the beneficial (e.g. probiotic) bacteria in the colon.
4.1. Origin and manufacturing processes
Prebiotic ingredients exist in nature in a variety of foods such as garlic, onions, leeks, asparagus, chicory, artichokes, bananas, human milk, wheat, maize and soy and are therefore part of a normal diet. Commercially available carbohydrates that fall in the concept of prebiotics are shown in Table 3.
Generally oligosaccharides of non-starch origin dominate the prebiotic arena with the exception of isomaltooligosaccharides that also get partly digested in the
vance to Food Safety143
Prebiotic Name Chemical formula Manufacturing process
Trans-galactooligosaccharides α-D-Glu-(14)[β-D-Gal-(16η′14 )-]n = 2-5 Enzymatic synthesis from lactose
Inulin β-D-Fru-(12)-[β-D-Fru-(12)-]n = 1->60 Extraction from chicory, leeks and artichokes
α-D-Glu-(12)-[β-D-Fru-(12)-]n = 2->60
Lactulose β-D-Gal-(14)-β-D-Fru Chemical modification of lactose
Lactosucrose β-D-Gal-(14)-α-D-Glu-(12)-β-D-Fru Enzymatic synthesis from lactose and sucrose
Fructooligosaccharides or oligofructose α-D-Glu-(12)-[β-D-Fru-(12)-]n = 2-4 Enzymatic synthesis from sucrose α-D-Glu-(12)-[β-D-Fru-(12)-]n = 2-9 Enzymatic hydrolysis of inulin β-D-Fru-(12)-[β-D-Fru-(12)-]n = 1-9
Isomaltooligosaccharides [α-D-Glu-(16)-]n = 2-5 Enzymatic hydrolysis & subsequent
transglycosylation of starch
Xylooligosaccharides [β-D-Xyl-(14)-]n = 2-9 Enzymatic hydrolysis of xylan
Soybean oligosaccharides [α-D-Gal-(16)-]n= 1-2α-D-Glu-(12)-β-D-Fru Extraction from soybean Glu: Glucose, Gal: Galactose, Fru: Fructose, Xyl: Xylose
upper gut. Higher molecular weight molecules of the isomaltose series, termed oligodextrans that have been manufactured by controlled dextran hydrolysis (Mountzouris et al., 2002b) are expected to display higher resistance to digestion in the upper gut whilst promoting saccharolytic bacterial metabolism in the dis- tal colon (Olano-Martin et al., 2000).
Prebiotic polysaccharides include inulin (i.e. storage carbohydrate consisting mainly from fructose moieties found in certain plants such as chicory) which is one of the most widely studied prebiotics (Gibson et al., 1995; Kolida et al., 2002) and high-amylose starch (Brown et al., 1997; Wang et al., 1999) that has not been included in Table 3.
Prebiotics can be produced by (Sako et al., 1999):
➢extraction from their natural sources (e.g. inulin from chicory and leeks, soy- bean oligosaccharides from soybean whey and mannanoligosaccharides from yeast cell wall).
➢enzymatic processes either as a result of enzymatic synthesis (e.g. oligofructose from sucrose and trans-galactooligosaccharides from lactose) or as a result of enzymatic hydrolysis (e.g. oligofructose from inulin and xylooligosaccharides from xylan).
➢chemical modification (e.g. lactulose from the isomerisation of the glucose moiety in lactose to fructose). Lactulose is used therapeutically for the treat- ment of constipation and hepatic encephalopathy and is therefore not used in food formulation despite its prebiotic potential.
Average prebiotic and in particular fructan consumption through the normal diet has been evaluated to amount to several grams per day (Van Loo et al., 1995).
They are legally classified as food or food ingredients in all EU countries and therefore can be used without specific limitations as ingredients in foods and drinks (Franck, 2002).
Whilst all of the prebiotics described in Table 3 are being used as food ingredients by the food industry in Japan, only oligofructose, inulin and trans- galactooligosaccharides are used by the European food industry. This could be due to the fact that for the above three prebiotics there is accumulating scientific proof of their function in vivo in animals and humans that could enable food man- ufacturers make various claims for their products. In addition they are produced in large quantities by companies based in Europe which provide a high level of technical support for food applications involving their products.
4.2. Physicochemical characteristics and properties
Commercially available prebiotics are supplied in the form of powder and/or solutions of variable purity and concentration level.
The physicochemical composition of the prebiotic carbohydrates is important not only for their effects in gastrointestinal physiology and function but also for their application in food science.
For example factors such as the type of sugar moieties that make the building blocks, the type of glycosidic linkages present, the structural arrangement and the molecular weight of the prebiotic molecule, determine their selective or non- selective fermentation by the intestinal microflora. In addition, these factors have an effect on the extent and the rate that prebiotics get metabolised by the bacte- ria. This in turn results in the development of different profiles of saccharolytic activity along the colon (Olano-Martin et al., 2000).
The differences in the enzymatic machinery found among bacteria of different genera, even among species of the same genus, could explain the large variation seen in the ability of bacteria to use different prebiotics as substrates for their growth. Furthermore, the physicochemical composition and properties of prebi- otic carbohydrates also affects the final products produced by the microbial fer- mentation (Cummings and Macfarlane, 1991).
Some simple sugars and oligosaccharides are known to be potent inhibitors of bacterial adhesion to epithelial cells by acting as receptor analogues to mucosal adhesion molecules (Kunz and Rudloff, 1993; Peterson et al., 1998; Kunz, 1998, Naugton et al., 2001; Lee and Puong, 2002). For example among the human milk oligosaccharides lacto-N-tetraose and lacto-N-neotetraose act as cell surface receptors for Streptococcus pneumoniae, fucosylated oligosaccharides are recep- tors for E. coli and sialated oligosaccharides are recognised receptor sites for influenza viruses A,B and C, Campylobacter pylori and Mycoplasma pneumo- niae (Kunz and Rudloff, 1993).
Finally the functionality of the prebiotic molecules in food systems will depend on their physicochemical properties such as viscosity, solubility, solution stability, crystallisation, freezing point depression sweetness, humectant proper- ties, heat and acid resistance. Therefore development of functional foods with prebiotics would have to account for the above in the research and development effort.
4.3. Legislation and safety issues linked with the use of prebiotics in foods
Prebiotics can be naturally found in many foods and are produced commercially by natural ingredients (Table 3). They are classified as food ingredients and their consumption is generally considered as safe with the restriction that an abuse in consumption could cause intestinal discomfort (Hammes and Hertel, 2002).
Food products that contain prebiotics must comply with the general legislation that governs the labelling requirements and claims for foods. Although all aspects of food labelling are to a very great extent harmonised by European legislation, there is currently no consistent legal framework regarding claims and particularly health claims within the European Union (Gibson et al., 2000). As a result, every product should comply in this respect with the national law of each European country that is distributed. This poses a serious drawback for the marketing of functional food products that relies heavily on claims linking consumption of the products with enhanced physiological functions or reduction of disease risk.
However development of such a legal framework is expected as a consequence of a recent proposal from the European Commission 2003/0165 (COD) on nutri- tion and health claims made on foods.
Japan is the only country in the world that has, since 1991, developed a legal framework that permits the commercialisation of selected functional foods under the term FOSHU “Food for Specified Health Use”. The legislation for FOSHU allows the presentation of a health claim for each approved product.
4.4. Physiological effects and health benefits
The potential health benefits linked with prebiotic consumption stem from the direct, indirect and systemic effects that these ingredients have on gastrointesti- nal physiology and function.
The direct effects could be attributed to the prebiotic function as soluble dietary fibre that promotes better intestinal function and relief of constipation (Roberfroid, 1993; Teuri and Copella, 1998, Van Loo et al., 1999). In addition, prebiotics contribute to less energy production since they are not digested in the upper gut but get fermented in the colon. In healthy humans fructo oligosaccha- rides were found to have an energy value of 9.5 kJ/g (i.e. about half that of sucrose) (Molis et al., 1996). Due to their non-digestible nature, prebiotic ingre- dients could contribute to lower glycaemia compared with equal amounts of digestible carbohydrates and therefore their inclusion could be considered in diets for diabetics.
Some prebiotic oligosaccharides could prevent attachment of pathogenic bac- teria to the intestinal enterocytes by blocking the bacterial lectins, thus acting as anti-adhesive (Kunz, 1998; Naugton et al., 2001; Lee and Puong, 2002).
The indirect effects that prebiotics have on the host result from the beneficial modulation of the composition and metabolic activities of the intestinal microflora in a way that it contributes to enhancement of the gut barrier function (Figure 1).
Research in humans and animals demonstrates that consumption of prebiotics stimulates and enhances the growth of probiotic bacteria in the intestinal microflora (Van Loo et al., 1999, Bouhnik et al., 1999). In humans the supple- mentation of diet with 10-15 g fructooligosaccharides or inulin was found ade- quate for a significant increase in Bifidobacterium (Bouhnik et al., 1999; Gibson et al., 1995).
Enhancement of the beneficial microflora results in stimulation of host’s immune system (Schley and Field, 2002), while inhibition of pathogens occurs as a result of competition for growth substrates and production of antimicrobial com- pounds, lactate and SCFA (Figure 1). Therefore, prebiotics represent an opportu- nity for the fortification of the indigenous microflora with its beneficial members through diet. This is of relevance to food safety since consumption of foods that have prebiotic ingredients could improve resistance to gastrointestinal infection.
There is accumulating evidence for a number of beneficial systemic effects for host’s health linked with prebiotic consumption. The most well researched
prebiotics so far are the fructans inulin and oligofructose. Results from research work under way, mainly in laboratory animals, indicate that prebiotics could have protective roles against: a) cardiovascular disease through the lowering of syn- thesis of VLDL cholesterol and blood triglycerides (Williams and Jackson, 2002), b) osteoporosis through the improvement of mineral calcium and magne- sium absorption (Scholz-Ahrens et al., 2001) and c) chemically induced cancer (Femia et al., 2002; Cherbut et al., 2003).
4.5. Future prebiotics with enhanced functionality
There is currently a need to increase our knowledge on the role of structure – function relationships for known and candidate prebiotics and to gather more sci- entific evidence to substantiate the potential health benefits of prebiotics. Once there is a better understanding of the above relationship, then “better” prebiotic molecules could be designed (Rastall and Maitin, 2002, Mountzouris et al., 2002b) and optimal combinations with other food ingredients could be consid- ered/developed.
Currently, a substantial amount of evidence clearly indicates that prebiotics improve gastrointestinal conditions (Arai, 2002). The prebiotics, as food ingredi- ents, should combine enhanced health function properties with good sensory and food technology properties.
Examples of desired properties for a prebiotic ingredient include: a) selective fermentation by beneficial bacteria, b) slow fermentability so that it persists and gets totally fermented to the distal colon, c) Inhibition of pathogen attachment to the gut wall, d) effective at low dosage, e) no side effects (e.g. flatulence, dis- comfort), f) good storage and self-life stability. Other desirable properties will depend on the food application in which the prebiotics will be used; that is, to heat and acid resistance viscosity, solubility, solution stability, crystallisation, freezing point depression sweetness and humectant properties.