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In 2017, the term prebiotic was redefined as “a substrate that is selectively used by host microorganisms that confer a health benefit” [10]. Prebiotics are often associated with carbohydrates of low molecular weight which are not digested in the human gastrointestinal tract and could positively improve the activity and composition of the microbiota of the intestine. However, the more recently definition of prebiotics opens opportunities to other compounds, such as polyunsaturated fatty acid, phytochemicals, linoleic acid, and phenolic compounds. Furthermore, it allows the utilization of prebiotics in many parts of the body besides the gastrointestinal tract and also other products, such as feed for poultry, aquaculture, and livestock [10]. In 2018, a group of scientists met again to analyse the advances in the prebiotics field, focusing on topics that affect research methodology, functionality, and geographical impacts [16]. Prebiotics are commonly highlighted in the studies, but the mechanisms of action and the health benefits need further evaluations [17].

Non-digestible carbohydrates, such as oligosaccharides, such as galactooligosaccharides (GOS), fructooligosaccharides (FOS), isomaltooligosaccharides (IMO), xylooligosaccharides (XOS), raffinose oligosaccharides (RFOs), mannan oligosaccharides (MOS), arabinoxylan oligosaccharides (AXOS), inulin, lactulose, and others, and polysaccharides (pectin, resistant starch, and dextrin) are considered food components with prebiotic properties (Table 2.2) [12].

The prebiotics with established health effects are separated into three groups: I) oligosaccharides (GOS, inulin, FOS, IMO, XOS, isomaltulose, RFOS, among others), II) polyols (xylitol, lactulose, lactitol, mannitol), and III) fibers (dextrins, cellulose, β-glucans, pectins) [18]. They have a diversity of chemical compositions, and the differences in the units (monosaccharides) and the type of glycosidic bonds in the non-digestible carbohydrates enable the prebiotics to be classified in many classes of oli-gossacharides [19].

The intestinal microbiota ferments the prebiotic compounds and can produce short-chain fatty acids (SCFA), such as acetate, propionate, and butyrate. These compounds can enhance many physiological effects, such as the function of the intestine, the absorption of minerals, the regulation of the metabolism of glucose and lipids, and also could reduce the risk of colon cancer [10]. Some health effects of orally administered combinations of a prebiotic substrate are reported in Figure 2.1 [10]. The consumption of prebiotics can favor the increase in the counts of beneficial bacteria and hinder the establishment of pathogenic bacteria, reducing the risk allergies and infections [18]. There are convincing and reproducible results in the literature of studies on animals that have consumed prebiotics demonstrating effectiveness in the reduction of risk or treatment of many diseases, e.g., irritable bowel syndrome, colon cancer, obesity, cardiovascular diseases and type 2 diabetes [16].

Table 2.2 Prebiotic substances, their source, and structure. Adapted from Sako and Tanaka [20].

Designation	Source	Structure
Lactulose	Lactose
Galactooligosaccharides	Lactose
Fructooligosaccharides	Sucrose
Inulin-type fructans	Inulin
Raffinose	Beet
Soy oligosaccharides	Soy extract
Xylooligosaccharides	Xylan
Chitin oligosaccharides	Chitin
Lactosaccharose	Lactose, sucrose
Isomaltooligosaccharides	Starch

Fru, fructose; Gal, galactose; Xyl, xylose; Glc, glucose; Fuc, fucose; GlcNAc, N-acetylglucosamine; NeuAc, sialic acid.

Among the most studied prebiotics, lactulose, galactooligosaccharides, fructooligosaccharides, and inulin can be highlighted. Lactulose is a disaccharide produced synthetically through the chemical isomerization of lactose in alkaline conditions. Lactulose also appears naturally in milk, is not metabolized in the human digestive system, and is preferably used by lactobacilli and bifidobacteria [20]. Lactulose proved to be a prebiotic compound when 10 g was consumed daily by healthy adults, resulting in increases in the bifidobacterial counts, and decreases in clostridia counts [21]. Furthermore, the consumption of foods with lactulose (4 g), Ca (300 mg) and Mg (150 mg) by healthy adults (n = 24) resulted in increased absorption of magnesium and calcium [22]. The improvements in Ca absorption was also reported in postmenopausal women after consumption of lactulose for 9 days [23].

Figure 2.1 Health effects of orally administered prebiotic substrates. Adapted from Gibson et al. [10].

Galactooligosaccharides are naturally present in milk of several mammals and are produced industrially through the enzymatic synthesis of lactose. They are commonly used in infant formulas to replace the bifido-genic effects associated with the oligosaccharides of human milk [24]. The potential of galactooligosaccharides has been documented in studies with humans and they show improvement in constipation, reduction of harmful enzymatic activities, reduction of cancer incidence, stimulation of bone mineralization, and reduction of secondary bile acid production [20].

Fructooligosaccharides and inulin stand out as a prebiotic functional component in food applications. Thus, fructooligosaccharides are produced in two different ways, through the partial hydrolysis of fructose polymers of vegetable origin or the transfer of the fructose portion to sucrose [20]. Fructose polymers, such as inulin, occur naturally in several vegetables and fruits. Inulin occurs as a natural reserve of carbohydrates present in plants of the Asteraceae family and is industrially extracted mainly from the species Cichorium intybus (chicory roots) [25]. The inulin degree of polymerization (DP) is in the range of 10-60, consisting of fructose chains that end mainly with a glucose residue [26]. The oligofructose are produced by a partial enzymatic hydrolysis of inulin with endoinulinase, which, therefore, has a lower DP than inulin, ranging from 2 to 8 [27]. Fructooligosaccharides are produced using sucrose as substrate and the transfructosylation reaction by β-fructosylfuranosidase, which is produced by Aspergillus spp. FOS have DP of 3-5, a terminal glucose residue in each molecule, and, therefore, are non-reducing carbohydrates. They have similar stability to sucrose at neutral pH, and are resistant to high temperatures (< 150 ^oC) [28]. The bifidogenic effect, the reduction of colon pH, the reduction of pathogenic bacteria, the reduction of putrefactive compounds, and the improvement of constipation by fructooligosaccharides have been reported in clinical studies [20]. Furthermore, improvements on the metabolism of lipids and absorption of minerals have been reported in studies with animals. In clinical studies, the improvements in calcium absorption have been confirmed, but the effects on the metabolism of lipids were inconclusive [20].

Soy oligosaccharides are composed of stachyose, raffinoses, glucose, sucrose, and fructose, and have been isolated from soy extract [29]. The stachyose and raffinose contents in soy oligosaccharides are generally 24 and 8%, respectively, while the glucose, sucrose, and fructose content are 55%. Raffinose, in a pure form, is commercially available and produced using beet syrup [30]. Bifidobacterium strains usually grow in media containing soy oligosaccharides, stachyose or raffinose as unique carbon source, as they generally have α-galactosidase activity, which hydrolyses oligosaccharides [31]. Raffinose and soy oligosaccharides have been demonstrated bifidogenic properties in clinical studies with a daily effective dose of at least 0.5 oligosaccharides equivalent [20].

Xylooligosaccharides are xylose oligomers that show DP of 3-8 and are commercially obtained from xylan using partial enzymatic hydrolysis and endoxylanase [32]. Xylan is a type of hemicellulose and can be found on the cell walls of plant in association with pectin and cellulose. Xylooligosaccharides are obtained from plants with high concentrations of xylan, such as cotton seeds and bagasse [33]. Xylooligosaccharides can be fermented by Lactobacillus spp., Bifidobacterium spp., Peptostreptococcus and Bacteroides vulgatus. The bifidogenic effect has been demonstrated in humans where the minimum effective dose of xylooligosaccharides was 0.4 g per day. Stimulation of mineral absorption and relief from constipation have been reported in studies with rats and humans, respectively [20].

Chitin oligosaccharides are constituted by N-acetylglucosamine (GlcNAc) oligomers and can be produced using chitin from shrimp and crabs, which are subjected to a partial acid hydrolysis [34] or by using bacterial chitinase [35]. The consumption of chitin oligosaccharides is associated with improvements on the intestinal microbiota, and antimicrobial and immunomodulatory activities [36].

Maltooligosaccharides and isomaltooligosaccharides, palatinose oligomers, α-glycosyl saccharose, lactosaccharose, nigerooligosaccharides, gentiooligosaccharides, and chitosanoligosaccharides are other commercially available oligosaccharides. Although these compounds have not necessarily been used as prebiotics, they may have bifidogenic activity. Furthermore, prebiotic agents are possible fractions of oligosaccharides obtained from a partial hydrolysis of non-starch polysaccharides, such as acacia gum, guar gum, and wheat bran [20].

Recently, new sources are being explored in order to discover or isolate new prebiotic compounds [37]. For example, the consumption of a water extract of Hirsutella sinensis (medicinal mushroom) by mice fed a high-fat diet (HFD) was able to reduce inflammation, obesity, and insulin resistance. Furthermore, the consumption of a fraction of the high molecular weight polysaccharide (> 300 kDa) obtained from the water extract was able to reduce the body weight (50%), the metabolic endotoxemia, intestinal permeability, insulin resistance, and inflammation. At the same time, P. goldsteinii counts were increased, which suggest that the health effects may be associated to the mediation of the intestinal microbiota and the increase in the probiotic P. goldsteinii [37]. The effects were dependent on the sensitivity of the bacteria to neomycin [38]. Therefore, polysaccharides from the H. sinensis mushroom can therefore be used as a prebiotic in the reduction of risk of obesity and its complications [38].

Several studies are still being carried out to identify new sources of compounds with prebiotic properties. Cereal grains can be used as sources of prebiotic compounds, such as wheat, corn, oats, and barley [17]. Fructose and fructooligosaccharides can be obtained using reusable bioreactors and biocatalysts with immobilized inulinase [39]. Aloevera, due to its antibiotic activity, can be a source of prebiotic compounds. Its fructans can increase the populations of microorganisms in a higher rate compared to inulin [40]. Algae such as brown algae (Osmundea pinnatifida, Ecklonia radiate) and red algae (Gelidium sesquipedale) can have laminarin as prebiotic compounds, resulting in increases in the number of beneficial microorganisms in the intestinal communities [41]. Dark roasted coffee beans and ground coffee have oligosaccharides that can have prebiotic properties [42]. Stevia rebaudiana can be a source of fructooligosaccharides and inulin, mainly due to the high concentration of steviol glycosides in the leaves [17]. The continuous use of cashew powder (Anacardium occidentale L.) was associated with increases in the counts of Lactobacillus species, suggesting prebiotic properties [43].

In addition to naturally occurring sources, prebiotic compounds may also be synthesized by microorganisms, and many industries aim to design or synthesize value-added biocomposites in a sustainable manner [44]. In this sense, the technology of microbiological processes that deal with mixtures of different substrates is valuable, such as agro-industrial residues and corn flour in solid-state fermentation systems [12]. Furthermore, Penicillium oxalicum may is being used for producing inulinase, which may be used for obtaining inulin. Agave salmiana spp. is composed of agave-fructans, which can increase the counts of Lacticaseibacillus paracasei and Lacticaseibacillus casei [17]. These are some examples of research involving new prebiotic substance sources; however, next-generation prebiotic substances are in high profile today and are continually being updated.